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Old 2nd February 2004, 06:26 PM   #1
cm961 is offline cm961  Canada
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Default DC offset in commercial amp

Hi guys,

I wish to eliminate the -1.5V DC offset at the output of my Yorkville PM-100 power amplifier. It has 43.8V rails and uses 4xMJ15015 output devices. So it's either gotta be class A or quasi-complementary right? I doubt its class A because its rated for 100W and doesn't get very hot. It is also a pretty old piece of equipment, perhaps that's what caused the DC offset.

Is -1.5V enough to cause a subwoofer to distort under moderate loading? I checked out the output waveform and it looks really clean except for the DC offset.

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Old 2nd February 2004, 07:09 PM   #2
JohnSz is offline JohnSz  United States
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Don't know any specifics on your brand/model of amp, but there are usually 2 pots per channel, on older amps. One per channel for bias and one for DC offset. Doesn't sound like you have the service manual, which this would be covered in. Look on the pcbs to see if there is any printing next to the pots. No expert, but think 1.5v is allot of offset, I think it should be down in the 0-40 millivolt range. 0 being the best. Sealed multi-turn pots are best, but many value oriented manufacturers go with the little cheap ones. If you have the cheap ones, consider replacing with multiturn, or at least clean them. There are some good pictures of pots in URL=http://www.tnt-audio.com/clinica/bias_e.html]this tnt article.[/URL]

On newer amps there is usually just two pots, for bias on each channel. The dc offset is compensated automaticly in the circuit.
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Old 2nd February 2004, 07:34 PM   #3
Netlist is offline Netlist  Belgium
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Hi Pete
It's hard to tell without schematic but here are some attempts:
According to Nelson a real class A amp has to be HOT, real HOT...sometimes %$@^]=) HOT
So yours is not a Real class A amp.
1.5VDC is not the end of the world but a bit high I would say.
Do both channels have the same problem? Try to measure when the amp is cold and again when warm (with speakers connected). Short the inputs while measuring.
Is there a difference?
Look inside the amp. Are there any trimmers, how much, where?
If it's rather old it could be time to replace some caps here and there.

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Old 3rd February 2004, 12:16 AM   #4
cm961 is offline cm961  Canada
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Hi guys,

Thanks for the replies. I checked and there's no pots inside. In fact the circuitry looks quite simple, not that many parts. Except there's about 7 or 8 power resistors which I would presume are for current sharing on the output transistors. So I'm going to go ahead and assume its quasi-complementary, because it doesn't heat up at all. Actually it doesn't really even have a heatsink, just an L-shaped piece of copper. So it looks like my only options are to replace the capacitors, or just live with the 1.5V offset. I think I'll just keep the power supply and case and rebuild a new amplifier into it. Oh btw, its mono. Perhaps I could install two amplifiers into it and make it stereo since it already has two connectors for input and two for output. Hmm, I love thinking up new projects! Not bad for $40 from ebay.

Problem is the 43.8V rails are too high for a GC I believe, so maybe I'll go discrete.

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Old 3rd February 2004, 06:34 AM   #5
tcpip is offline tcpip  India
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Default New amp! New amp!

Originally posted by cm961
Problem is the 43.8V rails are too high for a GC I believe, so maybe I'll go discrete...
The rails seem perfect for an ultra-low-distortion amp from Randy Slone's book. Fairly simple topology, BJT output stages, not too many components, and with output current limiting. And THD specs are about as good as anything I've ever seen anywhere. I'm told this is based on Doug Self's "Blameless" topology, with a few small improvements. Randy may even be able to sell you bromides for making the PCB for it... his PCB layout is about 6"x4", mono. The amp can output about 80W/ch into 8 Ohms.

Just thought I'd suggest what I'd have done.

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Old 3rd February 2004, 06:47 AM   #6
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If you are just looking for an excuse for a project (and a handy platform) then is sounds like you're set. But just to brush up your skillz you should fix what ya got.

With that much offset, look for broken/burnt ground wires or traces. Measure voltage between various ground points to ensure that is hasn't been lost somewhere. If all that looks good, take a look at the differential pairs at the input (assuming it uses one) and replace 'em.
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Old 3rd February 2004, 06:59 AM   #7
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Do as EchoWars suggests
If one channel is ok, you can easily compare the channel with DC-offset to the good working channel.
And since the offset is "low" (1.5V), I think you should look at the input stages
Free Schematic and Service Manual downloads www.audio-circuit.dk, Spare time company (just for fun): www.dupont-audio.com
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Old 3rd February 2004, 10:52 AM   #8
djk is offline djk
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How much gain does this amp have?

Hi-fi spec is 26dB, pro amps can run 32dB~40dB.

This amp probably has discrete transistors for the diff input.

What do you consider acceptable for unmatched diff inputs with no adjustment pot?


How about 37.5mV?

An amplifier with only 37.5mV off-set with 32dB gain will produce 1.5V off-set when the cap in the feedback loop to ground shorts.

I always use diodes across this cap.
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Old 3rd February 2004, 03:22 PM   #9
cm961 is offline cm961  Canada
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Wow thanks for the advice guys! Since its a mono amp I have no second channel to reference to, but I'll check all the ground points and differential pairs (if there are any). I would say that its a fairly low gain amp. In fact, I had to build a stage before it to boost the signal a little.

Will keep you updated!

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Old 3rd February 2004, 11:15 PM   #10
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Default Yorkville

Have you looked on the Yorkville web site for a schematic? If that one's not listed, you may find a similar one. Yorkville are pretty good at giving schematics. Most of the time there is one pasted inside unless someone took it already.
Dan Fraser
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