[anguswoodhead]

"installed the carnhill mic and line out transformers. The output transformer mounts on the same centres as the original. The original mic transformer was mounted on the pcb. It was required to drill two holes on the side panel to mount the carnhill. I've countersunk these and used hex head black screws so that it is neat.
One significant variation from the original circuit was in the type of caps used in the main signal path. I've replaced these with the same capacitor type that Neve originally used ( tantalum capacitors ).
I replaced the input load or zobel cap with the correct one for the carnhill transformer.
I tweaked the output stage bias to squeeze max level / min distortion from the output stage.

some performance data :

post mod compared to pre mod:
gain stays the same, noise stays the same
max output level @ 1% distortion improves by 3dB into a 600R load, no significant change into a bridging load

distortion at very hot mic levels distortion at 20Hz improves from 2% to 0.11%, this is a result of a better mic transformer
frequency response with original transformer exhibits a rising response of near 3dB at 40kHz, the modified unit freq response is within 0.3dB from 20Hz to 40kHz.
square wave testing shows an underdamped mic transformer whilst the carnhill with the correct loading cap is critically damped and shows no ringing or rounding.

I have replaced the front panel screws with neater looking black cap head screws. "