Class A amplifier bias - need your best guess

speakerfritz · 19th October 2010, 01:21 AM

Got an USHER 150 Watt class A amp which looks like it was made in asia so it's probably a knock off. No schematic anywhere. Its not a version 1.5.

Anyway the amp works. The left channel goes into a static storm after about 5 minutes at low volume, then the left channel cuts out until you crank up the volume. Then the left channel cuts back on.

So I open it up and check the bias and the left channel is 46.5 volts and the right channel is 45.5 volts. So I look for the bias adjust, finding it, I adjust the left channel bias to 45.5 volts. The amp now works fine and I'm not able to reproduce the static storm and left channel cut out.

Side effects of my adjustment are that the right channel bias dropped to 44.5 volts and the left channel's heat sink is warmer that the right channel.

Was thinking about adjusting the bias on the right channel but the pot seems to be locked due to slippage marks that probably glued the pot. So if I force the pot, I run the risk of breaking it. I don't want to get into replacing a pot...my PC board repair skills always result in circuit trace coming off, no matter how low of a heat setting I use.

So what do you think. Leave it alone since it now works. Or fine tune this some more til both channels behave the same. Same bias voltage, and same heat sink temp.

The amp has a single power supply. I don't understand why adjusting one channel drops the bias on the other channel. But it does.

I'm gong to guess that the unit was overseas and the bias was initially adjusted at 50hz, and never re-adjusted at 60hz.

Nelson Pass · 19th October 2010, 01:24 AM

Very unlikely that 50/60 Hz is the issue. A 50 Hz transformer
usually runs a little better at 60 Hz.

speakerfritz · 27th October 2010, 10:28 PM

update.

I would up checking the DC offset. The problem channel had a negitive DC offset while the good channel had a positive one.

I set them both to 000 - +001 and am going to watch it.

That setting dropped the output transister voltages to 45 volts.

Going blind I was shooting for 46 to 47 volts.

Now, is 0 DC offset the goal, or is there a question that they are in AB mode unless I use a higher DC off set value. I read somewhere that DC off set value of 50 puts the output stage in class A mode.

19th October 2010, 01:21 AM	#1
speakerfritz diyAudio Member Join Date: Jul 2008	Class A amplifier bias - need your best guess Got an USHER 150 Watt class A amp which looks like it was made in asia so it's probably a knock off. No schematic anywhere. Its not a version 1.5. Anyway the amp works. The left channel goes into a static storm after about 5 minutes at low volume, then the left channel cuts out until you crank up the volume. Then the left channel cuts back on. So I open it up and check the bias and the left channel is 46.5 volts and the right channel is 45.5 volts. So I look for the bias adjust, finding it, I adjust the left channel bias to 45.5 volts. The amp now works fine and I'm not able to reproduce the static storm and left channel cut out. Side effects of my adjustment are that the right channel bias dropped to 44.5 volts and the left channel's heat sink is warmer that the right channel. Was thinking about adjusting the bias on the right channel but the pot seems to be locked due to slippage marks that probably glued the pot. So if I force the pot, I run the risk of breaking it. I don't want to get into replacing a pot...my PC board repair skills always result in circuit trace coming off, no matter how low of a heat setting I use. So what do you think. Leave it alone since it now works. Or fine tune this some more til both channels behave the same. Same bias voltage, and same heat sink temp. The amp has a single power supply. I don't understand why adjusting one channel drops the bias on the other channel. But it does. I'm gong to guess that the unit was overseas and the bias was initially adjusted at 50hz, and never re-adjusted at 60hz.

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19th October 2010, 01:24 AM	#2
Nelson Pass The one and only Join Date: Mar 2001	Very unlikely that 50/60 Hz is the issue. A 50 Hz transformer usually runs a little better at 60 Hz.

27th October 2010, 10:28 PM	#3
speakerfritz diyAudio Member Join Date: Jul 2008	update. I would up checking the DC offset. The problem channel had a negitive DC offset while the good channel had a positive one. I set them both to 000 - +001 and am going to watch it. That setting dropped the output transister voltages to 45 volts. Going blind I was shooting for 46 to 47 volts. Now, is 0 DC offset the goal, or is there a question that they are in AB mode unless I use a higher DC off set value. I read somewhere that DC off set value of 50 puts the output stage in class A mode.

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