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:
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:
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!