Hi,
I replaced outputs of a Sansui R-70 amp. I was going to set bias but too low bias value at service manual confused me. Service manual shows 1.5mV across 2*0.33R emitter resistors, which means 3mA quiescent current. Has anyone an idea about that is because of topology or manufacturer's decision for being at safe side. Yes, heatsink is not that big but current setting seems too low somehow.
Thanks.
I replaced outputs of a Sansui R-70 amp. I was going to set bias but too low bias value at service manual confused me. Service manual shows 1.5mV across 2*0.33R emitter resistors, which means 3mA quiescent current. Has anyone an idea about that is because of topology or manufacturer's decision for being at safe side. Yes, heatsink is not that big but current setting seems too low somehow.
Thanks.
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The topology and value of emitter resistor suggest something around 70ma as optimal but as you say, are they playing (ultra) safe or is the heatsinking not up to the job. In practice you won't hear any difference once you get above a couple of ma but 3ma sounds ridiculously low tbh. Turn it up a little and check the stability vs temperature asit warms and also how hot it ultimately all gets.
It's probably just a typo. 15 mV would be more like it (giving 45 mA). As suggested, you can try going warmer, very cautiously. It's always a compromise between crossover distortion, heatsink size and the risk of secondary breakdown.
As explained to me, most of these ran far leaner than optimum. Not knowing on the effectiveness of the thermal compensation, I would stay conservative. 15 sounds good to me.
I did notice. Almost every schematic service manual from that era has at least one major error in it. Anti-copy or just sloppy?
Thank you guys. After opinions from you and some tests, I settled with 25mA across both emitter resistors that results ~40mA. Nothing alarming yet and heatsink runs quite cool. I also didn't forget to replace that ridiculous electrolytic input capacitor with a film type of course. Now, amplifier is sounding really nice after repair and mod.
Right on the money ! That old sansui most likely uses old planar Sankens.
They are optimally biased at 35ma-40ma each.
About half the bias of the ON/fairchild devices.
My old Z3900 service manual says 13ma/per .33R (26 total)for a warranty repair.
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