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Apt Holman Preamplifier Review (vintage Audio)

rdenney

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What do you mean "in the playback chain" and what difference does it make in term of signal routing? You feel the tape monitor button degrade the signal?
No, it doesn't degrade the signal because it's all passive switching (except for the buffer). It's a control convenience and understanding issue. If I'm using a PEQ for room correction, I want it on all the time, no matter what the source. If I put it in a tape loop, I'll have to leave that tape loop in "monitor" mode all the time, which means I'm listening to what's being piped to the record bus. That's the Record Selector, or the dubbing controls to route the source to the record bus. The return from the tape loop is activated by the "listening" button being set to Tape 1 or whatever (or by use of the tape monitor switch if there is one). On my Onkyo preamp, it means that the nice lighted controls to show the source basically always said "Tape" and I had to use other controls to select what was being fed to the tape loop (and therefore the EQ). Each preamp handles this a bit differently, and it can get confusing.

And if I want to listen to a tape in the second tape loop, now I have to dub the second tape loop to the first tape loop, and still listen to the first tape loop's monitor input.

But if it has a processor loop, everything--tape monitors and sources alike and their buffering--get switched before going to the processor. So, the tape handling works as we expect it to, without having to run everything through a tape loop and then "dub" that tape return signal to the line amp. And it means the tape loop can be used for what it's supposed to be used for--recording stuff, including the ability to listen to a tape monitor for those recording systems able to monitor what's being recorded (both my tape decks do that, and so does any typical computer USB sound card that I might be using, say, for needledrops).

It's especially complicated for me, because I have a multi-tape/multi-processor signal router in the tape loop of my preamp, and believe me, the buttons on that thing are confusing enough. I need that to support computer recording and two tape machines, plus a switchable processor output at unamplified line level for going to my external headphone amp. But if I'm just listening to a CD, I unswitch the tape monitor and all that stuff is bypassed including any effect of the tape loop buffering, but the PEQ is still happily making room corrections in the processor loop.

In terms of connectivity, they are similar. The output from the preamp to tape loops and to the processor is just switched line-level audio from the input source. The difference is that the signal goes through all the tape loop switching before it gets to the processor loop switching. The return from the processor loop goes to the line amp, which is what feeds the external power amp.

Edit: pics...

The Record Selector is on the left--it determines what is fed to the record bus to go into the tape loop. The Source Selector next to it determines what goes into the line amp (through the processor loop, which is called the "eq loop"--see the button just left of the volume knob).
mc101_selectors.jpg

(borrowed from a for-sale listing that sold a long time ago)

"Tape" as an input on this is a separate line-level source, for a tape playback system only. For playing tapes from something in the tape loop, you have to press the "tape monitor" button just to the right of the source selector.

Rick "who has lived for years at a time with EQ in a tape loop, until preamps that had separate processor loops got cheaper" Denney
 
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restorer-john

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No, it doesn't degrade the signal because it's all passive switching.

Rick, tape loops routinely use resistors to loop out the signal, so as to not load the connected source. As such, there is a hit to S/N even if you looped straight back in with a cable and signal levels are different to the line inputs. And not all line inputs are created equal, some are padded down like CD and tuners.

1637623014103.png


The Holman buffers the rec-outs. That introduces noise when looped back in.

Tape loops can be anything from identical to a line input, to degrading the signal significantly.
 
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rdenney

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Rick, tape loops routinely use resistors to loop out the signal, so as to not load the connected source. As such, there is a hit to S/N even if you looped straight back in with a cable and signal levels are different to the line inputs. And not all line inputs are created equal, some are padded down like CD and tuners.

View attachment 167460

The Holman buffers the rec-outs. That introduces noise when looped back in.

Tape loops can be anything from identical to a line input, to degrading the signal significantly.

I can't tell looking at the B&K if there is a resistor hiding somewhere between the record switch and the record out pins on the ribbon cable back to the back-panel. There probably is, to buffer the output so that the tape deck input load doesn't affect the impedance of the other source inputs, particularly when turned off. But it's probably just a pair of resistors.

But I don't see anything that could be separate resistors for the line inputs. All the switching is done at line level between the RCA plug inputs, and then the selected signal goes to the Tape Monitor switch and then to the EQ output. Don't see a resistor there, either, but there may be one. The EQ return is switched (downstream from the EQ) between the EQ return and the EQ loop input. Then, it goes to the mono/stereo switch, the volume pot, the balance pot, and then to the line bypass switch that either switches in the "aux amp" or a straight wire. Then, it goes to the Tone Engage switch that either switches in the tone amps or a straight wire. Finally, it goes to the High Filter switch, which just switches in an RLC circuit if engaged, shunting high frequencies to ground. The output goes to a relay that sends it either to the pre-out plugs or the headphone jack. Near as I can tell, there is no headphone amp--the headphone output is run directly off the aux amp, but cannot be shared with the pre-amp outputs.

But all I have is a block diagram in the user manual, plus what I can see on the boards.

Again, a real selling point for the Holman is the detailed documentation.

Rick "all line-level inputs seem to be treated the same" Denney
 

Doodski

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But all I have is a block diagram in the user manual, plus what I can see on the boards.
There are schematics of the gain circuitry to pre-out but that is it. I found a <.pdf> that looks pretty decent but not much to see there.

EDITED : to pdf instead of jpg
 

rdenney

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There are schematics of the gain circuitry to pre-out but that is it. I found a <.pdf> that looks pretty decent but not much to see there.

EDITED : to pdf instead of jpg
I've seen that, but it's a "Pro10", which was the predecessor to the "Pro10MC", which was the predecessor (I think!) to the "Pro10MC Sonata". The MC-101 Sonata certainly does not have those LM833N opamps on it--they are all TL072's--so I wasn't sure what else I could trust on it. The TL072's have two or three times the slew rate, among other advantages. And the schematic seems quite simplified compared to what I'm seeing. But we should take this to my thread on the B&K--we are straying further from the Holman.

Rick "apologies to Holman review readers" Denney
 

JeffS7444

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In case anyone's interested in building an analog preamplifier, Douglas Self has a few designs out there and all have tone controls!

Alas, this one appears to be n/a:
http://www.signaltransfer.freeuk.com/preamp.htm

But Preamplifier 2012 (w/MM+MC phono) as published in 3 issues of Elektor can be had as Gerber files:
https://www.elektormagazine.com/magazine/elektor-201204/19844
https://www.elektormagazine.com/magazine/elektor-201205/19863
https://www.elektormagazine.com/magazine/elektor-201206/19885

And PCBs for a slightly more recent design (line only) as published in Linear Audio #5 can be had here:
https://www.diyaudio.com/forums/analog-line-level/280458-doug-self-preamp-linear-audio-5-a.html
 

david moran

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I always wanted to try out a Holman preamp but it never happened. It was really appealing in its features.

iirc Tom Holman was also active in splainin about Transient Intermodulation Distortion which is probably when we transitioned from "transistor amps sound like crap" closer to "straight wires with gain". One of the Audio and HiFi greats.
>> active in splainin about Transient Intermodulation Distortion

well, sort of, meaning he and others demonstrated how it was bogo

>> which is probably when we transitioned from "transistor amps sound like crap" closer to "straight wires with gain".

nah
 

restorer-john

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Pounds, and unpopulated, that''s not a cheap preamp.

I agree, it's complete and utter theft for just the PCB.

I just laughed at the price. And I built a lot of audio gear back in the day. PCBs, even full size preamps and integrated/power amps were reasonable, projects were public domain and support was huge. I see none of that.

But then again, different era I guess. I grew up with Electronics Australia and Electronics Today International, with a huge proportion of the projects coming out of good old Australia. :)
 
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Doodski

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Pounds, and unpopulated, that''s not a cheap preamp.
Ahh gotcha. I was helping a peep with crossovers and making threads and reading about AP and making dinner and skimmed and scanned that web page and thought it was a complete kit. I must not multitask! :facepalm::D
 

restorer-john

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Ahh gotcha. I was helping a peep with crossovers and making threads and reading about AP and making dinner and skimmed and scanned that web page and thought it was a complete kit. I must not multitask! :facepalm::D

Excuses just won't cut it around here dude-ski. ;)
 

Mauro

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At that time I had a dual mono Hafler 220 with Vandersteen 2c speakers and an Oracle turntable. You could get amazing sound, actually, amd then the first gen of horrible sounding CD players kinda played havoc with the 2 channel world.
You might find this Vandersteen 2c measurements interesting..just to push you a little bit more on nostalgia ( ..or not? ):
 

smurfer

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It depends on the implementation. Some do degrade the signal. This Holman has active buffered tape outs which increase the noise very slightly, but offer a decent drive to the processor, whereas unbuffered tape outs load the source.

Personally, I prefer a separate rec-out selector which can route any source to the tape outs, all sources are actively buffered AND can be set to off. Better implementations also shut down the phono stage altogether when it is not being used as it can put noise into the line inputs.

Yamaha CX-1000 (internet pic)
View attachment 167455

John, I'm new here on ASR but I believe we have crossed paths on AK forums. I've been lurking here for a little while and have to say that I've noticed your posts and more and more here lately, as posts providing some important and correct technical analysis/feedback. In other words I'm really grateful to see you active here!
 

rdenney

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To those who update and modify the Apt Holman:

This is a curiosity. The TL072 JFET-input op amp is a good device, but rather long in the tooth, and so I wondered if there was any sense in using a newer replacement that might have better performance. One I see that really is a pin replacement (meaning: that uses a JFET input) is the OPA2132. The only issue I see with it is that its slew rate is faster (20 uV/s rather than 12), which means more possibilities for ringing in the ultrasonic range. They also pull a bit more current which can be a big deal in circuits that have a lot of them. But they have a lower noise floor by a significant (even if not important) margin, and could potentially improve S/N. I know that Tom Holman said that his design wasn't amenable to op-amp replacements, but I also know that he said that when the (then) Burr-Brown OPA series chips were much more expensive, and he may not have considered them. They are cheaper now for do-it-yourselfers, and the Apt Holman doesn't really have that many of them.

Obviously from previous posts, my interest is in my B&K preamp which uses the same op-amps, but with experts who have restored the Holman available in this thread, it seems a good place to ask. It's probably a better idea with the Apt preamp, just because it doesn't use a 100K pot for volume. The B&K has two 100K pots for volume and balance, so the noise from the op-amps is probably less than resistor noise in any case.

Rick "an opportunity to learn more than anything" Denney
 

Digital Mastering System

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To those who update and modify the Apt Holman:

This is a curiosity. The TL072 JFET-input op amp is a good device, but rather long in the tooth, and so I wondered if there was any sense in using a newer replacement that might have better performance. One I see that really is a pin replacement (meaning: that uses a JFET input) is the OPA2132. The only issue I see with it is that its slew rate is faster (20 uV/s rather than 12), which means more possibilities for ringing in the ultrasonic range. They also pull a bit more current which can be a big deal in circuits that have a lot of them. But they have a lower noise floor by a significant (even if not important) margin, and could potentially improve S/N. I know that Tom Holman said that his design wasn't amenable to op-amp replacements, but I also know that he said that when the (then) Burr-Brown OPA series chips were much more expensive, and he may not have considered them. They are cheaper now for do-it-yourselfers, and the Apt Holman doesn't really have that many of them.

Obviously from previous posts, my interest is in my B&K preamp which uses the same op-amps, but with experts who have restored the Holman available in this thread, it seems a good place to ask. It's probably a better idea with the Apt preamp, just because it doesn't use a 100K pot for volume. The B&K has two 100K pots for volume and balance, so the noise from the op-amps is probably less than resistor noise in any case.

Rick "an opportunity to learn more than anything" Denney
The OPA1656 CMOS opamp will work in most locations in the Apt. It's significantly better than 2132. One location needs a OPA2156 due to the high common mode voltage (near upper rail). OPA2156 is R-R In/out. Both '56s have 100ma output current which is much better than older opamps. I have not 'chip rolled' the Apt, but have done a Carver C2 with all 1656s and had great success. The chip was quite tolerant of layout imperfections and I had no stability problems with the 1656s mounted on SO to DIP adapters.
The APT-Holman uses emitter followers (CC) in several places. This accounts for the high 2nd harmonic distortion. I am considering either replacing these with opamp buffers or complementary feedback pair buffers (see Self - Small Signal Audio Design Chapter 3).
These changes should yield sub .001% THD.
 

david moran

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you're entitled to your opinion. Even if it's wrong.

Well, the TMI thing you can look up --- as a concern, here one day, gone the next.

Do go on (not sure you are into substantiation of assertions, though) about xistor amps sounding like crap and then not, or whatever the claim is or was.
 

don'ttrustauthority

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But if you already have a great, well kept, 67 Camaro with all the amenities you need (AC/PB/4 wheel disc brakes, electronic fuel injection [or perhaps an LT4 engine] that you have upgraded over the years, then I personally wouldn't get a new one. I'd just keep driving & upgrading what I have. & enjoy the music. At some point I'll add digital to the mix. A large part of it is "Because I can". And want to.
Does it take unleaded? Have seatbelts? Here I am with Amir.
 

david moran

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Does it take unleaded? Have seatbelts? Here I am with Amir.

One superb design from a decade later which some here might want to keep an eye out for is a preamp explicitly modeled on (and featuring several small improvements over) the Apt: the dbx CX1. (For current users or anyone else interested, I have schematics and OM, which I wrote.)
 

anmpr1

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Just some misc comments from one who was there then:
- Jpn speakers were awful then and not much better afterward; I was interviewed by the WSJ about it and had to say 'No one knows why'

- American speakers, some of them, were and remain superb --- Allisons, AR, ...no JBL at the time.

The Holman work was done in the 1970s and carried into the 1980s and a bit beyond, before audio and its market began to change so drastically.
David Moran, of the Boston Audio Society? Welcome. I've followed the BAS for years, often with a smile. As I recall, the typical BAS 'type' speaker was the 'polite' variety. The 'Boston' or 'New England' sound. More Henry James than Howard Lovecraft, for sure.

Allison? I recall Peter Aczel and David Rich, certainly not uncritical practitioners, panned them. In fact, I don't even think they ran the review, out of courtesy. Why kick 'em when they are down? Allison, a serious practitioner who got his start at AR, offered a variant of the AR formatted sound. AR was viewed (heard?) by many as a quite substandard loudspeaker. At least when compared to anything 'live'. That is, a live performance. It certainly had something new from a late 1950s perspective--low bass in a small cabinet; yet the rest was characterized by closed down and muffled sonics. Am I being harsh in my characterization? It's why Lansing sold so many L100s... that speaker, for all its faults, didn't sound like it was buried inside a large quilt blanket.

Japanese speakers were awful? I recall Harvey 'PT Barnum' Rosenberg building a cost no object variant of his Futterman OTL for a select few owners of Stax electrostatics--the floor standers, not the headphones. They probably didn't think they were unfortunate. But I know what you mean. No one really liked Japanese speakers, even the misguided folks who sprung for Yamaha's NS1000-- those who still think it feels (sounds?) good to them. What do they know, eh?

The 'Holman' gig was definitely a benefit to the audio scene, albeit because he and his company were based in practical engineering, and not due to any 'magic' the designer brought to the table. In the late '70s Holman came up with a lot of 'esoteric tests' that razzle-dazzled folks. For his part, Aczel was 'intrigued' by Holman's discussion of 'cartridge inductance interaction', and 'the spectral content of square waves passed through the preamp stage after suitable RIAA preemphasis' and so on and so forth. But eventually he (and Holman, if I understand it correctly) came to the conclusion that that was all a chimeric chase. What mattered was distortion, FR, getting the RIAA right, and build quality at a price point.

What makes this preamp 'special' is that it was designed and built by someone who was seeking the best engineering at a reasonable price. It's as simple as that. FWIW, I wonder how another 'Boston' area preamp, the DB Systems, might compare to Holman's? I think you can still buy a new DB preamp. Is Dave Hadaway still plugging along? Hopefully it is the case. We are all getting older. If I need a new preamp, I'd seriously consider the DB. Just because.
 
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