Volume pedal that is controlled by exp pedal

[Thewoodguy]

Hey guys noob here! So I'm wanting to build a buffered volume pedal that is controlled with an expression pedal. I will be using this project as a crash course to building pedals. I have a friend that is going to help me with the schematics and such.

My big question is it seems like instead of using a traditional pot, I should use a ldr type setup. I'm just a bit confused on how they work. If I was to use a 25k pot what ldr would replace that? If any one knows please post some part numbers and simple wiring schematic.

Thanks guys

[pinkjimiphoton]

look at a morley volume pedal. i think peeps are thrown by the description. you just want a volume pedal controlled by light, right?
25k is pretty low of a value, btw. you need a higher impedance unless it's gonna be between effects or ya may get some tone suck.

but what you're describing, i think, is a morley style volume circuit. it's fairly easy,. you need something to light an led, and an ldr to ground basically. you can almost get away with making it passive. it's pretty simple. hollah back if i'm understanding you.

[Thewoodguy]

look at a morley volume pedal. i think peeps are thrown by the description. you just want a volume pedal controlled by light, right?
25k is pretty low of a value, btw. you need a higher impedance unless it's gonna be between effects or ya may get some tone suck.

but what you're describing, i think, is a morley style volume circuit. it's fairly easy,. you need something to light an led, and an ldr to ground basically. you can almost get away with making it passive. it's pretty simple. hollah back if i'm understanding you.

I was planning on it having a buffer so there is no tone suck. And it seems that 25k is normal in a buffered volume pedal. The concept is, I use a exp pedal to controll different things manly though midi. I would really like to just have one rocker pedal on my board. I will be getting the boss es8 truebypass looper/midi switcher. You can per patch tell it what your exp pedal controls. Because most exp Jacks send out 3-5 volts to the exp pedal, then the exp pedal reduces voltage. I was thinking that sold be able to control the ldr?

[pinkjimiphoton]

if you're using it to control midi, there's no tone suck possible as midi is just data. if you need a control pedal for midi, pretty much any passive pedal will do. there's no audio with midi whatsoever. midi just exchanges information from a controller to a slave as to how to do settings.

so if i'm following you, you want an expression pedal to be able to remotely control a buffered led/ldr based volume pedal circuit? that should be cake. all you need to do is use the exp pedal to vary the voltage to the led, controlling it's brightness. again, i'd look at a morley for the general idea of the circuit, cuz for all intents i think a morley volume pedal is what you're describing, but you want it in a box so you can assign your control pedal to it (or another midi addressable box) in your t/b looper, correct?

[Thewoodguy]

Yes you got it. I have a Morley that I've been looking at. But they have a shutter in between the led and ldr. I was hoping to try a ldr with the led, seems easier for prototypes. Just really need a recommendation of some ldrs to try out. Also do any of you guys have a favorite buffer circuit you like?

[PRR]

Remote controlled volume, that does not suck, is perhaps one of the hardest problems in small audio.

> a crash course to building pedals

Build an LPB, then a FuzzFace.

[ElectricDruid]

Remote controlled volume, that does not suck, is perhaps one of the hardest problems in small audio.

It can't be that hard, surely? There are a metric ton load of VCA designs in the world. Pick one. Attach the expression pedal to the CV. Done. No?

If you want it in ten parts or less or some such, then I agree, it gets a lot more difficult. Some things just need a few parts to build them well and shortcuts finish up being corners cut.

Tom

[pinkjimiphoton]

seems to me he already has the control pedal part. all he needs is the application, which was why i suggested a morley style circuit. it's fairly easy to implement and only requires a few parts, tho the wiper aspect is weird. a vca seems even easier.

[Thewoodguy]

I will look into the vca with my guy this weekend. I have built a few easy pedals over the years, like the lpb. This is the first one to where I don't have anything to go by. So crash course really means, design and first thing with something other then a standard pot.

Thank you guys for the help. I'm going to draw up a schematic soon with my buddy and I'll post it to see what you guys think

[PRR]

> It can't be that hard, surely?

Non-suck VCA are few.

Looking at a druid supply site's "VCFs & VCAs" page:

'3046 isn't a VCA but a basket of bits to build a VCA.

'3080 has poor dynamic range even in guitar work.

LM13700 is nearly the same, a hair better if you implement input diodes.

'570 compander is IMHO an oddity, has to be wrestled to use for anything except telephony.

SSM 2044 is interesting that it has the backward topology which can reduce input compromises. "Filter" to VCA conversion is surely just replacing Cs with Rs(?). CV is small and referenced to "GND" which may not be zero in a single-supply application.

V2164 is actually pretty slick. Needs a recovery opamp. The reference design assumes bipolar supply, an annoyance in a single-9V world. Not obvious: the reference input impedance is 30K, which is a bit low for some guitar-cord points. The control pin impedance of 5K seems low if expression pot is 100K; but this is moot because the control voltage is 33mV/dB, VERY sensitive, meaning a large pad to accept 5V or whatever CV. The bipolar to single-9V conversion is complicated by the fact the CV is (I assume) referenced to the "GND" pin..... ah, wait, Fig 6.4 shows single-supply implementation and "0dB = Vcc/2" CV is a reasonable value (but "33mV/dB" suggests +/-136dB around zero??).

For non-wide-range volume control, a simple current-steering VCA works. But a non-thump implementation is a lot of discrete parts. There used to be a bazillion radio/TV chips to do this, all now long obsolete.

Heck, this is the 21st century. He says the manual (footual?) control on a pedal he has seen puts a vane between an LED and a Photo-R. I've worked with hobby micro-comps which had code to read a pot and code to control a servo-motor, which can work a vane. Going through the processor you can "warp" the numbers to smooth the nonlinearity of CdS, and apply a desired linear/audio "taper" to ankle-angle. Cost could be comparable/less than some of the older VCA chips. CdS is low-low-distortion and can be low-hiss at guitar level.

[ElectricDruid]

Nice review of the standalone VCA chips. I think you're being a bit harsh on the 3080 and 13700, but I agree they're not hifi. That said, there's a ton of famous synthesisers that use them, and a tonload more famous pedals too, and no-one's complaining about those.
Both the 3080 and the 13700 output a current, just like the SSM2164, so strictly they should be followed by the I-to-V op-amp too, but often people just seem to use a resistor to ground. The 2164 supply voltage is +/-4V minimum, so +9V operation is a bit close to the limit. The CV range gives a useful -100dB to +20dB from 3.3V(-100dB) down to -0.66V(+20dB). It seems like a odd range, but if you put a CV mixer in front of the CV input (which you probably need anyway because of the 5K impedance) then it makes sense.
Personally, I'd probably use CA3080 if I wanted a guitar VCA (Probably introduces a little distortion, but we'll call it "warmth", ok?!) since it's the only single VCA chip*, and I'd use 13700 if I wanted stereo. Both of them are easy enough to use single supply, and they cope down at 9V pretty well. There's some R.G.Penfold circuits based on them, I remember.

HTH,
Tom

*There's also the SSM2018 and the CA3280 , but these are rare and expensive beasties, so I'll ignore them.

[samhay]

>Heck, this is the 21st century. He says the manual (footual?) control on a pedal he has seen puts a vane between an LED and a Photo-R. I've worked with hobby micro-comps which had code to read a pot and code to control a servo-motor

If you are going to use a uC to read the treadle pot, then a recent thread threw up some dual supply digital pots wth 5V digital and +/-18V analog that might be good for this application if 7 or 8-bit resolution is good enough for smooth volume swells.

[ElectricDruid]

If you choose to go the OTA route, have a look at this:

http://www.nutsvolts.com/uploads/magazine_downloads/11/May%202003%20Ray%20Marston%20-%20Understanding%20And%20Using%20OTA%20OP-Amps.pdf

Figure 7 is a 13700 VCA for +9V single supply, which would be perfect.

There's similar for the CA3080 in Figure 14 in Part1:

http://www.nutsvolts.com/uploads/magazine_downloads/11/April%202003%20Ray%20Marston%20-%20Understanding%20and%20Using%20OTA%20Op-Amps.pdf

HTH,
Tom

[PRR]

> 3080 and the 13700 output a current, just like the SSM2164, so strictly they should be followed by the I-to-V op-amp too, but often people just seem to use a resistor to ground.

'3080 and kin have near rail-to-rail compliance. Dumb resistor loading works fine if you tolerate (or can use) the high output impedance, or use the EF on the fancier parts.

SSM2164 *appears* to be the Blackmer/dBx/THAT current-diff cell, which has output compliance only a few tenths of a Volt. For nearly all uses, it needs a I-to-V converter after.

Implementation tips for this type of gain control.

Yes, much the same either way except the '3080-etc can use an emitter follower which at some times and places is cheaper than a gain-amp. (Specially with '13x00 parts; or if you have a boat-load of BJTs.)

[rumbletone]

Yes you got it. I have a Morley that I've been looking at. But they have a shutter in between the led and ldr. I was hoping to try a ldr with the led, seems easier for prototypes. Just really need a recommendation of some ldrs to try out. Also do any of you guys have a favorite buffer circuit you like?

LDRs are cheap - perhaps just buy a couple different ones at the electronics store and experiment. Many of the specs relate to how fast they respond (likely not relevant for this application - doesn't need to be super fast for level control) or max dark resistance (often measured in Mohm), also likely not an issue if you're only looking for 25k resistance.

In experiments I've done, depending on the LED (which colour, for example) typical LDRs will, when the resistance in series with the LED sweeps from say 0 ohms to 10kohms (though I usually put a 300-500R resistor in series, to ensure I don't fry the LED, and assuming 9vDC power source), I get from near 0 resistance to 30-50k max from the LDR - which may be enough depending on how you implement the volume control circuit. If your expression pedal is 25k (rather than 10k), the max resistance will increase.

As for what circuit to attenuate volume, you could  start with any decent buffer (or boost) and perhaps just implement the LDR as part of the master volume at the output of the circuit - for example, a voltage divider with the LDR controlling how much is 'dumped to ground'?

Or perhaps use 2 LDRs, with 2 LEDs - and depending on your expression pedal setup,  have the 9vDC come into the wiper of the expression pedal pot and then have one LED (again, with a few hundred ohms series resistor on each LED so the LED doesn't fry) connected to each outer lug so as you sweep the expression pedal one LED gets brighter and one gets darker - so one LDR increases resistance while the other decreases - just as if you were using using pot to control the output.