Thanks for that explanation Wyn, makes sense to me now. This circuit just doesn't lend itself to widely adjustable gain, it should really be optimized per cartridge.
A couple of things occurred recently that got me off on this track. 1) had some unexpected cash come in, so decided to put some of it into this project and do it right w/better parts, enclosure, hardware, etc. 2) while rummaging through my parts to see what I already had on-hand, came across an old rotary switch, which got me thinking about selectable gains.
So I set about in LTSpice to see what gain resistor values would be needed for 10 x 1dB steps, and that's what exposed the FR issue. I know you had brought it up earlier, but I had forgot about it while concentrating on the 34db gain.
I've run all the sims now, and I can still do it but only w/in a 6-7dB range before the cap values interfere. The plots below are all run with 22uF output caps (the largest physical size that readily fit in the box), and 4 different values of bias cap:
- 100uF - green
- 68uF - blue
- 56uF - red
- 47uF - cyan
First is at 34dB gain. The 100uF remains essentially flat to below 20Hz, but the others show some droop, with the 47uF worst at @ 0.35db down.
View attachment 1313935
The droop improves as gain is reduced and eventually all go flat to 20Hz, down to 28dB gain, which is the last good looking plot.
At 27dB gain, the 100uF cap has pronounced peak, @ 0.4dB, but the others still look good enough.
View attachment 1313936
At 26dB gain, even the 68uF is peaking, with the 56 close on its heels.
View attachment 1313938
I won't bother to show 25dB.
If I had my druthers I'd go with 68 or 56uF, but neither is available in the better audio caps (Silmic or Muse), and I don't know what characteristics are most desirable for this use: low ESR, low impedance, DA, DF, etc. so I'll probably stick with 100uF. Do you have any thoughts on polymer electrolytics? They seem to spec well in most regards.
With 22-24uF limit on the output caps, I can still do 7 gain steps (34-28dB), and will try it.
Good to know I'll finally have a use for all those polystyrenes, thank you!