How to improve the distortion of this amp?

[mtoc]

Hi folks, my friend bought a power amp from a small manufacturer locally, it's single ended, non-feedback, discrete stuff, two big toroidal trannies inside (450W each), diamond output stage, using 6 pair of 2SC5200/2SA1943, output power is about 100w @ 8ohm, here's the FFT result they send us, gosh, there're lot of distortion between 90 - 900Hz compared with 1k - 20kHz (but, there's still a peak at 2kHz), what reasons do you guys think that may course these distortion? How do we tweak it to make it lower distorted in those area?

 

[Craig Sawyers]

It is mains harmonics.  You must be in the UK because they are at intervals of 50Hz.  Nothing to do with signal distortion.

[kevin gilmore]

This is typical of all power amps with unregulated power rails. There is a solution (see uberamp2) which requires a lot more of a lot of stuff. You can mess with the filter settings in audio precision to make much of that appear to dissapear. 

Still 100db down is better than .01% thd, 3rd harmonic invisible, typical of bipolar outputs done that way. About the best you can do without feedback or lots and lots of current mirrors

reducing distortions in circuits is not an easy thing, best to start with something decent to begin with.

Edited July 10, 2016 by kevin gilmore

[Dusty Chalk]

It's funny, I've been frequenting guitar forums a lot lately, so I read this as wanting to improve the quality of the distortion of an amp (as in more, not less).  @ self

[Bespav]

what reasons do you guys think that may course these distortion? How do we tweak it to make it lower distorted in those area?

Ground loop, imho.

Or badly designed feedback.

Or bad IPS/VAS supply.

Or some kind of combination.