Hi gentlemen;
A tale of another AHP that I got a while back. Sticky-note on the top said one channel out. Finally opened it up a few weeks back.
It had been "violated"!! Some kind of mods to the phono stage. The volume pot unwired, reversed 180 degrees so you could get to the contacts and wired between the phono stage and an output RCA. All other functions "unwired"
Spent a few days reversing the mods before trying to figure out any problems. Here's where the sweep capability of the Amber 4400 analyzer proved a godsend. One channel was fine - filters and tone controls seemed to work as they should. The other channel showed quite a level/frequency imbalanced across the audio range. Traced the problem to a bad cap in the tone control stage. Removing it from the board showed electrolyte escaping from the bottom but hidden from view. This prompted me to replace all the electrolytics. In the process, did find one other old-timer spewing it's innards.
After this, both channels looked identical in their "operation" with the tone/filter controls. Now on to "better op amps" and power supply.
Of course I've entertained removing as many of the electrolytic coupling caps as I could and this leads to a few questions:
1. Can anyone suggest a simple way to lower the output offset from the discrete phono stage - getting 2.2 V at the input to C17 and 1.6V to C117.
2. A number of the coupling caps are to there to not allow any output offset voltage from the "older" op amps to reside across the potentiometers - volume and balance. I can get under 1mv with the newer op-amps. Would this level be OK across a pot or do we really have to have zero volts to not cause any (longer term) "scratchy" noise problems for the pots??
3. Finally - for now; what is the function of IC11 (A&B) in the output stage? It is apparently called a negative impedance amplifier?? that Holman indicated helps supply some of the signal current to the output. Never heard or seen this before - can someone please shed light on it's function.
Thank you VERY much
Charles
A tale of another AHP that I got a while back. Sticky-note on the top said one channel out. Finally opened it up a few weeks back.
It had been "violated"!! Some kind of mods to the phono stage. The volume pot unwired, reversed 180 degrees so you could get to the contacts and wired between the phono stage and an output RCA. All other functions "unwired"
Spent a few days reversing the mods before trying to figure out any problems. Here's where the sweep capability of the Amber 4400 analyzer proved a godsend. One channel was fine - filters and tone controls seemed to work as they should. The other channel showed quite a level/frequency imbalanced across the audio range. Traced the problem to a bad cap in the tone control stage. Removing it from the board showed electrolyte escaping from the bottom but hidden from view. This prompted me to replace all the electrolytics. In the process, did find one other old-timer spewing it's innards.
After this, both channels looked identical in their "operation" with the tone/filter controls. Now on to "better op amps" and power supply.
Of course I've entertained removing as many of the electrolytic coupling caps as I could and this leads to a few questions:
1. Can anyone suggest a simple way to lower the output offset from the discrete phono stage - getting 2.2 V at the input to C17 and 1.6V to C117.
2. A number of the coupling caps are to there to not allow any output offset voltage from the "older" op amps to reside across the potentiometers - volume and balance. I can get under 1mv with the newer op-amps. Would this level be OK across a pot or do we really have to have zero volts to not cause any (longer term) "scratchy" noise problems for the pots??
3. Finally - for now; what is the function of IC11 (A&B) in the output stage? It is apparently called a negative impedance amplifier?? that Holman indicated helps supply some of the signal current to the output. Never heard or seen this before - can someone please shed light on it's function.
Thank you VERY much
Charles
Is this something everybody has memorized, except me?
APT Corporation Holman - Manual - Stereo Pre Amplifier - HiFi Engine
> 2.2 V at the input to C17
That's a Feature, not a Flaw.
It keeps a known-polarity DC voltage on C17. The rails are very generous so the <2V difference in headroom will never happen in real life.
1mV on pots is often fine. How does it *sound*?
The NIC is "explained" on manual page 16. I worked it out without peeking and then saw their explanation is correct, if not super-clear. I have never seen that outside obscure corners of telephony, but it can work to reduce the current needed from the tone-amp.
APT Corporation Holman - Manual - Stereo Pre Amplifier - HiFi Engine
> 2.2 V at the input to C17
That's a Feature, not a Flaw.
It keeps a known-polarity DC voltage on C17. The rails are very generous so the <2V difference in headroom will never happen in real life.
1mV on pots is often fine. How does it *sound*?
The NIC is "explained" on manual page 16. I worked it out without peeking and then saw their explanation is correct, if not super-clear. I have never seen that outside obscure corners of telephony, but it can work to reduce the current needed from the tone-amp.
Thanks for your response.
I'd like to ELIMINATE as many electrolytics in the signal path as possible.
Pre sounds great now but I understand that DC across the pots can cause noise over time.
AND it IS the BEST manual I've ever memorized!
I'd like to ELIMINATE as many electrolytics in the signal path as possible.
Pre sounds great now but I understand that DC across the pots can cause noise over time.
AND it IS the BEST manual I've ever memorized!
Eliminating the electrolytics, while a noble pursuit, is difficult, problem prone, and has no sonic advantage over just replacing them with good quality low DA, low ESR caps. Very few of the caps in that circuit are completely unnecessary.
Yes, fantastic manual. Always loved that format. Unlikely you'll see anything like that new today.
I recently re-capped an AHP, replaced all of the push button switches too. If I have any issue with that design it's the physical assembly. What a major pain. But a large benefit of the design was low manufacturing cost at the expense of serviceability. It wouldn't have been nearly as successful a product if it had cost more. It was positioned as a giant killer, and it was/is exactly that.
Oh, and go find better tone controls and that interesting mode control anywhere else.
Hello Seanc,
I'm sorry to dig this thread up, but I would like to know what kind of connector you used to allow for the easy removal of the daughter cards on the APT Holman preamplifier ?
And, are you satisfied byt the modification ? Or did it introduce problems ?
Thanks a lot in advance for your answer.
Have a nice and bright day !
I'm sorry to dig this thread up, but I would like to know what kind of connector you used to allow for the easy removal of the daughter cards on the APT Holman preamplifier ?
And, are you satisfied byt the modification ? Or did it introduce problems ?
Thanks a lot in advance for your answer.
Have a nice and bright day !
Tom Holman
He was the designer of the original APT products and formed the company, but had also worked for Henry Kloss at Advent, designed the phono pre section while at Advent, and did the audio system for the Advent Videobeam projector.
He was the designer of the original APT products and formed the company, but had also worked for Henry Kloss at Advent, designed the phono pre section while at Advent, and did the audio system for the Advent Videobeam projector.
Samtec IDSS series ribbon cables.
1 ea: IDSS-11-S-04.00-G-ST8
2 ea: IDSS-23-S-04.00-G-ST8
These are unusual pin counts (11 & 23). NONE of the usual vendors has them in stock, so I obtained direct from Samtec.
They are VERY flexible. Sockets made reassembly & a subsequent disassembly orders of magnitude easier than the original, very stiff, permanently soldered, cables. Very glad I did this upgrade.
And THX for theaters, later THX for home theaters, while at Lucasfilm. And TMH Labs. Pretty full career, certainly not represented well here.
THX stands for Tomlinson Holman eXperimental.
I was in some EE classes with him at the University of Illinois at Urbana, in 1974.
I was in some EE classes with him at the University of Illinois at Urbana, in 1974.
ok, i would have guessed that.
i was thinking you were going to throw me a curve ball.
🙂
I wonder what happened to the engineer(s?) he worked with at APT.
I'm thinking they were pretty sharp, too ....
i was thinking you were going to throw me a curve ball.
🙂
I wonder what happened to the engineer(s?) he worked with at APT.
I'm thinking they were pretty sharp, too ....
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Thank you Seanc !
I try hunting them.
Did you replace the original cable ? And if you replace, use a standard "computer" grade ribbon cable ?
Have a nice day
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Yes, I replaced with the Samtec cables listed above.
IDSS - Slim Body Single-Row IDC Socket Assemblies, 0.100" Pitch | Samtec
IDSS - Slim Body Single-Row IDC Socket Assemblies, 0.100" Pitch | Samtec