The feedback cap(s) is/are apparently shagged in my NAP140, leading to a massive DC offset.
The amp must have been serviced at some point, because there are different feedback caps in each circuit - one is a bright red tant (47uf/6.3v) and the other is a dull maroonish tant (47uf/6v).
I'm gonna replace them, but am wondering - has anyone used anything other than tants in this application? I've had great success with Silmics in the preamp feedback circuits and wondered if they would work here too? Perhaps it's better sticking with some new 47/6 tants from maplin?
I use silmics as the feedback caps in my 110, initially they sounded worse than the ageing tants they relaced. It was only after I replaced the coupling caps (tants for tants) did the silmics sound OK. I did try silmic feedback with silmic coupling, but this sounded awful in my system.
The silmics are still in there, I am very pleased with the sound. I have recently got hold of some CES 47uf/35v axial tants, but canít really be bothered swapping them for the silmics.
I tried silmics for coupling too, with results similar to yours - awful. It was mushy and flat. I did put a BC128 in place of the input coupling tant (next to the signal input connector) and it sounded great... so much so that it stayed.
I've just replaced all the other tants with like-for-like tants from Maplin; the feedback tant is now a 47uf/50v Silmic II from RichardH.
One point of note: I had a massive DC offset between channels (-420mV and -.3mV) before swapping the feedback caps. Now both are around 12mV. I noticed that one of the old feedback tants (which were different colours to begin with) had the polarity wrong! It was reversed on the channel that read -420mV! Sheesh, whoever opened it up last made a mess of that one - I'm surprised it didn't explode! I double checked the correct polarity with a photo from impulse7.com just to make sure ;)
Anyway, I'll let you know how it sounds in a few days.
I found it was only the replacement of the coupling and feedback caps that improved the sound. The other 10uf tants are in the SOA circuit, I donít know what the other 47uf tant does.
If you find the Silmics have changed the character of the sound too much. You could try new tants in the coupling position to bring back the naimness.