Amplifiers Rescue Gods

Another Carlsbro CP1000 Mosfet/HH VX900.  

I think these were built when Carlsbro owned HH.   This one had DC at the output when gains were increased rising to 14V!  Some people would have thought it was just noisy pots when they hear the pop/crackles and then blown drivers.  I straight away suspected a 15v rail for opamps leaking in somewhere..  Disconnected input board at ribbon connector and DC offset vanished. Examined input board and found blown tracks.  This could happen if the ribbon cable isn’t seated correctly i.e. too far over or some sort of high voltage shoved up the rear!  There is full rail voltage around 80v going through this cable on some of the pins..  Anyway, re-built input board.  Opamps and diodes tested fine.  Problem solved.  

Next main board needed refurbishment.  Caps drifting and high esr.  Changed caps.  Input DC blocking cap and feedback caps changed to higher value for a lower phase shift at bass frequencies.  

In the output blocks there are caps that also need to be changed. These caps live in a heated environment.  

Next a bit of experimenting:  input low pass filter caps changed to a higher value.  220pf was used to reduce RF coming in.  opamp feedback loop comp cap was increased in size which resulted in 20khz being around 1db down.  (This is fine for me).  I also increased miller cap slightly in value and lead comp cap.  Yes, this will reduce slew rate limit figures but resulted in a very smooth and powerful amplifier.  Square wave testing with various small capacitive loads and no oscillation.  

I don’t recommend fiddling with feedback comp caps unless you know what you are doing.  Oscillation can easily occur blowing amps.  There are too many amps where in the mistaken notion of chasing the lowest possible distortion at high frequencies, the final result is more often than not a marginally stable amplifier that breaks into oscillation, or rings, with real world loads.  Very stable amps with good phase margin sounds sooo much better.