Over the last years I spent much time studying microphone preamplifier topologies. This resulted in more design ideas that I'll ever be able to build (or at least I have a need for). Imprisoned thoughts don't help much so I decided to draw some schematics in case anybody's interested in building something new next year. Here we go:Design A: A_r1.pdf
Based on the famous topology firstly presented by Cohen, this is a pretty simple no-nonsense circuit. It uses PNP input transistors instead of the ubiquitous NPN for lower noise. In addition to this, a negative bias at the base of the input transistors improves headroom and provides correct bias for the input coupling capacitors even if phantom power is switched off. I've built this design with some minor changes with great success. Just recently had a classical recording in one of the largest concert hall of Switzerland using 16 channels of those.Design B: B_r1.pdf
An attempt to skip electrolytic capacitors (and DC servos). Needs many expensive precision components, but could be pretty stunning performance wise--we're talking about 110 V/us slew rate and around -130 dBu EIN (150 ohm source, 20 kHz) here, right? Don't expect an output offset in the uV range, but about any line input should be able to eat one or the other mV.Design C: C_r1.pdf
An extension to the first design--gain is now distributed amongst two stages. This will greatly reduce distortion at high gains. In addition to this, the frontend is modified for even lower noise. If I'd got asked for a mic preamp optimised for ribbon and moving coil microphones, this would be one possible answer.Design D: D_r1.pdf
That's an elaboration of the "shared gain" topology of which I have already posted two (less detailed) schematics. I can give reference to Steve Dove for the basic idea. The gain setting with the linear pot is pretty neat (D_r1_gain_law.pdf
). Overall a simple yet versatile and good performing circuit.Design E: E_r1.pdf
A solution for all that stocked 2SK170's. Given proper care to the implementation, this design will provide a very low noise figure, essentially limited only by the transformer's DC resistance. Likely very pleasing for dynamic and ribbon microphones. Includes a gain trim option.Design F: F_r2.pdf
A fusion of the transformerless and transformerbalanced topologies used here. Fully balanced and wide gain range yet simple to implement. Note very low quiescent current. Expect low distortion and noise over a very wide gain range. Revision 1 suffered from oscillation, revision 2 corrects this.Design G: G_r1.pdf
Based on the high performance AD797 IC opamp, with a 1:2 input transformer. Fully balanced for very good input CMRR, doubled slew-rate and high maximum output level. Only few parts needed and hence a simple build.
I believe that all designs are well thougth through and conservatively rated regarding stability and component tolerances. Nobody's perfect though, so I recommend breadboarding/prototyping and appreciate any comments.
If someone builds one of these, please let me know. And if there's some serious interest in one design, you might talk me into designing a PCB for it. Not promised though.
That's it for today. Two to four other designs in preparation!