I wrote this here but probably it was the wrong place. I think it's better to open a discussion.
I have an audiosector premium kit based on the LM3875 (Audiosector by Peter Daniel). The input impedance is set by R2=22K. I need an higher Zin because my tube premp likes higher Z. Have somebody tried an higher R2 value (like 50K-75K)?
I have an audiosector premium kit based on the LM3875 (Audiosector by Peter Daniel). The input impedance is set by R2=22K. I need an higher Zin because my tube premp likes higher Z. Have somebody tried an higher R2 value (like 50K-75K)?
Remember R2 is in parallel with the actual "internal" Chipamp impedance.
Should check the datasheet but faintly remember it was 150k or so.
It means impedance seen by preamp is less than just R2.
Just thinking aloud, using R2 100k (it might increase Chipamp offset voltage somewhat) plus internal Z would end up showing, say, 70k or so to preamp.
Is it happy driving that?
Best solution is to add a unity gain buffer at amp input.
Your choice ; single FET, bipolar transistor or Op Amp.
Using , say, a TL072 which has FET input, you can easily get 1M input impedance which any Tube preamp can drive.
No.
+ and - inputs are internally related for feedback purposes but to external drivers they look completely independent.