This page provides information about several of my mic preamp designs. If you wonder about some of the strange names—these are all mountains in the Italian speaking part of Switzerland.
Advanced design using four discrete operational amplifiers. Transformer balanced input and both impedance and transformer balanced output. Features a phantom power, polarity and “20 Ω Mic” switch. Overall gain range −10 dB to 67.5 dB in 2.5 dB steps (transformer balanced output with 6 dB more level). A few pictures can be viewed as well.
Third file lists resistor values (red column) for a dual wafer 24 position gain switch.
Gerber files for this design can be downloaded here: sassariente_r1.zip
An elaborated implementation of design D listed in the Design Collection paragraph below. No electrolytic capacitors and DC servos in the direct audio signal path. Overall gain range 7.5 dB to 65 dB in 2.5 dB steps. The third file lists resistor values (red column) for the 24 position gain switch. Check the pictures.
Slew rate is typically 110 V/μs and EIN has been measured at about −129 dBu for a 150 Ω source (20 kHz bandwidth, 65 dB gain) and −135 dBu with shorted input.
Frequency response is nearly independent of gain setting. Only at the highest gain and frequencies there is some indication that the first stage runs on somewhat reduced loop gain, although the −3 dB bandwidth of the first stage is still above 1 MHz. Note the fantastic low-frequency response—a direct result of having only one AC coupling capacitor pair in the signal path.
The THD+N plots show good performance as well. The residual consists mostly of noise, except for the highest gain where a faint of third harmonic starts to become visible above 5 kHz.
As can be noted from the last file CMRR degrades at low frequencies. This is mainly a matter of matching the input coupling capacitors C105 and C106. If possible the matching should be as good as 0.5 % or better (e.g. by the addition of small correction capacitors).
Gerber files for this design can be downloaded here: monte_generoso_r1.zip
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.
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.
Design A: A_r1.pdf
Based on the famous topology presented by Cohen in an AES PrePrint, 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 (see some pictures).
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/μs slew rate and around −129 dBu EIN (150 Ω source, 20 kHz bandwidth) here, right? Don’t expect an output offset in the μV range, but about any line input should be able to eat one or the other mV.
This design has been implemented, see Monte Generoso above.
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. 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). Loading of the first stage deserves some attention in this topology—see D_r1_loading.pdf. Overall a simple yet versatile and good performing circuit.
Design E: E_r1.pdf
A solution for all that stocked 2SK170s. 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 instability, 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.