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 Forum index » DIY Hardware and Software » Les Hall's Projects including eChucK
The Karplus Strong eChucK DIY Project
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Inventor
Stream Operator


Joined: Oct 13, 2007
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PostPosted: Tue Sep 15, 2009 5:42 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, dan, I am driving the chip with a level-shifted VCO. This is because we want to be able to vary the clock rather than use a fixed clock or one that is only adjustable by potentiometer. You'll see the VCO input gets buffered, then it drives the control input of the LM13700 VCO itself, followed by a level shifter and then into the OX1 pin. The inverter chain that would normally drive the oscillator just does some buffering in this case. It's not documented in the datasheet, but I think it will work OK.

Les

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loss1234



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PostPosted: Tue Sep 15, 2009 6:53 am    Post subject: Reply with quote  Mark this post and the followings unread

hmmm

as of now I can NOT get this to work. It could be my fault.

first problem is the 13700 is bleeding really badly into the BBD chip.
this may be because it is on +/-12 and is a very strong signal.

(even when it is not connected to the MN3207 I can still hear it)

the other problem is that the 13700 does NOT seem to be making the clock chip
create a clock.

I could be doing something very wrong.

I will keep at it, but so far, My MN3201 makes a clock when I clock it with a resistor and a cap but when I try it with the OTA, i get nothing.


when you tested this, did you run it on +/- 12 or +/- 15?

thanks

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loss1234



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PostPosted: Tue Sep 15, 2009 6:55 am    Post subject: Reply with quote  Mark this post and the followings unread

Ok

I got the MN3201 to clock

still have the bleed though

more later

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loss1234



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PostPosted: Tue Sep 15, 2009 7:09 am    Post subject: Reply with quote  Mark this post and the followings unread

i am giving up for now

when I started adding BYPASS caps (in the proper polarity and value) the clock coming OUT of the mn3201 DIED

very odd behavior

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loss1234



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PostPosted: Tue Sep 15, 2009 7:40 am    Post subject: Reply with quote  Mark this post and the followings unread

ok last comment

I am now able to get a better clock out of the MN3102

i switched the 33k on the I in to a lower R

HOWEVER there is still far too much bleed at this point


i cant seem to find a way to get rid of it....for now

However this could be a problem with BREADBOARDING and not something that would ever be an issue on a PCB>


INVENTOR...please let me know what kind of results you have had

thanks so much

dan

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Stream Operator


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PostPosted: Tue Sep 15, 2009 3:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

Dan,

Are you feeding the LM13700 directly into the MN3102 chip? It can't take that. You should be powering the LM13700 on +/- 12 or +/- 15 Volts and the two BBD chips on +5 from the regulator. I'm guessing that you did not include the level shifter perhaps, the little circuit with the 2n2222 transistor?

Can you describe in greater detail what you mean by "bleeding", I don't quite understand.

I have not prototyped this, I'll be ordering parts soon to build it.

Les

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loss1234



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PostPosted: Tue Sep 15, 2009 4:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

actually i included all of it


I have been building a lot of BBD circuits so i know what the chip needs for voltage and offset.

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loss1234



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PostPosted: Tue Sep 15, 2009 4:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

well when i say bleeding i mean that the vco is bleeding into the output.

it is driving the chip but BBDs are so sensitive that i think having a clock with such high rails on the same breadboard is causing problems


maybe i did something wrong.


but i have built quite a few other circuits this week successfully.


it is possible that driving the lm1700 off lower rails would help

when you get a chance, just try the vco, mn3102 and mn3207 and see if you have any problems

thanks so much ( i really do want to get this working!!!)

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Stream Operator


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PostPosted: Tue Sep 15, 2009 4:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

What do you mean by "bleeding"?
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loss1234



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PostPosted: Tue Sep 15, 2009 4:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

you are not supposed to hear the clock. (unless i am missing something)

i hear SO much clock i cant use it. no matter how much i use trimmers to null it.
(this is after the offset and with the 2n222 etc)


that is what i mean by bleeding.

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PostPosted: Tue Sep 15, 2009 4:37 pm    Post subject:   Reply with quote  Mark this post and the followings unread

thanks!
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Stream Operator


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PostPosted: Tue Sep 15, 2009 4:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

OK, this is a debugging issue and is not to be unexpected. Someone mentioned the concern before. The solution might be to put a cap from the input of the LM13700 VCD to ground, right after the BBD stage. I'm not sure what size of cap would do. This will add a second filter that will hopefully quench the clock noise. Hope that helps, let's work out the details together, be patient! Smile

Les

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loss1234



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PostPosted: Tue Sep 15, 2009 5:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

no problem man

just wanted to let you know!

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PostPosted: Wed Sep 16, 2009 3:53 am    Post subject: Reply with quote  Mark this post and the followings unread

Well, I just ordered the parts for the prototype KS board this morning, so I should be able to begin prototyping next week.

About the clock noise, it occurs to me that we should also put a small cap in parallel with the feedback resistor of the loop summing amp. That, and a cap on the input of the VCF should help the problem, though I do not know if that will solve it.

Les

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PostPosted: Thu Sep 17, 2009 4:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Upon revisiting Dan's observations, it occurs to me that "bleeding" is indeed the correct term for what might be going on here. Dan mentioned that removing bypass caps actually helps the problem, and that it might be an issue with breadboarding limitations.

That seems to be the case. The datasheet for the LM13700 does not describe the frequency of oscillation, but the cap is so small that I gotta think it's in the 100 kHz region or thereabouts. That won't fly too well on a breadboard with all the parasitics and stuff, so here's what I would do if I were Dan:

Slap a bigger cap on the VCO...

Slow that puppy down, all the way down to 10 kHz, which is the minimum frequency that the part supports, creating the maximum delay. Sure you won't be able to fully test it across the frequency range, but hey what do you want, it's just a breadboard.

Just this morning's observations, true or false...

Les

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PostPosted: Tue Sep 22, 2009 3:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

Update: I got my DigiKey parts yesterday, still waiting on SmallBear parts but they take a little longer. I'm patient. So soon I will be able to breadboard the Karplus-Strong board.

Some recent ideas:

1. The datasheet for the mn3207 shows an active filter after the delay line that removes this clocking noise apparently. I could just unbuffer the stim input, feeding it directly into the KS summing amp, then use that opamp for the active filter. That might be the best thing to do to properly quench the clock noise.

2. Since the KS and BS boards will be the same size, one could stack up a sandwich of one of each, BS on top and KS below. These "bricks" would each create one voice of a sort of eChucK-style rearrangeable synth, in which you place the bricks on a table and wire them up with jumpers. The boards are also made to be panelized for use in a more conventional synth cabinet.

3. We seem to have thought up a third companion board to the KS and BS boards. It's an SS board, for Song Sequencer. If we make this one, it will perform song-length macroscopic sequencing of the various voices. See the Song Sequencer thread under DIY/Developer's Corner for details.

Les

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PostPosted: Tue Sep 22, 2009 5:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

I had an interesting thought which led to the block diagram and illustration posted below. The idea is to return to an eChucK style of synth with these little boards (BS, KS, and SS).

The boards will stack into bricks with spacers. On top are the BS controls, and on the right of each brick are the KS and SS controls. Wiring posts for clip leads are on the bottom of the boards. Power comes in on the left.

There is a power supply / mixer board stack in the middle that mixes the various sequenced voices and has analog i/o with the netbook which runs ChucK and Audacity.

The bottom illustration shows a single "brick" or board stack that creates one voice at one point in the song. More SS boards can be added for multiple appearances of the voice in the song.

Just the latest thoughts...

Les


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PostPosted: Mon Oct 05, 2009 12:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

How's the project going? Did you prototype it?
I bought a few delay chips today. So I'll test it out next weekend.
Quote:
1. The datasheet for the mn3207 shows an active filter after the delay line that removes this clocking noise apparently. I could just unbuffer the stim input, feeding it directly into the KS summing amp, then use that opamp for the active filter. That might be the best thing to do to properly quench the clock noise.

I looked up the datasheet, but I didn't see an active filter. Could be that the datasheet was written in japanese, but there was no schematic. Do you have a link?
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PostPosted: Mon Oct 05, 2009 3:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

wmonk, I am still waiting for the chips from smallbear. I better check my mailbox. Here is the datasheet for the LM13700, it has the filter in it but not much description.

Les


LM13700.pdf
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LM13700 DataSheet

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PostPosted: Mon Oct 05, 2009 3:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well, that's a LM13700, not the MN3207 you mentioned Wink
An OTA based filter is nice, but I'm not sure if it's ok to use it to remove the clock signal. What is the frequency range of the clock signal? If high enough, there is no problem, as the audible frequencies will pass through.
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PostPosted: Mon Oct 05, 2009 3:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

Oops wrong chip. Here it is. The clock is 10kHz to 200 kHz. Hmm, you know if I just set it up so that the clock is above 20 kHz all the time, nobody's going to hear it anyway! Smile Good thinking wmonk!

Les


MN3207.pdf
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PostPosted: Mon Oct 05, 2009 3:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

Nobody's going to hear it? I think the overtones will make a difference in sound. But if you set up the this filter at around 25kHz, there is no big problem. You use another filter with a much lower frequency offset in the signal, so there aren't many high frequencies left.
I'll check out the datasheet. Thanks Smile
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PostPosted: Mon Oct 19, 2009 12:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Today I got around to doing some prototyping on the KS board. I hooked up the power, made the 5V regulator circuit work, wired up the oscillator and it's bias control opamp, then - no sound! Blue Hell (Jan) helped me diagnose the problem. The feedback resistor was the wrong value (should be 10k, not 20k), and also I had the positive and negative inputs reversed. I should be more careful when I create schematics. Oh well, at least it works now.

The oscillator cap is way too large, as expected since the datasheet had little or no description of the circuit. It only oscillates from 200 Hz to 2kHz, so the cap needs to be way smaller. Also the output is a triangle wave, which makes sense in retrospect, and that's OK because the level shifter and clock chip will square it up and fix the duty cycle.

Overall good progress was made and I'll work on the rest of the circuit soon.

Les

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PostPosted: Mon Nov 09, 2009 12:51 am    Post subject: Reply with quote  Mark this post and the followings unread

How's the prototyping going? Got some extra delay chips last saturday. Now I've to get another breadboard to test things out. Any updates on the schematic?
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PostPosted: Mon Nov 09, 2009 2:00 am    Post subject: Reply with quote  Mark this post and the followings unread

wmonk wrote:
How's the prototyping going? Got some extra delay chips last saturday. Now I've to get another breadboard to test things out. Any updates on the schematic?


wmonk, the prototyping got put on hold because of the EM09 festival, but I do have some updates for everyone, which I will discuss here.

I was able to prototype the VCO. It didn't work so I discovered that pins 3 and 4 were incorrectly reversed. That is, 3 should be 4 and 4 should be 3. In other words, I got the inputs to the OTA backwards. The circuit will not oscillate if the pins are not correctly connected.

Also the capacitor of the VCO, as expected, is way too large. The datasheet did not provide any frequency formula or text description of the circuit, so I knew it would require adjustment. I believe it needs to be 100 or 1000 times smaller, something like that - I forget.

At EM09 I spoke with Rob Hordijt (sp?) who happens to be an analog guru and quite familiar with Karlus Strong. He drew me a schematic of this freaky way of making the loop such that the tonal quality of the circuit is superior. I will try to draw out his schematic soon, and I'll be pm-ing him for details as we go.

Last night in the chat I spoke with ScottG, and sort of evangelized him onto the project. It turns out he is a big KS fan from his FPGA music creation experiments (which sound great BTW). ScottG will likely take over the breadboarding and I will continue with the schematic capture and layout.

In addition to helping out with the breadboarding, ScottG showed me a web page which describes using a comb filter at the output of the KS loop. Here is the page:

http://www.fpga.synth.net/pmwiki/pmwiki.php?n=FPGASynth.DigitalWaveguide

Scroll down to the illustration in the section marked Phase 2 for a block diagram. It has separate controls for pitch, timbre, and get this - pickup position! How cool is that?

So in summary, we have significantly improved the quality of the design, but at the same time this puts us a step back in schedule. Hopefully with ScottG doing some prototyping we ill move forward more quickly. Well, that's the update. Cheers!

Les

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