Originally Posted by bace
But have you done any measurement of the actual gain?
No, I can't due my home studio treatment (slow like a cathedral) except a SM7 and a little rane, all my gear are in a storage box.
I did a -75/4pm gain because is the max gain that I use (SM7) for very low thin sources. I did not intend to do it "scientifically" getting different measurres, just try this change and take action quickly.
Yes, I was adjusted the bias but not with a scope. I did not have it when I made.
Now I maintaing 2 issues that need learning and work. Currently only have 2 issues , I will try when I have my space "clear".
- Hum. I did the proposed ground change but is not enough.
- Distortion peek at HF changing TL072 that others IC's need bypass caps to power rails. But... TL072 is not in signal path, seems absurd, but in my case is an improvement of 6dB's.
Disclaimer: I always say "My" pre73, because I think there is less noise pre73 or different sonic characteristics depending on the batch that you purchased.
For example, when I took stock BC transistors, I measures the HFE and were very very different: 100, 200, 400 only a pair with 600, not compensated. The same with the ESR (too high) of one 22uf capacitor that I have yet to change.
I'm not saying this is bad, but denotes a disparity in each unit according to the lottery of components.
Maybe that's why there are two user groups: pre my stock is perfect or good, the other group, a bit disappointed, that do not like or need to change it.
When I asked for months if it did not seem noisy, was a controversial thread. Surely you pre73 is better than mine and I just had bad luck.
I spent weeks reading about changes, but I did not understand, if I want a Neve then I will buy a Neve.
I do not want wanna-be-Neve, I only wanted to improve aspects "measurables" for my cheap mic-pre that I bought only for push a SM7. It's that simple.
The improvement proposed by the Mr. Williams I think is simply the best of all, the most effective.