I need help for small modifiaction

[rotation]

Hi,

i don't want to use 500R trimmer in my next build, circuit is this:
http://www.twin-x.com/groupdiy/displayimage.php?album=322&pos=0
I thought about simply adding 500R (value of open trimmer-full gain) and 160R in // (or calculate single resistor). Am i right? I want to avoid this trimmer because i never need it.
I have a question regarding the global NFB in this preamp, i searched Google and this forum pretty well before asking; there are // caps and resistors for each gain position, this makes some kind of filter (it's called frequency compensated NFB). I'm wondering why we need a filter here and what it does compared to resistors only, for example like 1M6 resistor from anode to grid of E88CC.?
There is "Orange" version of this preamp with only one 22n in // with resistors from 220ohm to 30Kohm (in 12 steps). Something is telling me that this should work too, but is not so well calculated as in original.
Is there any internet site where i could read about this. I could only find some descriptions of NFB in power amps or phono preamps.

Miha

 

[abbey road d enfer]

I thought about simply adding 500R (value of open trimmer-full gain) and 160R in // (or calculate single resistor). Am i right? I want to avoid this trimmer because i never need it.

You can install just a 120R resistor. The trimmer was necessary because they wanted to be able to swap the modules in case of failure, so they wanted to have the same exact gain.

I have a question regarding the global NFB in this preamp, i searched Google and this forum pretty well before asking; there are // caps and resistors for each gain position, this makes some kind of filter (it's called frequency compensated NFB).

This cap is there to give better stability; this is a phenomenon called phase margin.

I'm wondering why we need a filter here and what it does compared to resistors only, for example like 1M6 resistor from anode to grid of E88CC.?

The cutoff frequency of the filter is well outside the audio range. With 1M6, you have little feedback, so there's almost no risk of instability, but when you reduce the closed-loop gain by applying more NFB, there's a risk of not satisfying the stability criterion. Most of the times this phase-lead cap will solve the problem.

There is "Orange" version of this preamp with only one 22n in // with resistors from 220ohm to 30Kohm (in 12 steps). Something is telling me that this should work too, but is not so well calculated as in original.

A single cap may cover a range of gain, but fitting the right cap for the the right gain value is better practice.

Is there any internet site where i could read about this. I could only find some descriptions of NFB in power amps or phono preamps.

Miha

Google for Bode stability criterion.

 

[rotation]

So this cap in NFB takes care of in/out signal "phase relation"? I don't really understand if this filter delays NFB signal, or it cuts it at certain frequency.
I found some articles about this, hanks for help!

Miha

 

[abbey road d enfer]

The amplifier has phase lag at HF. The cap creates phase-lead in the FB network. The NFB signal is now "more in-phase" with the input signal.