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Old 2nd March 2016, 05:43 PM   #1
nzst205 is offline nzst205  New Zealand
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Default Class A or AB

I am trying to determine if this amp is Class A or AB from the attached schematic. Is anyone more knowledgeable than be able to give me any guidance please ?
There is large DC Offset +25mV to -234mV between outputs which I assume I can correct with VR001 and VR004, am I on the right track ?
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File Type: pdf Jamo-AVR793 SCHEMATIC page 1.pdf (66.0 KB, 222 views)
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Old 2nd March 2016, 05:56 PM   #2
Mooly is offline Mooly  United Kingdom
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VR601 and VR604 are for bias current adjustment which is calculated from the volt drop across R635/636

There doesn't seem to be any offset adjustment and its hard to deduce how 'good' the offset should be. 200mv at the speaker terminal is very high (but non damaging).

A mismatch in the characteristics of the 2SA970 input transistors could be a contributing factor.
Its almost certainly biased into Class AB but technically it can be biased from Class B all the way to Class A (not recommended, it would fry)
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Old 2nd March 2016, 08:06 PM   #3
nzst205 is offline nzst205  New Zealand
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Great input thank you I think I'll see if I can match 2SA970 a bit better, hoping that would help. If I interpret what you're saying I could bias it more towards AB by using the two trim pots and measuring drop across R635/636, any pointers as to the sorts of measurements I should target ?
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Old 2nd March 2016, 08:12 PM   #4
Mooly is offline Mooly  United Kingdom
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I would guess that this would be biased to around 100ma for optimal results, so that's 22 millivolts DC across either of the two 0.22 ohm emitter resistors. No signal and no speakers connected for the measurement.
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Old 2nd March 2016, 08:16 PM   #5
nzst205 is offline nzst205  New Zealand
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Just what I was looking for, thank you so much, I'll give it a try and see if it stops the heat building up and the cooling fans coming on.
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Old 3rd March 2016, 10:37 AM   #6
erikb is offline erikb  Netherlands
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It's also possible that C605 has become leaky. That would directly translate into DC offset.
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Old 3rd March 2016, 10:52 AM   #7
AndrewT is online now AndrewT  Scotland
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Remove any load from the output.
Short the input to signal ground/return.
Measure the voltage drop across the two collector load resistors R646 & 647.
They should be passing the same current and thus drop the same voltage.
Measure the voltage drop across the mirror emitter resistors R610 & 611. Again they should be the same.
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Old 6th June 2016, 09:19 AM   #8
nzst205 is offline nzst205  New Zealand
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Default Should these be matched better


Still working away on this, repaired Power board Caps (Jameco) dropped way down.
Now back on to the AMP and it's still getting very hot, found huge variation in main power transistors, 2SC5200 (NPN) HFE Varying 60-71 between channels and 2SA1943 (PNP) HFE ranges from 90 - 104 between channels.
These are well within the Min/Max specified on the Toshiba Datasheet.
My question is does such variation matter in this circuit at the power stage ?
Any guidance appreciated.
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Old 6th June 2016, 09:34 AM   #9
AndrewT is online now AndrewT  Scotland
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paralleled output devices will pass different currents if they are not matched.

Upper side NPN and and lower side PNP devices do not need to be matched. Negative FeedBack (NFB) will help to attenuate the small errors introduced by unmatched upper and lower device parameters.

NFB CANNOT attenuate the differences in currents in paralleled devices.

You need to match both Vbe and hFE in paralleled devices at the operating temperature and at the operating currents. This is not easy and not quick.
It is usual to match at the operating bias current and with very low Vce to control the temperature rise. This usually proves to be a "good enough" method.
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Old 6th June 2016, 09:52 AM   #10
nzst205 is offline nzst205  New Zealand
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Tks Andrew, how close would you suggest I target, 18% is not close enough ?
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