Eurorack Hooting Owl

Hey, new to this forum so we’ll see how this post does.

I wanna make the hooting owl Sam showed off from the December 1972 issue of Practical Electronics, and I wanna make some changes to the circuit. I’m still fairly new to electronics, I can use a soldering iron and stuff, but I’m clueless when it comes to the values and modifying circuits so any help would be greatly appreciated.

As the title suggests, I’m looking to make the hooting owl but in a eurorack module. Therefore I’ll need it to work on 12V, and I won’t be using a speaker, just a 3.5mm output instead. I also want CV inputs for the trigger, and also for the 3 pots Sam used in his video. I’m guessing the values of whichever resistors are replaced should be the same for the pots, e.g. a 10k resistor being replaced by a 10k pot but I may be wrong. I also don’t know which resistors to replace, so if someone could tell me which ones I should replace and why that’d be amazing. I’m considering a volume pot before the output too, not sure if it’s worth doing though. This will be my first custom eurorack module so we’ll see how it turns out!

I’m sure you guys will be able to help out, so I look forward to your responses.


Are the schematics posted anywhere?

CV control of something that’s currently controlled by potentiometers can be trivial, or really complicated (see e.g. this thread and the links in it).

Here’s the schematic, page 43

I’ll have a look at that thread, I have wondered how exactly to go about adding CV inputs to the same things as pots.

Thanks for the response!

Sam I think replaced R2, R5, and R8 with pots. Or maybe R3, R6, and R9, but the oscillators are symmetric so that should work out the same, I believe. You’d probably want to go both higher and lower in pitch so I’d say use a pot that’s something like 2x the resistor, so maybe 250k or 500k for R2 and R8, and 100k for R5. You could try vactrols in parallel with the pots for CV, might work OK. For output you don’t need anything to the right of C8, but you might need an amplifier (op amp and a couple resistors) to get it to synth level. I’d just try it and see if it works on +12 V or not, my guess is it will. The “trigger” is just turning power on and off which is a pretty brute force thing to do and not very interesting — if it were me I’d treat it like an oscillator, have it on all the time and plug it into a VCA.


That reply is great thank you, I didn’t think about going up AND down in pitch so having a wider range is a good shout! I’ve heard of vactrols but don’t know what they are, so I’ll look them up now as well as how to do the CV inputs. As for the 12V I’ll give it a go, I don’t imagine it’s hard to go from 12V down to 9V for the circuit if need be anyway. I’ll need to research opamps some more to figure out how to get the signal to synth level, and as for the trigger I’d rather have a CV in than leaving it on as for now, I don’t have any VCA’s (just starting out with eurorack so I’ve literally just got a case, power, sequencer, pico drums and disting ex). It’s something I’ll look into, if just turning the power on and off is all it is then I should be able to do that fairly eaily.

Thank you so much for the response!

So I’ve been looking into this more and I need some help choosing the right values. I’m guessing it’s not super important to get exactly the same resistance values, it’ll just make the circuit sound a little bit different right? Not sure about the capacitors though, what happens if I use a different value capacitor? Can I get away with using a 47uF cap instead of 64uF etc? And I’m struggling to find a modern equivalent of the BC108 transistor, I’m not sure how to go about picking the right transistor for the job.

I was planning on buying a few kits of the most common resistors, capacitors, transistors and diodes in the hopes that I’d be able to start projects off and buy the odd IC or whatever one-off when needed, but how strict do I need to be of the values in schematics?

They’re still being produced, but that circuit should work with pretty much any small-signal transistor, and the modern equivalent BC548 is widely available and a lot cheaper.

Changing the C2 through C7 will just change the pitch. C1 I think is just to smooth out the start/stop and 47 µF should be fine, or if you have a 22 µF you could put that and the 47 in parallel to get 69 µF. Similarlly C8 the exact value’s not critical.

Thanks for the info both of you. I’ll have a look at the BC548 and I’ll try not to delve too far from the values of the capacitors. C2-C7 changing pitch is fine because I don’t need an exact pitch, but I’ll try to keep C1 and C8 as close as possible. Could someone please explain how the “smoothing out” works? I’m aware a common use of capacitors is to keep a voltage consistent, but how do different values affect that consistency? Really I ought to do an electronics course online so if anyone knows of a free course that covers more than just what each component does that’d be great. Thanks guys!