Quote from: Jungle-Jim on February 11, 2015, 07:23:49 AM
Thanks guys
I will have a think what to do with this amp. I have learnt an enormous amount from working on it, but perhaps it's just a noisy amp. I have other SS amps around, and I'm telling you this one has an annoying hiss. It has been a 'project' - with the fantastic help of you guys - to see if we could quieten this amp. I guess if I got it to a technician maybe the source of the hiss could be found, maybe he'd just say it's a hissy amp.

Criticisms of my recording - unfortunately I could only use my phone, so that's not a great start. Yes I put the phone near the amp, but there's already noise and hiss from the phone, so there was no point in having the phone at a distance where you couldn't separate it's own hiss from the hiss of the amp. Believe me from one metre the hiss on this amp is noticeable.

I will put the amp back together, and play it a few more times, and think about what to do.

For the time being perhaps we should close this case - and to quote Spinal Tap - 'best left unsolved'.

I plan to return to this forum soon with my next project - pimping up a Roland Micro Cube.

Thanks
Jim

I wish I'd seen this last year as I could have been helping you.
Considering IC1a and IC1b in your posted circuit diagram you can change the 1Meg feedback resistors to 100k,  the 100k resistors to 10k, increase your added 22pF to 220pF and increase the 10nF not to 100nF as you would expect  but to 47nF for a better sound.
All these ideas can be seen in the later model Award-Session HiFlex140 preamp circuit and found in the S100R112 circuit diagram on the Award-Session site. My own SG30 is now very quiet.

Regards,
Steve

The standard Peavey circuit shown gives a voltage gain without speaker of ~41 but with a nominal 4 ohm speaker this varies approximately between 19.2 and 36 because speaker impedance is greatest at resonance (typically 38-40 ohms at ~110Hz with a closed back amplifier) and also increases from the nominal value as frequency increases from about 600Hz. Now damping characteristics of the amp also change with frequency and speaker impedance. At 4 ohms speaker impedance damping factor will be 0.9 and amplifier output impedance 4.5 ohms (a reasonably good match) while at speaker resonance, i.e. 40 ohms the amplifier output impedance will only rise to 5.5 ohms and damping factor will increase to about 7.3, which is a good thing as it stops the speaker flapping about at resonance.
Look at Rod Elliot's stuff from ESP, Australia for more info on how to calculate all this. I've set up a spreadsheet to convert an Award Session Rockette 30 and am getting quite good results for a 'tube type' sound.
Hope this is useful.
Steve Mitchell