I'm having strange problems with a PS Audio Model 2 power amp running hot at the output transistors with no signal applied. The DC offset trims normally to less than 5mV and everything sounds fine. The schematic is the center one here in post 24: PS AUDIO Model IV preamp, Model Two power amp, and Elite schematics .
The output transistors had blown and I replaced them with proper (not fake) MJ15003 and MJ15004 ON Semi replacements. The drivers also blew which have been replaced with NTE 185 and 184 equivalents. I have two of these amps and have done this before and not run into this problem. The main heatsink runs about 45 degrees F hotter with the new transistors installed, at 160F. I was able to bring that down to 115F raising the bias point of Q5 by paralleling a 33k with the 6k8 which is quite a drastic change. I can't perceive a sound difference after doing this. The other amp sits around 115F.
Would anyone have any idea what could be causing this change and what might be a better solution? I'm running the output rails a little high at +/- 41V instead of 36V and the regulated supplies are running at 36 instead of 32, but that doesn't seem to be a problem in the other amp.
Thanks for any suggestions.
The output transistors had blown and I replaced them with proper (not fake) MJ15003 and MJ15004 ON Semi replacements. The drivers also blew which have been replaced with NTE 185 and 184 equivalents. I have two of these amps and have done this before and not run into this problem. The main heatsink runs about 45 degrees F hotter with the new transistors installed, at 160F. I was able to bring that down to 115F raising the bias point of Q5 by paralleling a 33k with the 6k8 which is quite a drastic change. I can't perceive a sound difference after doing this. The other amp sits around 115F.
Would anyone have any idea what could be causing this change and what might be a better solution? I'm running the output rails a little high at +/- 41V instead of 36V and the regulated supplies are running at 36 instead of 32, but that doesn't seem to be a problem in the other amp.
Thanks for any suggestions.
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Using the higher power supply voltages shouldn't be a problem. Make sure Q5 has not been damaged. As Nigel has said, set the bias current with R13, for 100mA current BV or BVV with no signal.
Take care,
Doug
Take care,
Doug
Thanks, so I'm on the right track it seems. What I don't understand is that the design has no pot for the bias adjustment. I guess they hand picked the resistors to set it at the factory. I noticed the later model variants have a pot in place of R13.
A resistor is cheaper and more reliable than a pot.
However, if you change drivers or outputs the bias can change and so needs realigning.
However, if you change drivers or outputs the bias can change and so needs realigning.
Yes, you are on the right track. Replace R13 with a 5K pot for bias adjustment. I have repaired dozens of these amplifiers and never seen one without the bias pot, you must have a very early model. I am not surprised that using NTE parts for the drivers resulted in a change in bias setting, I believe that originals are still available from Mouser.
These amplifiers are a bit under-designed for their power output, which is why they tend to burn out their power transistors. Running the rail voltages higher will make this issue worse. If you want to make a huge improvement in reliability and improve sound quality, add another heat sink with a second set of output transistors, with base and emitter resistors, to each channel, in parallel to the originals. Try to match the new transistors to the original parts (Vbe), and you may never have to replace them again.
Take care,
Doug
These amplifiers are a bit under-designed for their power output, which is why they tend to burn out their power transistors. Running the rail voltages higher will make this issue worse. If you want to make a huge improvement in reliability and improve sound quality, add another heat sink with a second set of output transistors, with base and emitter resistors, to each channel, in parallel to the originals. Try to match the new transistors to the original parts (Vbe), and you may never have to replace them again.
Take care,
Doug
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Another thing to keep in mind here is that PS Audio used very cheap single-turn trim pots in these units, I highly recommend replacing them with quality multi-turn types. The same thing applies to the internal bridging switches going intermittent, so watch out for that. As well, I have seen a lot of electrolytic capacitor failures in the the power supplies due to aging, so I recommend recapping the amplifier as well.
Take care,
Doug
Take care,
Doug
Thanks very much for the detailed replies Doug, these are really good sounding amps, but like you said, maybe a little underdeveloped. I'll be running them bi-amped into Acoustat Xs, probably with a passive high pass crossover going into the high end ones. I've run these for years in mono with Acoustat 3s, and they were always on the verge of meltdown if run hard. I think the bi-amp approach, while only half the watts on paper (given stock rails: 40W x 2 for each amp, vs. 160W) will be far more robust. If I blow the outputs again, I'll definitely try the multiple output approach.
One last thing, one of the offset trim pots needs to be hard one side to zero out. Would it make sense to adjust R5 or R6 to center it up a bit?
One last thing, one of the offset trim pots needs to be hard one side to zero out. Would it make sense to adjust R5 or R6 to center it up a bit?
You're very welcome!
I originally had PS Audio model 2s driving Acoustat model 2 speakers, and replacing outputs was something I had to do every few years. Over time I moved to Acoustat 1+1s, slowly upgraded the amplifiers, and then built completely new ones based on the Model IICplus design, with 4 sets of output transistors per channel and a lot of other upgrades. My current amplifier has 56 volt rails and enough power to fully drive the speakers in stereo mode. In 17 years I have never had an output device failure with this design, and the amplifier stays just warm to the touch no matter how hard I drive it.
I have a friend who uses Model IICplus amplifiers in mono mode to drive his Mirage front and center channel speakers of his home theatre. Replacing outputs on his amplifiers was a routine thing until I doubled the number of output devices in his amplifiers, and he hasn't had a failure since.
Has the offset pot always been set close to one side? If so I would consider checking the matching of Q1 and Q2 to eliminate the issue. If it changed when you repaired the amplifier, then I would try to find better matched transistors in the output stage to correct things. It may also be caused by a different part value being off, but I would only replace R5 or R6 if they are significantly different in value from each other.
The PS Audio amplifiers always sounded great with Acoustats, and the IICplus is the best sounding of all the model 2s by a significant amount. Any modifications that you can do to bring your amplifiers closer to that design will bring an improvement in sound.
You should probably also consider upgrading the audio path capacitors in the amplifiers if you haven't already. Makes a nice difference.
Take care,
Doug
I originally had PS Audio model 2s driving Acoustat model 2 speakers, and replacing outputs was something I had to do every few years. Over time I moved to Acoustat 1+1s, slowly upgraded the amplifiers, and then built completely new ones based on the Model IICplus design, with 4 sets of output transistors per channel and a lot of other upgrades. My current amplifier has 56 volt rails and enough power to fully drive the speakers in stereo mode. In 17 years I have never had an output device failure with this design, and the amplifier stays just warm to the touch no matter how hard I drive it.
I have a friend who uses Model IICplus amplifiers in mono mode to drive his Mirage front and center channel speakers of his home theatre. Replacing outputs on his amplifiers was a routine thing until I doubled the number of output devices in his amplifiers, and he hasn't had a failure since.
Has the offset pot always been set close to one side? If so I would consider checking the matching of Q1 and Q2 to eliminate the issue. If it changed when you repaired the amplifier, then I would try to find better matched transistors in the output stage to correct things. It may also be caused by a different part value being off, but I would only replace R5 or R6 if they are significantly different in value from each other.
The PS Audio amplifiers always sounded great with Acoustats, and the IICplus is the best sounding of all the model 2s by a significant amount. Any modifications that you can do to bring your amplifiers closer to that design will bring an improvement in sound.
You should probably also consider upgrading the audio path capacitors in the amplifiers if you haven't already. Makes a nice difference.
Take care,
Doug
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I don't remember how long the offset's been like that, probably from the last time that side's outputs blew. My reasonably cool amp was pulling 390 mA total at Bv so it looks like that one is biased twice as hot as it should be? When measuring the 100 mA, is that just through the outputs or the total draw including the drivers?
The diodes instead of emitter resistors looks like an easy mod. What's the theory on that one? My amps date from about 1980/1981.
The diodes instead of emitter resistors looks like an easy mod. What's the theory on that one? My amps date from about 1980/1981.
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I usually use the voltage drop across the emitter resistors to calculate the current flow. If the amplifier uses diodes instead of emitter resistors, you cannot use that method so I would instead measure the total current flow. That may be why the bias spec for the IICplus is 120mA instead of the 100mA that all the other versions use, as it's easy to disconnect the BV wire in those amplifiers to insert a meter to measure total draw.
The diodes are said to decrease distortion, and were used by a few different manufacturers in the 80s. There is a thread about it on this site somewhere, but I don't have good luck with the search engine here. The idea never really caught on, and if you are using multiple output devices in parallel, then you need to use emitter resistors anyways, to equalize the current between devices, so I personally don't use them.
Take care,
Doug
The diodes are said to decrease distortion, and were used by a few different manufacturers in the 80s. There is a thread about it on this site somewhere, but I don't have good luck with the search engine here. The idea never really caught on, and if you are using multiple output devices in parallel, then you need to use emitter resistors anyways, to equalize the current between devices, so I personally don't use them.
Take care,
Doug
So that's 100 mA one channel only? I've seen elsewhere mention of 30 mA for MJ15003/4. Is it run higher than typical in these amps?
Thanks again.
Thanks again.
Yes, 100mA, one channel only. I use the bias spec only as a guide, and do my final adjustments by temperature. 115F sounds like a reasonable bias point, so you could measure that channel's bias and set the other amplifier to be the same.
Take care,
Doug
Take care,
Doug
Thanks again for the quick and informative replies in this thread. I've got both amps up and running beautifully now. I had an issue with one channel's bias setting which had me baffled for a while until I started measuring all the components. It was literally the last component I would expect to blow on the whole board that was at fault: the one watt 2.7 ohm R3 had gone open. I almost felt stupid meauring it because it's in the input section and is way overrated for its job. It goes to show that you can never assume anything. Installing pots in place of the R13s was a really good idea.
When I measured the other amp (which incidentally requires installing a separate Bv or Bvv feed wire to the heat sink for each channel) One channel was 92 mA and the other was 250 mA, even though the transistors were all of the same manufacturer and type and the bias resistors were within a few percent of each other. It's been running like this for years.
Everything's running cool now in my 4-channel Acoustat X biamp setup with left and right channels handling the low and high frequency transformer sections of the interfaces respectively. I put a passive crossover consisting of a 0.05 uF in series on the highs' inputs. I'll see if I can blow the outputs again, and if so, I'll upgrade to dual output transistors per phase.
When I measured the other amp (which incidentally requires installing a separate Bv or Bvv feed wire to the heat sink for each channel) One channel was 92 mA and the other was 250 mA, even though the transistors were all of the same manufacturer and type and the bias resistors were within a few percent of each other. It's been running like this for years.
Everything's running cool now in my 4-channel Acoustat X biamp setup with left and right channels handling the low and high frequency transformer sections of the interfaces respectively. I put a passive crossover consisting of a 0.05 uF in series on the highs' inputs. I'll see if I can blow the outputs again, and if so, I'll upgrade to dual output transistors per phase.