I stuck an echo / delay effect on it output and I found the result so fun that I decided to make an original module out of it. I used and I modified the famous Chinese kit with pt2399 (available on aliexpress, eBay, etv …), in particular by adding a CV input for a control of the delay time.
Here is a first sketch, I will re-check and re-test all that, then I will come back with a more presentable diagram and with some examples of sounds obtained, for those who would be interested on this.
I made this simple echo and breadboarded it based off datasheet and some mods from muffwiggler or something. works great. if you go thru the trouble of adding that delay circuit you might as well add an external audio input while you’re at it.
The cv controls are kinda questionable and interact with eachother but fun none the less
Interesting — is that your CV input design? I don’t recall seeing it before. Sam has a somewhat different CV circuit in the Triple Spashback module.
Presumably R9 and C5 control the tom decay time? Have you tried a pot in place of R9?
What’s the reason for such large resistances in the op amp gain stages? Matthew Skala, in his infamous “Design mistakes” article:
But these high impedances should be avoided where possible, among other reasons because of an effect called Johnson noise … And in very high impedance circuits, this noise voltage may be enough to set the overall noise floor of the circuit. It’s usually better to aim for lower component values in op amp circuits in order to bring the level of Johnson noise below that of other noise sources… as a rough rule of thumb, I usually aim for impedance more like 10k or 20k on any op amp input where I don’t need to go higher.
Thank you for all your good work I have already build the same circuit for my module dedicated to the delay effect but here I think that the feedback is not necessary for using with my tom drum.
Thank you for your feedback and explications. I have see this cv design mod in a design of synthrotek, what do you suggest for the best cv control, a cv with a vactrol or this mod with a transistor ?
Yes no problem, more later when I have finish my job. For moment I have just test it on my breadboard, with the tom circuit and the classic pt2399 delay (without the time cv control and without signal amplifier input/output). It was really fun ! I’m very excitate to build the my shematic’s version whith all mods !!
Thank you for your tip about the decay mod, I have replace the 10k resistor with a potentiometer + a cap to GND; this work a little bit but no really good results. (It interacts on the pitch and creates mostly distortion)
This is a T-bridge circuit, so pitch and decay are coupled, and it’s difficult to modify it for independent controls over each… Unless someone have a solution to adding a decay control for this drum part??
I have found this schematic from an analog bass drum workshop. Maybe this would be working with my circuit ? (with some ajustements of course)
This is my last version of this project. Finally I have choose a simple but really usefull cv control with a vactrol. And I have improve input/output amplifiers (NE5532) with the design by @CTorp. Sound is good !
Thank at all for your great help !! I come back to you with some sounds extracts when I does have completely finished this one !
Is the buffer IC2B necessary or can I remove it ???
Well, not from an engineering perspective; the PT2399’s VCO is current controlled, and you can control it very precisely with some rather trivial circuitry. Vactrols aren’t precise at all, especially not if you just run a voltage through them, and CdS photoresistors use banned materials, but if you’re fine with that I’m not going to stop you from using it
For more on different voltage control options, see this article and this thread:
I have finish my tom drum module when we playing with the delay we can make nice beats and with a cv input we can have so funny and organic sounds like bleep and bloops
I have already build the same circuit for my module dedicated to the delay effect 
