Just a general comment:
Indeed, effective manual writing is quite a task! Long ago I worked in a CSIR department where we had to develop measuring instruments for clients, obviously to be accompanied by a service manual. Some of us were given a whole course in this, and I recall one of the "stunts" of the lecturer: We had to take each other's manuals and try to understand/service the instrument (which of course was designed by the other person). Some red faces followed!
There were indeed manual writers with special training, renting out their services.
Indeed, effective manual writing is quite a task! Long ago I worked in a CSIR department where we had to develop measuring instruments for clients, obviously to be accompanied by a service manual. Some of us were given a whole course in this, and I recall one of the "stunts" of the lecturer: We had to take each other's manuals and try to understand/service the instrument (which of course was designed by the other person). Some red faces followed!
There were indeed manual writers with special training, renting out their services.
Thanks Planet10 and Dimitri 
I see now, it is not so easy to make audio gear, many test to be passed. Is there any specific characteristic of "sonics" of audio gears which have passed all these test?
One good design I see is on Motorola AppNote AN1308 by Andrew Hefley.
My dumb question. If he follows the right procedure in making audio amp, will he still did this?
http://www.diyaudio.com/forums/showthread.php?postid=421553#post421553
I see now, it is not so easy to make audio gear, many test to be passed. Is there any specific characteristic of "sonics" of audio gears which have passed all these test?
One good design I see is on Motorola AppNote AN1308 by Andrew Hefley.
My dumb question. If he follows the right procedure in making audio amp, will he still did this?
http://www.diyaudio.com/forums/showthread.php?postid=421553#post421553
apt 1
if anybody wants info about the Apt 1, i was the final amp test tech at apt for 5 years, and the customer service amp tech also for the last 2 years that Apt was in business. also, i have my original copy of the service manual with schematics..... there were also 9 revisions of the amp boards (plus one prototype that i built in Apt's last few months in business.... replaced the input stage with an op amp, which reduced the noise floor and distortion even more).
i wouldn't mind getting my hands on one again...... had one, but got in so much financial difficulty, i had to sell it.....
btw the output devices were originally MJ15022/23, and they had to be preselected to betas of less than 75.
if anybody wants info about the Apt 1, i was the final amp test tech at apt for 5 years, and the customer service amp tech also for the last 2 years that Apt was in business. also, i have my original copy of the service manual with schematics..... there were also 9 revisions of the amp boards (plus one prototype that i built in Apt's last few months in business.... replaced the input stage with an op amp, which reduced the noise floor and distortion even more).
i wouldn't mind getting my hands on one again...... had one, but got in so much financial difficulty, i had to sell it.....
btw the output devices were originally MJ15022/23, and they had to be preselected to betas of less than 75.
"btw the output devices were originally MJ15022/23, and they had to be preselected to betas of less than 75."
Phase Linear did this too. Why?
"(plus one prototype that i built in Apt's last few months in business.... replaced the input stage with an op amp, which reduced the noise floor and distortion even more)."
I would be interested in seeing that. I always wondered how the APT output stage would sound with a Carver style front end (with a better opamp) and driven from the APT high voltage tier with the Baker clamp.
Phase Linear did this too. Why?
"(plus one prototype that i built in Apt's last few months in business.... replaced the input stage with an op amp, which reduced the noise floor and distortion even more)."
I would be interested in seeing that. I always wondered how the APT output stage would sound with a Carver style front end (with a better opamp) and driven from the APT high voltage tier with the Baker clamp.
the low beta was to keep the output devices from going into common mode conduction if accidentally driven by high frequencies (such as pushing th FF button on a tape deck with the volume up). the junction capacitance is proportional to beta, so the higher the beta the "slower" the transistor.
measurements on the "rev 11" amp were
thd -85db at 100 watts measured at 2khz
noise -95dbV A weighted.
average measurements were usually around -70-75db thd and about -85 dbV noise.
the reduction in distortion actually was a result of reducing the "real estate" (circuit board area) of the input stage, thereby reducing the magnetic interaction between the power supply rails and the input stage. during final test we had to maneuver the power supply wires around to null out the distortion. the reduction in noise was due to the fact that the op amp used had a lower noise figure than the discrete transistor diff amp.
measurements on the "rev 11" amp were
thd -85db at 100 watts measured at 2khz
noise -95dbV A weighted.
average measurements were usually around -70-75db thd and about -85 dbV noise.
the reduction in distortion actually was a result of reducing the "real estate" (circuit board area) of the input stage, thereby reducing the magnetic interaction between the power supply rails and the input stage. during final test we had to maneuver the power supply wires around to null out the distortion. the reduction in noise was due to the fact that the op amp used had a lower noise figure than the discrete transistor diff amp.
Hi Jed,
This is most interesting. If you are not tied down by confidences, may I ask?
You say that lowest distortion was achieved by reducing interaction between power supply rails and input stage. Was this due to signal on the power supply rails? I would imagine that that would be small enough (magnetic induction wise) not to affect even an input stage. Here I am thinking of power stage current drawn - that would understandably generate measureable magnetic fields. Do you mean that anywhere within the dimensions of a practical amplifier, this could be a danger (magnetic field wise) irrespective of placement of the printed circuit? How would distortion (as separate from the signal current, which could presumably result in undesired feedback of the signal itself) be introduced?
Thanks in advance.
This is most interesting. If you are not tied down by confidences, may I ask?
You say that lowest distortion was achieved by reducing interaction between power supply rails and input stage. Was this due to signal on the power supply rails? I would imagine that that would be small enough (magnetic induction wise) not to affect even an input stage. Here I am thinking of power stage current drawn - that would understandably generate measureable magnetic fields. Do you mean that anywhere within the dimensions of a practical amplifier, this could be a danger (magnetic field wise) irrespective of placement of the printed circuit? How would distortion (as separate from the signal current, which could presumably result in undesired feedback of the signal itself) be introduced?
Thanks in advance.
at full power the power supply rails feeding the output devices generate a magnetic field proportional to the instantaneous current they carry (each rail having a magnetic field equivalent to the half-wave of output current it carried, since the amp was class AB). in the Apt 1, these magnetic fields were somewhat contained by the twisted wiring in the wiring harness, but where the wiring broke out to the modules, you had the - rail on one end and the + rail on the opposite end of the module. one wire had to run the length of the module without being part of the twisted pair, and therefore having no cancellation of the magnetic field. the input stage was in the center of the module board, taking up about 4 square inches, the high impedance node of the VAS was also stretched out along 2 inches of the top of the input stage "real estate" and so was also susceptible to magnetic interaction as the + rail or the - rail wire ran past it. the "rev 11" modification used a tl072 op amp to replace the input stage, and also placed both the op amp input stage and the VAS on a 1" square board with a ground plane for additional shielding (the original amp boards were single-sided). this arrangement also removed the requirement to "null" the effects of the rail wires.
a better way to eliminate interaction would be to change the layout of the amp modules so that the rails never come anywhere near the input stage(which if you look at a copy of the board layout and physical layout of the amp is near impossible without making drastic changes in the whole form factor of the amp.
another way would be to reroute the power supply wiring so that it comes to the amp modules directly from the center area of the amp, going perpendicular to the amp board.
a better way to eliminate interaction would be to change the layout of the amp modules so that the rails never come anywhere near the input stage(which if you look at a copy of the board layout and physical layout of the amp is near impossible without making drastic changes in the whole form factor of the amp.
another way would be to reroute the power supply wiring so that it comes to the amp modules directly from the center area of the amp, going perpendicular to the amp board.
Apt Amp Question
I'm wondering if anyone has any ideas re a problem I've encountered with my Apt 1 power amp. The LH channel led indicator goes red (overload)after some playing time. The sound quality does not seem to be degraded in any way. Any thoughts would be appreciated. I'm not sure if anyone is still servicing this device.
Sorry if htis is not the right place to post this question.
I'm wondering if anyone has any ideas re a problem I've encountered with my Apt 1 power amp. The LH channel led indicator goes red (overload)after some playing time. The sound quality does not seem to be degraded in any way. Any thoughts would be appreciated. I'm not sure if anyone is still servicing this device.
Sorry if htis is not the right place to post this question.
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.....
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.....
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?
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.
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.
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.
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.
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
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
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.
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.
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.
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.