Nyquist aliaser Rate

soggybag · February 25, 2008, 02:56:58 PM

I'm working on a project using JC Maillet's Nyquist Aliaser. I want to create a S&H. The JC's project uses an audio rate sampling clock. I'd like to slow it down to LFO speed. can anyone give me a pointer?

http://www.lynx.bc.ca/~jc/nyquistAliaser.html

brett · February 25, 2008, 07:36:35 PM

Hi
at a guess, I'd say that the 0.1uF cap under the sampling rate variable resistor (pot) controls the frequency of the oscillator. An increase from 0.1 to a few uF might work.
cheers

Eb7+9 · February 25, 2008, 07:42:09 PM

the ocillator needs an extra mod in addition to increasing the charge cap value, or it gets stuck in a dead state ...
sorry I can't share that trick just yet ... a little reasoning on how the circuit works will lead you to it ...

btw, any way to get in touch with MarkM ??

jc@lynx.net

soggybag · February 25, 2008, 08:18:02 PM

Thanks Brett for the replies. I tried a 1uf cap in that spot and it didn't seem to work.

I had some luck with running the FSH-1 at 9v. I was planning on giving that a try this evening. I was thinking about following it with an op-amp buffer similar to what you have.

Eb7+9, it took about a year to realize that your Nyquist aliaser was what I had been looking for a few years ago when I brought up the subject of creating a 9V S&H. in this topic.

http://www.diystompboxes.com/smfforum/index.php?topic=36497.0

At that time I had gotten close. But I gave after much frustration. I'm trying to get back to this again and get it work.

Processaurus · February 25, 2008, 08:27:58 PM

http://geofex.com/FX_images/stutter.pdf

this has a square wave lfo with a variable duty cycle.

soggybag · February 25, 2008, 11:59:30 PM

Thanks for the suggestion. I think I'm going to give the simpler LFO a try first. The Stutter circuit has a lot of parts. I think I should be able to get by with a single op-amp. Plus I'm suspecting the S&H needs a very short sharp spike rather than a square wave, which I suspect is generated by the Stutter circuit.

The Tone God · February 26, 2008, 01:12:00 AM

You can check out Crazy Larry which is an aliaser with an opamp PWM LFO so you can control the frequency and duty cycle. There is also a gate to switch off the clock at low levels but you can leave that out if you want.

Andrew

soggybag · February 26, 2008, 02:05:36 AM

The crazy Larry sounds interesting. Where can I find this?

After a second look the Stutter pedal appears to use two op-amps for the audio path. The other two seem to make up the oscillator, integrator and a wave shaper made of the two diodes. It reminds of an LFO with wave shaper from the Stompbox Cookbook.

I just whipped up a the clock from the FSH-1 on the breadboard tonight. It worked pretty well from a single 9V supply. The rate was a little too fast with a 1M pot. The rate pot is listed as 2M which is hard to get. I don't have any of these lying around.

I hooked up an LED and could get it to pulse. I looked at the output on the scope and it seems to be above ground. The LED seems to be lit and gets brighter with each clock pulse. Which will be a problem. The clock into the sampling FETs will need to keep the gate voltage a certain amount below the drain, if I'm correct in my understanding of how this works.

I'm thinking of taking a tip from the Nyquist aliaser and using the other side of the op-amp as a buffer. It looks like the 100K trimmer on the second op-amp in the Nyquist is used to adjust the level of the clock output?

The Tone God · February 26, 2008, 02:18:48 PM

Quote from: soggybag on February 26, 2008, 02:05:36 AM
The crazy Larry sounds interesting. Where can I find this?

The

function my friend.

Second hit. FX-X entry.

Andrew

Eb7+9 · February 26, 2008, 06:46:12 PM

Quote from: soggybag on February 25, 2008, 08:18:02 PM
Thanks Brett for the replies. I tried a 1uf cap in that spot and it didn't seem to work.

I had some luck with running the FSH-1 at 9v. I was planning on giving that a try this evening. I was thinking about following it with an op-amp buffer similar to what you have.

Eb7+9, it took about a year to realize that your Nyquist aliaser was what I had been looking for a few years ago when I brought up the subject of creating a 9V S&H. in this topic.

http://www.diystompboxes.com/smfforum/index.php?topic=36497.0

At that time I had gotten close. But I gave after much frustration. I'm trying to get back to this again and get it work.

that's going back a while now ...
I remember when I came up with that - was the winter I was living in a cabin on the Cowichan river
just across from this big Cottonwood tree where the Bald Eagles like to hang out when the salmon are spawning
inspiring times, having these big birds cruising through ...
them guys disappearing now ...

that's a good thread, people having noise-source problems with the FSH-1 should give it a careful read ...
just in case anybody's wondering that noise source is flawed ...

darron · February 27, 2008, 02:08:27 AM

it's not an LFO, is it?

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Nyquist aliaser Rate