This is a pre-amplifier to boost low level audio signals from an instrument (eg electric guitar) up to Eurorack levels, and an envelope-follower to track the amplitude of the audio signal.
It can also be used to insert an external effects pedal into a patch, by routing a line-level signal (eg via the AUDIO OUTPUT module) to the pedal’s input jack and the pedal’s output back via this module.
The INST IN input is a high-impedance input suitable for guitar pickups, including single-coil.
The GAIN knob sets the pre-amplifier gain in the range x1 to x100, to boost an input level of say 100mV up to 5V.
The SIG OUT ouput is the amplified audio signal.
The ENV OUT output is a control voltage envelope which tracks the amplitude of the audio signal. Not perfectly, as shown later.
Circuit
The first op-amp stage (A) is a non-inverting amplifier, with the gain set by VR1 (the GAIN knob) and R2 (gain = 1 + VR1/R2). C1 and R1 control any tendancy for the amplifier to self-oscillate at high-gain settings. The output goes via R5 to the SIG OUT output.
The next two op-amp stages are a non-inverting voltage-follower (B) and an inverting amplifier (C) with gain set by R3 and preset pot VR2 to match op-amp B. The diodes D1 and D2 select the positive peaks from op-amp B and the negative peaks from op-amp C to pump up the capacitor C3 with a rectified version of the audio signal.
After each peak C3 bleeds to ground through preset pot VR3 quite quickly, but is immediately pumped back up by the next peak.
The final op-amp stage (D) is another voltage-follower, presenting a high input impedance to C3. The output goes via R6 to the ENV OUT output, and also to the LED D3.
Note that both outputs can hit 11V at high gain settings with an over-excited guitarist.
Construction
The whole thing is built to fit 4HP module width on a single 7 hole x 31 hole piece cut from a 9cm x 7cm perfboard, leaving room for the 1/4″ jack socket on a flying lead. All components are wired point-to-point with green 30 AWG kynar wire.
A TL074 quad op-amp was used here, which was found to amplify a guitar quite cleanly without additional pre-amplifier components.
The 10-pin boxed power connector and the low-profile electrolytic capacitors (directly onto the pins of the power connector) should be soldered first, for easier access before the other components are fitted. The 10-way boxed power socket is slightly wider than 4HP so must be shaved down with a stanley knife.
The 2-pin header for the input jack and the two preset pots are fitted on the opposite side of the board to the other components, so they can be accessed when the front panel is fitted. The flying lead to the 1/4″ input jack has to route past the power cable.
Adjustment
The two preset pots VR2 and VR3 adjust the response of the ENV OUT output.
The scope traces below (different notes at different timebases) illustrate the compromise between creating a smooth envelope with minimal audio-frequency component while accurately tracking sudden changes in the audio signal.
Adjust VR2 so that both positive peaks and negative peaks in the audio signal (green trace below) contribute evenly to the envelope level (yellow trace).
Adjust VR3 to produce a smooth envelope; higher settings will cause an over-long decay slope if the audio signal stops abruptly.
Ideas and rethinks
The simple input is suitable for a range of low-level signals including guitar pickups, dynamic microphones and line-level outputs, but not high-voltage devices such as piezo pickups because there is no over-voltage protection. To add this protection, clamping diodes might be added as shown here.
As explained above, the envelope is modulated by some audio frequency signal. The final op-amp stage might be improved with heavy high-cut filtering.
Untried suggestion: this might work best as an inverting amplifier with a filter in the feedback loop, so diodes D1 and D2 would have to be reversed to invert the pulses to C3. However, the input impedance of this stage would affect the discharge behaviour.
This module works quite well together with a Sonuus G2M (early model) pitch-to-MIDI convertor to send MIDI “note on”, “note off” and “bend” messages. The MIDI module sets VCO pitch, and the ENV OUT from this module controls a VCA to add the guitar dynamics back to the signal.
It’s also unexpectedly useful for audio input from a drum machine (which is also clocking a sequencer), to route the drum sounds through the synthesizer’s VCF with the ENV OUT adding a bit of CV movement to the filter. ENV OUT also works as a slightly unpredictable gate triggered on the louder drum sounds.
ShedSynth 