CV Doubler Box

[JehuJava]

I think I'm ready to built my first project and I'm hoping you guys can point me in the right direction.  I would like to build a box that takes the CV output of a Moog MP-201 (0-5V) and doubles it (0-10V).  I have a bunch of Moogerfoogers that the 0-5V works perfect, but I have one pedal that takes 9V CV.  I did the 9V/volume pedal trick and it works, though I would love to be able to control that pedal with the MP-201.  So...is there a way to do this?  Maybe even include a pot to control the amount of increase and/or "calibrate" the output?  I've seen a bunch of voltage doubler circuits on the internet, but they all seem to be for other applications.  I don't know enough to be able to take what I need from those schematics and make this work properly.

[PRR]

[Mac Walker]

To expand on the above circuit -



You want a gain of 2 to change your 0-5VDC input to 0-10VDC signal.....so this circuit would give you a range of 1:1 to 2:1 with full adjustment of the pot.

[ashcat_lt]

And if you want it to go all the way down to 0V then you need an opamp which can actually get to the bottom rail.  And if you want it to get all the way up to 10V you need an opamp which can actually get to the top rail, or else a top rail that is "higher enough" than 10V to allow for the opamp's headroom limit.

Thus the lm324 and 11V rail.  That opamp will go down to the bottom, but will only get yo within about a diode drop of the top.

[JehuJava]

Thanks PRR. This is pretty easy to follow and easier that I thought it'd be. A couple questions:

The last resistor before output, it reads 100. Is that 100k or 10k like the rest?

When hooking up the pot, pin 1 is the input and pin 2 is the output. Pin 3 still ground?

All three grounds (V1-, Lm324 GND, and pot) go back to the power supply negative?

[JehuJava]

To expand on the above circuit -



You want a gain of 2 to change your 0-5VDC input to 0-10VDC signal.....so this circuit would give you a range of 1:1 to 2:1 with full adjustment of the pot.

Gotcha! R1 being 0k turned down and 10k turned up. So Av = 1+0 turned down and 1+1 turned up.

[JehuJava]

And if you want it to go all the way down to 0V then you need an opamp which can actually get to the bottom rail.  And if you want it to get all the way up to 10V you need an opamp which can actually get to the top rail, or else a top rail that is "higher enough" than 10V to allow for the opamp's headroom limit.

Thus the lm324 and 11V rail.  That opamp will go down to the bottom, but will only get yo within about a diode drop of the top.

Just curious. What would happen using even higher power source, say 18V (arbitrary)?  The circuit would still function as intended because of the Av=1+(R1/R2)?  Think I might have some power supplies laying around but not sure what they're rated at.

[tubegeek]

The last resistor before output, it reads 100. Is that 100k or 10k like the rest?

It's 100 ohms.

When hooking up the pot, pin 1 is the input and pin 2 is the output. Pin 3 still ground?

Middle pin is the wiper - that's got the arrow symbol. Wiper connectds directly to one of the outside pins and then the circuit connects to the outside two pins as well.

If you want the output to get "louder" as you turn clockwise, then you want to increase the resistance in the pot - so connect the wiper to the terminal closest to the clockwise end of the pot.
(Pin 1, pin 2 and pin 3 are somewhat ambiguous in the world of pots. Get used to thinking about "wiper," "clockwise," and "counterclockwise" instead.)

[/quote]
All three grounds (V1-, Lm324 GND, and pot) go back to the power supply negative?
[/quote]

Look again at the drawing:

The ground connections are:

- the sleeve of each jack
- 1Meg resistor ("Pull-down" resistor to prevent pops)
- 10K resistor (part of the negative feedback voltage divider)
- LM324 GND pin.

The pot does NOT go to ground, it goes to the inverting input (marked -) of the LM324 and to the output of the LM324.
Usually with a battery-operated unit, V- goes to the ring terminal of a TRS input jack, so that it is disconnected from ground when the TS plug is removed from the input. Like an on-off switch, but sneakier. The sleeve of the plug connects R (ring) to S (sleeve) when the plug *is* inserted which allows the battery power to reach ground and bring the power connection for the unit "live." Unplug it, the battery shuts off, preserving its life.

[JehuJava]

That was informative, tube geek. Thanks.

[ashcat_lt]

You can use any supply between about 11V and the limit of the opamp, which is probably something like 30 or 36.

The real question is what to do with the other three opamps!

[PRR]

> last resistor before output, it reads 100. Is that 100k or 10k like the rest?

No. You are driving a load, the input to what you are controlling. I don't know -what- that load is, but it is probably 100K or 1meg.

If you have 100K output resistor and 100K load, 10V from the opamp becomes 5V at the control input. You've gained nothing.

10K is a lesser loss but still severe.

For *extreme* precision (0.1%) that resistor would be zero.

However.

1) Unless you are controlling musical pitch, you don't need even 5% precision.

2) This is a Stage Effect, not a white-coat lab. Accidents happen. Cords get plugged wrong. The opamp output can be back-fed unexpected signals, up to a Loudspeaker signal.

If you take a chip pin "outside" the power rails, it conducts like crazy, and can be killed.

So whenever possible, opamp Ins and Outs should have some resistance to limit the current when bad stuff is accidently connected.

On reflection:

The output resistor (shown 100 Ohms) can be 1K. Won't significantly reduce the CV, the small loss will be trimmed when you adjust the trimmer, and 1K is very traditional in Moog/ARP synth control paths.

The input 10K could possibly be on the other side of the 1Meg, from 1Meg to opamp +In. But the difference is 1% which is small and trimmable.

With these values:

Input pin can tolerate 10mA out-of-bounds current. With 10K resistor that means 100 Volts of bad-stuff will not harm the input. If you have 120V in audio connectors you have worse problems than protecting a 39 cent chip.

Output pin 40mA max. 1K will tolerate 40V of bad-stuff forced in. This is roughly a 100 Watt loudspeaker level. I don't suggest you try it, but if the mistake happens, the booster has a chance of surviving. And obviously signals from other pedals, mix-board sends, etc will do no harm.

[JehuJava]

Thanks PRR, that was a huge lesson for this novice. I really appreciate you raking the time to explain that. FWIW, the pedal is the Frostwave Resonator...the MS20 filter clone.

Well, let me re-read that a couple of times and play with some math to make sure I'm following. Then off to buy the parts. I'll probably breadboard it before I assemble it for real. I'll post my results when they happen. Thanks guys!

[JehuJava]

Hey guys. I just wanted to post that I finished the breadboard mock up and this works perfect. ...and this was my first electronic DIY project.

For the sake of simplicity and learning how the breadboard layout works, I used just a 10k resistor instead of the 10k trimmer pot. 5VDC at the input and 9.9VDC at the output. LFO voltages tracked perfect as well.  Next up, check it with the trimmer then mount on perfboard.

Thanks for your help!!