[quote author="Harpo"]I would miss a pad at the input section cause your actual mics could change.[/quote]
I don't think a pad will be nescessary. For this application I have always needed 30-50dB gain with different mics. I should be able to get gains as low as 0dB with the That chip.
[quote author="Samuel Groner"]I've simulated and experienced a lot on transformerless mic preamp RFI protection and I must admit that it is a tricky business. A simple LC filter as shown on your schematic is very dangerous. Some microphones (e.g. the modern Schoeps variety) has very low (and presumably inductive) output impedance which will give you a very high Q, resulting in monstrous peaking (15 dB and more). [/quote]
So far I have only used a pair of ferrite beads, so I don't think they do much harm (or have much effect for that matter). The peak would be at a very high frequency at least.
This can even lead to instability problems as the output will feed back to the input at high gains. Solutions include a resistor (about 100 ohm) in parallel with the inductor or a snubber network (see the 9k schematic).
Nonetheless I'd prefer the suggested CM choke. I'd go with about 50 uH, add 100 ohm resistors in parallel (again to control peaking) and use two 1 nF capacitors to chassis afterwards. Farnell 926-5783 or 926-5791 look like suitable parts for the CM choke.
Here's the 9k input stage:
The cut-off frequency of your suggested filter would be a lot lower than the 9k filter, or the filter in my schematic. Still far above the audio range, but why?
Regarding loading: you could try a discrete buffer inside the output stage loop if you don't want another opamp.
I didn't quite get the "inside the output stage loop" part. What do you mean? I have considdered just using an emitter follower after the 1512 chip.
[quote author="JohnRoberts"]
I typically target -3dB change between pan pot centered and hard panned L or R. The values shown only change about 1.5 dB. There are probably multiple combinations of values that meet that criteria but 4x 13k resistors around a 10K (linear taper) pot works if my early morning calculations are accurate. I also show about 10 dB of loss so you need 10 dB of makeup gain in following stage.[/quote]
Yes, four times 13k looks fine to me also. I hadn't noticed your 43k note at first, but I got the same number. So either it's correct, or we're both wrong :grin:
PS to maintain proper polarity from input to output you can just swap +/- inputs for odd number of inversions. In a console you also want to be sure all inserts and such are in proper polarity wrt inputs.
No inserts and stuff needed here - I have to keep it small and simple...
Thanks for the help everybody!
Best regards,
Mikkel C. Simonsen