May change the left hand pair to see if that is the problem. It's odd that the left channel distorts earlier though and suspect this is due to output transistors. Happy with the results as better than OEM spec. Happy with the FR, although I could roll off the bottom end a little earlier. I thought this forum may be interested some of the tests I've put it through using the QuantAsylum QA-401 and QA451. After 30 years, I eventually decided to try designing and building some replacements based on the Bob Cordell and Douglas Self books. The output stage PCBs were damaged in the early 1980s I've I had this amp kicking around since then. This is a project I did last year - a mid 70s Luxman L-85v refurbishment with a re-engineered output stage. I guess it would be more informative to put the spectrum plot in such posts. As it was working, I hadn't noticed the two dodgy 2SC1345 Q101/102. I've not checked distortion of that separately so could be the source of some of the -80dB / 0.01% THD. Presumably the output impedance is essentially that network summed up with the emitter resistor and transistor Re?įor the preamp, I only recapped it rather than made any mods. I've not changed any of the output wiring or protection circuit which includes relays and inductors. I chose that from balancing optimum modelled distortion (which if I recollect correctly was minimal at 160mA) and real world idle temperature. Should I lower it further? I'm running with 100mA quiescent which according to LTSpice gives me 87mA in Q16 and 7mA in Q14. I thought I was being brave choosing 0.27R instead of the 0.33R used in the OEM design.

They didn't appear to have any thermal bias compensation either whereas I've mounted Q13, Q14 and Q15 on the main heatsink.

The OEM design was topside too so may be one of the reasons why these amps go bang. Hi for the tip on the trim pot - good point, if it goes high we want no bias spread rather than maximum spread.

Luxman amp