FilterFx w/cv influence, a culmination

[Matthew Sanford]

Mostly working, here's an open chord snippet . It's got fender amp reverb on full, mostly to show the fast to slow rate change. The excessive amounts of wires and unreliability of the cv pot & switch has me making PCBs for those off board controls, will be better.


 I ended up off with CMOS and went back to LM358s. I thought back to DiffEQ on about output resistance while reading the data sheet again, so 1M to ground solved that. Realized for 2 green LEDs in series (diy vactrol & indicator) that's a drop of 4 volts or so, so changed gain and bias to accommodate about 6.5v top end. Starting to think I should use one that gets closer to the V+ rail, but later.

On that end, I messed in Falstad for this scheme. The 4.5v bias goes through 1M Rb so a 3.3M tacked to ground gets first side to 3v bias...well, on the 3rd try PCBs I got. I plan to pull the FilterFX apart at some point, I raised its vactrol cars from 470R to 1k, wrong choice...and other fixing.

Still, moving on, I'm going to make boards finally for the StompLFO and One Shot to live in. They'll send all the CV to everything!!!

[jorg777]

Why are there vactrols between the control pots and the LFO chip?  Seems redundant.  Sometimes vactrols are used to add voltage control to a circuit that expects a variable resistance, but the StompLFO directly accepts control voltages (and anyway, your control pots are variable resistances, which, connected as voltage dividers, could be wired directly to the chip).

I don't quite follow why you are switching between the two sections of a dual pot; nominally both sections will be the same setting.

I think you need pulldowns on your outputs; also the sleeve contacts on the jacks seem to be floating.

The Sync input needs a current-limiting resistor.

[Matthew Sanford]

Jorg, you're right on much. Tom mentioned way back when about just using CV voltage direct to the pins; only, I want any wave coming in, and as I currently do it they top around 6.5 volts or so when I think is already too much for the pic input. I feel like I should have trim pots for the CLRs in there, just if bigger stuff will be used then adjustments can be made. When I started I was deadset on 0v being included in the wave yet I realize now as I'm using vactols I really should get it set so the Vf is the min voltage. Good note about a series resistor from sync, I'll add it before I order those boards (and correct the pots since I think w/backside placement they are full at CCW, zero at CW).

For the dual pot, prior to it and the NPNs the two inverting amps in the LM358 produce complementary signals. So the switch is to be able to change which goes between 1&2 or 2&3 for each pot, so then they increase/decrease at the same time or in complementary fashion (say depth goes down when freq or offset goes up). For the output jacks, they are offboard, and next to the signal pad there is a pad to tie the sleeve to the groundplane.

I appreciate all your insights! I have a couple things to add to that PCB and correct before ordering, new to that, and I had to go through 3 versions last as I didn't really do all I should prior to ordering...but it was a better way to learn, from the mistakes...I guess? Otherwise I'm working on little PCBs for the dual and switch but with vactrols there, and even thinking NPN and led indicator so it's just wires to carry signals, power, & ground (and extra pads to daisy chain them), well, and wires to tie to effect pot.

As is it works, though one side is fully off before the other starts, so I'm going to look at other op-amps and possibly fixing values, etc., to get it so they waves cross in time instead of the gap between.

[moid]

I can't offer any advice about your schematic, but I did enjoy the sounds you were making with the pedal! I didn't know of this circuit before, it looks really interesting... how many of those sounds are achievable with the standard filter FX, or are they only possible with your mods? Do you have another device affecting the LFO via CV in this clip or are the random changes just one of the options of the circuit? Would you have any idea how slow or fast the rate pot can go? Could it do really slow 6 second peak to peak swells or hit audiorate for pseudo ringmod sounds?

[Matthew Sanford]

All the sounds are the FilterFx, get one! But keep the vactrol CLR at 470R, I dummied in 1ks which isn't right (I think why bandpass/low pass settings both sound like low pass)The change of rate is the part I added.

So my build is screwy; the waveform cv seems always on, reducing it's pot turns everything down, even the off on the on-off-on dpdt has one light on! Otherwise I think I put CV on the depth for a bit...and the little Fender amps reverb was all the way up.

It can take other CV in but this was just internal LFO triangle-that can go as slow as 10 seconds but gets a bit janky. The StompLFO chip has a very wide range.

Check it out!

[Matthew Sanford]

New Falstad

Not trying to bring it back up again, just want to leave this ↑ link here for later. Thinking on more voltage = vactrol more on, I figured putting first inverter at bias of 4.5 gets that side up to around 8.8v on 9v supply using TL072 (per data sheet is [V-]+2, [V+] +0.01), biasing the other side with 7 volts keeps it complimentary. This all should keep the lights on, smoother wave from turning the effect pot from 0% to 100% - I'll try it later though!

[ElectricDruid]

I can't offer any advice about your schematic, but I did enjoy the sounds you were making with the pedal! I didn't know of this circuit before, it looks really interesting... how many of those sounds are achievable with the standard filter FX, or are they only possible with your mods? Do you have another device affecting the LFO via CV in this clip or are the random changes just one of the options of the circuit? Would you have any idea how slow or fast the rate pot can go? Could it do really slow 6 second peak to peak swells or hit audiorate for pseudo ringmod sounds?

I can answer some of these questions.
The FilterFX circuit uses the StompLFO chip, which includes both a "random levels" and a "random slopes" waveform. Random levels is the classic S+H jumping about, random slopes is like a triangle wave that doesn't know where it's going next.
The Rate pot goes from 0.05Hz (20 seconds) to 25Hz, so you can easily do the 6 second swells you mentioned, but you wont get up into ring mod territory. To be honest, a vactrol-based filter is wrong for that anyway, since the vactrols won't respond quick enough, even if the StompLFO could do it. You'd need a LM13700-based state variable filter for that, I'd say. There are plenty of those about...

[jorg777]

Jorg, you're right on much. Tom mentioned way back when about just using CV voltage direct to the pins; only, I want any wave coming in, and as I currently do it they top around 6.5 volts or so when I think is already too much for the pic input. I feel like I should have trim pots for the CLRs in there, just if bigger stuff will be used then adjustments can be made. When I started I was deadset on 0v being included in the wave yet I realize now as I'm using vactols I really should get it set so the Vf is the min voltage. Good note about a series resistor from sync, I'll add it before I order those boards (and correct the pots since I think w/backside placement they are full at CCW, zero at CW).

For the dual pot, prior to it and the NPNs the two inverting amps in the LM358 produce complementary signals. So the switch is to be able to change which goes between 1&2 or 2&3 for each pot, so then they increase/decrease at the same time or in complementary fashion (say depth goes down when freq or offset goes up). For the output jacks, they are offboard, and next to the signal pad there is a pad to tie the sleeve to the groundplane.

I appreciate all your insights! I have a couple things to add to that PCB and correct before ordering, new to that, and I had to go through 3 versions last as I didn't really do all I should prior to ordering...but it was a better way to learn, from the mistakes...I guess? Otherwise I'm working on little PCBs for the dual and switch but with vactrols there, and even thinking NPN and led indicator so it's just wires to carry signals, power, & ground (and extra pads to daisy chain them), well, and wires to tie to effect pot.

As is it works, though one side is fully off before the other starts, so I'm going to look at other op-amps and possibly fixing values, etc., to get it so they waves cross in time instead of the gap between.

OK I sort of see what you're getting at.  You want to modulate the Stomp LFO's inputs with an external CV, with adjustable gain and polarity.  That pile of vactrols, dual pots, op amps and transistors is really getting in your way rather than helping here.  What you really need is just an attenuverter per signal.  No need for all the expensive parts.

[Matthew Sanford]

By jove, I think you've got it! That, an attenuverter, should work great to feed the CV inputs of the StompLFO! I had just ordered the PCB from JLC, so on that "New Falstad" link I have up there I actually put trimpots for the pullup resistor from the 4.5 volt bias to bring it up to 7 volts, sort of similar to attenuverting?

On the other side, I have other effects to get the CV coming from the StompLFO (and putting internal triangles too) that will influence pots set as voltage dividers and variable resistors, like in Christine. I ordered a panel of other pcbs to have those pots and switches on, but threw npns and vactrols on there too so any CV (with enough current) can power it with out all the other junk. I've been making my own vactrols, so not expensive, works out to $0.30 each. So for those I'm still planning on the switches and pots for level, well, for now. THank you though, you've given me direction on where to proceed to improve! Eventually will program something, but learning this side.

For info, here are the PCBs that will come to me Global Direct Standard Shipping eventually...

[moid]

I can't offer any advice about your schematic, but I did enjoy the sounds you were making with the pedal! I didn't know of this circuit before, it looks really interesting... how many of those sounds are achievable with the standard filter FX, or are they only possible with your mods? Do you have another device affecting the LFO via CV in this clip or are the random changes just one of the options of the circuit? Would you have any idea how slow or fast the rate pot can go? Could it do really slow 6 second peak to peak swells or hit audiorate for pseudo ringmod sounds?

I can answer some of these questions.
The FilterFX circuit uses the StompLFO chip, which includes both a "random levels" and a "random slopes" waveform. Random levels is the classic S+H jumping about, random slopes is like a triangle wave that doesn't know where it's going next.
The Rate pot goes from 0.05Hz (20 seconds) to 25Hz, so you can easily do the 6 second swells you mentioned, but you wont get up into ring mod territory. To be honest, a vactrol-based filter is wrong for that anyway, since the vactrols won't respond quick enough, even if the StompLFO could do it. You'd need a LM13700-based state variable filter for that, I'd say. There are plenty of those about...

Thanks Tom - wow 20 seconds! Something to put a synth pad through maybe... but it doesn't matter if it can't go as fast as ring mod if the MS20 filter I'm working on can do that for me I mean, having two filter pedals to play with will obviously improve my guitar playing skills immensely, so I think I know what I need my birthday present to be; now to find a relative that wants to buy me your filter FX kit!