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PCB pattern for René Schmitz "late" MS20 VCF?
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andrewF



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PostPosted: Tue Dec 04, 2007 4:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

Takeda San has a good version of the MS20 filter with very compact PCB.
I built it a few years ago and reckon it is very good.

how`s this for tight?


http://www.aleph.co.jp/~takeda/radio/MS20clone2E.html
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v-un-v
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PostPosted: Tue Dec 04, 2007 5:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

yeah- seen that before. nuts! Smile
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Clack



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PostPosted: Wed Dec 05, 2007 1:45 am    Post subject: Reply with quote  Mark this post and the followings unread

v-un-v wrote:

If the MS20 can be cloned (or at least part of it), it should be done ASAP!



sort of done here

http://www.analog-synth.de/synths/ms20klon/ms20klon.htm

uses circuits from Rene Schmitz and Yves Usson

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ericcoleridge



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PostPosted: Wed Dec 05, 2007 12:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

andrewF wrote:
Takeda San has a good version of the MS20 filter with very compact PCB.
I built it a few years ago and reckon it is very good.


The Takeda San VCF is a clone of the "early" Korg 35 MS20 VCF (versus the "later" OTA ala Rene Schmitz). The Korg 35 VCF was also used in the MS10, X911, and I think MS50 also. But at some point the MS20 switched to a 13600 "discrete" OTA design which was housed on a daughter board. FWIW, I believe this OTA design is probably "the" MS20 filter, as it is different from other Korg instruments.


I'm a little embarrassed to say I couldn't get this Takeda circuit working when veroboarding it a few nights ago. For one, I think the PCB pattern is for Japanese transistors with different pin-outs from typical BC- or 390X transistors; I had a hard time bending the leads to accommodate this difference in such a small space. He probably even mentions this in his description. Do you remember this, or am I mistaken?
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andrewF



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PostPosted: Wed Dec 05, 2007 4:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

I do remember, the transistors used are very generic I have used them in all CGS circuits without problems.
The 2sc and 2sa pinouts are ecb , as in - you are holding the transistor, pins down, writing side facing towards you and reading from the left -
emitter, collector, base.

You could try mounting the transistors on the copper side, looks ugly, but plenty of space.

Thanks for the info on evolution of Korg filters, is it known when Korg changed to the OTA design? I might crack open my MS20 one day and have a look, though I suspect it is the early version.
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ian-s



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PostPosted: Wed Dec 05, 2007 5:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

andrewF wrote:
is it known when Korg changed to the OTA design


I guess my MS20 had an early filter because I got one of the first ones to arrive in NZ. I'm curious about how different the two versions sound.

I remember my MS20 could get a little fatiguing on the ears after a while.

The MS50 filter sounded better to my ears.
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Clack



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PostPosted: Thu Dec 06, 2007 2:12 am    Post subject: Reply with quote  Mark this post and the followings unread

is there any circuits showing the discrete ota? or was it a 'secret design' covered in black stuff ?
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yusynth



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PostPosted: Thu Dec 06, 2007 7:37 am    Post subject: Reply with quote  Mark this post and the followings unread

ericcoleridge wrote:
The Korg 35 VCF was also used in the MS10, X911, and I think MS50 also.


The Korg 35 was not used in th MS50. Although the MS50 VCF is also based on a Sallen and Key structure, the voltage control of the frequency is achieved with a current driven diode ring. This is the same principle as used in earlier Korg monosynth models such as the Korg 800DV and Korg 700.

The Korg 35 is a very cheap and dirty design and it results in a natural tendancy to distort (that's why g2ian felt an hearing fatigue after a while).

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ericcoleridge



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PostPosted: Thu Dec 06, 2007 10:56 am    Post subject: Reply with quote  Mark this post and the followings unread

andrewF wrote:
is it known when Korg changed to the OTA design? I might crack open my MS20 one day and have a look, though I suspect it is the early version.


I've read that there is no serial number correspondence to the change in filter design. However, the daughter board version VCFs are labeled in the service manual as "new," KLM-127 PCBs versus the "old" PCB which housed everything on one PCB.

It's pretty easy to see if your MS20 has the daughter-board or not. Just remove the left end panel and look inside. If there's a PCB on top and perpendicular to the main board (which is flush against the control panel), then you have the later VCF.

Mr Clack wrote:
is there any circuits showing the discrete ota? or was it a 'secret design' covered in black stuff ?


No, the schematic for the OTA VCF is in my service manual. However, the "early" Korg35 VCF was considered a secret for a long time because Korg wouldn't release the design of the chip. There's a link to this story on Takeda San's MS20 clone page. But, apparently now the design is known. In fact, there is a company in the UK that re-manufactured and sells this Korg35 IC on e-bay.
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PostPosted: Thu Dec 06, 2007 11:11 am    Post subject: Reply with quote  Mark this post and the followings unread

The original Korg 35 schematic was generously given to someone, I can't recall who by K. Ishi (IIRC), and this person either has or had it on the web. I've got a copy of it somewhere; it could probably be found with a bit of Googlery.

In the meantime, a redraw and quite extensive analysis can be found in Tim Stinchcombe's doc about it here:

http://mysite.wanadoo-members.co.uk/tstinchcombe/synth/MS20_study.pdf

Cheerios,
Scott

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PostPosted: Thu Dec 06, 2007 3:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

Scott Stites wrote:
In the meantime, a redraw and quite extensive analysis can be found in Tim Stinchcombe's doc about it here:
http://mysite.wanadoo-members.co.uk/tstinchcombe/synth/MS20_study.pdf

Scott --

Thanks for the reminder about Tim's analysis. I just finished going through his writeup. His analysis of the OTA-based version agrees with the one I did the other day. A minor point -- the 2 Oct/V response is obvious just looking at the schematic, since the divider at the converter transistor is 47:2 rather than the usual 100:2. The rest of the scaling is probably just done elsewhere in the box.

I also agree about the 6 dB slope of the HP. Again this is obvious from a glance at the schematic, since the output is from the same stage as the input. One stage cannot produce anything other than a 6 dB slope. I haven't finished the analysis yet, but I imagine that the correct 12 dB response occurs at the output of the other buffer. So is this how an actual Korg filter works (6 dB), or is there just an error in the schematic?

I plan to keep looking into doing a 3d order version. It also seems to me that the character of the filter comes from the distortion of the diodes in the feedback path, so it might be interesting to look at putting a more "interesting" nonlinearity in the feedback path.

Very Happy

Ian
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PostPosted: Thu Dec 06, 2007 3:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

It's surprising that the HP response would only be 6 dB - it's a very agressive filter, and has a pretty fair auditory "scoop" to it.

Wasn't there some version of a four pole S-K filter that Korg did? Seems to me there was. Seems to me like one of the "Greek Letter" Korg's had one. Delta or Lamda or Sigma or something like that. Maybe EFM copied one - can't quite remember why I think that may be the case.

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PostPosted: Thu Dec 06, 2007 3:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

OK, yeah - it was the Korg Delta.
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PostPosted: Thu Dec 06, 2007 7:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

Scott Stites wrote:
It's surprising that the HP response would only be 6 dB - it's a very agressive filter, and has a pretty fair auditory "scoop" to it.

Hmmm ... well I guess you could measure it ... but ...

Of course if you have the resonance turned up, the slope is very steep and you may not perceive the ultimate 6 dB rolloff.

I looked at just connecting the output at the other buffer, but that doesn't work, because then the output is proportional to the feedback gain. But it was pretty easy to figure out how to make the proper dual to the lowpass, so with a big enough switch the unit could be set up for 12 dB in either mode.

Now the question is why Korg didn't do it that way. Does the 6 dB response sound better? thinking

Very Happy

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PostPosted: Fri Dec 07, 2007 5:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Of course if you have the resonance turned up, the slope is very steep and you may not perceive the ultimate 6 dB rolloff.


You're exactly right. I tried it last night (didn't measure, just listened) and the slope was not nearly as sharp as I'd always imagined - mainly because I rarely use the filter without a fairly high resonance setting.

I wonder if the slope contributes to the wonderful resonant effect I've been yammering on about with the high pass.

Why Korg did this, I don't know. Would this particular design (OTA version) be made 12 dB HP by inserting the input into the first capacitor, then running serially from the first stage into the second capacitor (where the "normal" high pass input is)?

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PostPosted: Fri Dec 07, 2007 7:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

Scott Stites wrote:
You're exactly right. I tried it last night (didn't measure, just listened) and the slope was not nearly as sharp as I'd always imagined - mainly because I rarely use the filter without a fairly high resonance setting.
Why Korg did this, I don't know.

Well I have some idea now.

Quote:
Would this particular design (OTA version) be made 12 dB HP by inserting the input into the first capacitor, then running serially from the first stage into the second capacitor (where the "normal" high pass input is)?

Bingo! That's how it works out exactly. Very Happy dancing Very Happy

I started out by just poochkicking the Korg design and working out the proper dual design to the lowpass. [Dual design: swap R <-> C to swap hi pass <-> lo pass.] Not totally obvious how to do this, since the R is an OTA. But drawing up exactly what you said and working out the math does give the hi pass.

I now have it whiteboarded and it seems to work pretty much the same as my state-variable unit. That's with linear feedback. Now it's time to play with the nonlinearities. Twisted Evil

As to why Korg didn't do it this way? Probably because it doesn't matter that much as far as the sound, but possibly more importantly because it takes a five-pole switch to convert the response.

Very Happy

Ian
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PostPosted: Sat Dec 08, 2007 3:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

Scott Stites wrote:
When the resonance is high, but is not swept, but rather is tracking the same voltage that's controlling the VCO that is feeding it, that's where the real magic begins to happen for me. ...

Don't get me wrong, if I want very extreme "weird" it'll supply it, but I really like the effect of mid level input signal, and very high resonance tracking the V/Oct input, usually in the HP mode - it's unbelievable the timbres it can produce. ...

However, the resonance effect tracks the input well over a very wide range, much wider than the V/Oct response of the filter itself. That's another thing that makes the effect so nice.

Scott -- Just for clarification, when you have a signal in and you crank the Q way up, is it up high enough that the unit would oscillate by itself if the signal were removed? I think the answer is "yes" based on some of the things I have been looking at and on the fact that in Rene's design oscillation starts with the control about 1/3 the way up.

Ian
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PostPosted: Sat Dec 08, 2007 3:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Ian,

Absolutely. At the point where resonance becomes fairly apparent with a signal present, removing the signal will leave the filter self-oscillating. In fact, I use that for effect now and then by just cranking the input level all the way down.

Cheers,

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PostPosted: Sat Dec 08, 2007 4:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

Scott Stites wrote:
Absolutely. At the point where resonance becomes fairly apparent with a signal present, removing the signal will leave the filter self-oscillating. In fact, I use that for effect now and then by just cranking the input level all the way down.

Yeah, that's what I thought. Very Happy

At that point you are actually looking at a pair of non-linearly coupled oscillators, with interesting dynamic phase-locking behavior. The fact that you are doing it with a filter circuit would seem to be irrelevant. You are coupling two oscillators. I don't think this has been previously discussed much (if at all) in the EM context. Cool

There is an interesting issue wrt levels here. From the Korg circuit, I conclude that they most likely use a signal level of about 1V, at least at the filter core. (Maybe they have some gain after the filter?) I think this because (1) they use a 100:2 attenuation at the OTA inputs vs the usual 1000:2 ratio used in 5V circuits, and (2) because that is the level where, simultaneously, the feedback circuit saturates and the feedback gain is a bit above 2 (the oscillation threshold).

So I doubt that you could drive the original Korg as hard as you are driving Rene's. If I am correct about this, for the Korg to operate at 5V levels the same way that it does at 1V would require thirty diodes in the limiter rather than 6! Along with larger OTA input resistors.

I have been looking at a version similar to Rene's (except using linearized CA3280 OTAs) with different feedback elements. With back-to-back 7.5V Zener diodes, the circuit saturates politely and gives a nice filtered output at about 4.5V, as one would expect for a garden-variety filter. It oscillates at about the same level.

But with the LEDs, it has a fairly "dirty" filtering action (there are always large steps in the waveform) and, as you have pointed out, many crazy non-linear effects, including sub-harmonics and even (over a limited range) chaotic oscillations. Surprised

You and Rene have come up with something really unique here, IMO. It looks way beyond the Korg filter to me.

thumleft salut thumleft

Ian

EDIT: I should have been clearer that the circuit I am looking at is the HP dual of Rene's design, not Scott's version with the input to the second stage.
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PostPosted: Sat Dec 08, 2007 6:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

@ Ian

You mention using an LED to introduce a bit of dirty clipping into the waveform.. the Blacet 2100 oscillator does this:
http://www.blacet.com/VCO.html

The output is labeled as VC Wave, and if you click the image of the module in the link above, you get some nice shots of the board itself. The LED's (he uses two) that are used are in the middle rear of the board, one resister in from the outermost trim-pot:

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First thing I built, sounds fantastic.
My favorite is the VC Wave, especially in the bass range.
Meaty. =]

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PostPosted: Sat Dec 08, 2007 7:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

ENDIF wrote:
You mention using an LED to introduce a bit of dirty clipping into the waveform.. the Blacet 2100 oscillator does this:
http://www.blacet.com/VCO.html

The output is labeled as VC Wave, and if you click the image of the module in the link above, you get some nice shots of the board itself. The LED's (he uses two) that are used are in the middle rear of the board, one resister in from the outermost trim-pot:

Is this part of the oscillator core? scratch
Looks more like an add-on waveshaper to me. Very Happy

Ian
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PostPosted: Sat Dec 08, 2007 9:29 pm    Post subject: mp3 Reply with quote  Mark this post and the followings unread

Here is a demo showing a few of the sounds this system can generate. A SAW wave is input, and the input amplitude and frequency as well as the filter frequency are fixed as the resonance is set at different values, starting at the high end and coming down in steps. Scott wasn't kidding, you get all kinds of strange and wonderful sounds from this. Note that there are several regions of chaotic behavior.

Very Happy

Ian


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PostPosted: Sun Dec 09, 2007 6:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

frijitz wrote:
But with the LEDs, it has a fairly "dirty" filtering action (there are always large steps in the waveform) and, as you have pointed out, many crazy non-linear effects, including sub-harmonics and even (over a limited range) chaotic oscillations.

you are talking about LEDs in the feedback path!? (sorry, i did not follow this thread in detail, because i simply don't understand the half of the discussion). i recently built a wasp vcf clone and i just did this: i use a switch to chose between 1n4148 diodes and green LEDs. similar results: ill sounds with LEDs and the filter starts to selfoscillate. i will upload some sound samples soon.
http://electro-music.com/forum/viewtopic.php?highlight=wasp&t=20500

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PostPosted: Sun Dec 09, 2007 8:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

fonik wrote:
you are talking about LEDs in the feedback path!?

Right. A limiter to give stable symmetric oscillations, but that also does some interesting wave shaping if the filter is simultaneously oscillating and receiving an input signal. When I first tried this I had no level control on the input, which is partly why the LED behaved the way it did.

Quote:
i recently built a wasp vcf clone and i just did this: i use a switch to chose between 1n4148 diodes and green LEDs. similar results: ill sounds with LEDs and the filter starts to selfoscillate. i will upload some sound samples soon.

Interesting! (Although not really analagous). I thought the wasp could not oscillate, since it is a state variable design. scratch

Anyway, I'm thinking about putting in VC resonance because of all the wild effects I'm seeing. I also want to look a more general nonlinearities, like patching some waveshaper modules into the feedback path. Twisted Evil

Very Happy

Ian
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PostPosted: Sun Dec 09, 2007 11:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yes, that sample is exactly "it" Ian! If I set the frequency of the sawtooth to a fairly high pitch, I can use the cutoff control, starting at its lowest settings, to dial in what I can only describe as "subharmonics" - they are pitched lower than the fundamental, but it's an entirely different thing than just a simple sub-oscillator. As the cutoff is dialed close to and above the fundamental frequency, the timbre becomes more "reedy", and higher than that, almost bell-like, but with a reedy quality.

On my dual module, I put in a three-position switch to control how the cutoff controls work. The first position allows me to set the cutoff of both filters separately. The second position allows me to control them in tandem. In other words, if filter "A" is adjusted, filter B will track that. If I adjust filter B, it sets the offset from filter A's cutoff control. So, I can select a fixed spread, and move that spread around with filter A's control.

The third position allows filter A's cutoff to control both filters' cutoff frequency just like the second position, but the filter B position will now move the filter A cutoff frequency in the opposite direction that filter B is being adjusted - this was intended to allow the "B" control to act as a sort of "bandpass" adjustement when using the filter in high pass->low pass mode (cascade bandpass), while filter A's control sets the center frequency of the bandpass. In the end, I find it's useful for making all manner of bizzarre noise when cranking it in this mode Laughing

My page about it is here:
http://mypeoplepc.com/members/scottnoanh/birthofasynth/id10.html

Back to the nonlinear function and how useful it is, here's a sample of what turned out to be a longwinded, repetitive Klee piece. It's a single sawtooth going through the high pass section with the resonance cranked up, then through the low pass section to take off the high frequency hash. This is with the fundamental frequency set fairly high - the low portion is the filters non-linear reaction to the signal. The end result is a very thick sound that would have you think it's not just a single sawtooth. It is passing through the Dim C to liven it up, but still, there's a lot of frequency there that wouldn't be there if that high pass wasn't kicking in that wonderful resonance.

The full long-winded thing is here:

http://mypeoplepc.com/members/scottnoanh/sitebuildercontent/sitebuilderfiles/klee_key2.mp3

Like I say, fairly long winded. The Klee is driving the whole thing. The MS-20 filter voice is controlled by a CV output of the Klee and the keyboard. In order for the nonlinearity to track the voice, I patched the keyboard voltage into its V/Oct input, and the Klee voltage into one of the modulation inputs. If that tracking voltage isn't there, the filter's voice will "crack", so to speak, which is a neat effect, but not what I was after here. In the fadeout of the longer piece, you can hear me crank the "A" control while the two filters are controlled in tandem. I had to fade it there, because the D8 was out of room.

The non-linear feedback idea sounds very interesting.

Cheerio,
Scott


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Single sawtooth through dual MS 20 (resonant high pass to low pass) with Dim C thrown in.

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