On LED Flashlights…

By Bryan Geyer

The innate connection between a flashlight and “Audio Tech
Talk” becomes apparent as soon as you try to inspect any part of your sound
system that lies beyond the front panel. In recognition, here’s some personal
guidance about the most vital tool you’ll ever need: A modern single-cell LED
flashlight.

First, learn why the light emitting diode (LED), a
semiconductor device that dates to the late 1960s, didn’t become abundant in
flashlights until some 30 years later. What a curious delay! What happened? And
what changed? Well, a modern “white light” LED won’t fire until it sees a
threshold bias of about 3.2 volts minimum, and 4 volts or more works better.
But a fresh single cell battery produces only ~ 1.2 to 1.7 volts, dependent on
its chemistry (see footnote*), so multiple batteries would be needed to
fire a solitary white light LED. Clearly, using more batteries than before
didn’t advance the state of flashlight design, so miniature incandescent lamps
endured. Then, barely before year 2k arrived, integrated circuit “boost chips”
appeared. These new silicon ICs could efficiently elevate the energy from a
single storage cell to the operating level needed to fire a white light LED.
The basic design concept was old, but implementing it in IC form was
breakthrough genius. The entire boost function, consisting of the surface-mount
chip plus some tiny passive peripherals, could now be packed into a bundle
smaller than the size of two stacked dimes. A practical single AA cell LED
flashlight was suddenly viable, and its launch assured immense advantage over
anything using an incandescent bulb. A lone LED could now provide more light
(with selectable lower levels), draw less current, offer high resistance to
shock and vibration, and assure decades (not hours) of maintenance-free service
life—all while selling at competitive mass-market prices.

Product development was intense, with many players engaged.
Some failed, some flourished, and several smart trans-Pacific sources became
dominant. There’s now a wide array of LED flashlights and multi-LED lanterns on
the market. Many designs serve niche applications, but my intent here is to
feature a basic everyday flashlight—a modestly priced model that’s appropriate
for general purpose use and practical in-pocket carry. A flashlight that…

…is short (≤ 3.1" long), skinny (≤ 0.75" diameter),
and uses only a single AA size cell.

…is blindingly bright at full output, with multiple lower
levels to conserve current drain.

…has long throw capability, with enough side spill to see the
borders of your path at night.

…is obvious in operation, with a single butt-end control
button for all switching functions.

…offers instinctive battery exchange: unscrew head and
replace AA cell, button end up.

…has solid mechanical integrity; built to withstand the
rigors of moderate abuse.

…if supplied with a pocket clip, such clip must be readily
removable and leave no evidence.

… meets the IP68 waterproof standard (up to 30 minutes at
depths of ≤ 2 meters).

…is cheap enough stash extras (toolboxes/cars/bikes/garage)
and give as gifts.

There aren’t a lot of candidates that meet all of my
objectives. Many come close, then fail the “side spill” criteria. That’s
because flashlight marketers always hype throw (range), and inexperienced
buyers focus on that feature; they haven’t yet grasped the importance of spill
(beam spread). The two parameters (throw and spill) are mutually exclusive;
more of one means less of the other. A very tight beam will deliver impressive
reach, but hot spot fixation invites personal risk when you can’t see the edges
of a narrow path from just one step away.

My pick as a good “everyday” single AA cell flashlight is the
Fenix E12 Version 2.0. It meets all of my criteria, and you can buy it
here: https://www.fenix-store.com,
or here: https://www.batteryjunction.com
The Fenix E12 V2.0 flashlight offers three selectable outputs: 5, 30, and 160
Lumens. It uses a cool white LED, and has an ultra-thin (3mm) optical front
lens—all very desirable. The lowest level is appropriate for checking your
watch in a darkened theater or finding a keyhole at night. The mid-level
setting, 30 Lumens, assures a practical balance between output and capacity.
It’s great for all of your “fix it” tasks, also for walking at night. The full
160 Lumen output provides a brilliant spotlight, but keep it brief! Apply only
in 10 to 20 second transient bursts. At this level, supply current is consumed
at a rapid rate, and there’s significant internal dissipation (heating). As
with many LED flashlights, these output levels are all electronically
regulated, so the lighting intensity stays constant; it won’t droop as the AA
cell ages. Full expiration becomes suddenly apparent; it’s dependent on the
drain that’s required for the level you’ve selected. When the supply current is
insufficient for a given level, the output will drop to the next lower level.
When it’s too low to maintain the regulated 5 Lumen minimum, the output drops
to a useless glow, then goes out.

The Fenix E12 V2.0 flashlight comes with an attached (by
compression) pocket clip that I dislike, so I trash it. This is easy to do with
the aid of pliers—just grab the clip and pull it off. It will readily detach
and leave no evidence that it was once mounted on the flashlight body. This
flashlight is so small and light that a pocket clip isn’t of practical benefit;
however, removing the clip will then expose a perfectly cylindrical body, and
round stuff that’s not restrained can roll off flat surfaces. If this annoys
you, consider mating a wrist strap to a rear lug. Straps that have a small
pigtail loop at the end will pass through the lug without impeding the control
button.

BATTERY OPTIONS: AA cells are readily available in the
variants (3) noted below*, but you’d do well to reject the use of
alkaline (manganese dioxide) LR6 cells. These conventional cells are very
popular, but too risky to merit informed consideration. When an alkaline AA
cell approaches expiration (output ≤ 0.7 volts), it will progressively
destabilize and form a caustic compound (KOH, potassium hydroxide) that will
penetrate the outer shell and attack anything that’s within reach. The
consequent damage can be severe. This is an inherent flaw—all alkaline cells
present this same inevitable hazard—so don’t use alkaline AA cells.
Instead, select either of the other options. I use the non-rechargeable
lithium-iron disulfide Class FR6 cells, e.g., Energizer’s “Ultimate”**.
Others prefer the rechargeable nickel metal-hydride (NiMH) Class HR6 cells.
Both options are safe, don’t leak, and work well. Non-rechargeable lithium AA
cells exhibit extended operating life, perform well in cold ambients (alkaline
cells don’t), weigh about 40% less than the alkaline equivalent, and can
tolerate long term (10 years) passive storage with minimal (5%) net
self-discharge. Buy in bulk, stow for use as needed.

* The nominal AA cell output voltage,
when fresh (or freshly charged), is…

…1.5 volt for a non-rechargeable alkaline (manganese dioxide)
IEC Class LR6 cell.

…1.2 volt for a rechargeable nickel metal-hydride IEC Class
HR6 cell.

…1.6 to 1.7 volt for a non-rechargeable lithium-iron
disulfide IEC Class FR6 cell, e.g., Energizer “Ultimate” AA cells.

IEC = International Electrotechnical Commission

 ** I buy on-line, from Battery Junction;
refer… https://www.batteryjunction.com/energizer-aa-ln91-24pk-battery.html

BG (December 2022)

My “Battery” Advice…

By Bryan Geyer

Re. “battery,” let’s
nail the definition first. A single AA or AAA voltaic storage cell is actually not
a “battery”; it’s a cell. A “battery” is a combination of two
or more cells. So the sign at your local hardware store that directs you to
the Energizer and Duracell AA and AAA “battery” display is
technically in error. (That’s excusable, but the prices they post are not. You
can do lots better at…https://www.batteryjunction.com.)

Yes, I realize that
ramblings about AA and AAA (size) DC storage cells is distinctly outside my
assigned specialty (audio nitty-gritty) here, but this is my 30th tech paper
for this site, so host John Puccio (he of the chocolate digressions) has kindly
allowed me some off-topic slack. The outcome of this liberty might well be more
pertinent for you than anything in my previous 29 posts, so I say hey,
listen up—this is stuff you need to know!

I’m an especially
heavy user of both AA and AAA voltaic storage cells. However, I have not
purchased any standard alkaline (LR6 class) AA or AAA cells since the late
1970s. Instead, I use only lithium (FR6 class) non-rechargeable AA and
AAA cells (aka L91 & L92 cells). I do so because these lithium iron disulfide
(LiFeS2) cells present a stack of compelling advantages over archaic alkaline
(manganese dioxide) cells, not the least of which is the fact that their
extended life generally makes them cost-competitive in all but a few benign
applications. The service life advantage of a lithium AA cell—when compared to
its alkaline equivalent—varies, dependent on how it will be utilized. In very
low drain service (e.g., a TV remote control) the lithium battery will outlast
an alkaline cell, but not by much. However, lithium longevity becomes
appreciable when used in higher drain service; e.g. in a flashlight or lantern,
or in photo strobes, RF transmitters, and in some portable audio equipment.
There’s a real 2X to 3X service life advantage, as compared to an alkaline
equivalent, in a great many common applications. For example, I’m seriously
into photo-macrography (close-ups) and also long lens wildlife photography
(mostly birds). These endeavors often involve using two or more electronic
strobes that can support rapid and repetitive high-power bursts, and the
related current drain is normally more than wimpy alkaline cells can sustain.
This is especially so in colder weather because alkaline cell output declines
rapidly when ambient temperatures approach +40˚F, whereas lithium AA or AAA
cells will still retain most of their native room-temp. punch.

While long service
life is nice, maybe more important is the fact that lithium L91 AA cells will
NEVER LEAK, even when discharged and left in your equipment for ≥ 5 years.
(Been there, done that.) Conversely, all alkaline cells (of any brand)
will eventually expel their noxious corrosive electrolyte (it’s KOH, potassium
hydroxide) when they’re at or near a depleted state.

Further, lithium AA
cells are ~ 40% lighter than alkaline equivalents, based on size AA comparison.
That’s a helpful benefit, especially when you consider packing sufficient
back-up spares.

The best place to buy
Energizer non-rechargeable AA-size lithium (L91 type) cells is Battery
Junction; see…https://www.batteryjunction.com/energizer-l91aa.html,
where they’re $1.60 ea. + tax and shipping. The current Ace Hardware “walk in”
price for Energizer Max alkaline AA cells (in an 8 pack) equates to
$1.25 each + tax.

FR6-class lithium
cells will retain ~ 94% of their capacity after 15+ years of storage, so it’s
feasible to maintain extensive backup stock without concern. Alkaline cells
degrade more rapidly. They’re specified to be storable for ~ 5 years if kept in
cool dry environs, and then be down by only ~ 10%. However, I think that’s a
rather idealistic projection.

The story is the same
for AAA size cells. Energizer’s lithium (FR6 class) AAA cells are known as type
L92, and their listing at Battery Junction is…https://www.batteryjunction.com/energizer-l92-bulk.html.
The lithium cell benefits and characteristics that are noted above also apply
for the smaller AAA (L92) size lithium cells.

Regarding Travel & Safety: The
perilous warnings that you’ve heard about lithium battery fire hazards
are real, but they trace entirely to the use of multi-cell rechargeable-type lithium-ion batteries, as commonly used today in cellular smart ’phones, digital cameras, and portable computers (also Teslas). A non-rechargeable (single use) L91/L92 type lithium (LiFeS2) cell does not
pose the same fire threat. However, to assure that the federal safety
mandate has clear and effective public compliance, ALL lithium power
sources are now lumped as posing such fire hazard. As a consequence, you
are not permitted to ship lithium cells without a related exterior
warning sticker, and shipping by air (or by sea) is banned within the
USA. Regardless, when you are traveling by commercial air service, you
can then carry on equipment
that utilizes lithium cells. This means that all of your
battery-powered “carry on” gear (your cell ’phones, computers, digital
cameras, strobes, flashlights, et al), whether using lithium cells or
not, are welcome aboard the passenger compartment of the aircraft.
Packing such devices in your checked luggage is permissible only if they
are “…completely powered off and protected to prevent unintentional activation or damage.” This latter restriction is subject to the whim of the TSA agent on duty, so you’d best “carry on” your battery-operated devices. Further, all back-up stocks of uninstalled “…lithium metal and lithium ion batteries are always prohibited in checked baggage and must be placed in carry-on.” And: “When
a carry-on bag is checked at the gate or at plane side, any spare
lithium batteries must be removed from the bag and kept with the
passenger in the aircraft cabin.” (On your person, not in your
shoulder bag or briefcase.) Obviously, these last requirements don’t
pertain to L91/L92 cells, but they directly apply to any backup
rechargeable-type li-ion camera body batteries that you might have in your possession. The relevant TSA publication is here…https://www.faa.gov/hazmat/packsafe/more_info/?hazmat=20 (Issued 5/31/2018).

Re. NiMH
Rechargeable Cells: In
closing, please take note that many AA and AAA cell users rely on rechargeable
nickel metal-hydride (NiMH) cells as their preferred alternate to alkaline
cells. NiMH rechargeable storage cells work quite nicely for many applications
(far, far better than the older nickel-cadmium rechargeable alternative), but
the self-discharge rate for NiMH cells still makes them suspect for general
usage. A fully charged NiMH cell reads about 1.2 Vdc when under load at the
start (versus ~ 1.55 Vdc for a fresh alkaline cell), but it will sag to ~ 0.9
Vdc after just 6 to 8 weeks of passive storage. Conversely, a stock
non-rechargeable L91 (AA) or L92 (AAA) lithium battery will read ~ 1.6 to 1.7
Vdc initially when under load regardless of whether it’s new or has been in
storage for several years.

--BG (May 20, 2021)

On Upgrading What’s Analog…

By Bryan Geyer

THE POSIT—Recent
technology advances make it practical to upgrade select analog functions in
many conventional stereo systems. It’s now possible to…
…dramatically extend the range and enhance the
linearity of low bass response.
…materially reduce the mass of the main
speaker enclosures.
…clarify the midrange response by isolating it
from low bass modulation.
…eliminate archaic high-level passive
crossovers that reflect only low order filter slopes.
…enable precise line-level active
crossovers with full 4th order (-24dB/octave) attenuation.

BACKGROUND—The
pursuit of good sound is evolutionary, but the conviction that high fidelity
= big speakers persists. Low frequencies have long wavelengths (28 feet at
40 Hz), so big transducers, in big enclosures, have always been considered
essential to reproduce, propel, and propagate big bass. Regardless, monstrous
monkey coffins that require prominent front-and-center placement look absurd in
domestic living rooms, so reasonable compromise is vital. The usual course is:
(a) downsize the speakers, or (b) always entertain outside, or (c) cram the
system into a “man-cave”, even though the main LR offers superior acoustics
(lower Schroeder frequency), or (d) use headphones only. These are odious
options; let’s pursue better.

WHAT’S CHANGED—In the
mid ’90s, the push to popularize home theater became dependent on finding a
practical way to recreate Godzilla’s footfall while squeezing the mass out of
the additional speaker enclosures. This paradox looked hopeless until design
guru Bob Carver, then at Sunfire, proposed his astonishing new (1997)
self-powered “True Subwoofer” as a viable solution. Market acceptance was slow
until Sunfire’s patent grip was softened; then the trade piled on. The new subs
could pump out pure (low harmonic content) 30Hz sine wave bursts at pressure
levels exceeding anything previously envisioned. Today, relatively compact
subwoofers with only a single 10" or 12" diameter driver (e.g., https://www.jlaudio.com/collections/home-audio-e-sub/products/e112-gloss-home-audio-e-sub-powered-subwoofers-96279)
can produce lower, louder, and cleaner bottom bass (the two bass octaves
below 80Hz) than the very best, biggest, and most expensive full range
floor-standers ever built by anybody, anywhere.

Tempering this marvel
is the reality that these new self-powered subwoofers excel at just one thing:
They can handle those two bass octaves better than any conventional woofer.
They’re able to do that because they’re expressly designed for a single,
solitary mission. They have extremely deep, ultra stiff cones, long and
compliant surrounds with extended x-mass capability, and they’re driven by high
power class D amps that apply carefully contoured equalization. Unlike the
woofers in a costly full range floor-stander, the “sub” doesn’t have to produce
any appreciable output above ~ 140Hz. Conversely, a conventional full range
floor-stander is designed with the expectation that it must also cover the full
low-to-middle bass range, commonly extending to frequencies approaching ~
800Hz, and do so with good linearity. That’s a stringent additional assignment.

TODAY—If the intent
is to achieve optimal fidelity, we need to reconsider how stereo systems should
be configured. Clearly, paired stereo subwoofers* should handle the
bottom bass octaves. The main stereo loudspeakers can then be reduced in size
to reflect the fact that they will no longer have to reproduce those
subterranean sounds. Stand-mount main speakers with 6" to 8" drivers
have proved absolutely optimum. Excellent bookshelf-size speakers, like Harman’s
Revel Performa M126Be (https://www.audioholics.com/bookshelf-speaker-reviews/revel-performa-m126be),
or the Revel M106 (https://hometheaterhifi.com/reviews/speaker/bookshelf/revel-performa3-m106-2-way-bookshelf-monitor-loudspeaker-review-part-one/),
come to mind, and they confer welcome decor relief. The related subwoofers are
best placed near the front corners of the room, at or near floor level, where
they’re generally easy to accommodate. There’s simply no need, anymore, for
monstrous four to five foot tall main speakers that are packed with multiple
10", 12", or 15" woofers. At least, not unless you’re personally
committed to high-end electrostatic-type speakers, such as those
championed by Sanders Sound Systems. (In that event, please see https://classicalcandor.blogspot.com/2019/09/on-elephants-in-room.html.)

In addition to the
cited decor benefit, separating the low bass from the upper bass presents a
compelling aural upside. In a conventional setup, the main woofer and its power
amplifier handle both signals, so there’s often some mild modulation blur of
the upper bass or lower mids when the score calls for a sudden burst of big
bass thunder. Routing that sound directly to the low-pass amplifiers, inside
the self-powered subs, means the main speakers will now be unaffected. The main
power amplifier will never see those low bass signals when they’re appreciably
below the crossover notch. Eliminating this potential midrange modulation is
one of the most vital rewards that you can claim when you add subwoofers and
install an external active crossover. Some listeners feel that the midrange
benefit is even more audible than the subwoofers’ bass extension. Your own
impression might be a bit program-dependent in sensing these complementary
improvements, but it’s clearly evident when the score fits**.

MODERNIZING
CROSSOVERS—A further concern is that conventional full-range speakers impose
the need for accurate driver-level crossover networks. Such filters are
fussy to design and awkward to assemble. They’re wholly dependent on precisely
defined R/L/C components that must pass high current signals to low impedance
loads. Tight tolerance passive parts of this unique nature are rare—most have
no other fundamental application in the scope of the electronics trade, so they’re
often difficult to obtain in the exact values that are needed. The inevitable
consequence of this difficulty is compromise, and compromise breeds inaccuracy†.
Even when they’re effectively designed and constructed, these high level
crossovers often prove inadequate because they’re typically just first order or
second order (-6dB or -12dB/octave) filters with gentle attenuation slopes.
Full fourth order (-24dB/octave) passive crossover networks would be more
suitable, but they become impractical (too complex, physically prohibitive) for
high level (low impedance) applications.

The solution to this crossover dilemma is to move that
function from its classic position, between the power amplifier outputs and the
various driver inputs (where it must process high level signals), to an
earlier point, prior to the power amplifier, where the components will
see only lower line level signals.This
preferred location assures ready access to lots of stable, tight tolerance
R/L/C components of virtually any desired value. When combined with active
op-amp circuitry it’s then possible to form highly selective Linkwitz-Riley
type filters with full 4th order attenuation slopes. The penalty implicit with
inserting the crossover in this line-level position is that a separate power
amplifier will now be needed to connect each crossover output (high pass and
low pass) directly to its designated driver. That means operating in “bi-amplifier”
mode, a form of connection that confers useful advantages; e.g., better load
(driver) damping; also the ability to independently dictate the signal
amplitude sent to each power amplifier.

A fully external
(meaning separately enclosed and powered) active crossover controller is
invaluable when adding supplementary self-powered subwoofers. The active
crossover’s high-pass outputs can feed the main stereo power amplifier +
main speaker system, while its low-pass outputs feed the line inputs on
the self-powered subs. (The active crossover thereby supersedes any built-in
low-pass filtering that might be a part of the sub’s internal circuit, so the
subs should then be operated in their selectable “bypass mode”.) This setup
facilitates convenient adjustment of the “mains-to-subs” dominance (the
relative output levels of the main speakers compared to the subs) from a
single, central location. Without this crossover controller you’d need to crawl
to each individual subwoofer and separately adjust their input sensitivity
controls—and repeat that crawl every time you want to tweak the mains-to-subs
settings or alter the relative channel balance.

An external active
crossover controller with variable frequency capability will allow the user to
position the crossover notch at a point that’s optimum for the main speakers in
use. For most subwoofer setups, this will involve a setting that’s somewhere
between ~ 76Hz and 100Hz. (Lower crossover frequencies are not advisable,
regardless of the main speakers’ perceived bass capability.)

Loudspeaker systems
with internal line-level active crossovers + self-powered class D amps
are already in common use as near-field monitors; also as desk-top setups.
Designs of this nature offering higher power output have also appeared, e.g.,
the “Kii Three”. Clearly, this trend will accelerate. We’re going to see lots
of analog circuitry integrate with the loudspeaker, and some of the speaker
makers are likely to merge with those that currently make power amplifiers. Of
course, vacuum tube power amps—already a moribund species—will then fade forever.
Ditto for big solid-state quasi-class A power amps and heavyweights that
resemble engine blocks. Niche products will always exist, but it’s doubtful
that those who make them will prosper and survive, so be wary about where you
hitch your wagon.

THE PREAMPLIFIER—Refer
https://classicalcandor.blogspot.com/2019/12/on-controlling-volume.html.
Amen.

Footnotes…
*Paired stereo subwoofers (not a solo “shared
bass” subwoofer) are essential for acoustic advantage; refer https://classicalcandor.blogspot.com/2019/03/basics-about-bass_20.html.

**Re. “when the score fits”…is a phrase
that brings to mind a subwoofer quirk that you need to know. Many (all?) of the
companies making self-powered subs provide the useful ability to select
automatic “signal sensing” as a means of activating the sub’s internal power
amplifier; i.e., to auto-awaken the sub from its passive sleep state. In sleep
mode, the sub stays on, but it dissipates minimal power until it perceives that
it’s time to rumble. The sub then goes into active mode, and remains active
until there’s no input signal for an appreciable duration, e.g., 20 or 30
minutes. An initial problem is sometimes evident with respect to the maker’s
signal-sensing level. Bear in mind that the sub can only hear very low
frequency signals because its line-level input path is through a low-pass
filter. When there’s persistent heavy bass (as with virtually all pop music
today), the sub will awaken quickly. But when the bass content is evident only
where Mozart scored it, that sub might not awaken (go active) until you’re a
third through an extended concerto. Classical music just doesn’t exhibit the
same pounding bass line that persists with pop. Unfortunately, the folks that
make subs think classical music is extinct, so they normally set the signal
sensing to trigger at a higher (less sensitive) level than what’s optimum for
classical format. In many cases the original factory setting is as much as 14dB
to 16dB too high (too insensitive). So—when you buy a sub, communicate with
your supplier. Have the maker set your subs to auto-activate when The Lark [is
barely] Ascending. (I bow to Mr. Vaughan Williams.) If they fail to do
this you’ll then need to return your subs to the source, and have a factory guy
readjust the trip point (warranty return, maker’s expense). With all of the
subs that I’ve seen, this setting is always an internal adjustment. Some subs
might now make this tweak externally accessible, but internal-only is more
logical because it’s a “set once” consideration, and external access invites
misuse. Home-based DIY readjustment is definitely not feasible. The internal
class D “plate amp” has complex multi-layer boards + tiny surface-mount (not “through-hole”)
components, and that makes this simple task too risky when no schematic is at
hand.

Be assured that there’s
no downside related to increasing the subs’ activation sensitivity. When your
external active crossover controller’s low pass outputs (to the subs’ inputs)
cease sending a source signal (for the specified time lapse) to the subs, they’ll
automatically revert to their dormant sleep mode, exactly as intended. The only
instance in which this might not happen is if your crossover controller
suddenly became intolerably noisy—with noise so annoying that you’d be fully
absorbed with that fix before noting that the subs didn’t go to sleep.

†It’s only natural that those who own big
full-range floor-standing speakers would assume that the internal high level
crossovers buried inside their costly purchase are both optimum and accurate.
Regardless, their faith is likely misplaced. Accuracy is certain to be deficient
in the case of passive networks that have accumulated appreciable use because
passive high level crossover components drift. High level operation = high
level stress. (Refer paper headed “On Crossing Over,” at https://classicalcandor.blogspot.com/2020/01/on-crossing-over.html.)

BG (April 2021)

On Rational Selection--Part II…

By Bryan Geyer

In Part I, I listed the most important specifications that
should be reviewed when selecting an appropriate power amplifier. These truly
critical parameters included…

            …size and
appearance.

            …input
impedance (Zin).

            …output
impedance (Zout).

            …power
output capability.

            …voltage
gain; i.e., input sensitivity.

Now let’s look at some other specs that merit attention.
These aren’t quite so vital as the essential considerations that were listed in
Part I, where appropriate compatibility was the main objective. Instead, these
specs reflect basic quality-related issues, such as…

SIGNAL-TO-NOISE (s/n) RATIO: Freedom from hum will be
highly dependent on power supply design and grounding issues. Freedom from noise
will generally trace to circuit design and component selection variables. It’s
desirable to exhibit a wide spread (e.g., a difference of 100dB or more)
between the value of a given output signal (the modern reference is 1Vrms) and
the residual detected when there’s no input signal (input shorted). This
represents the amplifier’s innate signal-to-noise (s/n) ratio. It’s commonly measured
through a filter (it’s “A-weighted”) to emphasize audible frequencies
rather than ambient hum or response beyond the audible spectrum. If the consequent noise reading is 10µVrms, the
related s/n would be -100dB relative to a 1Vrms reference. A fine quality solid-state
power amplifier of current design will meet -110dB (re 1Vrms) s/n ratio. A 90dB
s/n ratio is certainly not state-of-the-art, but it’s relatively acceptable. A
s/n ratio that dips to -80dB (0.1mVrms re. 1Vrms) means that you’ll start to
hear some annoying background hiss from your loudspeakers, dependent on their
efficiency. Of course, all vacuum tube power amplifiers produce poor
signal-to-noise ratios—it’s an inherent limitation.

QUIESCENT DC OFFSET VOLTAGE: Well, it’s just nuts to
neglect checking your power amplifier for DC offset. It’s easily measured with
almost any DC voltmeter, and it’s vital that offset be minimized. Do it! Check
this column for background info.: https://classicalcandor.blogspot.com/2019/10/on-power-amplifier-dc-offset.html

FREQUENCY RESPONSE, DISTORTION, & DAMPING FACTOR:
These specs were once of vital concern in expressing the efficacy of a power
amplifier. That’s no longer the case today because virtually every solid-state
power amplifier exhibits near-perfection with respect to its innate frequency
response and distortion performance. Ditto for damping factor, as that
parameter is directly related to a solid-state amplifier’s ultra-low output
impedance; refer Part I of this paper. As a consequence, you can safely skip
these three specifications when ranking the merits of solid-state power
amplifiers. Any divergence will be too small to justify analysis. Conversely,
there are operating characteristics that you should research, such as…

INPUT OPTIONS: If it’s your intent to apply balanced
interconnects, make certain that the required XLR sockets are provided. I don’t
personally recommend the use of XLR balanced interconnects in a normal home
stereo installation (refer “On Noise, Coax, and Control” at https://classicalcandor.blogspot.com/2020/08/on-noise-coax-and-control.html);
however, you might want that option. Balanced in/out connections are of great
benefit when operating in noisy environs, but typically present no measurable
advantage in benign home environs. XLR plugs and sockets derive from an archaic
vintage (see https://classicalcandor.blogspot.com/2019/10/on-equipment-interface-options.html),
so they’re grossly oversized and generally awkward to accommodate.

Most modern power amplifiers now offer both XLR (balanced)
and RCA-type (unbalanced) inputs, although some older units might lack the XLR
option. Confirm (view back panel photo) that you’ll get what you want. To the
best of my knowledge, there’s just one power amplifier on the commercial market
that provides XLR balanced stereo inputs but NO unbalanced (RCA-type) stereo
inputs, and that’s the Benchmark AHB2. They offer short RCA-to-XLR adapter
cables to accommodate users that require unbalanced input compatibility.

POWER LINE PROVISIONS: Verify that any power amplifier of
interest will be compatible with the power line provisions available at the
site where the amplifier is to be installed. This implies more than merely
confirming that the AC supply voltage and power line frequency are compatible—it
should also encompass the AC current drain (AC line load) compatibility. Refer
“On Assuring Adequate AC Power” at https://classicalcandor.blogspot.com/2020/05/audio-tech-talk.html.

TURN-ON and TURN-OFF PROVISIONS: While it’s always
possible to turn your power amplifier on/off manually, via a switch that’s
wired in series with the power supply, there are potentially more convenient
options to consider. Here are some popular automatic control options…

(1)  Turn on/off
via a low voltage (12-15V) DC control signal: This is a popular turn-on option.
It’s commonly offered on many modern power amplifiers and on other peripheral
components.

(2)  Turn on/off
via signal sensing: The amp senses an audio input, then turns on. The turn on
sensitivity setting is generally variable, via a back-panel switch or pot, so
that background noise won’t be sufficient to falsely trigger operation. The off
function is generally based on a time delay; i.e., when no input signal is
detected for a specified period of time (e.g., 5 minutes) the amplifier then
turns off.

In either case, it’s helpful to have a power amplifier
that connects the input signal path only after a momentary (relay-controlled)
sequential delay. That permits the initial DC power-up surge to “settle out”
first, so that there’s no audible turn-on “thump”; i.e, no sound output until
the amplifier’s internal DC power supply has stabilized.

IN THE FUTURE: As you consider these issues, do bear in
mind that the days of a stand-alone stereo power amplifier look limited. The
current design trend is to fully integrate the power amplifier with the
loudspeaker system. Future audio systems will reflect that preference,
especially when listeners learn to appreciate that…

…it’s more accurate to use active—rather than
passive—crossover networks to feed the drivers in a speaker system with their
intended passbands.

…low bass frequencies (those < 60 Hz) can be more
accurately produced by separate self-powered subwoofers than by an integrated
driver that must also handle all of the mid-bass frequencies (to 600 Hz) as
well. The best top quality speaker systems will no longer include multiple
large diameter woofers because we’ve learned that the classic all-in-one-box
approach is an inferior and outmoded way to implement a full spectrum sound
system.

BG (February 2021)

On Rational Selection–Part I…

By Bryan Geyer

It’s nice to assess
the quality and capability of a new power amplifier in advance of buying it;
however, don’t expect to fulfill that research solely by listening. A listening
trial will reveal almost nothing about the amplifier’s performance
compatibility. You simply won’t hear insufficient voltage gain, or hear
the loss that accrues from a poor impedance match. Lots of critical factors
can be deficient, or significantly less-than-optimum, without imparting any
audible evidence. In truth, it’s not even possible to detect, by ear, whether
your test amplifier has sufficient power output capability. (A power amplifier
should be capable of providing some +2dB to +3dB more undistorted power output
than your loudspeakers can safely handle; refer commentary herein. That value
can then be precisely determined by reference to the related product
specifications.)

A solid-state power
amplifier’s sonic character is almost entirely defined by the intervening
physical and emotional filters that separate the listener’s aural perception
from the amplifier’s output. Notable intervening screens include the
loudspeaker system’s crossover network, the fidelity and character of the
drivers , the acoustical nuance of the listening environs, and the shifting
vagaries of your own personal mood. Most especially, the final sound will be
the result of preconceived expectation; i.e., what your mind says you will
hear. These compound and complex factors will cloud the aural linkage between you
and the output of that new power amplifier. Any impression or conclusion that
you extract will forever remain the product of this imprecise and subjective
exposure. In essence, this is why listening is not an accurate or reliable way
to appraise audio equipment. Do seek more rational options.

Audio component
evaluation is best accomplished by research and inspection rather than by
listening. You can pretty well determine everything that you need to know about
a given component (let’s assume a power amplifier), by proceeding through the
four steps listed below. The data is freely available; you just have to become
conversant with how to utilize it effectively. Doing so will certainly require
more commitment than passive listening, but it’s likely to be less taxing than
interpreting all of the frothy adjectives that you see in a Stereophile product
review.

(1)  Check for published reviews on the
product. Focus on the factual test data + photos, and the related technical
commentary. Skip over the author’s tiresome aural assessment; it’s pretentious
nonsense. The Audioholics website is generally more useful than most because
they excel on the tech stuff and minimize such fluff as “has expansive
soundstaging, explosive dynamics, and exceptional transparency”, or claims
that “it produces natural transient attacks, a generous, almost tube-like
sustain, and take-your-breath-away decays that produce the sensation of
floating on a cloud.” (These latter phrases were as extracted from an
on-line Stereophile product review.)

(2)  Examine a photo of the amplifier’s
back panel; it’s often available on the maker’s website, or in a published
review. (Interior photos sometimes turn up as well. They can be informative
too, but only if you already know a lot about what you see.) The back panel
photo will give you a good grasp of the various input/output options and the
connector detail, plus such features as the turn-on options, provisions for
line level input trimming, bridged mono operation, and an understanding of how
the line cord connection is implemented, and how the cord is dressed. This
latter information is useful if you intend to substitute a replacement line
cord; refer footnote.*

(3)  Briefly consider price, then size,
then appearance. Eliminate the Dan D’Agostino ilk first. Gaudy products with
preposterous prices are obsession traps. Don’t get snared; just move on. Review
product size next. Nothing needs technical analysis if it won’t fit within the
space allotted. Ditto if it exhibits a pretentious blue-glow-bloat and says
McIntosh. Mac’s products just don’t fit my concept of contemporary
design; they look more like archived relics from the 1939-’40 NY World’s Fair.
Of course, you might disagree, but do reject, up front, all of the equipment
that you feel presents undesired esthetics. Appearance matters—especially
if it sports a prominent display panel.

(4)  Last, conduct a patient technical
assessment of the pertinent performance specifications. Some of the more
critical parameters that apply to power amplifiers follow, with comment about
how the specification can affect operation. The primary objective is to select
an amplifier that will perform in a manner that’s fully compatible and
complementary when it’s integrated with your existing components, so you should
also be well acquainted with their operating characteristics as well; review
those specs too. Lots of this detail will become more apparent as we get into
the individual parameters, so let’s begin now with…

INPUT IMPEDANCE
(Zin): Whatever the value, assure that the noted Zin is consistent with what
fits your need. Explanation: In audio electronics, effective (lossless) signal
transfer requires that a low source (output) impedance (Zout) should feed into
a much higher load (input) impedance (Zin). Of course, these low/high terms are
relative, so their absolute value is flexible. If a preamp exhibits an output
impedance of 50 to 100 Ω, that’s low, so almost any load impedance ≥ 10kΩ is
then high enough. But if the Zout climbs as much as 2.5kΩ (as with a passive
preamp using a 10kΩ attenuator), you’ll then want the load to be some 30kΩ to
50kΩ or higher. In general, when considering a power amplifier, don’t settle
for anything with Zin < 30kΩ because you’d start to limit your source
options. And yes, some hi-end power amplifiers actually exhibit Zin as low as
10kΩ.

OUTPUT IMPEDANCE
(Zout): Low is desirable; higher Zout can breed issues. But how low/high? Well,
it depends on what you’ve got. If it’s a solid state preamp, its implicit Zout
of 50 to 100Ω is always fine. A vacuum tube preamp (typical Zout ≈ 400 to 500Ω)
might be low enough too, if the ensuing load impedance is high enough (10kΩ) to
prevent any significant interactive loss. A load impedance (Zin) that’s some
10X to 20X the source impedance (Zout) is a good guideline, so a really low
Zout provides more freedom with respect to what might later constitute an
acceptable load. When this ratio can’t be conveniently satisfied, an active unity
gain buffer (UGB) stage can be inserted. The UGB is designed to provide a
relatively high Zin (say ≥ 75kΩ), and a very low (50Ω) Zout, so that it can
serve as an idealized “brick wall” to isolate some interim stage that has a
higher-than-optimal output impedance; e.g., a passive preamp using a 25kΩ
attenuator (worst case Zout ≈ 6.3kΩ).

BRIEF DIY ASIDE: If
you’re a competent electronics buff with elementary circuit design smarts, you
can create and construct your own very capable UGB, just as I and many others
have done. Refer my paper headed “On Noise, Coax, and Control” for
tutorial guidance on DIY UGB circuit design. The requisite semiconductors,
passive components, connectors, and suitable through-hole stripboard stock (https://www.mouser.com/datasheet/2/58/BPS-DAT-(ST6U)-Datasheet-1282874.pdf)
are readily available, at reasonable cost, from Mouser Electronics (https://www.mouser.com). And here…https://goldpt.com/enclosures.html…are
two very elegant mini-box enclosures** that have internal support rails
compatible with mounting the designated ST6U (1/16 inch thick) stripboard
stock. Cutting that glass/epoxy (tougher than phenolic resin) stripboard to the
desired size is best done with a portable Milwaukee M12-series saw (https://www.homedepot.com/p/Milwaukee-M12-FUEL-12-Volt-3-in-Lithium-Ion-Brushless-Cordless-Cut-Off-Saw-Tool-Only-2522-20/305663849),
and the related abrasive cut-off wheel (https://www.homedepot.com/p/Milwaukee-3-in-Metal-Cut-Off-Wheel-3-Pack-49-94-3000/306599198).

BACK TO OUTPUT
IMPEDANCE: Power amplifiers represent a dramatic example of the implicit
advantage of low output impedance. A solid-state power amplifier will exhibit a
Zout that’s naturally very close to zero; i.e. it’s generally ≤ 0.1Ω. That
value will assure an ideal damping factor (≥ 100) and virtually no
response-altering interaction with the loudspeaker load. As a result, the sonic
character of the output will be shaped almost exclusively by the loudspeaker
components alone, without related nuance traceable to the driving amplifier.
Conversely, a vacuum tube power amp will exhibit a much higher Zout (> 20X
higher) because its transformer-linked load interface is incapable of better
coupling. As a result, the tube amplifier will directly interact with its
speaker load (the Zout and Zin values will be of similar order) to create a
unique “sonic signature” that can vary from the speaker’s natural voice. The
higher Zout will similarly degrade the tube amp’s potential damping factor.
(NOTE: Benchmark Media Systems offers a white paper concerning the importance
of high damping factor; refer https://benchmarkmedia.com/blogs/application_notes/audio-myth-damping-factor-isnt-much-of-a-factor.)

POWER OUTPUT: This is
the highlight specification for power amplifiers. First, verify that the stated
watts are measured according to the industry standard code; namely that they
reflect Volts rms (root mean square), not peak volts, and that a specific load
is noted (e.g., 8Ω), and that the bandwidth extends from 20Hz to 20kHz.
Obviously, more Watts = more power, and more power output is always good. The
power output rating should also be listed for 4Ω loads too, and the closer that
4Ω rating gets to = 2X whatever power was listed for 8Ω, the better. Amps that
can’t come within ≥ 85% of doubling their 8 Ohm power rating when full output
is applied across a 4 Ohm load might be a bit current-limited; maybe too wimpy
to meet the peak dynamic transients in Respighi’s Pines of Rome when it’s
played at sound pressure levels approaching 90dB (C-scale weighted).

Always assure that
your amplifier’s full power output capability is at least some +2dB (1.6X) to
+3dB (2X) > the rated power that your loudspeaker system can safely
tolerate. This will assure that the loudspeakers are never exposed to a clipped
input signal when driven to levels that are within their rated maximum power
limit. Outside of physical abuse, nothing can be more potentially injurious to
a loudspeaker system than consistently clipped drive signals, and nothing
sounds worse than peak level clipping.

VOLTAGE GAIN: This is
an important parameter that’s often overlooked. Power amplifiers typically
exhibit different voltage gains. They generally vary between ~ +23dB and +30dB,
with most of them ranging between +26dB and +29dB. This is an intrinsic design
parameter, not the consequence of random variance, so it’s an important user
consideration, and it deserves your attention.† The actual gain, expressed in
dB, may not appear as a listed spec, but you can readily derive it from the
published “input sensitivity” spec. Just do the math: P = EE/R, where P
is the power in Watts, E is the AC signal in Vrms†† (EE =
E-squared), and R is the load resistance in Ohms. A power amplifier that
requires 1.0Vrms to produce 100 Watts into an 8Ω load has a voltage gain =
28.28X; otherwise expressed as +29dB.*†

INPUT SENSITIVITY: Is
simply a measure of the input voltage needed to drive a power amplifier to a
specified output level (generally, but not necessarily, to full output). It’s
precisely the same parameter as voltage gain (above); it’s just restated after
somebody else does the math. The “input sensitivity” is = 1.0Vrms when it
drives the same amplifier to 100 Watts output across that 8Ω load.

The term “input
sensitivity” brings to mind a fundamental concern: You want to assure that your
power amplifier has sufficient internal gain that source signals, at their
highest levels, are sufficient to drive your power amplifier to full output
when your volume control is at zero cut (volume full up). Of course, if you’re
using an active preamplifier between your source and the power amplifier, then
copious extra gain—likely some +8dB to +12dB, and maybe even more—is available,
with virtually all of it unneeded. However, in the event that you’ve modernized
your system to eliminate that superfluous high level gain stage, then do assure
that your CD player’s output, when driven at the maximum standard 0dB record
level, is sufficient to push your power amplifier to full output when volume is
set at zero cut. Let’s take an example: My own CD player (a discontinued Onkyo
C-7000R) puts out ≈ 1.98Vrms (let’s say 2Vrms) across my 20kΩ volume control
when driven at the industry standard 0dB maximum recording level (at 1kHz), and
the channels match to within 0.2dB. I’m confident that your CD player’s output
will be very close to same. (A good quality CD test disc with 0dB record levels
at 1kHz [actually 1.001kHz for some obscure reason] is easily purchased. Mine
is “Denon Audio Technical CD” #38C39-7147, and the 0dB test tones are on
bands #18 [left] and #19 [right]. This is a professional grade test disc from
1984; it’s now scarce, but sometimes pops up on Amazon. Equivalents are
commonly available. Boston Audio Society member DB Systems lists a compendium
of test CDs at http://www.bostonaudiosociety.org/db_systems.htm.)

My power amplifier (a
Parasound A23, circa 2014, now superseded by A23+) has a voltage gain = +29dB,
and the specified full output is 125 Watts/channel into an 8Ω load. As a
result, I can be certain that a CD source signal of 2.0Vrms will be more than
sufficient to drive my amp to full output. How do I know that? Well, 125 Watts
into an 8Ω load means that there’s 31.6 Volts of source signal impressed across
that load. Subtracting 29dB of gain (0.035 x 31.6Vrms) tells me that the input
need be only 1.1Vrms to drive my amp to full output. So I’d have a full 0.9
Volt margin relative to the potential 2.0Vrms input. (The volume control serves
to prevent input > 1.1Vrms, hence avoid clipping.) OK, but what if my power
amp had only +23dB of voltage gain instead of +29dB? Well, with volume then
turned full up (zero cut), that would mean that the amp would need 0.071 x
31.6Vrms = 2.2Vrms input to reach 125 Watts into 8 Ohms. With just 2.0Vrms of
drive from the CD player I’d be close, but still some -0.9dB short of the
2.2Vrms minimum required to produce 125 Watts across 8 Ohms.

The implication is
clear: Pay attention to a power amplifier’s voltage gain specification.
Be certain that your power amp can always reach its fully rated output
(regardless of what that output might be) when your CD player hits the highest
signal level that the CD recording process permits; i.e., the 0dB record level.
Here is a summary of the recommended amplifier gain consistent with stated
output levels, assuming an 8Ω load and a CD player that puts out ~ 2.0Vrms when
fed at the standard 0dB maximum record level.
 
            +26dB minimum amplifier voltage gain when
rated output is ≤ 200 Watts.
            +27dB minimum amplifier voltage gain when
rated output is ≤ 250 Watts.
            +28dB minimum amplifier voltage gain when
rated output is ≤ 300 Watts.
            +28.5dB minimum amplifier voltage gain when
rated output is ≤ 350 Watts.
 
These minimum gains
assure that a 2.0V input signal will drive the power amplifier to the cited
output. More gain is fine, but only up to a point. Hold amplifier gain to
+31dB or +32dB tops. If your gain was any higher you’d then be forced to
unduly retard your volume control setting (to the 9 o’clock arc) when listening
at more moderate levels, and that’s not good either. Indeed, that would
replicate the penalty that you accept when retaining a conventional active
preamp. The preamp’s high level boost of some +8dB to +12dB is grossly
excessive, and that unwanted excess prevents good volume management. Dump that
archaic active preamp! Instead, get a passive equivalent, and buy (or build) a
better volume attenuator than the $15 Alps RK271-series dual pot that’s inside
most of the hi-end active preamps.

My own DIY 20kΩ
stereo volume control uses a stepped 24 position double-deck switch (Goldpoint
type V24C-2, see https://goldpt.com/compare.html),
with 23 fixed ±0.1% tolerance metal-film (low noise) 1/4 Watt resistors per channel
(46 total), all carefully hand-soldered in position. It provides precisely
-2dB-per-step attenuation over 70% of its full -60dB span, and its calibrated ±0.1%
accuracy assures far better channel tracking than the crummy 2dB or 3dB ∆ that’s
specified for premium Alps pots.

That’s quite enough
for this session. In the next paper we’ll continue with this same subject, and
discuss some additional parameters that you should review when perusing power
amplifier specifications.

BG (September 16,
2020)

*There are often good
reasons to seek a replacement line cord. You might need a shorter or longer
cord, or one with an angled (90˚) socket, to reduce the rear clearance.
And you might choose to utilize a 14 AWG line cord instead of the supplied 16
AWG cord. (Don’t consider 8-10-12 AWG cordage. There is no electrical benefit,
and fatter gauge power cords become difficult to route.) USA market amplifiers
utilize a molded SJT-type cord, with a Nema 5-15P plug and a IEC320 C13 socket
(C13L or C13R if it’s angled). Simply order a replacement (molded, SJT grade,
14 AWG) cord of the length that you want, with socket and plug as desired. This
site-- https://www.pchcables.com/expocoandad.html--offers
many existing stocked options at very reasonable prices, and the quality is
excellent. This site…https://www.stayonline.com/cordbuilder/molded-cord-configurator.asp…will
custom-build your cord to your own personal specifications. Just fill in the
options on the form; pop-up photos will guide you in making your selection.

Ignore all chatter
about ultra-costly replacement line cords that allegedly improve the sound of
an amplifier. That fable traces to a pervasive human weakness that we know as confirmation
bias (https://en.wikipedia.org/wiki/Confirmation_bias).
The effect is persuasive, and groupthink-infused (https://en.wikipedia.org/wiki/Groupthink)
zombies on sponsored websites keep this promotion pumped—as do the retail
dealers. Upgraded line cords are among the most profitable of all products
stocked. Sales are good, and returns (sometimes not permitted until a mandatory
“burn-in” cycle has elapsed) are easily resold. Here is an array…https://www.thecableco.com/cables/power-cables.html?limit=all…of
318 assorted line cords for sale; prices range from ~ $100 to $12,000
each. Of course, science says that the mythical emperor still has no clothes. A
substitute line cord will produce no audible change, at least not until some
other user says “Hey, can’t you hear that tighter definition in the mids?”

**The Goldpoint
#EN1-C enclosure size is quite sufficient if you’re content with using standard
metallized polyester film input and output coupling capacitors. However, if you’re
going to use premium Vishay MKP 1848 series polypropylene coupling capacitors
(they’re big!) you’d need the slightly wider Goldpoint EN2-C enclosure.

†When comparing
different power amplifiers in a listening trial, don’t expect to freeze the
volume control at some fixed position and assume that you’re thereby providing
a fair comparison. The amp with the higher gain will always win, even when decidedly
inferior. A difference of just 1dB is enough to sway all votes.

††Use of the
designator “E” to represent voltage stems from the classic tutorial use of the
term “electromotive force” to describe the voltage function. It’s akin
to pressure when expressed as a plumbing analogy.

*†Do keep a “Decibel
Table” handy, and review how to read it. Here’s an on-line pdf copy of the “Handbook
of Electronics Tables and Formulas”, 6th edition (1988); refer decibel
table, pp. 32 to 36…
https://redcxem.ru/files/Handbook_of_Electronics_Tables_and_Formulas.6-th_ed.1986.pdf.
Especially note the scaling difference between voltage
gain or loss and power gain or loss. A voltage gain of 2X =
+6dB, whereas a power gain of 2X = +3dB.