I wanted an analogue chorus-pedal for my modular stomp-box and after acquiring a few MN3007’s from Hong Kong, I decided to build the Hollis Zombie Chorus as a starter-for-ten – which I did – and I was a bit disappointed with the result.  It may be that the PCB design was sub-optimal or some component was dodgy but I didn’t like the sound much and I had a problem with an excessive audio click when bypassing which I couldn’t eliminate.  So I downloaded a few schematics of MN3007/MN3207-based chorus pedals and decided firstly that they are all much of a muchness and secondly that the Hollis design was pared-down a little too much for my liking.

I added a few components back in (mainly from the design ethos of the Boss CE-2 with a bit of Pearl Chorus thrown in for good measure) and came up with a design which (for obvious reasons) I have called the Hybrid Chorus.

Here’s the schematic:

121SV

The Hollis version is notable by the fact that there is no active mixer for the clean/delayed sound and the design is d.c. coupled internally.  In the Hybrid design I have added Q1 which has freed-up an op-amp to act as an active mixer.  C4, C8 and C9 a.c. couple the circuit internally.

Here’s the prototype:

Hybrid Chorus Board

 

I paid very careful attention to supply and grounding in the PCB design to try and eliminate clock noise from the output.  The PCB design allows for R27 (somewhere between 10R and 100R) and for C21 to be a largish value.  In the end R27 was not required and 22uF is good for C21 instead of 220uF as originally planned.  Originally, I had a 10uF in for C13, but found that the bias voltage was not stiff enough.  Increasing C13 to 100uF cured the problem.

So we have an analog chorus similar in topology to a Boss CE-2 with quite a rich, subtle sound; no clicks or pops when bypassing and almost imperceptible clock noise in the output.  Creamy! 

 

One Response to Hybrid chorus with MN3007

  1. mictester says:

    To get rid of the clock clicks completely you need to separate the bias voltages for the op-amp audio stages from the bias for the LFO. Also, the bias for the LFO (to pins 2 and 5 of the LFO IC) should have equal values to prevent the LFO output hitting the supply rails (try two 10 to 22k resistors).
    Another issue is that John Hollis used a compromise bias scheme for the MN3007 – instead of 10k and 12k, use a 10k preset and 10k at each end. You can then tweak the bias (taken from the wiper of the preset) to make the onset of clipping in the delay line symmetrical.
    Finally, the MN3007 is designed to work at 15V for best noise and minimum distortion specs. It’s well worth using two PP3 9V batteries in series and dropping the supply to your board with a 7815 or an LM317 to get exactly 15V.

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