output transistor biasing question?

[crippledchicken]

hi there, i have an onkyo m-282 power amp which has no pots for dc-offset, or quiescent bias adjust. have these more than likely been set with resistors? or some other means? i've just installed the mjw3281a/mjw1302a outputs settling in and sounding nicely. according to my emmiter resister measurements, the bias is 45ma. per side. should this be enough? or maybe increased a little for optimum performance? has pretty heavy duty heat sinks and power supply. i don't want to go over board or any thing, just wondered if a little increase might be of benefit. if resisters are used for biasing, how would i locate which ones? in other words, what points on transistors would the connect to? and could pots be installed in place of them. any ideas, appreaciated crippledchicken

 

[subwo1]

That figure sounds fairly standard for a commercial circuit from a Japanese company.

 

[Jan Dupont]

Many commercial power amps has resistors to set the Bias.
Properly for two reasons: Pots ate expencive and can fail, and resistors prevent mis-adjustment by the users (if the try to tweek it).

45 mA seems fair, but as you say, your heatsinks are big so you can try to doubble it or so (I personally don't think that you will hear any difference)

 

[crippledchicken]

thanks for the replys guys, i'll probably just look around inside just as a learning experience, and try to figure how they have biased it for future reference. anyone know of any good links, with schematics of various output stages? i could maybe learn from. thanks again, crippledchicken

 

[boholm]

http://sound.westhost.com/projects-1.htm

http://www.tcaas.btinternet.co.uk/

http://www.epanorama.net/links/audiocircuits.html#amp

Knock yourself out

. . oh of course - where are my manners : http://www.passdiy.com/amps.htm

 

[crippledchicken]

thanks boholm, that helps alot crippledchicken

 

[ChocoHolic]

...you may find an additional but smaller transistor on the heat sink, which is not a driver transistor.
Typically this transistor is used for the thermal compensation and bias adjustment. This transistor normally has some kOhms between base and emitter. If you reduce the value of this resistor you will get a higher bias. But be careful...
And recheck if the bias is not increasing with temperature.

In fact the crossover distorsion may put harm to the sound.
But I cannot offer a rule of thumb.

Two days ago I improved my NAD receiver this way.
The original idle current was less than 15 mA. Sound was poor.
(Sorry to all NAD employees in this forum..., but it was definitely not matching your reputation...)
In first step I went to 50mA. Sound was OK.
Then I increased the idle current to 170mA, which also represents
the thermal limit of the heat sink....
WOW! Not expected but true, now it seems better than
all my previous amps and receivers! More details, much less agressive and at the same time more precision! Seems like I have underestimated the crossover distorsions up to now.
And may be that the friends of class A amps are not just crazy, but
simply have sensitive ears..... just changing my mind these days...

This NAD receiver uses fast "Ringemitter" BJT output stages (2SC5242 & 2SA1962).
....curious about the behaviour of your jw3281a/mjw1302a....

Bye
Markus

 

[crippledchicken]

hi ChcoHolic, thanks for the tip i'll check into that. as far as the mjw3281a/mjw1302 transistors at first, they sounded a little weak in the bass but, after running them in for awhile the bass is sounding pretty full and tight. they seem to be running without alot of heat just bareley warm. i've also changed the drivers to the mje15035/4 drivers which are a little heavier duty, and higher gain than the originals. seems to be no heat problems any where and i think it has a little more dynamic slam, and better transient response for sure. if they turn out to be fakes, and blow up! i'll be sure and post the heads up hehe. thanks crippledchicken