Polytone Minibrute II LM391 and bias issues.

[mkiijam]

I am working on a Polytone Minibrute II that had a ton of crossover distortion. I am able to scope the output of the driver IC and reduce the crossover distortion to minimum, but I am concerned this might end up in thermal runaway at some future point from what I have heard about this amps.

According to the schematics I have found the driver IC should be a LM391N-80 but there is a LM391N-100 installed. I have read that this discrepancy might cause problems?

I have also read that these amps are supposed to be run so "cold" that they will have crossover distortion. Any thoughts on all of this?

 

[JohnRoberts]

I used a lm391(?)  in a modest power stage for a consumer product (hifi audio delay) I designed for Bozak decades ago.

I had to attach an output stage bias transistor to the same heatsink as the power devices to improve thermal tracking.

This was decades ago so I don't recall all the minor details but yes, the cookbook application design is not as hifi, or thermally stable  as you would hope.

JR

 

[PRR]

> should be a LM391N-80 but there is a LM391N-100

Same thing, higher voltage spec.

There is some reason the '391 chip never got popular.

Putting the on-chip bias a mile away (OK, 2 inches) from the drivers it must track was ignorant.

This seems to be a version *with* added tracking diodes.
http://el34world.com/charts/Schematics/files/polytone/Polytone%20378%20Power%20Amp%20Schematic.pdf

I would not sweat mild crossover in a stage-amp.