This is an interface module which combines multiple functions.  It is a 3 input mixer with a spring reverb with both mix and dry output.  I have a clip indicator set for maximum current through the springs.  I also have an A-440 reference standard as a fourth input to the mixer with a separate output. 

There is also an AC coupled attenuator for driving line level outputs

I mounted the reverb tank as far away from the power supply transformer as possible. There is very little hum at full volume.  I have all of my filters on this end of the cabinet which works well with the reduced depth of the cabinet. 

The Clare M-991 call progress tone generator works well for generation of an A 440 Hz reference tone.  Here is a scope image of the output after gain to +/- 5 volts.

The frequency measures 440.295 Hz.  The deviation is calculated by the equation:

     cents =  1200 × ln(f_b/f_a) / ln(2).

In this design, the ratio of 440.295 Hz to 440 Hz is 1200 × ln(440.295/440)/ln(2) = 1.1603 cents which is within the tolerance of tuning 'by-ear'.

DJB-002 FrontPanelExpress design file

DJB-002 schematic

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