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Old 27th August 2015, 10:14 AM   #1
pinco is offline pinco  Croatia
diyAudio Member
Join Date: Dec 2004
Location: Croatia, Trogir
Default triple bias

I have problem with bias, triple output, drivers and predrives are mounted on main heatsink in first prototype. And I have a positive drift with classic VBE, amplifier is undercompensated. When I move predrivers on the one heat sink, current was stabilise relatively quickly on nominal bias, cca 10 min. But when I start with a load, 1/3 of power 30 min, bias was down cca 30-40%, amplifier is overcompensated.... In first 100% compensation is not enough, and then predrivers out of main heatsink 100% compensation is to much.
I put a resistor, in bias sensor emitter, 7R5 was OK, bias is stable without and with load, and with short time of load cca 10 s. But, when is cold start, need 60-90 min. without load to stabilize bias, it start with just few mA.... I hawe feeling that VBE is somehow slow with the resistor in sensor emitter, sensor is on top of the MT 200 output transistor.
I reed a book from Cordell and Self, but none use only resistor. With Cordell circuits, different degree of compensation I lost it. Any ideas how to put the right amount of compensation without compromising other characteristics. I tried the idea bonsai NX amplifier, with the NTC and tranistor sensor. It is superfast with SMD, but I have overcompensated amplifier again, and it is the same with any position of trimmer for compesation, predivers on main heatsink.

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