Simple 2 Chip Drum Machine

[Cynosure]

[Cynosure]

Someone made this into a PCB and has cases for it too. I feel like a grandfather

More info: https://www.muffwiggler.com/forum/topic-174599.html

[blue hell]

[gasboss775]

I really like the bass drum sound but still a bit unclear on its mechanics. 3.8H is a big inductor!
I'm thinking that the 3.8H inductor and 82n capacitor form a series tuned circuit, which in conjunction with the 4069 wired as an inverting amplifier, you get a tuned band pass filter with a centre frequency of:

Fc = 1/(2*pi*sqrt (L*C))

Fc = 1/(2×3.14×sqrt (3.8 × 82 × 10^(-9)))

Fc = 285 Hz approx which seems a wee bit high for a bass drum but the inductor probably has a huge tolerance so the figure could be at either side of this.

Now, I have a 500mH inductor which I believe are still made as spares for wah wah pedals.

Now to get the same Fo = 285 Hz, we need to rearrange the formula;

C = 1/ (2*pi*Fo)^2×L

C = 623.7 n , so probably use a 680n, though might be better with an even bigger cap for lower Fo, will try a 680n then maybe a 1u cap instead.

[IGR]

You are near right, but.. Original LC has damping factor approx. 0.00734 and bandwith of 4,2 Hz. Yours will have damping factor 0.05 /bw 31,83 Hz. It will not so readilly oscillate as you need.

[IGR]

I just thought try small power transformer primary, this should be about 5H.
Check Roland TR 77 schematic too, there is 16H inductance in BD tank, of course LC circuit in parallel. Good luck.

[gasboss775]

IGR I did wonder about this, though I was thinking in terms of Q ( resonance ) only because I am more familiar with that concept.

The mains transformer primary might be worth a try, thanks for the suggestion.

[IGR]

I just breadborded this circuit- primary 3W mains transformer in parallel with 1uF cap, excited by piezo pickup. I got simple passive stompbox, much better then all those cigarbox with piezo. It must be followed by high impedance preamp, otherwise nice boom disappears.

[gasboss775]

I'm going to experiment with this over the weekend, I like the idea of using inductors in music circuits, they are quite unique and quirky.

Craig Anderton made a passive filter using one half of a transformer a bank of capacitors selected by a rotary switch and a pot to vary the resonance.

The CR78 drum machine had a few inductor based percussion voices and I think a lot of these early beat boxes used LC circuits in their percussion voices.

[IGR]

I agree, inductors are piece of magic, partly unpredictable, hard exactly reproductible due core alchemy.

[Cynosure]

I am glad to see that people are still playing with this idea.

I used those huge conductors only because I had them. I had just taken apart an old EQ that was filled with them. They are big things about 1" in diameter.

You should be able to replace the LC parts with RC instead and get the same sound.

[blakeAlbion]

I am very interested in this topic. Where does one buy inductors similar to those in pre-1980 drum machines?

[blakeAlbion]

Huh I see some things here. https://www.electronicsurplus.com/search/inductor

[blakeAlbion]

Uh... but some of these inductors are like 4H, 7H, 15H. Yow. I guess I could build the circuit and then wind me an inductor with some wire and keep wrapping until it sounds good.

[Steveg]

Hi blakeAlbion,
Remember that two inductors in series add their inductances together.
FYI here are some photos of the inductors in Mosc's Moog 914 filter bank: https://electro-music.com/forum/viewtopic.php?highlight=filter+bank+moog&t=14857
Maybe you could talk to analogmonster about how he winds inductors:
https://electro-music.com/forum/viewtopic.php?highlight=filter+bank+moog&t=58576
http://www.analog-monster.de/mmt914_en.html
You can get off the shelf inductors form places like Element 14 in values up to a at least 10H: https://au.element14.com/c/passive-components/inductors/fixed-value-inductors

[blakeAlbion]

Thanks! I saw Mouser has a number of larger inductors, but wow they are big and heavy and high-current. I’m putting this project off for a bit, but I have this Hammond Rhythm box with the rhythm logic, but the voice box is missing. Sigh. So I will circle around to this topic sometime this winter. I will try my hand at transistor-based analog drums.

[Steveg]

Jurgen Haible made a version of the 914 using simulated inductors instead of iron and wire but he used a complex circuit. A simpler gyrator design can be found here: https://sound-au.com/articles/gyrator-filters.htm
The Moog filter uses inductors to ground so gyrators should work perfectly.

[Cynosure]

I don't think you need to use inductors. I was just playing around with them because I had some I took out of an old EQ. They are being used as low pass filters, so you can just use a resistor and capacitor instead.