Apt Holman Preamplifier
The Apt Holman Preamplifier. enlarge.
Rear, Apt Holman Preamplifier (about $300 used). enlarge. This free website's biggest source of support is when you use these links, especially this link directly to the Apt Holman Preamplifier at eBay (see How to Win at eBay), when you get anything, regardless of the country in which you live. Thanks! Ken.
The Apt Corporation's Holman Preamplifier is the most well thought-out stereo preamplifier ever designed.
It was designed at the very height of recorded music's reign as the dominant form of home entertainment (home video was still years away), and was designed by none other than the brilliant Tom Holman, the inventor of 5.1 audio and the inventor of THX motion picture sound.
More important than its stellar laboratory performance is the fact that this preamplifier is designed to do exactly what's needed to make music, and a music reproduction system, sound great. This preamplifier even has a patented variable stereo expansion circuit, a foreshadowing of Tom Holman's later exploits into multichannel sound.
It uses many ingenious and patented circuits. For instance, in addition to the stereo control, a brilliant volume control system actually varies the amplifier's gain for the best noise and distortion performance, instead of merely attenuating a larger gain as do other amplifiers. It allows us to control the level over a huge range with ease, and always keeps the channels perfectly balanced.
The unique tone controls adjust more precisely than ordinary controls. They are individually calibrated for flat response at their detents.
The Holman preamp is so well thought out that its precise bandlimiting not only prevents distortion in other components, it even worries about the other signals coming into the unit that aren't selected! Its internal wiring is shielded electrostatically with alternate grounded conductors, and the selector wafer switches use several decks and grounds to keep everything separated and without crosstalk. Unselected inputs are properly terminated, not left hanging as they are in most other preamps.
What goes on inside this preamp goes far beyond exotic preamps today, and even more importantly, what makes the Apt Holman Preamp sound so great is that it's designed to perfect the interfaces to and from all the other parts of a system. In other words, rapt attention has been paid in its design to ensure that signals coming to and from this preamp are well conditioned and matched to ensure sonic perfection from the entire system in the real world, not just on a test bench.
The name "Apt" comes from its definition as "an appropriate and intelligent solution to the task at hand." Newer preamplifiers today are mostly 25-pound billet aluminum cases filled with fluff: ordinary analog circuits controlled digitally with showy vacuum fluorescent displays, but missing many of the important control features and superior audio performance of the Holman Preamplifier. Instead of chasing idiotic things that consumers think are important but aren't, like what kind of wire's inside, the Apt Holman Preamplifier concentrates on what's far more important, like where those wires are connected and the analog circuitry around them.
The Apt Holman Preamplifier sounds great because it's ultra-quiet and the channels stay balanced even at the lowest volume control settings. It's extra quiet because its special gain control circuitry means that it rarely needs to apply any gain at all; it's usually working only as a buffer as opposed to most preamplifiers, which always need to be operating at significant gain, regardless of the volume control setting.
Its outputs are always non-inverting, meaning that polarity is preserved.
The best part about its sound is the lack thereof: it's so quiet, it's the first preamp that let me hear the iPod touch's audio amp shut-down automatically after several seconds of paused operation!
I can't hear the internal transformer hum or buzz unless I hold my ear closer than a few inches to the preamplifier.
The tone controls are just right for delicate optimization.
For serious headphone listening, use electrostatic headphones like a Stax headphone and driver system fed from the regular preamp outputs, but if dynamic headphones like the Ultrasone Edition 8 or Sennheiser HD 800 are good enough for you, the Holman Preamplifier has a separate headphone amplifier capable of enough output to deafen a drummer.
Mechanical and Electrical
The case is heavy-gauge sheet metal, with pointy front-panel corners and four 3M stick-on 3/4" rubber feet. The knobs are solid aluminum with engraved indices with one hex set-screw each. The passively-lit pushbuttons are plastic and I wouldn't trust them, except that they're the same kind as I've seen on professional studio equipment of the same vintage, and as I've used in my own designs.
The only light is the bright red power LED; the other indicators are passive.
It's got a 10-foot power cord, five rear-panel outlets rated for a kilowatt, and a double hand-signed schematic diagram on the bottom. Holman thought of everything.
Rated Specifications top
Tom Holman has many pages of specifications and how to measure them in the Apt Holman Preamplifier Service Manual. I'd recommend you read this cover-to-cover if you're interested, as it not only explains many of the novel circuit innovations, it even includes several sample projects to build in order to measure the excruciatingly high performance and low noise of this preamplifier!
2 Phono inputs.
The first has selectable resistance and capacitance, while the second is fixed at 47kΩ and 50 pF.
100 mV maximum at 1 kHz, inverse RIAA, warp-proof.
The right channel of each of the two phono inputs is internally adjustable ±2 dB for perfect channel balance.
All line inputs are 50 kΩ in parallel with 330 pF.
Unselected inputs are terminated with 2.2 kΩ.
Maximum input level: 10 V RMS (+20 dBV or +22 dBm).
3 line-level inputs.
2 tape monitor loops.
1 external processor loop, for a total of six line-level and two phono inputs!
Gain, Polarity and Volume Control
18 dB maximum gain from line inputs.
Phono preamp gain: 36.5 dB at 1 kHz: 5 mV for 2 V output, 1.25 mV for 0.5 V output.
All outputs are non-inverting.
Volume control rated to track left-right to within 1 dB over the entire range. I'm impressed: this is a very important parameter, and this is the only preamp I've ever seen which bothers to specify it.
The multiwiper 10 kΩ volume control has 32 clicks, or 31 intervals, at 10º per click, or room for 36 clicks in 360.º
The volume control uses individually trimmed thick-film resistors to ensure precise channel-to-channel tracking.
Two MAIN outputs, fed in parallel. Provisions on the circuit board to allow your technician to insert a fixed resistive pad in the path of the MAIN 2 output if this optimizes the gain structure for your power amplifier.
330 Ω source impedance, 5 kΩ/3,000 pF maximum load.
Two tape outputs: 100 Ω source impedance, 10 kΩ/1,000 pF maximum load.
One external processor output.
One ¼" headphone output from a separate amplifier.
2 V RMS for rated specifications.
7 V RMS maximum undistorted.
If loaded with 2 kΩ, 3 V RMS maximum.
10 V RMS from the separate headphone amplifier.
20 - 20,000 Hz ±0.5 dB, with ultrasonic and ultrasonic filters active.
RIAA EQ ±0.5 dB 30 - 15,000 Hz.
Selectable 15 Hz Infrasonic filter (rear panel switch)
Within 0.5 dB at 20 Hz, -3 dB at 15 Hz and > -30 dB at 5 Hz.
Group delay 4 mS at 50 Hz.
If the filter isn't selected, the -3 dB point is 8 Hz.
Ultrasonic Filter (automatically engaged when Tone Controls active)
Within 0.5 dB at 20 kHz, -3 dB at 40 kHz, and -18 dB at 100 kHz.
Group delay 7 µS.
Interchannel delay: < 0.5 µS. (the limit of audibility is 10 µS.)
When the tone controls are defeated, the -3 dB point is 150 kHz.
High Filter (only active with Tone Controls selected)
-3 dB at 8 kHz.
Less than 0.01% THD at rated output (2V) and lower.
Less than 0.01 % SMPTE intermodulation distortion (60/7k Hz 4:1) at rated output (2V) and lower.
Less than 0.005% DFD with 19 kHz and 20 kHz mixed 1:1, at rated output (2V) and lower.
Less than 0.006% (-84 dB) TIM with 3.18 kHz square and 15 kHz sine mixed 4:1 peak-to-peak, low-pass filtered at 6 dB/octave at 100 kHz, at rated output (2V) and lower.
Noise (all A-weighted RMS)
Phono, measured with standard IHF cartridge
-74 dB down from 5 mV RMS at 1 kHz. Typically -76 dB.
Noise figure: 4 dB, 2 dB typical.
-90 dBV output noise with volume control at -20 dB.
-106 dBV equivalent input noise.
-95 dBV output noise with volume control at minimum.
-95 dBV output noise with volume control at -20 dB.
Typically equal to or less than the the wideband noise spectrum.
< -65 dB at 1 kHz.
< -45 dB at 20 kHz.
< -90 dB from unselected inputs.
When tested, feedthrough from unselected inputs was inaudible.
120V AC, 50-60 Hz. (240V AC with internal wiring change)
95-135 V AC OK. Below 95 V AC won't trigger the unmute relay. (190-270 V AC at 240 V setting.)
Rated less than 15 W power consumption.
Rated 100 mA RMS current consumption at 120V AC.
125 mA (1/8 A) internal slo-blo pigtail fuse.
I measure 9 W power consumption.
10 foot unpolarized power cord, but oddly the accessory outlets are polarized.
Two unswitched and three switched outlets, rated 1kilowatt maximum.
15.04 x 3.30 x 9.32 inches (382 x 83.8 x 236.7 mm), WHD overall.
10.5 pounds (4.8 kg).
12 pounds (5.4 kg), packed.
Hand-made in Boston, Massachusetts, USA.
It's attached to the bottom, hand-signed by the man who inspected your preamplifier, and hand-signed by the man who tested it.
Bottom, The Apt Holman Preamplifier. enlarge.
* corrected for inflation in 2011.
The Apt Holman Preamplifier. enlarge.
I used a state-of-the-art Rohde &Schwarz UPL for these measurements.
Unless mentioned otherwise, they are measured with the Holman Preamplifier set to unity gain (about noon to 1 o'clock on the volume control), measured from the AUX 1 input to the MAIN 1 output, and loaded with the 200k Ω load of the Rohde &Schwarz UPL.
The traces are color coded for the Left Channel and for the Right Channel. When they don't lie on top of each other, it's due to channel imbalance.
Frequency Response: +0, -0.25 dB top
Frequency Response, tone defeated, Apt Holman Preamplifier.
It's flat to +0, -0.25 dB from 20 - 20,000 Hz, and that's including the protective DC and RFI filters. This preamp isn't designed for lab flatness; it's designed to make the system sound great by removing inaudible out-of-band crud that can cause audio distortion.
I'll cover the response of the filters and tone controls in the Tone Control section.
9.1 VRMS measured at 0.1% THD, tone controls defeated.
8.5 VRMS measured at 0.1% THD, tone controls victorious (engaged).
See also the Headphone Amplifier Maximum Output Levels.
THD, Apt Holman Preamplifier, 200 mV RMS at 1 kHz.
Distortion harmonics, Apt Holman Preamplifier, 200 mV RMS at 1 kHz.
THD, Apt Holman Preamplifier, 1V RMS, tone defeated. (unchanged with tone active).
THD, Apt Holman Preamplifier, 220 mV RMS, tone defeated. (unchanged with tone active).
I used less averaging for the readings in this trace, so don't read this as noise.
THD versus level at 1 kHz, Apt Holman Preamplifier, tone controls defeated. (unchanged with tone controls active.)
SMPTE (60/7k Hz 4:1) IMD versus level, Apt Holman Preamplifier.
DFD, 19/20 kHz 1:1 at 1V, Apt Holman Preamplifier, tone controls defeated.
DFD, 19/20 kHz 1:1 versus level, Apt Holman Preamplifier, tone controls defeated.
Same as above, but in dB: DFD, 19/20 kHz 1:1 versus level, Apt Holman Preamplifier, tone controls defeated.
Gain versus signal level, Apt Holman Preamplifier, tone controls active.
Gain versus signal level, Apt Holman Preamplifier, tone controls defeated.
These look sloppy more due to noise than any real nonlinearity; these aren't digital preamps and therefore should have no problems with linearity at every level; that's why they're called analog. The first trace was made with only 32 generator-sourced levels, while the second was made with 16 readings averaged per level, and measured at 640 levels, taking over an hour to plot.
The Attenuator: Gain & Volume Control top
The most important thing a preamplifier does is amplify. The most critical part is the volume control, or attenuator, which can lead to distortion and channel imbalance.
These are measurements of the various steps of the attenuator (volume control). I measured these with the tone controls defeated, at 1 kHz, from the AUX 1 input to MAIN 1 output. These are the average of both channels, and were measured with a precision of 0.001 dB.
17.84 dB maximum gain is a linear amplification of 7.798x.
Tracking (Left-Right channel balance versus volume control setting) top
Right channel level versus left channel, vs. volume control setting. (positive means image moves to right.)
The stereo image never varies as the volume is changed. Poorer amplifiers often skew the stereo image several dBs or more at the lowest volume settings, while the superb attenuation circuit of the Apt Holman Preamplifier keeps the channels balanced to within about a third of a dB at every setting.
The balance control attenuates the lesser channel, leaving the other alone. It works exactly like that: the one channel varies by less than 0.03 dB, while the other is attenuated.
Taking the detent as a reference, here is the imbalance imparted by the balance control at various positions:
Measured as driven from the muted output of the Rohde &Schwarz UPL, with its 5Ω unbalanced source impedance:
* The tone controls come after the volume control. The output noise was measured with the tone controls defeated. When they are active, they can add an inaudible amount of extra noise as shown in the last column. When active, setting the treble control up or down all the way can vary the noise by a few dB either way, and the high filter can drop it a half dB.
To convert these dBV readings into signal-to-noise ratios, add the voltage of your signal. For instance, a CD player's maximum output is +6 dBV, so with -104 dBV of noise, there is a 110 dB signal-to-noise ratio. I also measured the unweighted noise, AC-coupled in a 22 kHz bandwidth, and it was exactly 3 dB greater than the A-weighted values.
Channel separation versus frequency, Apt Holman Preamplifier.
This is measured in the right channel looking for leakage from the left channel. It measured the same with the channels reversed.
The impressive thing is how much separation we have when we realize that as soon as the Stereo Mode Control is take off its center detent, that separation goes out the window by design. It is not trivial to maintain such deep separation and have a variable control to mix the channels.
Idle Input Rejection
When one input is selected, but other inputs have signal, how much feeds through to the output?
With a 1 V RMS input at 1 kHz to the AUX 1 input, here's what fed-through to the output when other inputs are selected:
On the schematic drawing on the bottom of the preamplifier, Tom Holman labels the tone defeat switch positions as "defeat" and "victory." In the defeat position, they are out of the circuit.
Tone Circuit Gain
The tone control circuits have very slightly less gain than in their defeat position. This slight change is inaudible, measured at 1 kHz, and is the gain relative to the gain in the defeat position:
Oddly, Tom Holman included trims for phono preamp balance and tone control flatness, but something must have fallen out-of-tolerance to cause the whopping 0.4 dB loss. I'm kidding; no one is ever likely to hear 0.4 dB.
The tone controls have internal calibrations to ensure that they are flat when set to their detents.
Frequency Response, Apt Holman Preamplifier, tone controls active.
Frequency Responses, Treble Control, Apt Holman Preamplifier.
For this and the next sets of curves, the controls are each set to full cut, 9 o'clock, 10:30 o'clock, the center detent, 1:30 o'clock, 3 o'clock, and the full boost position.
Frequency Responses, Bass Control (normal ">--"), Apt Holman Preamplifier.
Frequency Responses, Bass Control (shelving "=>-"), Apt Holman Preamplifier. (roll mouse-over to compare to normal mode.)
Filters measurements top
Frequency Response, Normal (left) and 15 Hz filter (right), Apt Holman Preamplifier.
The low-frequency response is the same with the tone controls either defeated or engaged, for all inputs and all outputs.
The 15 Hz filter is selected with a rear-panel switch.
Same as above, different vertical scale.
Frequency Response, High Filter (lowest trace), Normal (middle), and Tone Defeated (top), Apt Holman Preamplifier.
The 8 kHz High Filter is only active when the tone controls are engaged. If the tone controls are active but the High Filter isn't selected, then the middle 40 kHz filter is active.
If the tone controls are defeated, then the -3 dB frequency is 150 kHz as seen in the highest curve.
Same as above, different vertical scale.
There are three buffered outputs: TAPE 1, TAPE 2, and EXTERNAL PROCESSOR.
Gain, as measured from the AUX 1 input, is -0.30 dB, and the channels are balanced to within 0.015 dB.
Here is the relative frequency response measured from AUX 1 to TAPE 1 OUT:
Tape Output Frequency Response ref. 1 kHz, Apt Holman Preamplifier.
If selected, the 15 Hz filter protects these outputs. This curve is without the 15 Hz filter.
I measured the PHONO 1 preamp at the TAPE 1 output. I set the phono preamp to 47 kΩ and 50 pF, and drove it from the 5Ω source impedance of the R&S UPL.
Gain at 1 kHz is 35.64 dB left, and 36.03 dB right channel, an average numerical gain of 61.91x.
A-weighted noise, with inputs terminated by the the 5Ω source impedance of the R&S UPL, measures -86.36 dBV left, -90.30 dBV right. (Unterminated, noise measures about -80 dBV.)
Maximum input level at 1 kHz: 140 mV RMS at 0.1% THD, which leads to an 8.7V RMS output.
Phono frequency response (showing RIAA curve), Apt Holman Preamplifier.
Phono THD at 1 kHz vs. input level, millivolts, Apt Holman Preamplifier.
At the 120 mV maximum tested input level in the curve above, the output was 7.4 volts.
This has all been with 200 kΩ output loading. This preamplifier's MAIN outputs are only rated to drive 5k Ω loads, not 600 Ω loads, but let's see what happens anyway.
Not shown in these traces is that the overall gain drops 3 dB when loaded with 600 Ω due to the finite output impedance.
Frequency response, 1V RMS into 600 Ω load, Apt Holman Preamplifier.
At 600 Ω, the 20 Hz response is a little less than when unloaded.
THD, 200 mV RMS output into 600 Ω load, Apt Holman Preamplifier.
I used less averaging for this trace; this isn't noise. This is the same distortion seen unloaded.
THD, 1 V RMS output into 600 Ω load, Apt Holman Preamplifier.
This is the same distortion seen unloaded below 1 kHz, and it rises a bit at higher frequencies, still well below any significant value, compared to unloaded.
This is excellent performance for a load almost ten times more than rated.
The power-off mute relay is triggered also by a front panel switch. The switch turns white when you've set it to mute. The switch's indicator doesn't change when the power logic activates the relay.
The mute relay selects ground instead of an op-amp's output to feed the 330 Ω build-out resistor. Depending on the source's instantaneous level, it could click very slightly.
I measure -128 dB or more attenuation.
The output does not mute automatically when headphones are inserted; you have to hit it yourself if you like.
The headphone plug may get in the way of your fingers grabbing the volume control.
Headphone Amp Gain
Measured at 1 kHz relative to main outputs:
Headphone Amp Source Impedance: 120 Ω ±1% at 1 kHz
Let's look at the voltage drop at 1 kHz under load using the data above:
Let's calculate that the unloaded source is actually 0.0104 dB greater than it is loaded by 100 kΩ. Then R source = (R load/attenuation ratio) - R load.
After all this, we've confirmed a well matched 120 Ω source impedance just like it says on the schematic, ±1%.
Here are maximum output levels, measured both channels driven to 0.1% THD:
Headphone Amp Frequency Response
This of course includes the response of the preamp, from which the headphone amp is fed internally.
Headphone-output frequency response, 100 kΩ load, Apt Holman Preamplifier.
Headphone-output frequency response, 75 Ω load, Apt Holman Preamplifier.
Headphone-output frequency response, driving Beyer DT 990 (600Ω version), Apt Holman Preamplifier.
Headphone-output frequency response, driving Ultrasone Edition 8, Apt Holman Preamplifier.
Headphone Amp THD versus Level
This of course includes the THD of the preamp, from which the headphone amp is fed internally.
Headphone-output THD vs. level, 100 kΩ load, Apt Holman Preamplifier.
Headphone-output THD vs. level, 75 Ω load, Apt Holman Preamplifier.
Headphone-output THD vs. level, driving Beyer DT 990 (600Ω version), Apt Holman Preamplifier.
Headphone-output THD vs. level, driving Ultrasone Edition 8, Apt Holman Preamplifier.
Apt Holman Preamplifier. enlarge.
Leave the MODE knob in the middle at STEREO, and leave the L > R and R > L switches down for normal stereo.
For mono, turn the knob to the left. The left-side of the control slowly narrows the image from stereo into mono. Controls like this were popular in the 1960s because people, used to mono, were sometimes uncomfortable with the full stereo effect.
Turning the knob to the right expands the stereo image. This doesn't excite me as much as I thought it would; it mostly just sucks out the center of the image.
The L > R and R > L switches select what input channels go to what output channels.
Here's what they do:
If you'd like more detail, Tom ran out of panel space to label these more clearly as MONITOR INPUT SELECT RIGHT and MONITOR INPUT SELECT LEFT, which is what they're really doing.
If you lift the L > R switch, it puts the left channel into the right channel. The left channel remains in the left channel. You now are listening to the left channel (in mono of course) in both channels.
Lifting the R > L switch does the same thing, playing the right channel in both channels.
Lifting both switches at the same time simply flips the channels: left goes to right, and right goes to left.
These two switches allow the six line-level stereo inputs (including the tape and EPL loops) as up to twelve discrete mono inputs! This feature was popular in the 1960s, when mono tuners, phonographs and tape decks had but one output connector.
This switch controls the turnover frequency of the bass control.
Lifted, the "shelving" position, the bass control has somewhat less effect than more common bass controls. It affects the lower frequencies without altering the lower midrange as much as ordinary bass controls.
The lower setting controls only the deepest bass. Its effect is even more subtle than the upper position.
Use this filter to remove noise from scratchy records or AM radio.
This is a 12 dB/octave low-pass filter at 8 kHz. It is only active with the tone controls active.
I never use this filter. It's too sharp and at too low a frequency for use with better recordings; the Quad 34 has much more useful filters for good recordings.
Any time the tone controls are active and this filter isn't selected, a 40 kHz filter is selected instead.
See curves above at filters.
Rear, Apt Holman Preamplifier. enlarge.
The infrasonic filter switch is on the rear panel, near the phono inputs.
Press it IN to remove record warp and turntable rumble. In some digital classical recordings, it will remove low-level infrasonic noise from passing trucks and air conditioning originally present in the recording venue.
The purpose of this filter is to remove subsonic noise that itself is inaudible, but which often can cause speakers or other components to distort. If you see your woofers floating in and out, this filter will stop this excess motion without affecting the music, and stops any Doppler or intermodulation distortion that this cone flutter otherwise could cause.
Leave this filter OUT otherwise. It has no audible affect on good recordings, and reduces infrasonically-induced distortion on records or some classical recordings.
It is an 18 dB/octave high-pass filter -3dB at 15 Hz.
When selected, it is active for every input and every output. The tone controls need not be active, as they must be for the High Filter.
MAIN 2 Output
There are places on the circuit board for our technician to replace jumpers with a resistive pad for the MAIN 2 output, if needed to optimize the gain structure for your power amplifier.
Be sure to hit this when you put in your headphones. The output to your power amplifiers does not mute automatically.
If using the Holman Preamplifier in a laboratory, it might be handy to have a licensed technician readjust the the volume control knob so that unity gain is straight-up.
With two Holman Preamplifiers, you can synthesis and control four output channels from just two input channels.
Apply all your inputs to the first Holman Preamplifier. Use it for all your tone controls, filters and source and tape selections. Leave its Stereo Mode in Stereo.
Connect the first Holman Preamplifier's MAIN 1 output to one power amplifier and your two front speakers.
Connect the first Holman Preamplifier's MAIN 2 output to any line-level input of the second Holman Preamplifier.
Connect the second Holman Preamplifier's MAIN 1 output to the power amplifier for your two rear speakers.
Rotate the second Holman Preamplifier's stereo mode control to L-R, and start by setting it to about unity gain or a bit less, and keep its tone controls flat. The tone and filter settings of the first Holman Preamplifier are fed automatically to the second Holman Preamplifier.
Set balance on the first preamp. Set front-rear balance on the Volume control of the second preamp.
Leave the power switch of the second preamp ON, and plug its power cord into a switched outlet of the first Holman Preamplifier. Now the power is controlled by the first Holman Preamplifier, too.
See also the Apt Holman Preamplifier Service Manual.
Hey, for $300 or so today, the Apt Holman Preamplifier is an incredible steal for stereo music enjoyment.
The only thing missing would be to make it two inches wider to bring it up to the standard 17," and then adding a selectable presence EQ control:
Rockwell-modified Apt Holman Preamplifier. enlarge.
I designed and built this one-of-a-kind variation in 1983, complete with a presence tone control with selectable frequency and bandwidth. It now resides in das Museum der Tonaufnahmetechnik (The Museum of Sound Recording Technology) in Munich, Germany.
If you've found this report helpful, this free website's biggest source of support is when you use these links, especially this link directly to the Apt Holman Preamplifier at eBay (see How to Win at eBay), when you get anything, regardless of the country in which you live. Thanks! Ken.
AudioProz still services these, and still has original parts.
Owner Vince worked at Apt and knows these intimately.
Audio magazine, October 1978, pages 36 and 187.
Audio magazine, October 1979, page 42.
Audio magazine, October 1982, page 14, left side.
Audio magazine, October 1983, page 217 (talks about Preamplifier 2 and the stereophony circuits).
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