APT 1 power amp undeservedly forgotten - Page 8 - diyAudio
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Old 24th June 2008, 11:52 PM   #71
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probably not DC offset, unless it's not enough to trip the relay. could be a low level oscillation or a gain change. the first thing i would do for any of the above would be to replace all of the small and medium electrolytics. there really aren't a whole lot of them, but where they are could cause problems if they're dried out. another possible suspect for low level oscillation at temperature would be the "moxie" device, which is a thermistor on the input. it's purpose is a slow decrease in audio level at the input as the amp gets hot. it's part of the "idiot proofing" of the amp, to turn down the signal when the amp is being run too hot (presumably overdriven). i.ve seen those cause oscillation, as well as the bias transistor bypass cap being open, or one of the bias transistor leads getting hung up on the clip that holds it to the heat sink. electrolytics in any of the current sources on the amp board could also be causing problems. and also check and see if any of the little black clip-on heat sinks have slid down over the leads of the transistors they're clipped to...

interesting, even after 23 years, how all of the information floods back in while i'm giving hints of what to look for. it's like i was there just yesterday.....
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Old 16th October 2008, 06:48 AM   #72
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I just bought the Apt 1 a couple of days ago and am loving it. However, I have one concern that is going to appear rather stupid on this forum. I cannot avoid having a horrible thump when I unplug the amp (no power switch on it). Turning off the pre-amp, which wasn't recommended to me, yields the same result. I am assuming that I am no meant to leave the set-up on at all times. What am I doing wrong here?
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Old 16th October 2008, 09:00 AM   #73
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If it thumps, the speaker relay is not working properly.
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Old 16th October 2008, 11:25 AM   #74
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the speaker relay should disconnect the speakers immediately upon power loss. if not there's a problem with the relay circuit. a secondary problem could be dried out caps in the low voltage power supplies and the DC servo. if it thumps when the preamp is turned off, i'm guessing that it's not an APT-Holman preamp, which has a mute relay to prevent turn-off thump from occurring when the preamp is shut off.

as for the caps that should be replaced in a re-cap of the Apt-1, all of the electrolytics on the amp boards and the protection board. there are a few on the display board, and you might as well do those too. check the heat sink grease on the output devices and make sure it's not dried out. check the tightness of the output device screws. while there was no actual torque spec for them, we used a screwdriver with a 2" diameter ball handle to tighten them, and there's no lock washers on them. overtightening may damage the mica washers. undertightening poor thermal behavior. make sure the little to-92 heat sinks are solidly on the transistors, and not hanging off the transistor leads (which among other things can cause oscillation). i'm trying to remember whether there were 2 or 4 of those little heatsinks per channel..... on either the VAS transistors or the VAS cascodes..... hmm.....i'll get back to you on that one.... it's in the service manual.
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Old 18th October 2008, 03:10 AM   #75
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My thanks to both of you!
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Old 19th October 2008, 04:56 AM   #76
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one other thing to check. there were two different heat sinks used in the APT-1. the first one was a bolt-together fin assembly, the second one was an aluminum extrusion. if your amp modules use the bolt-together heat sinks, check the tightness of the bolts. after 20-some-odd years, thermal cycling may have loosened them a bit.

take out the original big caps under the protection board and shake them. if you hear bits of dried electrolyte rattling around inside, replace them..... btw, we used a standard banana plug with an 8 ohm 2 watt CARBON COMPOSITION resistor wired between the plugs to discharge the caps (discharge between the red wire and the ground wire, and then between the blue wire and the ground wire at the connection points on one of the amp modules). i put carbon comp in all caps because a carbon or metal film 2 watt will burn open instantly without discharging the caps completely.

check the operation of the 8 ohm- 4 ohm switch, and check the operation of the bridging switch (which can get oxidized contacts making ir noisy). my copy of the service manual is in storage, so i don't remember the procedure for checking the load impedance indicator.... IIRC it was to run the amp at half power 200 hz into a 5.1 ohm load and check that the load impedance LED lights (or it might have been full power into a 5.1 ohm load (at least a 200W resistor)). the load impedance indicator adjustment is on the protection board.
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Old 8th November 2008, 12:40 PM   #77
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I have four of these amps,

Unfortunately 2 have failed.

I guess with all the past experience in electronics I can fix them. But finding time is always a problem. Happy to see that these amps still rate so well.

so much to do so little time.


repair 2 or sell ?
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Old 16th February 2009, 10:38 PM   #78
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Boy I have to enthusiastically agree with you about the Apt-Holman power amp.
I up graded my system about 14 years ago. replace my pair of NAD 40WPC integrated (used in bridged mode) which I never like all that much, I thought they were way over hyped by the Audiophile community. the felt cheap and a tad noisy, so-so everything else. I first thought that my dislike was because I sold my beloved Marantz 7C and pair of Marantz 8B's that I drove my Kef 103ab speakers with; but later I found that I could easily find a dozen receivers at the Goodwill that would blow the audio darling NAD 3140's off the shelf.;
Enter the Apt-Holman system. Nicely built and darling of the "Audiophiles on some what of a budget" Apt preamp and 2 unknown to audiophillia Apt-Holman power amps
I had heard the preamp several time but I just did get me excited as i hope it would. the guy I got it from suggested I have it modified at Audiostar in Bellevue, WA which made the Apt preamp really come alive! wow! well worth the the $200 mod.
The amp's, well they are much better than the stock preamp ever was. I have run them bridged on the double speaker system (about 4 ohms) cranking out better than 350WPC for several years and on several other systems. The Apt-Holman power amps are a true find
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Old 5th March 2009, 11:58 PM   #79
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Default Apt 1

I am very gladdened to see your interest after 30 years! So I grabbed an un-updated unit we had around and measured it briefly today. I thought it would be interesting to see how it behaves 30 years after it was made!

Apt Power Amp 1 s/n E03049 probably made in 1979. Measured in March, 2009. Unmodified or repaired insofar as I am aware.

Measured with Prism DScope III s/n 00126. This is a digital unit and the sample rate was set at 192 kHz for its FFT. The THD+N band limiting filter was set to its highest value of 80 kHz.

The load was a Dale 8 Ohm 250 W 1% non-inductive wirewound resistor.

The output of 28.28 Vrms is 100 W in 8 Ohms.

The THD+N at 100 W was as follows:

20 Hz 0.0038%
2k Hz 0.004%
10k Hz 0.011%
20k Hz 0.015%

As I recall it the specification was 0.01% so it is slightly out of that tolerance after 30 years.

The noise floor was -78.4 dB below 2.83Vrms or about 3 mV. This too is a little out of spec (from memory) which was -80 dB re 1 W (+20 dB of output for 100W means 100 dB dynamic range, better than true 16-bit audio, but not as good as true 20-bit audio).

So all in all, pretty darn good. The performance was better than the cosmetics as this unit had taken a beating.

It is likely that replacing the smaller electrolytic caps, particularly the power supply ones, would bring it back into spec I'm guessing.

I also grabbed a modern monoblock power amp off the shelf which shall go unnamed for reasons that will get to be obvious. Here is its THD below clipping, at about 45W or so.

20 Hz 0.046%
2k Hz 0.09%
10k Hz 0.63%
20k Hz 0.77%

The noise floor with the level control down was -79 dBr and with it up -72 dBr.

Yikes. What are these "modern" designs all about?

The performance "secret" in the Apt was to design for a low distortion open loop (with a THD+N meter looking at open loop linearity), get it to have low distortion open loop, then close the loop. This means the 2nd stage, the big voltage gain one, dominates, and the cascode connection helped a lot here.

As time goes by I'd be happy to post schematics, layouts, etc., but I don't think it can be reproduced today since some of transistor types are no longer made. It is hard to stabilize a triple Darlington for instance, and it had a 400 MHz ft pre-driver and a 300 MHz driver (both complementary pairs) driving 4 MHz ft output devices so that their poles could be neglected. I don't know where to get such today.
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Old 6th March 2009, 12:36 AM   #80
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Whoa... welcome!

Yes, there is still interest in this amp after all this time. The diyaudio user ostripper has just recently created a DIY variant in this thread for instance.
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