Easy analog synth project

[Cfish]

Wanted to show a little project I put together.

Is a 16 note analog synth, using Ken Stones schematic using a 74c922 IC to matrix the keys

The EFM VCO1c schematic

And the single transistor VCA I have seen a few places on this sight.

Nothing fancy. But a great first synth project.

Will try to throw in a link to a YouTube video on the project.

https://youtu.be/N4v_UEWV_Z4


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(Update) the key lag in the key matrix circuit talked about in this project was drastically improved by decreasing the capacitor values at pins 5 and 6. The data sheet recommends a x10 proportenment with a minimum 100n and 10n. When I tried it at the minimum values with a quick rigged set of momentary switches it worked well, but may require higher values for other switches.

Thanks for everybodies help with that little snag.

[Cfish]

The UJT in the VCO is available through many vendors on eBay, and I believe Newark has them available too.

I purchased all of my components on eBay.

[PHOBoS]

nice project,. sounds pretty good for such a simple circuit, although slightly out of tune in the higher octaves.
But the bass sounds great, might have to make me something like that myself. I think the only part I currently
don't have is the keyboard decoder IC.

Oh and what is the collector of the transistor in the decoder circuit supposed to be attached to, I assume the positive rail ?

[Cfish]

it tracked a little better once I sat down with a tuner and adjusted the trims.
I would say 3 usable octaves.

Yes the collector of the transistor for the gate ties to the positive rail.

[Cfish]

Here is un update to what I have done with this project.
Instead of the 74c922 circuit to matrix the keys, I finally got a simple volt per octave key board circuit working.



This got rid of my key lag issue and you can scale it to a larger keybed if you want. I'm using it in a set of bass pedals.

( UPDATE ) I used this in a set of bass pedals and a 2 octave controller all winter long, with no problems. Turns out the radiant heat in my house is very stable at 72 degrees. Spring came and the heat is off. The house got up to 78 degrees and the span drifted substantially.

I ordered some op-amps with better temperature drift ratings to try out.

I have high hopes for the OP177 op-amp. At a glance it appears it may plug right in the IC socket and minimize the drift issue. they are about 3 dollars each at DIGI-KEY

(Another update) the OP177 opamps were a great success.

Build it with what you have, but if you need it instrument perfect these OP177's are awesome.

I had to use the heat gun to get a drift I could register.

[L´Andratté]

Hey Cfish! Supernice synth circuit, you compiled there, I think it sounds very good/raw. I actually think I have to check it out myself! Need to get the 2646s first! Banzai says they have them.

Som3where in the video you mention a low pass filter, I didn´t get that (no native English/American speaker), would you explain?

Probably will use push buttons, need to find some playable and durable ones...

Thanks for posting

[Cfish]

Thanks L'andratte. The low pass filter I was using was just a 100k poteonometer with a .47 uf capacitor to ground. Was wired like a guitar tone knob.

would love to see some posts here about your take on the project. Good luck with it.

[Cfish]

This is what I wound up doing with this project.

Really wasn't what I started out to do. But I'm really happy with it.

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[Cfish]

This is the pt2399 delay circuit I wound up using in it.


Here is a link to the video of the almost finished project.

Sorry the amplifier I was using was crap.

https://youtu.be/h1drX1jdUEE

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[Cfish]

To sum it all up

It works great for such an easy project.

It takes a few minutes to warm up the vco, then it stays pretty stable. I did use a matched pair of 2n3906s and bonded them together. Not sure if it helps in this particular situation or not. That's beyond my knowledge level.

I could get about 2 octaves in tune then it fell off quick. The one octave on the bass pedals is perfect on the tuner.

The trimmers on the resistor chain volt per octave key circuit I used in the pedals need to be 10 turn. I started with single turn and got it in tune. But it was a pain, and the gate signal was so touchy to get right. The 10 turns made things a lot easier.

I increased the low pass filter capacitor values more than I ever have in the pt2399 delay circuit, and added another low pass on my input buffer.
I think it's my favorite delay circuit yet. It sounds warm and natural even at long delay times.

[Cfish]

I decided to try this basic idea on a larger scale keyboard using the volt per octave schematic I used in the key pedals.

The span is drifting way to much with small temperature changes.

i ordered a bourns trimmer for the span adjustment. These cheep Chinese ones do seem to drift more with temperature. Fingers crossed

The bass pedals did not have a substantial temperature drift issue, however I was only trying to keep one octave perfect.

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[Cfish]

I have recently came back to this project and am working on a desktop synth using 2 of the VCOs and 2 of the VCAs along with 2 simple transistor envelope generators, and a simple VCF. All passive components.

Should be fun

Here is the board as it looks now. That is actually almost completely populated. So I'm thinking of throwing on a 2 channel ping pong sequencer.

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[Grumble]

A ping pong sequencer? Whats that and what donyou do with it?

[Cfish]

Hi Grumble

Just something to ping pong between the 2 VCOs.

This is what I came up with.

Am trying to do this as an exercise in (getting what I can from discrete components).

Only using circuits from my memory and trying to give them good purpose.

Might do a sample and hold too.

(Edit) I just noticed something. When I was testing this circuit for values to get acceptable speed range output, I used an LED in series with each 470 ohm resistor. I forgot to make value adjustments when I removed them.

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[Cfish]

Well, my memory wasn't so good.

LOL I apparently made the same mistake on both VCOs and can't seem to figure it out.

Have built that VCO schematic more than 10 times and never had it fail to work.

It should be obvious, however it's iluding me.

When I find it I'm gonna feel stupid!!!

[Cfish]

Found it.

And yup, stupid. Flipped B1 and B2 on the UJTs. I kept checking that, and inverting it when I flipped the board back and forth.

[Cfish]

A little on the project using this concept.

All on one board, 2 VCOs 2 VCAs plus 2 Descrete EGs I posted in another place here.

Look at that board, a child could do it.

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