My Noise Toaster picks up radio stations (finished + photo)

[sndbyte]

Hi,

I just finished building the Noise Toaster and notice that it is picking up radio stations. I removed U3 (the speaker amp) to see if the problem would go away, but it made no difference... it still picked up a radio station (I am using the external output in this example since the speaker obviously does not work without U3).

Any ideas on what could be causing this problem?

I've built 3 other MFOS projects (WSG, Soundlab and Soundlab+) but never had this problem before.

Thanks in advance for any advice!

[sndbyte]

Update: I may have isolated the problem. I first removed all LM386 amp components but that did not stop the radio interference.

I then got out my scope and also listened to different points on the pcb. I listened to the output of the square and ramp waves and there was no radio interference at all. I also listened to the output of the noise circuit and could not hear the problem.

I then listened to the filter. The inputs to the filter were fine, but the output of the filter (pin 1 of U2) had the radio interference. So it looks like the filter is the problem.

I'll next check each filter component and perhaps replace each. I have already tried 2 different LM324 chips but that did not make any difference.

[sndbyte]

Update:

After isolating the problem to the filter, I checked all components, all solder joints, panel wiring, ground points, etc... Everything seems to be assembled correctly but I still have the radio interference problem.

So I decided to breadboard the filter according the schematics. The filter worked but I got the same radio interference. Now I wonder if this filter design is just prone to this problem and if other builders of the Noise Toaster have this problem. I could not find it mentioned in the forum.

Because I obviously do not want to listed to the radio on my synth, I may just try to take the filter out of the synth and try replacing it with another filter. I'll try the WSG filter design and see if I can get that to work with the other Noise Toaster modules.

[Blue Hell]

It has to pick up the RF from somewhere .. likely candidates are the input, output or power wires .. you could try ferrite beads in those.

[sndbyte]

Update

This weekend I decided to try breadboarding a number of filters to compare them with the filter and it's RF interference.

First I breadboarded the Noise Toaster filter from the schematics on the MFOS site. I got the RF interference just like I do on the one I built on the PCB.

Second I breadboarded the WSG filter, and I also got the RF interference.

Third I breadboarded the Soundlab filter and again I got the RF interference.

I was surprised to have the same problem with all these filters I suspect the reason why is that I live about 1 mile or so from a large radio antenna. I noticed I was usually picking up one of the more popular local radio stations. So I wonder if I took my Noise Toaster someplace away from my home location... maybe it would not have any RF interference problem.

Note that the RF interference happens for each of these filters when you have the cutoff frequency at it's highest level (fully clockwise).

I purchased a Snap Ferrite Choke yesterday and placed it on various wires but that did not seem to make a difference.

So for the time being I have installed a fine adjustment pot (10k) on the filter in series the regular 100k cutoff pot. That way I have a little more control in tuning out the RF.

[elmegil]

There is a thing known as a faraday cage which is essentially a wire mesh that's grounded that could possibly help (though I'm not sure how you'd apply it if experimeting shows that it does.

I'd take something like a wire colander, connect a ground wire to it (alligators or other clips, grounding it to the circuit), and lay it over the top of the circuit to see if that helps.

I'm not 100% certain that a half sphere will do the job, you might want to put a sheet of metal under it and set your circuit up on something nonconductive....

Just a complete stab in the dark.

[THeff]

Hi sndbyte,

If you are sure that the RF pickup is coming from the filter you may want to try putting a very small capacitor (4.7-10pF) across R35 (U2-A pins 1 & 2) or across U2-A pins 2 & 3. You can put the caps in both places if you want. This lowers the bandwidth of the opamp so don't make the caps to large or they will limit the audio performance.

-Theff

[sndbyte]

Thanks for the suggestions. I did not try the metal colander since I do not have one but I did try the capacitor idea but unfortunately that did not work. When I tried the capacitor I got a humming noise and the RF interference did not go away. Maybe my only solution is to move away from the large radio tower...

[sndbyte]

I finished this just last night. Never got rid of the RF interference but that probably is due to where I live. I can tune it out using the Cutoff Fine pot.

Anyhow, here are the mods I made to the original Noise Toaster:

1. The CV mod as suggested on the MFOS webpage

2. The External Input mod as suggested on the MFOS webpage

3. Replaced the 100k volume pot with a 50k audio pot with a limiting resister at one end (this stops the feedback I'd get if the volume was turned all the way up).

4. Adjustable Noise level mod, which is just a pot to control the noise level when the noise is switched on.

5. Added a Cutoff Fine control pot. This is just a 10k pot wired in series with the 100k Cutoff Coarse pot.

And here is a photo of the completed box:

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[elmegil]

That is beautiful work!

[jaidee]

Very nicely finished!

Any chance of some more photos from other angles and showing the output?

What sort of enclosure have you used?

[sndbyte]

Thanks for the comments. Here are some more photos so you can see the in/outputs.

The case is a plastic case I purchased at Radio Shack with an aluminum faceplate. I painted the case with a grey hammered metal spray paint. The panel design was done on my computer using LibreOffice (draw) and printed on cardstock. I then glued the paper design to the faceplate and sealed it with a clear matte acrylic spray.

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[Andromeus]

[sndbyte]

I added the pot between X4 on the PCB and S4 (the noise switch). I'm pretty sure I used a 10k pot, but it may have been a 100k pot. I think it is a mod that is useful. You can adjust the amount of noise rather than just switch it on and off.

I tried a similar mod to the VCA but found it was not very useful or effective.

[Skrog Productions]

Hi , that's a nice clean tidy build there , great stuff.
No eating chips before touching the panel , can you post up an mp3 of it ?.

Dave.

[sndbyte]

Thanks for the feedback. Unfortunately I do not have an mp3 of it. I don't even know how to record an mp3 on my computer and never bothered to figure out how to do it.