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Class A bias figure in M60 service manual vs M65 service manual

B

Bob Chieldon

Active Member
Both service manuals are available online for the M60/M65 and I have read that the M65 is basically the M60 with a few changes to make it less likely to Chernobyl if something does let go.

I noticed that the manual for the M60 gives a bias figure of around 110mV +-10mV across the emitter resistors when the class A function is activated whereas the manual for the M65 merely says "more than 20mV" from a baseline class AB bias of 10mV +-1mV.

What gives?

110mV gives about half an amp from each output transistor, of which there are three four and so a total class A bias current of around 2 amps, which gets us roughly 20 watts (peak, if I've understood the calculation correctly I think) in class A. EDIT: I got that wrong, there are only two per polarity not three. I assume then the total bias current flowing is the sum of the positive and negative side emitter resistors, which would give 2 amps instead on one and our expected 20w. Am I right in thinking is a peak figure?

A bias which is acceptable by M65 service manual proscriptions of lets say, 25mV would give a class A region of about a watt, which would make the amp out of spec according to it's specifications, surely?
 
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:lurk:

I think this is my first time ever using this emoji! (Which I'm sure is something the world needs to know.) The inquiry draws big curiosity from my geek side, but my got-a-theory-for everything side has got... doodly-squat. My cynical side imagines that Yamaha just didn't want anyone outside the Inner Sanctum to know what the bias should be, but that side has been out-voted.
 
chazix said:
:lurk:

I think this is my first time ever using this emoji! (Which I'm sure is something the world needs to know.) The inquiry draws big curiosity from my geek side, but my got-a-theory-for everything side has got... doodly-squat. My cynical side imagines that Yamaha just didn't want anyone outside the Inner Sanctum to know what the bias should be, but that side has been out-voted.
Click to expand...
Well, cynically speaking they dropped the amount of class A bias as the amps were cooking and didn't want people to cotton on to the fact that the amp has a peak output capability of ~13.5W instead of 20W.
 
Just checked, same story for the M80/M85.

The M80 service manual actually gives a 30w figure for the class A output in the specs section and calls for a standing bias voltage of 130mV across the emitter resistors.

The M85 manual makes not mention of the 30W figure in the specs section and again says "greater than 31mV" for the bias current in class A for a 15.5mV class AB bias.
 
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Bob Chieldon said:
Well, cynically speaking they dropped the amount of class A bias as the amps were cooking and didn't want people to cotton on to the fact that the amp has a peak output capability of ~13.5W instead of 20W.
Click to expand...
LOL … that’s probably the right explanation. Scummy way of handling an issue but I’m not surprised, at all … LOL again.
 
Bob Chieldon said:
I noticed that the manual for the M60 gives a bias figure of around 110mV +-10mV across the emitter resistors when the class A function is activated whereas the manual for the M65 merely says "more than 20mV" from a baseline class AB bias of 10mV +-1mV.

What gives?
Click to expand...
Sounds like a Yamaspiracy. Without being a design engineer or an expert at this stuff...I can only assume at this time. 1. Maybe it was just a clerical error in the SM language between the two models
2. Maybe they realized the left channel was a little weaker than the right and hushed it to avoid TSB's & recalls
3. Maybe both channels were balanced with the black gate caps and we're seeing the quirk after the re-cap

I do know my left channel creeps up to 75ish mV in class A while the right channel sits around 100 mV...on all 3 M-60 models in my stash.
Wanted to find an eloquent way of saying F if I know :biggrin::dunno:

EDIT: disregard item 1. I didn't notice the 3 posts above this, oops.
 
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To be fair, both channels in mine bias at around the same level, I.E. not imbalance.

I’m debating with myself if want my 6.5w back! :biggrin:
 
Questions: When ya push the class A button, what happens in the ckts to make it operate that way?

Also, what role does TR139 & 141 play in Class A?
 
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