Editor's note: On one of the several synthesizer-related lists to which I subscribe, Andrew Scheidler asked some questions about building a CV (control voltage) source. I replied that I was interested, too, and "Stooge" Larry Hendry soon stepped in with the requisite information. The following "article" is a slightly edited version of Larry's messages to Andrew and me. Thank you, Larry!

Postscript: Anne and J. Larry Hendry were killed on October 9th, 2005 while vacationing. The story is posted at www.wiseguysynth.com. We miss you, Larry.

Synthesizer DIY Project: CV Source

OK, you two (Remember, Larry was addressing Andrew and me. --Ed.), no hiding behind no electronic skills. This is so easy my wife could do it. Now, I am not going to give it to you 100%. I am going to make you think a little.

The circuit shown on the small schematic, below, is all you need for a nice, buffered voltage source that will be steady and not vary greatly with load.

I have selected a TL074 op amp for you guys because it is very common, and it has 4 amps in one 14 pin package. So, you can build all 4 of your voltage sources with one chip. So, I left the pin numbers off since this circuit is actually duplicated four times around this one chip (except the power supply connections and those 2 caps). If you only wanted 2, you could have used a TL072 8 pin chip instead.

Note: You could spend $24.00 on the potentiometers -- depends on how nice you want the pots.

Let's discuss some theory of the circuit. The op amp is wired in an inverting mode. This means that when you connect positive voltage on the input, you get negative out. You can connect amps to be non-inverting, but inverting is easier and has the advantage of summing which will be beneficial for Andrews "switches" idea.

As you see, I have a 100K pot connected from V+ to V-. 100K is not a magic number. But, they are common and big enough. 50 K will work OK, but it will cause your circuit to use a little more current.

Ohms law E = I * R. Or, in this case if we are solving for current, I = E / R. So, to figure out the current in the pot, we have 30 volts / 100,000 ohms = 0.3mA 50K pots would cause 0.6mA to flow in this circuit. Either is fine. Use what you have or can find. However, don't go too low or the current will be too much for the power rating of the pot. P = E * I

You will use resistors R2 and R3 to scale the output. The output voltage is simply equal to -1 * input voltage (R3/R2). So, if I want to have a total range of +/- 5 volts out (10 volt total), I need to have a ratio of R2 and R3 that divides by 3. We want nice large resistors so that we do not draw too much current from the pot. The more current you draw from the pot, the more non-linear it behaves.

So, lets say I use a 100K resistor for R3 and a 300K resistor for R2. When I have my pot turned fully counter-clockwise (toward pin 1), I am connecting +15 volts through R2 to my input. My output voltage = -1 * +15 (100/300) = -5 volts

When I turn the pot fully clockwise toward pin 3, I am connecting -15 volts to the input of the amp through R2. My output is now +5 volts.

See how easy that was? The size of the pot will not effect the voltage out of the circuit. Only the ratio of R2 and R3. You ask about R1 and R4. What gives here?

R1 is proper in the circuit to avoid offset. IN this simple circuit, you could just leave it out and connect the "+" input directly to ground. However, it is supposed to be sized to equal the equivalent parallel resistance of R2 and R3.

(R2 * R3) / (R2 + R3) is the formula for parallel resistors. So:
(100K * 300K) / (100K + 300K)
= 30,000,000,000 / 400,000
= 75,000

So, 75K is the correct value for R1. But, it is non-critical. So, anything close is fine.

What about R4. R4 is not needed in the circuit. However, if you connect the output of the amp to a jack, you run the possibility of shorting the amp output with a cable. The amp will huff and puff and die. So, R4 protects the amp from accidental shorts when plugging and unplugging or a bad cable. It needs to limit the current the amp will deliver to the short circuit. Something in the 470 ohm to 1K ohm range will do fine.

So, now you can select your own resistors for R1, 2, 3 and 4. or you can just use the 4 values I gave you to get +/- 5 volts. But what if you wanted 2 of the 4 circuits to deliver only positive voltage? Connect your pot from -15 (pin 3) to ground (pin 1) instead of to +15 (remember we are inverting everything). With no resistor changes, your output will be 0 to +5 volts. What to change that to 10V? Change the ratio of R2 and R3. If you leave R3 at 100K, you can decrease the size of R2. to get the correct ratio 15 volts in and only 10 out. You need to divide by 1.5 So, R2 = R3*1.5. You can adjust R1 according to your new values of R2 and R3.

Now what about switches? Sure.. this is why I had this circuit designed as an inverting amp. Because inverting amps are summers. That means we can sum various voltages at the summing node (the (-) input).

Now, you want a switch with 1v, one with 2v, one with 3v and one with 4 v? So, with those 4 switches you can select any voltage from 0 to 10 by using combinations. I think that is what I heard you say.

Each switch connects to -15V and then through a resistor to pin (-) on the amp. Use 100K for R3 like before. Now, figure your switch resistors. I can tell you. But, I want you to figure them. Or, you could even use a rotary switch.

You two chew on this a while and ask any questions you wish. They sell a small book about op amps at radio shack by Forrest Mimms. It think it is $3.95 and it will teach you all about these op amps.

Larry

Follow-up

There are plenty of smaller PCBs available from some of the electronic catalogs. Photo #1, below, is one I use quite a bit for small projects. The project shown is a dual reversing attenuator. It uses the same kind of parts your project will use. I also built the original pedal interface on one this size.

I do not like to work with plain perfboard or pad per hole. Some people do. I like to work with the ones that are designed for dual-in-line chips like these op amps that were are discussing. They have a layout smaller but similar to the layout on the MOTM proto board. Photo 2, below, shows a PCB like [one of you] had described. The connections of 2 and 3 pads together makes it easy with a few jumpers on the top to make up your circuit. You can do that with scrap resistor leads from your next kit.

You may be able to find them at Gold Mine Electronics. Radio shack sells something similar, though maybe not quite as nice.

Dual Reversing Attenuator on Proto Board

Small DIP Proto Board with Pots