Will the real MS-20 filter please step forward

[frequencycentral]

This might help if you just want simple series/parallel switching:

[Jordan A.]

Yes, if you want to mess with series/parallel on the fly a switch like that is cool, no stereo though...

Here is an idea for a switching scheme for two LFOs, this gets you a great many modulation options with two switches.  It's a little weird to look at so I'll go through it.

http://www.aronnelson.com/gallery/main.php/v/diyuser/IMG_0203.JPG.html

-The "sweep mode" switch is shown in the "normal" position, LFO A sweeps VCF A and LFO B sweeps VCF B, each with its own depth control (the pots wired as voltage dividers).

-In the middle position we have a "sync to LFO A" mode, LFO A sweeps both VCF A and B, the depth can be set separately for each VCF.

-In the bottom position we have an "add" mode, the sum of the two LFOs modulates both VCF A and B.  In this mode the depth control for LFO A controls how much of LFO A is fed to both VCF A and B, and the depth control for LFO B controls how much of LFO B is fed to VCF A and B in the same way.

-The other switch chooses an inverted or normal LFO signal to drive VCF B, this is handy in the "sync to LFO A" and "add" modes.

I think it's a pretty fun setup, I've used this exact scheme in a similar project and remain happy with it.  Again, any suggestions or improvements are welcome.  I used 100k linear pots and 100k for all unmarked resistors.

As for the LFOs themselves, it becomes a preference thing really.  The two tapLFOs would be super duper deluxe, one simple triangle only LFO with just a "reverse B" switch would still be fun for days.  My vote is for one "fun" LFO and one simple triangle only LFO .  The "fun" one could be the tapLFO, my vote is for an LFO I stole from Jurgen Haible, from his "son of storm tide flanger" that gives you triangle, square or sample and hold waveforms.  If you don't need voltage control of LFO frequency (I don't think it's all that important for this kind of project) an LFO that does that can be made with six opamps, so ten opamps total for the LFOs and inverters for VCF B.

jordan

[garcho]

@FC>UK
thanks for the scheme, that looks perfect. personally, i'd take that simplicity over the stereo option. Deluxe w/stereo for sure.

@Jordan
love those LFO thoughts, will look at those Monday, exciting!

[WhenBoredomPeaks]

This is the first time I've ever really wished I could curse on this forum. I just breadboarded Rene Schmitz's schematic and it sounds ____ amazing! I used two 9V batteries, a LM13700, two 2N3906s, two random GE diodes instead of the LEDs, 2K resistors instead of 1K8, and fed the CV1 in signal from the simplest dual opamp function generator I could find.
Because I didn't do the math on the LFO, my initial test started out with a triangle wave oscillating at like, 250Hz, which, of course, sounded like a ring modulator! With the resonance all the way up there was oscillation squeal but with the resonance turned down about 50% or so, the oscillation shrinks so far down it's hard to hear unless you turn your amp up way loud, and then it's still a fairly small signal. It sounds incredible, I'll record samples soon. With the LFO slowed down into tremelo territory, depending on how the resonance, frequency and CV in mix pots are adjusted, the filter sounded like a strange flanger or something. I was having so much fun playing through it I ran out of tinkering time! I played through a MOSFET boost before the filter, definitely needed extra gain. I haven't converted it to HP yet (waiting for more parts before I can breadboard both filters simultaneously    ). I can't wait to try it out with both, and with LFO CV more interesting than my square and triangle generator (which will work perfectly for ring mod/trem-ish sounds). Time to start looking for the simplest ADSR generator schematic I can find. I'll report more later, hopefully I'll have more time next week, otherwise it might be a month or two. Thanks for turning me on to this y'all, it's becoming my favorite project! Big thanks for the detailed help Jordan!

EDIT: Just realized ADSR doesn't apply here, but some sort of CV shaping outside of basic wave forms would be nice. Maybe a CV expression pedal...

What was the supply voltage of the LFO? When you use the filter with a +15V -15V supply do your LFOs voltage swing have to match that?

[garcho]

9V, because I used two batteries instead of a 'real' power supply.
There is a pot that bleeds the LFO's signal to ground.

[WhenBoredomPeaks]

9V, because I used two batteries instead of a 'real' power supply.
There is a pot that bleeds the LFO's signal to ground.

Wouldn't you get 18V by using two batteries? BTW how did you connected the two batteries? 9V/0V/-9V or 18V/9V/0V?

[garcho]

-9 0 9

[WhenBoredomPeaks]

-9 0 9

I've made an LFO based on a now discontinued MoltenVoltage IC but it can put out only positive control voltages. (from 0V to whatever depending on the supply voltage of the analog side)

Could i supply this filter with a voltage something like +24V +12V 0V so the LFO's control voltage could better match the filter's voltages. (i developed a fear from trying this without asking since i killed two other filters messing around with their suppply voltages like this wasting some vintage russian ICs, handmade pcbs and lots of hours)

[ml]

So, I'm using this project to learn to use Eagle.  I've got the LP and the HP on a board and want to implement the series/parallel switching next. 

Looking at FC's schematic I'm assuming that in addition to the DPDT, we'll need the opamp for the parallel situation to sum the two outputs (sorry, absolute beginner here).

I'm not sure what the Vref input going to the + pin of the opamp is, though.  Some help please?

[WhenBoredomPeaks]

So, I'm using this project to learn to use Eagle.  I've got the LP and the HP on a board and want to implement the series/parallel switching next. 

Looking at FC's schematic I'm assuming that in addition to the DPDT, we'll need the opamp for the parallel situation to sum the two outputs (sorry, absolute beginner here).

I'm not sure what the Vref input going to the + pin of the opamp is, though.  Some help please?

VREF is always half of the supply voltage. (it is 4.5V when you use a 9V battery to power the pedal)

btw i made that René MS10 in addition to Escobedo's, sadly i had to take the LM13700 out of the Escobedo to get the René work.
(caution the vero in that René thread is full of mistakes, there is an extra, non-needed cut around the IC, then you should put cuts under the CV resistors (the 100k ones) and the transistor orientation is probably wrong, i used 2n3906s)

[garcho]

we'll need the opamp for the parallel situation to sum the two outputs

Have you breadboarded this yet? You'll want that opamp in the front too, boosting the signal, at least for guitar.

Noob question: Is the opamp at the end of the circuit more important for buffering the output than summing the parallel filters?

Excited to hear how your PCB-ing goes.

[ml]

Well, I breadboarded until the point when I realized it wouldn't fit on my half breadboard. Then I decided to try to make a pcb, since I've just gotten a laser printer. 

I've got one filter laid out on a board and am trying to fit the other filter on it (the free version of Eagle does maximum 10 x 8cm).

Once a +/-15v power supply configuration is decided upon, I plan to put that and possibly two TAPLFOs on a daughter board. 

I can post the eagle files if it might be useful to anyone.  The only thing is that I'm using the 3080.  Easy to swap it in the schematic, though. 

[Strategy]

Please share your eagle files when the time comes, i'd be into some experimenting with that!

If people have success or need PCB's I can coordinate a limited run of pcbs through this group-buy thing:

http://oshpark.com/

Strategy

Well, I breadboarded until the point when I realized it wouldn't fit on my half breadboard. Then I decided to try to make a pcb, since I've just gotten a laser printer. 

I've got one filter laid out on a board and am trying to fit the other filter on it (the free version of Eagle does maximum 10 x 8cm).

Once a +/-15v power supply configuration is decided upon, I plan to put that and possibly two TAPLFOs on a daughter board. 

I can post the eagle files if it might be useful to anyone.  The only thing is that I'm using the 3080.  Easy to swap it in the schematic, though. 

[ml]

I'll definitely post the files once I have a working board!

In the meantime, for those who might find it useful, here's a video of a lecture in which the Rene Schmitz filter is discussed. The cap before the output is bigger in the video, but aside from that, it's the schematic we're working with.

http://blip.tv/abovenyquist/ems-2010-session-24-sallen-key-filters-part-2-3614551

[ml]

He starts talking about the MS-20 filter at around 8 minutes.

[Cliff Schecht]

I've tried to watch that guys lectures before but he just doesn't understand electronics well enough to make me want to really listen to him. He needs to be more careful about how he words things, some of the stuff he says can be either misleading or wrong.

The circuit itself is actually pretty simple. The way the circuit is inverting in the first stage and not in the second stage is a way to get a signal for the negative feedback without an additional inverting op amp. Instead they have the resonance pot which feeds into a high impedance "+" input. While this circuit is a fun way to save an op amp, I think for our purposes it would be smart to change the input stage to a non-inverting stage and use an inverting amplifier for the feedback stage (you could use a quad op amp and buffer the resonance pot before it sees the low input impedance inverting op amp stage). For me this fixes the problem of inverting polarity (which they obviously didn't care about in the original circuit) but we still have the problem of a very low input impedance to that OTA stage. Some older effects would definitely not enjoy a low input impedance very much.

Getting even more clever with the design you could you could again change that input stage to non-inverting (moving the first 10k to the "+" input) and buffer the input with an op amp. Then setup the resonance circuit as an inverting circuit (which then keeps the correct negative feedback relationship with that first stage changed to non-inverting). Keep the resistors in the feedback network large to keep input impedance high (SNR won't be a problem here in a feedback path). The last two op amps go where they already are in the filter schematic.

Ideally the second change would give you functionally the same circuit with a couple of the modern "niceties" that I expect from modern pedal designs. You guys can still go crazy with whatever LFO and power solutions you want, but at least make the heart of the overall filter circuit non-inverting! If you were to setup any sort of wet/dry knob this would cause problems for instance...

I can also redesign it to be a single supply circuit most likely without complicating the design too much, but I haven't really looked into this much. If there are enough requests to either change the power rails (to say +/-12V instead of +/-15V) or to try to go to a single supply design then I can figure out how to do this.. We could probably even get away with a single 9V supply although you lose some noise performance (lower SNR).

[garcho]

We could probably even get away with a single 9V supply although you lose some noise performance (lower SNR).

For a project on this particular forum, I think a single 9V supply justifies lower SNR. For the super mega deluxe pro platinum version maybe we should go all out and have 12 or 15 bipolar.

[Cliff Schecht]

So people have tested the Rene schematic with a 13700 and it didn't work properly? I'd much prefer this part over a 3080 for the OTA's. If it doesn't work I can run some simulations to figure out what needs to be changed. I suspect the 13700 has *slightly* different requirements for the ibias pin but it should not be a big deal to switch to the superior (and cheaper!) chip. I'll look into this as well as a single supply, the latter will be trickier to implement methinks.

[Cliff Schecht]

I got the circuit working pretty quickly in simulation with a single supply and the small changes I suggested. I think I will need to do go back to pen/pad to figure out what some of the resistor changes need to be to work correctly with the 13700 but I've certainly got a good starting point to do so. Also I didn't muck with the OTA bias circuit yet but this shouldn't be too bad to swap over to single supply either.

I'll post schematics and layouts as I finish them..

[Strategy]

Cliff thanks for jumping in on behalf of the 13700 that is my preference as well!

I got the circuit working pretty quickly in simulation with a single supply and the small changes I suggested. I think I will need to do go back to pen/pad to figure out what some of the resistor changes need to be to work correctly with the 13700 but I've certainly got a good starting point to do so. Also I didn't muck with the OTA bias circuit yet but this shouldn't be too bad to swap over to single supply either.

I'll post schematics and layouts as I finish them..