By Bryan Geyer
The preamplifier has
served as a standard link in the audio chain since the dawn of hi-fi. The title
doubtless derives from its essential function: To amplify (and equalize, when a
phono cartridge input is involved) the incoming low level source signals, and
boost them to full line level amplitude prior to driving the power amplifier.
The preamp also serves to manage source selection, volume control, and,
traditionally, lossy tone controls*. Further, all functions must be achieved
consistent with proper source loading, and should provide good output
isolation, i.e.: present low Zout relative to the load impedance.
Times change. Your
present source signals are likely to already be at full “line level”, and most
of them can already drive the power amplifier to full output directly, without
further amplification.** Injecting more boost from the preamp’s line level gain
stage (generally +8dB to +12dB, sometimes as much as +20dB) will simply force
the user to push the volume control down near the 9 o’clock arc, a position
where the attenuation taper is cramped, calibration is compromised, and stereo
tracking is inherently poor. In this event, you’d do well to entirely bypass
the preamplifier’s traditional line level gain.
Although there might
be no need to amplify any incoming line level signals, it’s still necessary to
provide for source selection and volume management, and to do both in a manner
that assures proper source loading and good output isolation. With due care, and
by imposing just one reasonable restriction (explained below), that challenge
can be entirely satisfied by purely passive means, without resorting to any
active circuitry. The consequent solution is normally named a “passive preamp”.
It might be more accurate to call such box the main controller.
The main controller
is a good place to get fussy. The input selector and the master volume
attenuator represent the sole tactile link between your fingers and your
auditory perception, so those parts merit top quality. Ditto the requisite
input/output connector jacks. The main controller is also the part of your
system that you’ll manipulate the most, so it should be prominently placed and
readily accessible. Compact size will prove helpful, so consider the advantage
implicit with the use of RCA-type in/out jacks. By using RCA jacks you can
mount four full stereo channels (L/R inputs x 4 + main L/R outputs = 10
jacks total) on a tidy 2 inch by 6 inch panel (see photo), whereas XLR
connectors are too massive for more than two channels. Given normal home
environs, the use of XLR connectors here would not confer the slightest noise
advantage. That fact is further assured by limiting the permissible output
connection cable length to 1 meter maximum, a minor concession that reflects
the above noted passive design constraint. This length limit is actually
imposed to assure that there’s no significant (-0.3dB) 20kHz rolloff arising
from shunt cable capacitance when a passive 10kΩ or 20kΩ log-taper volume
attenuator is positioned at its worst case (highest) Zout setting.
A value of 10kΩ to
20kΩ for the master volume attenuator is sufficiently high to provide a Zin
that’s fully compatible with any known solid-state source component. The latter
typically exhibit a low Zout, on the order of ~ 50-150 Ohms. Conversely, a 10kΩ
or 20kΩ load would not be suitable for a vacuum tube source component.
The typical cathode-follower output stage of a tube-type source will exhibit a
much higher Zout, hence need materially higher load impedance, i.e. ~ 50kΩ. And
a passive 50kΩ attenuator would then require active circuitry to provide a
tolerable (low) Zout—so forget about using a tube circuit as a source
component.
In order to maintain
good calibration accuracy, the loading on a 10kΩ or 20kΩ volume attenuator
should be on the order of some 5X to 10X the attenuator’s worst case (highest)
Zout setting. That Zout is, respectively, 2.5kΩ and 5kΩ. Both values are then
fully compatible with the typical input impedance of most solid-state power
amplifiers, where Zin is generally ≥ 30kΩ. The potential error is even less
when an active external crossover controller is the load, as Zin is then on the
order of some 75kΩ to 100kΩ. Check the specified Zin of your own equipment to
be sure that it presents a similar value.
There are a great
many different commercially available passive preamp designs on the market, at
prices ranging from $49.50 to insanity (~ $8k), with a very wide variety of
means (some quite bizarre) applied to set the volume level. My personal choice
is Goldpoint’s model SA4, as made by Goldpoint Level Controls, of Sunnyvale,
CA. Refer http://www.goldpt.com/index.html.
The price (order
direct, on-line) for a Goldpoint SA4 is $532 + tax and shipping. That expense
may seem steep, given the functional simplicity involved, but the general level
of excellence, choice of components, and the craftsmanship applied justifies
the maker’s tag. It’s an elegant product. The standard Goldpoint SA4 provides four
stereo input channels, utilizing RCA jacks. (There’s also a two channel stereo
XLR version if you insist on adhering to those bigger input jacks.) The volume
attenuator consists of a premium quality Elma 24 position double-deck switch,
with 23 laser-trimmed ±0.5% thin film nichrome low noise resistors per
channel.*** See the SA4 product page at http://www.goldpt.com/sa4.html.
Also, take the time to read the informative section about stepped attenuators
versus conventional volume control potentiometers…https://goldpt.com/compare.html. (It’s
way down at the bottom of that page.)
It’s a distinct
pleasure to utilize a fully calibrated stepped attenuator to control the
output volume. The design accurately exhibits exact incremental gain steps,
with closely matched stereo channel tracking and the visual ability to
precisely reset a given reference level. Even the very best of the continuously
variable rotary controls is crude and sloppy in comparison with this stepped
switch.
I recommend Goldpoint’s
basic 24 position stepped attenuator, rather than their newer 47 position
option. The former is basically a 2dB/step attenuator, with the last 28dB of
cut compressed into a tapered 5 step descent as you approach the fully-off
position. The net result is 62dB of total attenuation, most of it accessed in
gentle -2dB steps progressing from the fully-on position. This design is ideal.
In the past 7 years of using my own Goldpoint SA4 (with 24 position attenuator)
I have never wished for a control with finer resolution. I find 2dB/step to be
quite perfect. The mechanics are equally excellent. The switch mechanism is
quiet and reliable, and the rotation is very smooth, optimally damped.
BG (December
3, 2019)
*Tonal adjustments
are best accomplished by utilizing a separate external active crossover control
unit that directly loads the preamp and feeds the ensuing power amplifiers.
Active crossover controls facilitate variable selection of the desired low
pass-to-high pass crossover frequency, with adjustable damping and adjustable
boost/cut of the independent low/high passbands. This provides a cleaner, more
precise, and more logically managed means of altering the tonal nuance of the
system than previously possible with traditional tone control filtration.
**Do confirm that you
can drive your system to full output directly, without the need for
supplementary preamp gain. In most cases this will be true, but exceptions
happen; it’s dependent on your power amplifier’s internal voltage gain and on
loudspeaker efficiency. Power amplifiers exhibit different internal voltage
gains. Most designs range between +23dB and +29dB; refer spec. sheet, see “input
sensitivity” (or equivalent term). Power amplifiers with gain = +29dB (e.g.: 1
Vrms input produces 100 Watts output [28.28 Vrms] across an 8Ω load) are
inherently capable of reaching their full rated output capability when driven by
virtually any modern line level program source. Power amplifiers with internal
gain ≤ +24dB fall into an area that I consider marginal for use with a passive
preamp when driving low efficiency mini-monitor speakers. Try to stick with
amplifiers that provide ≥ +26dB gain.
***If you’re a
compulsive DIY perfectionist (me), you might consider buying a naked attenuator
(no resistors) switch from Goldpoint that’s made for axial lead resistors, and
rig your own attenuator.† That entails some careful work, and some fussy
ordering. Why would anyone do this? Well, you might want a different attenuator
value. (I wanted a 20kΩ attenuator. Goldpoint’s nearest standard is 25kΩ.)
Also, you might want resistors with a ±0.1% tolerance, whereas Goldpoint
trims their surface-mounted nichrome resistors to ±0.5%. If you chose to run
this DIY route…
(a) Contact Goldpoint for guidance on the discrete
resistor values needed for your custom attenuator; they have a programmed “app”
for their 24 position model.
(b) Order your resistors on-line from Mouser Electronics.
Specify TE Connectivity (brand), 1/4 Watt, axial lead, metal film (low
noise), ±0.1% tolerance, with thermal coefficient 15ppm.
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