Does a simple filter with reverse exist? The filter would close instead of open when a chord or note is struck.
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Just take an autowah and plug it in backwards!
...that's logical, right?
Do you mean a filter that sweeps down in frequency instead of up? One of the common mods for the old MXR envelope filter is to add an up/down switch. There is a Tonepad project for it, and a schematic at GGG: http://www.generalguitargadgets.com/wp-content/uploads/ggg_mxr_envelope_filter_mods.pdf (http://www.generalguitargadgets.com/wp-content/uploads/ggg_mxr_envelope_filter_mods.pdf) - the mod is the "reverse sweep." Smallbear sells the tonepad PCB if you want one (old school single sided PCB).
This mod is possible because of the interesting way the MXR does the filtering - it uses pulse width modulation of a clock signal at ultrasonic frequency, and that signal opens/closes a gate to effectively vary resistance and change the bandpass frequency of the filter. The reverse sweep mod inverts the clock, so the PWM makes the filter go down instead of up. I am building one of these using the Tonepad layout on perfboard (which is a little nuts). I haven't finished it yet, but I do have the clock running, which is sometimes tricky apparently, and modulating the filter.
Jimi, IIRC you have a Rissole, that has an up/down switch :)
Quote from: alanp on July 29, 2018, 03:22:46 AM
Jimi, IIRC you have a Rissole, that has an up/down switch :)
And I love it, I was just looking for something simple.
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Doesn't seem he wants a reversible sweep (up or down). He wants the envelope to act in reverse. Gotta be something like that out there. Can you do something similar with the slow gear?
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Pick a simple design you like and I'll take a look at the schematic and tell you what to change.
In simplified terms, all of these (autowahs, Slow Gear, compressors, etc) are examples of voltage control. For guitar pedals, typically the control signal comes from an envelope follower, something like a rectifier and smoothing capacitor that turns the input signal into a signal that follows the volume envelope. For synthesizers, the envelope may come from an envelope generator triggered by a key press.
For an autowah, the control is fed into a VCF, voltage controlled filter, that changes the filter frequency. The filter could be bandpass or low or high-pass. That simulates rocking a wah pedal as a note is struck. The up/down switch is like choosing to rock the wah back or forward.
For a compressor the envelope is fed into a VCA, voltage controlled amplifier, that changes the volume instead of frequency, to turn down the gain when the envelope is strongest.
The Slow Gear is an extreme variant of the VCA compressor that makes the volume ramp up as the envelope decays.
Quote from: midwayfair on July 29, 2018, 05:55:08 PM
Pick a simple design you like and I'll take a look at the schematic and tell you what to change.
(http://fuzzcentral.ssguitar.com/schematics/dodfx55b.gif)
Bean, that's cruel.
Here is the mojo schematic. You can practically feel the toan:
(https://s19.postimg.cc/6ivshds2r/DOD_fx55b_simplified.gif) (https://postimg.cc/image/xth3pauzj/)
Quote from: madbean on July 29, 2018, 09:06:54 PM
Quote from: midwayfair on July 29, 2018, 05:55:08 PM
Pick a simple design you like and I'll take a look at the schematic and tell you what to change.
(http://fuzzcentral.ssguitar.com/schematics/dodfx55b.gif)
My first recommendation is to replace all the chips with discrete components.
Quote from: sirbergersworth on July 29, 2018, 05:26:30 PM
Doesn't seem he wants a reversible sweep (up or down). He wants the envelope to act in reverse. Gotta be something like that out there. Can you do something similar with the slow gear?
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No, reversible sweep is the idea.
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(https://uploads.tapatalk-cdn.com/20180729/00abf96b56aa99a188410db0b50e1078.jpg)
(https://uploads.tapatalk-cdn.com/20180729/3593dce392bc4c21aea5e9a5b67bb835.jpg)
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Quote from: midwayfair on July 29, 2018, 05:55:08 PM
Pick a simple design you like and I'll take a look at the schematic and tell you what to change.
^^^^^^^^
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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.
Quote from: midwayfair on July 30, 2018, 01:36:47 PM
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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Quote from: jimilee on July 30, 2018, 01:59:32 PM
Quote from: midwayfair on July 30, 2018, 01:36:47 PM
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.
Quote from: Torgoslayer on July 30, 2018, 02:17:21 PM
Quote from: jimilee on July 30, 2018, 01:59:32 PM
Quote from: midwayfair on July 30, 2018, 01:36:47 PM
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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Quote from: jimilee on July 30, 2018, 02:28:51 PM
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.
Quote from: midwayfair on July 29, 2018, 05:55:08 PM
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.
(https://dzwonsemrish7.cloudfront.net/items/3F280G2g2T3A2s1v1i1d/Mirror%20Ball%20Schematic.png?v=90a93f68)
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 (http://www.tonepad.com/getFile.asp?id=113)
Quote from: reddesert on July 30, 2018, 09:55:09 PM
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 (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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Quote from: jimilee on July 31, 2018, 05:01:54 PM
Quote from: reddesert on July 30, 2018, 09:55:09 PM
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 (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.
Quote from: reddesert on August 01, 2018, 08:49:33 AM
Quote from: jimilee on July 31, 2018, 05:01:54 PM
Quote from: reddesert on July 30, 2018, 09:55:09 PM
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 (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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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
Quote from: jimilee on August 01, 2018, 10:58:06 AM
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).