Auto wah type filter with reverse?

[jimilee]

I was hoping for even simpler.

I'll give it some thought.

Initially I'd try putting the bottom of P3 to power instead of ground and reversing D3. There might be more to it.

This seemed less me the easiest to me, I wonder if the kraken is an easier one?


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[Betty Wont]

I was hoping for even simpler.

I'll give it some thought.

Initially I'd try putting the bottom of P3 to power instead of ground and reversing D3. There might be more to it.

This seemed less me the easiest to me, I wonder if the kraken is an easier one?


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or the DOD 440? It sounds great in reverse and is a fairly simple circuit.

[jimilee]

I was hoping for even simpler.

I'll give it some thought.

Initially I'd try putting the bottom of P3 to power instead of ground and reversing D3. There might be more to it.

This seemed less me the easiest to me, I wonder if the kraken is an easier one?


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or the DOD 440? It sounds great in reverse and is a fairly simple circuit.

That's the one I was searching for. Is there a PCB for this one?


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[somnif]

That's the one I was searching for. Is there a PCB for this one?

The "Sawse" 1590g board was based on the 440. Downside is it uses the unobtanium TL5C4/2 vactrol. GGG Used to have one as well.

Currently, I see Delyk has a "mini" board based on the circuit still for sale ( pcb.delyk.com/mini-boards/48-green-funk-pcb.html ). Its scaled for a 1590LB, so the control scheme is altered somewhat.

[jimilee]

Pick a simple design you like and I'll take a look at the schematic and tell you what to change.

Here is the dod 440 that jubal made, and I built. The reissue had a reverse switch. I've looked at schematics, but thus far haven't found one with a reverse switch.

[reddesert]

To have a switchable autowah, you need to get the inverse signal of the rectified envelope that is the control signal, which typically goes to either an LED (as in the DOD 440) or the input of a VCA (as in the SWAW).  And you have to be able to switch the inversion in/out. One difficulty is that some of these components aren't linear, like the LED/LDR, so if the forward envelope is the right amplitude to sound good, the inverted envelope might not be. Mark Hammer has written a bit about this on DIYSB.

For the SWAW, the control signal is coming from the emitter of the Darlington pair Q2-Q3 through R26 into the VCA. Trying to change that entire transistor pair setup into an inverting amp might be hard, so one approach could be to add a DC-coupled inverting op-amp after R26 that can be switched in/out of the circuit.

For the DOD 440, the rectified envelope is at the cathode of D2, passed through R11 (a current limiting resistor) to the LED. The R12-Level pot-R13 seems to be setting a bias level for the LED, essentially a sensitivity control. The LED is turned on by essentially (voltage at D2 minus ground). To invert it, you could try to have the LED turned on by (+9V minus voltage at D2). This would mean that you put in a switch that moves R11 and the LED to run from +9V to the cathode of D2, rather than from the cathode of D2 to ground. I'm not 100% certain if you'd have to change anything about the R12-Level-R13 arrangement.

Let me put in another pitch for the MXR Envelope Filter with the reverse sweep mod. This is a very simple mod that just adds an extra inverter into the control path. The stock circuit uses two CD4069 CMOS hex inverter chips and has one inverter left over, so all you need for the mod is a switch and some wire. It can be built on a stock PCB like the Tonepad PCB, and the original filter only has two knobs, so it's pretty simple. http://www.tonepad.com/getFile.asp?id=113

[jimilee]

To have a switchable autowah, you need to get the inverse signal of the rectified envelope that is the control signal, which typically goes to either an LED (as in the DOD 440) or the input of a VCA (as in the SWAW).  And you have to be able to switch the inversion in/out. One difficulty is that some of these components aren't linear, like the LED/LDR, so if the forward envelope is the right amplitude to sound good, the inverted envelope might not be. Mark Hammer has written a bit about this on DIYSB.

For the SWAW, the control signal is coming from the emitter of the Darlington pair Q2-Q3 through R26 into the VCA. Trying to change that entire transistor pair setup into an inverting amp might be hard, so one approach could be to add a DC-coupled inverting op-amp after R26 that can be switched in/out of the circuit.

For the DOD 440, the rectified envelope is at the cathode of D2, passed through R11 (a current limiting resistor) to the LED. The R12-Level pot-R13 seems to be setting a bias level for the LED, essentially a sensitivity control. The LED is turned on by essentially (voltage at D2 minus ground). To invert it, you could try to have the LED turned on by (+9V minus voltage at D2). This would mean that you put in a switch that moves R11 and the LED to run from +9V to the cathode of D2, rather than from the cathode of D2 to ground. I'm not 100% certain if you'd have to change anything about the R12-Level-R13 arrangement.

Let me put in another pitch for the MXR Envelope Filter with the reverse sweep mod. This is a very simple mod that just adds an extra inverter into the control path. The stock circuit uses two CD4069 CMOS hex inverter chips and has one inverter left over, so all you need for the mod is a switch and some wire. It can be built on a stock PCB like the Tonepad PCB, and the original filter only has two knobs, so it's pretty simple. http://www.tonepad.com/getFile.asp?id=113

Am I right in seeing that all you have to do is put pun 8 and 9 on a dpdt or spdt switch? That's crazy easy.


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[reddesert]

Let me put in another pitch for the MXR Envelope Filter with the reverse sweep mod. This is a very simple mod that just adds an extra inverter into the control path. The stock circuit uses two CD4069 CMOS hex inverter chips and has one inverter left over, so all you need for the mod is a switch and some wire. It can be built on a stock PCB like the Tonepad PCB, and the original filter only has two knobs, so it's pretty simple. http://www.tonepad.com/getFile.asp?id=113

Am I right in seeing that all you have to do is put pun 8 and 9 on a dpdt or spdt switch? That's crazy easy.

Yeah, that's it - you need a DPDT.  The Tonepad layout has pads to wire the switch to pins 8 and 9 (the extra inverter).

You could get extra sophisticated and try to tie the input of the inverter to ground or +9V when not using it, but this is just to keep it from using extra current / generating noise. I metered my build at drawing 3 mA, so it's not exactly a current hog.

Building this pedal can be a little subtle because you may have to change a resistor (R16 on the schematic) to get the clock to run, depending on the brand of CD4069 chip. There are comments in the Tonepad build notes about it. I think I used the stock value.

[jimilee]

Let me put in another pitch for the MXR Envelope Filter with the reverse sweep mod. This is a very simple mod that just adds an extra inverter into the control path. The stock circuit uses two CD4069 CMOS hex inverter chips and has one inverter left over, so all you need for the mod is a switch and some wire. It can be built on a stock PCB like the Tonepad PCB, and the original filter only has two knobs, so it's pretty simple. http://www.tonepad.com/getFile.asp?id=113

Am I right in seeing that all you have to do is put pun 8 and 9 on a dpdt or spdt switch? That's crazy easy.

Yeah, that's it - you need a DPDT.  The Tonepad layout has pads to wire the switch to pins 8 and 9 (the extra inverter).

You could get extra sophisticated and try to tie the input of the inverter to ground or +9V when not using it, but this is just to keep it from using extra current / generating noise. I metered my build at drawing 3 mA, so it's not exactly a current hog.

Building this pedal can be a little subtle because you may have to change a resistor (R16 on the schematic) to get the clock to run, depending on the brand of CD4069 chip. There are comments in the Tonepad build notes about it. I think I used the stock value.

Did you use the pcb that is sold at smallbear?


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[mcasemo]

Just adding one more I know of, Tonepad FSH-1 has a reversible sweep, up/down DPDT switch.

http://tonepad.com/getFile.asp?id=138

[reddesert]

Did you use the pcb that is sold at smallbear?

I used that layout, but I made it on perfboard rather than the PCB (not a totally sensible thing to do).