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 Forum index » DIY Hardware and Software
self-oscillating filters are ring modulating?!?
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jumunius



Joined: Apr 19, 2010
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PostPosted: Tue Jul 26, 2011 1:07 am    Post subject: self-oscillating filters are ring modulating?!? Reply with quote  Mark this post and the followings unread

So I've built a Buchla LPG, Papareil Polivoks, and CGS Steiner, and all of them sound great in normal ranges and then seem to ring modulate when they get to self-oscillation (sounds a bit like tuning a radio), at the highest regions of Freq and Res settings. Has anyone else experienced something like this?

I've tried to scope this but my scope only goes up to 10mHz and someone suggested it might be something in a higher frequency range.

I have about a dozen modules at the moment, all running off a .8 amp Power One HAA15.
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sneakthief



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PostPosted: Tue Jul 26, 2011 3:05 am    Post subject: Reply with quote  Mark this post and the followings unread

If you can hear it, your scope can see it. Unless you're a bat Wink
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jumunius



Joined: Apr 19, 2010
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PostPosted: Tue Jul 26, 2011 7:03 am    Post subject: Reply with quote  Mark this post and the followings unread

sneakthief wrote:
If you can hear it, your scope can see it. Unless you're a bat Wink


How do you know I'm not a bat?

What if what I'm hearing isn't the oscillation itself but, say, the product of two waves -- the self-oscillation and the mystery frequency above human hearing? Of course I can see the result of the two waves (and it just looks like noise) but so far I can't see the noise source alone, if you know what I mean.

Anyways, what would cause this same problem in 3 filters?

I use all the normal decoupling, ferrites, etc., as called for on the PCBs and I presume my power supply is well designed in this regard.
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sneakthief



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PostPosted: Tue Jul 26, 2011 8:18 am    Post subject: Reply with quote  Mark this post and the followings unread

First thing to do is unplug all the modules. Then plug in each one, one at a time in isolation from the rest and see if it still does this.

Another possibility is that 12 modules on a .8A power supply might simply be too much depending on the sum of their current draw.

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jumunius



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PostPosted: Tue Jul 26, 2011 11:13 am    Post subject: Reply with quote  Mark this post and the followings unread

sneakthief wrote:
First thing to do is unplug all the modules. Then plug in each one, one at a time in isolation from the rest and see if it still does this.


Well, that would be too sensible. Ok, will give it a try. I had a chance to try just the Polivoks before work this morning, and I still got the same effect, but the Polivoks is a crazy filter anyway so maybe it's not the best first test case.

Quote:
Another possibility is that 12 modules on a .8A power supply might simply be too much depending on the sum of their current draw.


Hmm, I'll have to do more calculations but my early guess was that I'd have plenty of room. By the way, when my supply is rated +/-0.8 amps, does that mean .8 on each rail? Or the sum of the two rails = .8?
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sneakthief



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PostPosted: Tue Jul 26, 2011 12:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

This says .8A per rail for the HAA15:

http://datasheet.octopart.com/HAA15-0.8-A-Power-One-datasheet-6656.pdf

Make an audio recording. I also have all 3 filters you mentioned and can let you know if they sound the same as mine.

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jumunius



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PostPosted: Tue Jul 26, 2011 12:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

sneakthief wrote:
This says .8A per rail for the HAA15:

http://datasheet.octopart.com/HAA15-0.8-A-Power-One-datasheet-6656.pdf

Make an audio recording. I also have all 3 filters you mentioned and can let you know if they sound the same as mine.


Excellent, thanks!
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jumunius



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PostPosted: Wed Jul 27, 2011 1:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Ok, now I feel a little dumb, and I don't think this is a fault of my DIY gear at all, but I still have questions about this. So it turns out that the problem I'm having comes when I'm clipping my audio ins to my interface. Embarassed Having to go through the process of recording the sounds made me realize that that is the issue. I hadn't done much recording yet since I'm still finishing building/troubleshooting my modules.

So my question is, is this still a common behavior for clipping of self-oscillation? I don't remember experiencing this so much any time before. I expect the normal sort of clipping sounds -- over-saturation, squared peaks, etc. I ask in case it's a problem with other gear, like my audio interface. Additionally, I've been having problems with noise coming from other gear, and have taken apart a bunch of my setup to remove the noisy gear for now. The noise from my other outboard gear is very different in character from the noise I hear when self-oscillating a filter, but I wonder if the two problems could be different reactions to the same source issue.

Ok, so there are samples in case you want to hear what I mean. The file contains clips from the Buchla and Polivoks, with no input, res set to max and frequency sweeping. Both run through a MOTM-190 VCA and into a Fireface 400. These are the only 3 modules attached to my HAA15.

During each clip I start at a lower gain, and then slightly increase gain midway through. What seems like a small increase on my VCA knob results in a 6-8db increase to my audio input, which then clips, resulting in the modulated signal you hear. (Note that I plugged in a Moogerfooger LPF and was able to get the same results from self-oscillating it, which means my HAA15 is not at fault!)

Thanks in advance for any advice.


SelfOsc_RingMod.wav
 Description:
self oscillation sample from polivoks filter and buchla LPG

Download
 Filename:  SelfOsc_RingMod.wav
 Filesize:  3.44 MB
 Downloaded:  426 Time(s)

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sneakthief



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PostPosted: Wed Jul 27, 2011 1:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Quick question: How much attenuation are you doing before the signal hits your interface?

Don't forget, modular levels are typically 10 Volts peak-to-peak (vpp) and your interface can only handle 2-3Vpp.

I've also noticed that in self-oscillation, some filters can put out as much as 15Vpp!!! Seriously. Who knows what the heck is happening inside your interface when it's getting voltages that high :S

It's no surprise you can't crank your vca.

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jumunius



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PostPosted: Wed Jul 27, 2011 1:45 am    Post subject: Reply with quote  Mark this post and the followings unread

sneakthief wrote:
Quick question: How much attenuation are you doing before the signal hits your interface?


Apparenty not enough. With a pad on the input I could get to about 10:00 on my VCA before it went to ring modulation. My meter says that's about 3v. If I push it all the way, the Polivoks goes to 11v or so. I think the jump of levels at self-oscillation can be so extreme that even if the levels seem fine when I start, one turn of a filter knob can send things over the top.

So if this is the case, how do people use an envelope with their VCA? Do they just set the gain to minimum and adjust CV accordingly? Or do people use more than one VCA or something else to attentuate in between?

Thanks for the reality check!
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sneakthief



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PostPosted: Wed Jul 27, 2011 2:10 am    Post subject: Reply with quote  Mark this post and the followings unread

I have a bank of simple passive attenuators. They're super easy to build and useful for so many things!

Re. your samples: it's normal in the digital world to end up with weird side-effects like that. Every ADC topology is a bit different and so there's going to be a myriad of fail conditions, none of which are pretty.

You should have seen the days when the first DAT recorders were around. It took a while for people to get used to the fact that they were so touchy when being overloaded, unlike reel-to-reels which would get beautiful and crunchy if you pushed them into the red Very Happy

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jumunius



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PostPosted: Wed Jul 27, 2011 8:52 am    Post subject: Reply with quote  Mark this post and the followings unread

sneakthief wrote:
You should have seen the days when the first DAT recorders were around. It took a while for people to get used to the fact that they were so touchy when being overloaded, unlike reel-to-reels which would get beautiful and crunchy if you pushed them into the red Very Happy


DAT... is that like those weird wax cylinders that played through the large cone?

Actually, I remember those days fondly, I guess I went "in the box" for so long that now that I'm using outboard gear again I'm learning it all fresh.

In my defense, I will say that at some point I was having a real problem with my Buchla wiring... when detached from the panel, I could play my deep switch like a theremin. I fixed those issues, and then hearing this weird ring mod / clipping everywhere made me think it was still build issues. That's the problem with DIY as a newbie -- at certain points you expect everything you do has errors, and anything weird you hear gets blamed on your build rather than other gear, patching, etc. Embarassed
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