Class A or AB

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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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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
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)
 
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.
 
Should these be matched better

:eek:
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.
 
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.
 
Andrew both measurements at IC=5.00mA IB=5.00mA. Looking at the specifications on the Peak Atlas Pro it indicates that the hFE is Min 3.0V and Max 9.0V.
It's more a question will such a gain variation 18% adversely effect this amp ?
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.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
....Looking at the specifications on the Peak Atlas Pro....
You have avoided the non-problem but the remaining one is that you have been misled into thinking that mA current testing can tell you much that is useful about the 15A current device in your hand.

I have a DCA 75 myself and beautifully designed they are, with their slick, compact graphics and curve drawing software. Too bad it's really only of use for small signal devices just like your DMM or those really cheap Ebay component testers. In electronic measurements and servicing, the curve tracer (a rare and mighty expensive oscilloscope and high power test jig) is the recommended way to test power transistor parameters. For me, that counts out proper testing and leaves simplified tests at fixed currents and temperatures as the only way to compare and match parts.

There are many repair threads that either are about or refer to both professional and rough methods of matching BJT output transistors - there's even a DIY, PC driven curve tracer kit by member locky_z here :) http://www.diyaudio.com/forums/vendors-bazaar/205733-intelligent-curve-tracer-3-0-release.html

In the real world, not many amplifiers are now built with matched components. Matching by using parts from the same tube package is now reliably good enough for output transistors on new equipment and replacements - as long as any transistors in parallel are all replaced at the same time. Getting more fussy than that will not improve your audio - it only calms anxieties about non-problems. Maybe you can't afford a tube of 25 transistors but you could ask your supplier to do that selection, if he buys in that form.
 
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