Tuesday 6 June 2023

Synthesizer Build part-53: RINGMODULATOR with AD633 for Eurorack.

This is the MFOS Ring-Modulator based on the AD633 analog multiplier chip and it sounds awesome. It's very small and will easily fit a Eurorack system so naturally no problem for a Kosmo system either. This ring-modulator is perfect for creating bell sounds and all sorts of timbres.

This is the first article in which I did not actually build the project myself. I was asked by one of our Facebook members, Justin Andrews, to make a stripboard layout for a schematic that he found. It sounded like an ideal little project for the website so I set out to make the layout. Justin built it up and it worked like a charm first go. The AD633 is a small DIP 8 Analog Multiplier chip and they can be a bit pricey. They cost about €22,- each. The MFOS article states they are cheap but they seem to have gone up in price. Make sure you get them from a reputable source though, not from AliExpress for a few dollars. Those will be 100% fakes! I looked around and it seems they are no longer in production but there are still electronics webshops who have them in stock so they shouldn't be hard to find.
The other chip in this circuit is a single opamp, the LF411. I don't know why this type was chosen over the normal µA741 and I suppose you can use a 741 if you wish. The pinout is the same. Only the actual opamp connections of the chip are used not the offset controls. The 43K resistor is a bit of a strange value. You can get away with a 47K too I reckon. Not even the 2,2µF input caps need to be specifically that value. They are DC blockers and in that function you can use 1µF upto 4,7µF without any problems. Together with the 100K resistors these caps form a highpass filter with a cutoff frequency of 1.6Hz if you use 1µF for the caps. It's even lower with higher values so no influence on the sound what so ever whatever value cap you use.
Here is the link to the full article on the Music From Outer Space website.

Here's the schematic for this project. As you can see it can hardly be simpler and there are no trimmers to set so no calibration necessary. If you read the article linked above you'll see that right at the top Ray Wilson says not to build this project as it has been superseded by newer ones but that doesn't mean this design doesn't work of course. Far from it in fact. It works very well. It's just very basic in its setup.
Both inputs, the Carrier and the Modulation input, are AC inputs. They have an electrolytic capacitor in series with the inputs so this ring modulator is only for audio range signals, not for CV. In the MFOS article it is stated in the Bill of Materials that these should be ceramic capacitors so in fact bi-polar, but I would just put in electrolytic capacitors. That always seems to work just fine and they are used in many other projects for the same function without any problems.

The circuit has two settings: Modulate and Multiply. Justin's experience was that the Multiply mode had a more choppy sound with more artifacts and harmonics. You can see in the scope screenshots at the bottom of the article why that is.
The circuit needs signal at synthesizer levels to work well. The input is meant for 10Vpeak-to-peak signals so if you want to use it for lower level signal you are advised to amplify those first to at least a few Vpp before they enter the ring modulator. 
The circuit is designed to run on +/-12V but I don't see why it wouldn't run normally on +/-15V either.

There is an updated version of this ringmodulator called the Sonic Multiplier which is more difficult to build and has a quad opamp in it and uses an internal sinewave generator with an LM13700. I have not made layouts for it but here is the link to that project on the MFOS website:

Here are the layouts I made for this project and they have been verified. Beware the size of the stripboard is only 16 strips by 26 holes. I see I made one little oversight in the layout design. It would have been better if I had put the power connector at the bottom instead of at the same side where the faceplate is meant to go. But it still works fine of course :) 
Beware the negative 12 Volt is the top connection and the positive 12 Volt is bottom connection of the power header.
Wiring diagram:

Stripboard only view:

Below are the cuts and wirebridges as seen from the COMPONENT SIDE! As always, mark the cuts on the component side and then stick a pin through the marked holes and mark them again on the copper side. Then you can cut them with a sharp hand held 6 or 7mm drill bit.

Bill of Materials. I typed this bill of materials in Notepad so it's a bit small:

Here are some pictures of Justin's work. He did a great job and made a really cool faceplate for it too, in Eurorack size. 

The finished module. Justin used waterslide decals for the faceplate artwork and sealed it in with a coat of clear lacquer:

Oscilloscope screenshots:
Here are some scope screenshot combining different waveforms in both Modulation and Multiplication Mode so you can see the difference in processing. I put multiple images together in one to save some space.

And finally a demonstration of the ring modulator in action.

So that's all for this article. Not much of a write up but then again there's not really much more to say about this Ring Modulator. It does its job and it does it very well. If you have any questions or remarks you can put them in the comments below or post them in the special Facebook Group for this website.

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