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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
CGS18 Drum Simulator -- Pitch Pot?!?
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jumunius



Joined: Apr 19, 2010
Posts: 346
Location: San Francisco, CA
Audio files: 13

PostPosted: Mon Sep 05, 2011 11:49 am    Post subject: CGS18 Drum Simulator -- Pitch Pot?!? Reply with quote  Mark this post and the followings unread

Hi, I'm fiddling with my Drum Simulator and am wondering if there is a good reason not to replace one or both of the 470k resistors with a 500k pot. In testing, doing this essentially offers a wide sweeping pitch change -- one voice goes from kick drum through toms to metronome-like click.

The only reason I can see so far is that the wiring invites RF and ground hum easily. Using shielded coax cable appears to minimize this. Can anyone else offer any other compelling reasons not to do so?

In other notes, I very much like the sound of bringing the trimpots out to the front, once you set the range per Ken's instruction. This offers variable decay. And if I let it go just a bit too high on the very max end, it seems like it does a kind of nice self-oscillation that could be useful on occasion, much like with a VCF.

And lastly it strikes me that if you need an extra pot space, you could consider using a single pot for "Harmonics" for both voices. Since the Harmonics pot goes from an LED cathode to ground, I assume that you can connect both LED cathodes to the same pot. Obviously you'd lose independent control of them, but that might not be a bad thing for some people.

In case anyone is curious, I've built a few other drum modules (Bass++, DS8, MPS) at this point and I'd highly recommend the Drum Simulator -- it's a little less versatile than some of the others, but what it does, it does very well. Even modding my Bass++ to use a sinusoidal wave doesn't quite produce the same result as the classic twin T design as used in the CGS18. Very pleasing, lots of "pop".
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Mongo1



Joined: Aug 11, 2011
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Location: Raleigh NC

PostPosted: Mon Sep 05, 2011 12:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Hi, I'm fiddling with my Drum Simulator and am wondering if there is a good reason not to replace one or both of the 470k resistors with a 500k pot. In testing, doing this essentially offers a wide sweeping pitch change -- one voice goes from kick drum through toms to metronome-like click.


Hi,

Yes- there is a reason -
The circuit is basically a Twin-T oscillator. The top T is the 470K and 1n caps. The second T is the 1M5 and 1n caps. Both of these networks have to be tuned together for the circuit to do anything useful. I you change the 470K, you'd also need to change the 1Ms as well. If you just modify one of the twin T's, you'll just get some weird filter-like effect out of the circuit.

Gary
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jumunius



Joined: Apr 19, 2010
Posts: 346
Location: San Francisco, CA
Audio files: 13

PostPosted: Mon Sep 05, 2011 12:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Gary, thanks for the prompt reply. While I see that it's not quite the same as properly modulating the pitch, I'd argue it produced a musically worthwhile effect. See the attached sound file of my "Kick" voice (1n/10n pairing), with 470k to ground replaced by 330k pot. It starts well higher in pitch than the original would since the 330k is obviously far off 470k. But the clicky stuff up high is kinda useful.

Even with the shielded cable there are weird hums up towards zero resistance. Not sure if that's the filter starting to self-oscillate or the effect of crosstalk.

But I'm glad you point this out since it reminds me that this proposed mod doesn't actually replace a properly tuned Twin T voice.


CGS_Kick_PitchMod.mp3
 Description:
"Kick" voice (1n/10n pairing), with 470k to ground replaced by 330k pot.

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 Filename:  CGS_Kick_PitchMod.mp3
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Mongo1



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PostPosted: Mon Sep 05, 2011 4:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
While I see that it's not quite the same as properly modulating the pitch, I'd argue it produced a musically worthwhile effect. See the attached sound file of my "Kick" voice (1n/10n pairing), with 470k to ground replaced by 330k pot. It starts well higher in pitch than the original would since the 330k is obviously far off 470k. But the clicky stuff up high is kinda useful.

Even with the shielded cable there are weird hums up towards zero resistance. Not sure if that's the filter starting to self-oscillate or the effect of crosstalk.


That sounds pretty cool! And lets face it - that's the real criteria we all want to meet.

As far as that noise starting as you approach 0, I could definitely see that happening. As you approach 0, the pot you've added is shorting the negative feedback loop to ground for whatever frequencies are able to pass through that part of the twin-T.

You might want to just a put some small resistor value in series with the pot so that it can never really reach 0 ohms. I guess the best thing would be to move toward 0 until you start seeing the problem, and then measure the resistance on the pot. If you put a resistor a little bigger than that in series, it should eliminate the problem.

Gary
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jumunius



Joined: Apr 19, 2010
Posts: 346
Location: San Francisco, CA
Audio files: 13

PostPosted: Wed Sep 07, 2011 1:26 am    Post subject: Reply with quote  Mark this post and the followings unread

Gary good suggestion was just in the midst of trying it when....

Wow, well I don't know what I did, but my "pitch" pot literally caught fire. Mind you, I'd breadboarded it previously and it worked. Then I soldered most of the mods together with a more "appropriate" pot choice and configuration and it caught fire. The circuit was working fine when I was testing one voice then the moment I switched to the other it went haywire.

I'm assuming there's something else going on here -- maybe I created a short when switching wires since there are still a few banana clips in place. I think I'll wait til morning to check this out.
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Mongo1



Joined: Aug 11, 2011
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PostPosted: Wed Sep 07, 2011 4:55 am    Post subject: Reply with quote  Mark this post and the followings unread

Wow, well I don't know what I did, but my "pitch" pot literally caught fire.


Hmmm, that's probably not something you want! Laughing

Generally speaking, when my project catches fire, I find that it's good to do something else for a while.

If it makes you feel better, a few weeks ago I inadvertently reversed the power leads to one of my modules. I found the problem quickly and fixed it, and then decided to go ahead and give it a try. I was rewarded with a big puff of magic smoke and a crater blasted in the top of one of the ICs.

This is an exciting hobby eh?

Gary
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