Thursday, 5 November 2020

Synthesizer Build part-36: DUAL VOLTAGE PROCESSOR.

 This is the Fonik Buchla Style Dual Voltage Processor. A very useful module for altering Control Voltages with four different functions.  Offset, Attenuation, Inverter and Lag control.

I wanted a Voltage Processor module in my synth for a long time and I was thinking of copying the ARP2600 VP, but that one is fairly limited in its options and when I saw this design I thought it would fit much better in my system. This module lets you alter the offset of a control voltage by 0 to +5V or -5 to +5V. It lets you attenuate and invert a control voltage by means of a Attenuverter and it has a Lag control that is a direct copy of the Lag control from the ARP2600, with a 1 MOhm potmeter and a 470nF capacitor (The ARP used a 100nF cap). This alters the slew rate of, for instance, a Squarewave and rounds off the corners turning it into a Sharkfin Wave. In fact it adds a 90° phase shift to the signal. Besides control voltages this module can also handle audio signals.

This module will work fine on either a dual 12V or a dual 15V powersupply so no problem for you Eurorack fanatics =). 
The circuit is primarily meant for control voltages but it can handle audio signals just as well. Even at very high frequencies it won't distort the signal. With audio you can use the Lag control to turn a Triangle wave into a Sinewave although with less amplitude. It won't be perfect but it's possible. It can also turn a 0V to +10Vpp signal into a +5/-5V signal by adding a -5V DC Offset voltage to it. The other way around works too of course, turning +5/-5V into 0V to +10Vpp signal. Very useful.
The circuit consists mostly of 47K resistors but you can actually alter the value of those and use for instance all 91K resistors. I actually did this as a test with the second part of this dual module and it didn't change the working of the circuit in any way. Just make sure you use the same value for all 7 resistors. Don't make them lower than 47K though. 
This circuit was designed by Chris MacDonald and modified by Peter Grenader and then further improved by Matthias Herrmann who added the Lag (Glide) control function. The only thing I did was adding the Offset switch, changing the potmeter values from 50K to 100K, changing the value of the Lag Capacitor from 1µF to 470nF and adding the 470 Ohm resistor before the Lag potmeter to eliminate noise issues.
The original schematic and a PCB design can be found in this original PDF and I made a new drawing from that schematic which is posted below. Like I just mentioned, they use 50K panel potmeters in the schematic but I didn't have those so I used 100K potmeters. Again, this made no difference what so ever. You must however use a 1 MegaOhm potmeter for the Lag control because this, together with the capacitor, forms a simple lowpass filter and these values are important to get the correct frequency response. The original schematic uses a 1µF capacitor for the Lag control but with testing I found out that this is way too much. So I changed it for a 150nF in the layout but that turned out to be not quite enough. (The original ARP2600 Lag control uses a 100nF capacitor.) In my own build I experimented with different values and I ended up using a 270nF and a 180nF in parallel to make a total of 450nF and that works fine. So I set the capacitor value on the layout to 470nF. I found that this gives the best Lag control response. Of course, if you don't have a cap of that value available, you can use an other one with a value close by. Anything between 300nF and 700nF will work fine and you can put two (or more) in parallel to create the value you want.
The trim potmeters are for setting the attenuverter mid point, but they don't have too much of an impact so you don't have to use multiturn potmeters for those. The normal ones will do fine.  I added a switch to the offset control so you now have a choise to offset a control voltage from 0 to +5V or from -5 to +5V. 
A little quirck I found, at least in my build, is that there can be a lot of noise on the output if the Lag potmeters are set fully closed (counter clockwise). Because this was the case with both sides of the Dual Processor I figured this was a fault in the circuit design so I added a 470 Ohm resistor between the Lag potmeter and R6. The value is low enough not to influence the Lag filter and it gets rid of all noise issues that I had.
The schematic drawing doesn't include any de-coupling capacitors but they are included in the layout. Just four 100nF ceramic caps on the power rails as close to the chips as possible. If you experience hum on the audio output you could even put some 10µF to 47µF electrolytic capacitors on the power rails. There's room enough left for that. Make sure they are rated 25V or higher and put one on the +15V to ground (negative pole to ground) and one on the ground to -15V (negative pole to -15) rails. I leave that up to you but for my module it wasn't necessary to include them. (The electrolytic capacitors are not included in the layout, only the de-coupling caps.)

Here's the schematic drawing which I re-made from the original, from the above linked PDF file. The Dual Voltage Processor consists of two of these circuits side by side with only the Ground as a common link:

Here is the verified stripboard layout I made for it. It's the same layout once repeated and mirrored to make it a dual module. You can easily cut the stripboard in half and fold it over, connecting the traces that need to be connected, together with some copper-wire to make it a Eurorack size.

Print only. Beware that some stripboards are sold with 56 instead of 55 holes horizontally. The layout is 55 holes wide. 

Bill of Materials:

Here's a video with a quick overview of the different functions. 

I watched a demonstration video about the ARP Odyssey and in it they showed the effect that the Odyssey's Lag control had on the filter cut-off control voltage. It made the filter make these 'Wah' sounds. And I'm very chuffed to see that the Lag control in this module has the precise same effect on a filter.

Here are some pictures from the build process:

In the picture of the panel (below) the 'Lag' control is still called 'Glide'. That's what it's called on the schematic but I chose to use the same term that ARP uses in the 2600.  I think it's a more accurate description because it actually creates a phase shift of about 90 degrees (see also the article about the ARP Envelope Follower). So that makes the signal lag behind the original in a small way. 

The picture below shows one side of the dual module wired up and the other side has not yet been wired up. The LEDs of that side are still mounted on the print (which was necessary for testing) instead of in the panel.

When the panel is in 'rest' position so to speak, all potmeters should be set fully counter clockwise and the switch set to 0/+5V. That way, any signal you put in will come out unchanged. You can then alter it by turning the controls.

Okay that's an other one done. If you have any questions please put them in the comments below or on the EddyBergman Facebook group. Please read the whole article before asking questions.

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DISCLAIMER: The author of this article does not accept any responsability for the correct functioning of this, and any other, module/project on this website. What you build, you build at your own risk. All project layouts are thoroughly tested before publication, it's up to you to replicate them and the author can not be held responsable for any mistakes made.