Will the real MS-20 filter please step forward

[garcho]

Thanks Jordan, I will try this out next week or the one after. After that I won't have any time until August or so, but I'll keep checking in.
I recently used the LFO from the Tremulous Lune to control an active filter, sounded great. Lots of shaping possibilities. Getting excited!

[ml]

Started following this thread as it was dying down and I'm now starting to breadboard the Schmitz filter to compare the 3080s to the 13700.

I'd like to find a solution for +/-15v and I'm looking at this MAX743 DC-DC converter: http://datasheets.maxim-ic.com/en/ds/MAX743.pdf

Does this look like a good way to go?  I'm pretty new to electronics DIY so any help is greatly appreciated!

[garcho]

Been moving real slow on this one...

I'm looking at Rene's schematic. My plan is to swap the CA3080 out for a LM13700. Anything I should know about that? I plan on leaving the 13700's diode biases floating, the buffer outs floating, and the buffer ins grounded. Any other concerns I should have?

CA3080
p2 - inverted in = p4 on 13700
p3 - non-invert in = p3 on 13700
p5 - amp bias = p1 on 13700
p6 - output = p5 on 13700

[garcho]

I was thinking of using the MFOS wall wart power supply for ±12V. Any more thoughts on power options?
Will swap the BC558C for 2N3906, yeah?

[garcho]

Ok, one more post, sorry about all this, I should've just waited.

Found these MS-20 schematics in this post. 

[ml]

Yeah, thoughts on power options.  I should have put this in my previous post, but the MAX743 dual output switch-mode regulator takes 5v and outputs +/-15v or +/-12v. Also it's not SMD so it looks good to me.

[Strategy]

I'm not breadboard enabled and have been in overtime at work but look forward to hearing the testing results so we can settle on the desired configuration for a pcb layout! I suspect the "rene schmitz but configured for LM13700" like garcho is doing will be the chosen solution

Will see if I can drum up an etchable version for the sake of testing since i am not breadboardy...(I know, I know I should be...)

Strategy

[Skruffyhound]

Subscribed. Interesting thread

[teej212]

ive said it before, but the circuit not being 9v will be a big turn off for many people. and if not a turn off, having 9v is undeniably a big plus, and the max743 circuit is quite simple, so why not?

i suppose while breadboarding its fine to use the +12/-12 supply thats from MFOS.  and tap lfo would be awesome in this circuit!

[garcho]

EDITS:

So 9V into a 78L05 into the MAX743? And then into two regulators?

How about current draw, charge pumps and this circuit - something to worry about? The 743 datasheet says at 15V it can deal with 100mA. That's gotta be plenty, right?

I thought I'd use the MFOS style ±12V for the breadboarding simply because those parts are laying around my 'lab', not because I think it's specifically an elegant solution for the PS.

I erased a few things because I just reread the thread, should have done that earlier, sorry for clutter

Strategy, when is your birthday? I'll send you a breadboard and jumper cables.  

[Cliff Schecht]

The MAX743 is not necessarily a bad solution but the implementation of a circuit using this is relatively bulky and I'll bet the part itself isn't cheap. I don't dislike Maxim parts specifically but I tend to only use their stuff if it meets a specific niche that nobody else does (which is IMO what they do especially well!).

Charge pumps aren't a bad solution for low current, non-critical applications but as I said before, you really want regulated voltage when you are running filters. You will hear the filter change cutoff frequency if the supply voltage drifts enough to cause the control circuits (for frequency and Q) to drift. I played around with a neat solution that uses a single coupled inductor to generate +/- power from a single 9V input but still haven't gotten around to building a real prototype version and trying it in an audio circuit.

[garcho]

Interesting problems with the power supply; good for the build threads. 

Sorry to unnecessarily clutter this thread with a thousand posts, but I have some questions.
Looking at Rene Schmitz's schematic:
What does the 'pitch' node get connected to? I don't know anything about synth circuits (i.e. gate vs pitch). I'm planning on only using the 'In' node, for instance, with my guitar.
What does the pot (47KB between +15V and -15V) directly below the 'pitch' node alter? Is that the frequency?

...for high pass you just do the old trick of grounding the input resistor, lifting the second caps grounded leg and inputting your signal there...

So the "real" input for 'FROSTY the FILTER' is through the grounding cap (1N) of the second OTA stage (O2)? Keep everything else the same, except for grounding the 10k 'In' resistor?
Then the output of the HP goes straight into what's listed on the schematic as 'In' (of the LP stage) and the 'Out' (again, of the LP stage) goes to amp?
Is the eventual goal: audio in -> buffer -> HP w/frequency and resonance pots, LFO/CV -> LP w/frequency and resonance pots, LFO/CV -> compressor -audio out?
2 separate LFOs?

[hoyager]

This thread has more info and a stripboard layout for Rene's OTA version

http://electro-music.com/forum/topic-43085.html

garcho - that 47kb is the for filter frequency

The filter can be switched from LP - HP using both inputs too but can't do both at the same time, you need 2 filters.

[Jordan A.]

@Garcho- The "pitch" input is for use in a modular synth where you have a calibrated 1 volt per octave control signal, usually from a keyboard controller.  In guitar world we don't have to worry about it.

You can see in the upper left of the schematic a junction of four 100k R's, an 1k8 R and the base of a PNP transistor, this is a summing node for control voltages, basically you feed a voltage to this node through a resistor and that voltage will sweep the frequency of the filter.  We can add any number of inputs to this node just by adding more 100k resistors, one is for manual control like you suspected and the others could be used for LFOs, envelope followers or whatever.  Those 47k pots could be anywhere from 10k to 100k without noticing any difference.

Your description of the signal path sounds good for the classic HP into LP setup.  I don't know if an integrated compressor will be mandatory, it's more of a luxury.

The power supply does seem to be a bit of a sticking point.  I suggested an AC wallwart/half wave rectifier bipolar supply earlier (like the MFOS "wall wart power supply"), and I still think it's a pretty good solution.  It keeps people away from mains wiring and works well enough, and it's barely more work/real estate than a charge pump.  I guess the main drawback is the special wall wart you need to carry around.

It's possible we could get adequate performance using a charge pump to get +/- 9 volts just like RG shows in the neutron documentation, but with two VCFs and two LFOs the current draw will be higher than the neutron for sure.  I don't have much experience with those, but that seems to be the most pedalboard friendly setup.

jordan

:edit: as for subbing a 13700 for the 3080s, just drop it right in and ignore the diode bias pins and darlington buffers.  PNP transistors can probably be whatever you have, 2N3906 will definitely work.  cheers!

[garcho]

 



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...

[Strategy]

Can't respond in great length but independent CV inputs for LP cutoff, HP cutoff, LP resonance and HP resonance will be crucial to the final version of the project so user's can insert whatever they want in parallel to the LFO's. In the example of the Frostwave Resonator I borrowed, I used the Frostwave FAT Controller sequencer (compact 2-channel CV/gate sequencer) to rhythmically modulate the cutoffs in a loopy fashion...it is endlessly musical and addictive

ADSR's from a modular synth, or anything would be good

As for foot control, it should be easy for most DIY'ers to use switching jacks to add expression pedal input to the desired controls? I have done this with many of my builds

Strategy

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...

[Strategy]

Now that I think of it adventurous users could plug in simple CMOS sequencer like in Vanishing Point, Seek Wah, and others

HexInverter has a compact easy sequencer project right now called SympleSeq (PCBs) that should run off +/-9V

I really think of this project as a core that can then be added on to, I still wonder if a version without any LFOs or any modulation is desirable so it's just a core! Somewhat modular for a pedal I guess but even without modulation it is a deep tone shaper

Strategy

[garcho]

Here's how I'm feeling at the moment. I'm gonna start small and work up to the pro version. It is definitely doable with Electric Druid's TAPLFO. Wouldn't even be that huge of a PCB. Off board wiring headache though - 16 knobs, 8 jacks, 6 toggles, and 2 foot switches! Also, at $10 each, the TAPLFO is probably a bit of an extravagant chip to double-up, but then again, a 16 knobber is extravagant no matter how you cut it. Unsure of how to work the parallel/series switching out without a patch bay; new territory for me. Regarding the LFO sync switch, I assume that one LFOs can sync the other through the external clock pin of the second TAPLFO, maybe that's not the case. The tap tempo would control both LFO rates. Also, still unsure of how to rectify the power supply situation with our +9V guitar pedal universe.
If I understand the conversion to HP filter correctly, it seems like switching between HP and LP would be a simple matter of wiring a 3PDT toggle switch for each filter:

Does that look right to anybody?
There would be 4 filter combinations that way (HP->LP  HP->HP  LP->HP  LP->LP). If I understand the circuit correctly and that switching works, I'll try those combos out and make clips of them eventually, and decide if it's worth it to add to the core version, or just leave it HP into LP.
What am I forgetting?

frosty the filter:
buffer -> HP -> LP -> out
8 knobs (2 freq, 2 res, 4 CV in mix)
4 CV ins
possible HP/LP switching
power?

frosty the pro (with TAPLFO):
buffer -> series or parallel -> out
16 knobs (2 freq, 2 res, 4 CV in mix, 2 LFO shape, 2 LFO speed, 2 LFO mix)
6 CV ins (2 wave distort CV ins for the LFOs)
series/parallel switch
2 HP or LP switches
2 native LFOs
2 LFO switches (freq or res control)
2 LFO external sync ins
LFO sync switch (so both LFOs are in sync together)
tap tempo foot switch for the LFOs
power?

[Jordan A.]

Nice work Garcho!  I think we can do HP/LP switching with two poles and two throws and use a cheaper switch, let's see if I can get a picture to show up-

http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/IMG_0202.JPG.html

Here is an idea for series/parallel/stereo operation using switching jacks and an additional opamp, hopefully it makes sense.  Unmarked resistors are all the same value, I used 100k.

-If we plug into "input A" and come out of "output B" we get VCF A cascaded into VCF B for series mode.

-If we plug into "input B" and come out of "output A" we get the output of VCF A added to the output of VCF B for parallel mode.

-If we plug into "input B" and run "output A" to one amp and run "output B" to another amp we get mono in/stereo out.

-If we plug one signal in to VCF A and another signal in to VCF B and use both outputs we get stereo in/stereo out.

-We can use either/or VCF independently.

-One quirk is that if we plug in to "input A" and come out of "output A" the output of VCF B will be added to the output, even though there is no input signal for VCF B.  This might be weird or confusing if VCF B is oscillating, but if the resonance control of VCF B is turned down it shouldn't be a problem.

-Another quirk is that it will make bypassing hard/weird, I have not given it much thought yet.

-Any improvements or suggestions are welcomed.

http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/IMG_0200.JPG.html

:edited for an output mode I forgot:

[garcho]

Bravo Jordan!

DPDT is way better, nicely done. I like the series/parallel ideas, too, thanks for drawing those out.
Do you have any desire for native LFO?
Any ideas on PS?
Would anyone be into having a miniature Bantam patchbay on their pedal? At the very least it would look hilarious...