Guitar Effects with MN3209 BBD

[Inventor]

Hi, my question is: What can I do with these four MN3209 Bucket Brigade Device chips that I just found, plus a 15 kHz to 25 kHz square wave Theremin?

I built a simple Theremin out of just a relaxation oscillator (oh, so relaxing) and it operates with sufficient sensing distance (about 4 to 6 inches) for mounting on the butt--end of a guitar. Then the player can wave their hand over it to create some effect. OK, good start there.

I also just discovered in my long lost chip collection, a nice unopened DigiKey packet with four little MN3209 BBD chips in it - what joy! I can't wait to gen up a differential clock circuit for this and drive the BBD with the Theremin, mounting the whole thing on my guitar.

Not knowing too much about how such delays are traditionally used to create tremelo, chorus, and other effects, I am asking what you would do with these little babies. I would like to use just one and save the other three for duplicates as this is a discontinued part.

Along the same lines, are other BBD chips still in production? I'd prefer to work with available parts if possible in the future. What would you do?

[goodrevdoc]

the MN3209 is only 256 stages, quite short for a BBD. Nevertheless, a suitable chorus/flanging type circuit could possibly be constructed. The datasheet specs it at .64-12.8ms being clocked from 10 to 100 kHz. You can of course clock it faster, but be sure to buffer the pulses to keep them nice and sharp, as most BBD's don't respond well to sloppy pulses. There are a few companies still churning out BBD's, most are higher staged with the intention of being used for analog delays. Shanghai Belling and Coolaudio come to mind, but there may be others.
-justin

[Inventor]

Thanks Justin! I built a first crude prototype of a circuit and tested it, it works! There is a lot of switching noise, however. I notice in the example circuit on the datasheet there are a bunch of low pass filters in the loop. I can only presume that this is to deal with all the switching noise. The next time I get motivation I'm going to transfer the design to a bigger breadboard, add a microphone, and put in some decent filtering to deal with this.

The circuit is an inverting amp followed by the BBD, then going back to the inverting amp. What type of effect is that, chorus or flanger or what?

It has a dial for nominal delay, one for feedback gain, and one for offset to decrease noise. I am thinking of ultimately making a circuit board using Eagle CAD software that one can attach to the guitar's pick guard and have a Theremin effect box like that. Cool idea, if I can stick to it and get the job done. But there is no hurry, is there?

[Uncle Krunkus]

Hey Les,
This Saphire Flanger is quite simple, and yet works really well. I built two of them into a rack unit, and I've still got the (proven) stripboard layout if you want a copy. I'm pretty sure the 3209 and 3207 are pin for pin, just a different number of stages. If not, it could be easily modified.

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[Inventor]

Thanks Uncle Krunkus! That's exactly what I was looking for! So, it is a flanger that I have built after all. A noisy one, without filtering and with insufficient buffering, but nonetheless a flanger. And a Theremin Flanger at that. I can modify your schematic to suit the application, changing the oscillator section to a Theremin and other changes.

I would also like to add a microphone section, but perhaps I should keep it simple for now.

I'm thinking of eventually making a flat rectangular circuit board in surface mount that would attach onto the surface of the scratch plate of a Strat or similar guitar. A patch antenna would slip under the strings and the rest of the circuit board would extend to the right (bottom) of the guitar scratch plate. That way the product could have no enclosure, just a circuit board that mounts to the scratch plate with double-sided foam tape, and have low cost mini-pots and be inexpensive.

Or not, I dunno. Certainly the Theremin's sensitivity needs improvement and I need to use a production BBD or a PIC (though I'd rather not PIC it). But these are things that can happen in good time with lots of care, attention, and dedication.

By the way, I also prototyped it in software using ChucK. It's described in the "Primal Sounds" thread in the ChucK forum if anyone wants that.

Thanks again, nothing like being part of a community of enthusiasts!

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[Uncle Krunkus]

Here's the stripboard layout if you want it.

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[Inventor]

Oh, that diagram is kinda cool. I'm not very good at following instructions though, I'll improvise quite a bit. But that filter with all its precious design values is a real gem in this application! It also has a clean bias method.

[Inventor]

[Uncle Krunkus]

If you find the effect a bit weak, you could try inverting the regeneration. In a strange way, this creates a kind of notch flanging on the higher frequencies, and acts like doubling the delay for the lower ones.

[bugbrand]

Hey Inventor

I think 3207s aren't too tricky on ebay - there's maybe some clone types about too?
A good place to check is Small Bear:
http://www.smallbearelec.com/home.html
They're good people to support!
They have original MN3207s or BL3207s (clones)
(strangely the clones are the same price as the originals!)

[Uncle Krunkus]

The MN3207 is actually the only BBD I can get easily. Wagner Electronics in Sydney have a big stash of them. Not sure why.

[Inventor]

Yes, Uncle, I tried that and you are right - inverting on the feedback makes the effect more pronounced.

bugbrand, thanks for the smallbear link, plus they have the BL3208A which is 2048 stages, nice. I need some other Smallbear parts, I like them, I think I'll place an order in two weeks when I have more cash.

Rgarding the Theremin, I tried a negativie impedance converter instead of a capacitance multiplier and it worked OK. I etched the antenna and gave it interlocking "E" pattern, cap on front and gnd on back but that didn't help much. Then I covered the antenna with electrical tape and it now works as kind of a variable touch sensor, not so much as a magical Theremin. I think a good Theremin has to have a transformer for the antenna or something.

[Kassen]

256 stages is little for a delay but it's plenty for Karplus Strong. How about combining the Theremin antenna for pitch with a nice button for throwing a short burst of noise in? Perhaps a pressure-sensitive button could be found for more subtle intonation in strumming?

You'd need a filter that would go along with the oscillator's modulation, I believe one trick (used by Boss in their BBD's) is to used PWM based on the clock to control the CV for the filter. Perhaps the most interesting part would be to create a versatile noise-generator that could be controlled by a pressure sensitive button and lead to suitable pluck and bow type excitation signals.

[Inventor]

Very Interesting, Kassen. I looked up Karplus Strong on Google and got the Wikipedia page. It does mention this:

[Inventor]

I tried squaring up the guitar signal and putting in a filter but it didn't sound like string synthesis. Perhaps I need to create a burst like kassen said. Anyway, I did a frequency sweep courtesy of ChucK and it has a wavy frequency response. When I touch on the touchpad the filter characteristics compress slightly, and when i take a screwdriver and wiggle the frequency pot, it sounds really interesting.

I just need to make the theremin modulate more and i'll be in business. I'm going to try a fixed capacitive negative impedance converter to remove the parasitic capacitance. we'll find out soon.

[Inventor]

wow, i figured out how to widen the frequency sensitivity of the theremin. i used a negative impedance converter with a fixed capacitor and a pot adjust, plus a capacitance multiplier on the antenna. It's a sensitive adjustment, but the NIC cancels out the parasitic capacitance leaving mostly only antenna capacitance which varies a lot. You still have to touch the antenna (or get close) but it does have a big effect on the um, effect. nice.

[loss1234]

does anyone know if the front end (and back) of this schematic work for LINE LEVEL (synth levels of +/- 5volts or higher?)

i cant seem to find any flangers that have specified LINE inputs.

thanks

[Uncle Krunkus]

Line levels are not that high.
They are only about 1V p-p.

[loss1234]

sorry i should say SYNTH inputs.

i suppose the problem is finding a way to patch it with modules in a modular

has anyone found a way to do this without companding?

thanks

[Inventor]

Just a little cross-fertilization and follow-up here. See this link:

http://electro-music.com/forum/post-263081.html#263081

For the project thread on the KS board that Kassen suggested. It has been months and I've done a lot of work on the project, only to re-read this thread and discover that it was Kassen after all who suggested the project in the first place.

Oh, and Andy would you mind taking a look at my KS board? I'd like your opinion.

Les