Yamaha M-4

[unclejed613]

overbias (biasing a class B into class A) can cause some distortion in the crossover area due more to lowered output impedance within the crossover region than to transistor nonlinearities.

[desperateaudio]

DJK

So I understand you clearly say with the 65v for now and turn the bias up a little correct? Or turn the voltage down with the voltage pot and turn the bias up which is what I think you saying?

Also would you be kind enough to give the equation on the amount of current per mV?

Thanks

[desperateaudio]

DJK also when you say cranking the bias up what do you think would be safe point?

Take no issue with the advise. I take on my own responsibilities as I paid next to nothing for this amp and have many others.

[unclejed613]

there isn't a voltage pot. but if you want to lower the rail voltages without getting another tranny, you could do it with a pair of series pass regulators. if they were adjustable, you would then have voltage pots.

[desperateaudio]

there is a voltage pot and it says voltage adjust right under it

[djk]

"overbias (biasing a class B into class A) can cause some distortion in the crossover area due more to lowered output impedance within the crossover region than to transistor nonlinearities."

I know.

There is an optimum bias point for distortion. As you increase bias the distortion will go down, dip, and then go up as you further increase the bias (when measured with an IM anlyzer). As I mentioned before, Yamaha made several amps around this time with class A switches that could be flipped in a matter of seconds, and it didn't make even a small difference (as long as you didn't run it out of power in class A).

Without an analyzer I would probably set the bias to 20mV (about 40mA per device) and be done with it.

"you could do it with a pair of series pass regulators"

Which would then add another 75W per channel of dissipation to the sinks (at ±30V rails), and they would run at the boiling point of water (for a short time).

"when you say cranking the bias up what do you think would be safe point?"

As mentioned before:

"If you just cranked the bias until the heatsinks would be about 60°C the amp would put out about 6.5W in class A. This would require about 130mV of bias. I would try this and see if I liked the sound, and could live with that much heat."

I don't think the heatsink will stand any more (and the output transistors life will probably be reduced).

[djk]

"there is a voltage pot and it says voltage adjust right under it"

Look at the schematic, it's for the front end and only supplies a few mA.

[unclejed613]

on the amp board or the power supply board? sorry, i don't have a service manual for it...

[djk]

"i don't have a service manual for it..."

ACD posted the link, 14MB.

"on the amp board or the power supply board? "

On the front end card, one per channel.

[jacco vermeulen]

The heatsinks of the M4 (B4) do not look very encouraging for Class A operation.
A suggestion might be to open up the bottom plate of the M4 chassis, similar to the openings under the heatsink as in the B4 model, and mount a small desktop processor fan under each heatsink wing.