R.A.Moog old school ring modulator clone

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CZ Rider
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R.A.Moog old school ring modulator clone

Post by CZ Rider » Sat Jul 26, 2014 1:38 pm

Interested in these old school ring modulators. R.A.Moog offered the Harold Bode licensed stand alone model 6401 and dual model 6402. Both models had extra circuitry to include a pre-amp and gate/threshold with final amplifier to work as a complete system. The basic ring modulator is a pair of audio transformers connected via a germanium diode "Ring".
The Harold Bode schematic here:
Image

The basic modulator circuit is all that would be needed in a modular system when processing Moog oscillators. So I set out to build just the basic modulator. The transformers I used are Edcore audio type 10K/10K with 20-20,000 Hz frequency response. First step was making the frame and mounting the transformers.
Here is the assembled Moog modular frame.
Image

Using the same type Vectorbord that Bob Moog used on modules, I first had to insert the Vector T-28 pins for the component layout. Here is the begining layout copied over from a component drawing. Can see the Vector P-91 tool for inserting pins. A small Vector note from the 60's reads this special tool can be purchased for 50 cents. :lol:
Image

For this circuit I only needed a CP3 type amplifier after the ring modulator to bring the output level back to the same level as the input. Since the ring circuit is a passive one the output is reduced to about half the input. There was extra room in this module frame, so I added two additional seperate Moog type reversable attenuator circuits.
Once all the pins are placed in the Vectorbord, soldering in the components is the next step. The board is removed and placed in a Panavise holder for soldering/assembly.
The two reversable attenuator circuits on top and ring amplifier on bottom.
Image

Assembly is easy as component placement and soldering is all done on the same side. No flipping the board over to solder like through hole construction. Never really liked that with a through hole board, and I find these are more enjoyable to put together. Once the component side is done the back point to point wiring is done. There is a color code where all red is +12 volts, blue is -6 volts and green is ground.
Back after soldering all the connections.
Image

Once the board is finished it can be set back in the frame and transformers wired in. Next the front panel needs to be wired/connected.
Image

Keeping this module R.A.Moog correct, the front panel parts used are mostly surplus NOS. Moog used these serrated washers behind each Switchcraft 112. This was probably for ground connection and helps keep the front threaded sleeve flush with the mounting nut. Mine are black oxide finish, but the Moog ones were plated. Moog also used military spec. Allen-Bradley pots and small Alco toggle switches.
Front panel hardware.
Image

For the reversable attenuators, I ended up selecting 10K linear pots with a center tap. Was not able to source the 25K type, and many early modules only used the standard 25K linear pot without a center tap. After testing, the 25K pots worked with a very precise center. However ther was a slight offset at the zero setting, noticable when using oscillators. There is no way to trim this out and just part of the circuit design using a pan pot between +/- sides. Trying the 10K center tapped pot worked much better as there was a ground at zero and a larger dead band. The 10K pots were from old EML stock and used in a few EML designs.
Image

Next I have to label the front panel. Perhaps in the future I will get a proper etched panel from Re:Synthesis. But for now using a Dotcom drilled blank and dry transfers for the panel graphics.
Transfering the lettering here.
Image

This type lettering turns out OK and is DIY.
Image

Hardware can now be mounted on the panel for final wiring.
Image

Circuit side of module all wired up. Top two circuits are the reversable attenuator, the mixer/amplifier below that and the germanium diodes at the bottom. All discrete circuits, very 1969!
Image

Another view of the component side.
Image

And a view of the back of the circuit. You can see it was tricky to mount those transformers and have room for the circuitboard and support brackets.
Image

Mounted next to the 3U wide 901 oscillator. Looks like it belongs there, and it sounds as good as it looks.
Image

So what does it sound like?
As one can imagine, something magical happens to a signal when you pass it through all that wire and iron in those coils. What comes out when two 901 oscillators are ring modulated together is difficult to put into words. While ring modulators are usually associated with bell like tones, and this can do that fine. The sound when the two sources are closely tuned together at different intervals resembles a chorus effect. It sounds bad ass! Interestingly, it gets that almost Minimoog like quality to lead tones I could never get on the modular.
Here is a youtube demo I put together. A little longer than I usually do a demo at just under 10 minutes. Begins with two 901's sine waves then I begin to patch in the different waveforms. I will edit the video later to include captions of what I'm patching. But here is the link.

R.A.Moog Modular old school ring modulator demo.

Really has a unique tone and sounds awesome!
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Re: R.A.Moog old school ring modulator clone

Post by thealien666 » Sat Jul 26, 2014 1:54 pm

Interesting, but YouTube indicates that the video is "private" and doesn't play... :(
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Re: R.A.Moog old school ring modulator clone

Post by CZ Rider » Sat Jul 26, 2014 2:04 pm

Ooop! Forgot to hit the publish button. :lol:
Try it now!

(Thanks for the heads up! That's what I get for multitasking.)
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Re: R.A.Moog old school ring modulator clone

Post by megavoice » Sat Jul 26, 2014 2:16 pm

Wonderful !!!
Would be nice to be able to do this on a Mini.......I tried immediately to come close to this on the Tonus, and it isn't far away............
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Re: R.A.Moog old school ring modulator clone

Post by thealien666 » Sat Jul 26, 2014 2:17 pm

It's okay now. Thanks.

This is one of the best sounding ring modulators I've heard. Very "musical". Image
Thanks for sharing ! Image
That little sequence sounds very close to the bass line from the instrumental piece "The Minotaur" by Dick Hyman... 8)

P.S. BTW, I've got the mod wheel pot on my Minimoog D that is showing signs of wear (worn carbon wiper and resistive surface). As you probably know, it's a 50K Audio (taper ?) Allen-Bradley type J pot. Does anyone know where I could get a "new" one for replacement ? Thanks for any advice.
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Re: R.A.Moog old school ring modulator clone

Post by nicholas d. kent » Sat Jul 26, 2014 8:41 pm

Great project! And thanks for the build pics. Love that sort of documentation.

For someone more beginner or not with a Moog Modular I thought this link might be of interest, though being passive there are level issues not faced with a more complex design-
http://cgs.synth.net/modules/cgsrr.html


Don't want to hijack the thread too far off topic, but I've got an EML 100 and a 101 (so I AM jealous in a tangent sort of way) that could really use some replacement center-taped pots - I have to re-check the values, but have found the problem ones incredibly hard to get into (based on a test of a non-center tapped one because it could be replaced with a newer pot), they have some extra sealant on them making them much harder to open up and they don't get better with "massaging" like some old pots do. My A&B ones in the EMLs are all Mexican made, not to draw any conclusions, but when I peeked into 60s Moog modules I remember seeing UAS stamped ones.

In any case I'd be extremely interested in purchasing or finding a source of any center tapped pots with values I'm after (don't have to be A&B but correct value and taper).

Looking at the previous post, while you do run the risk of a newer pot having a different feel when turning, a 50K Audio taper should be very easy to source some sort of a replacement of. Obviously check the specs of the shaft, my mini was heavily modified decades ago so I can't check what it's supposed to have

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Re: R.A.Moog old school ring modulator clone

Post by drogoff » Sun Jul 27, 2014 12:59 am

Very nice sounding! The 'scope waves show the non-linear distortion the diodes add. You wrote the transformers were 10k/10k, which sounds like 1:1 voltage. Using germanium diodes, with their lower threshold voltage, helps in the circuit but I wonder if the transformers were 1:10 and 10:1 - making the voltage on the diodes 10 times bigger to the non-linearity is a smaller fraction of the signal - if it would sound cleaner and more like the typical ring-mod. Of course, that might not sound as good!

That would also cut the current by a factor of 10 so I don't know if there would be enough to turn on the diodes. Also, I think a lot of the sound - and the flattening of the waveform - might be saturation in the transformer.

David

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Re: R.A.Moog old school ring modulator clone

Post by GregAE » Sun Jul 27, 2014 9:14 am

nicholas d. kent wrote:For someone more beginner or not with a Moog Modular I thought this link might be of interest, though being passive there are level issues not faced with a more complex design-
http://cgs.synth.net/modules/cgsrr.html


There's also this passive Ring Modulator offering from Synthrotek:

http://www.synthrotek.com/products/othe ... modulator/

Comes with everything you need except for a case to put it in.

- Greg

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Re: R.A.Moog old school ring modulator clone

Post by CZ Rider » Sun Jul 27, 2014 11:54 am

I should note the Youtube demo is only the ring modulator output. The real magic happens when that signal is recombined with the straight 901 waveforms in the Moog CP-3 mixers. The scope waveforms would literally dance with added saw/square/triangle waves from the 901's. This was the reason for the small discrete amplifier after the ring output to match the same 901 level for balanced mixing in the CP-3. Without the amplifier the ring would need to be mixed in at 10 while the other waveforms would need to be around 5 to be equal amplitude. I selected the amp/mixer input resistor to a 15K to roughly match the output levels of the 901's.

Here is a small MP3 link to the 901's ring modulating each other with the raw saw/square waves mixed back in via CP-3 mixer. The ring modulated signal interacts with the raw waveforms with these cancellations and additions making a chorus like effect that varies with the note frequencies.
Picosong link to ring modulated 901 lead/solo tone
(Drone in background, Line 6 echopro on Moog.)

In a way it is a subtle effect, but there is a waveform animation that is really pleasing to listen to. When you A/B the raw waves with and without the added ring modulated source, the difference is easy to hear. It adds this motion to the sound that is difficult to describe, like the waveforms are being torn and ripped apart when they interact with each other. I have been playing a Minimoog since 1974, and the Mini was always king of those lead/solo tones. As broad as the sounds are from the old Moog modular, they never really got anywhere near that special tone the Minimoog had. With this old school ring modulator, the modular surpasses any lead tone I have played on any synthesizer.

For anyone trying this circuit I do recommend using high quality audio transformers. Harold Bode used those UTC brand model S-20 modulation transformers. They are large, old, and can be expensive. Those Edcore ones I used were about 10 bucks each and made in USA. The frequency response on those small type vary, but are usually around 600-13,000 Hz. I have not A/B'd the cheaper versus the expensive ones, but would assume a higher fidelity range from the 20-20,0000 Hz ones.
The levels seem perfect for the 901's into those transformers. The 901's output is close to line level at roughly 1.5 volts P-P. I can drive them harder with outputs from the CP-3.

For those 50K audio mod pots, I do not have a tested source for those. Important in that type circuit that at fully CCW the resistance be 1 ohm or less. Otherwise you will get modulation bleedthrough. I have sourced different brands of 10K dual ganged pots for the CP-3 master output level. And the ones that measured 3-4 ohms CCW would have slight bleedthrough and not useable. Did recently purchase some PEK brand 10K audio pots, but can't seem to find them to measure if they actually have that 0-1 ohm resistance at CCW in the 10K audio. Can not recommend a pot brand without testing for this. But there are a few available today similar to AB type J pots. Kevin L. wrote that the expensive Clarostsat brand would not pass the test.
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Re: R.A.Moog old school ring modulator clone

Post by thealien666 » Sun Jul 27, 2014 1:19 pm

Thanks for the info. :D
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Re: R.A.Moog old school ring modulator clone

Post by Filipe » Mon Aug 04, 2014 7:21 am

This is SO COOL!
Thanks for sharing.

Filipe

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Re: R.A.Moog old school ring modulator clone

Post by grenert » Thu Aug 07, 2014 11:58 am

That sounds great! What are the specs or Edcor part numbers for the tranformers?
Thanks!
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Re: R.A.Moog old school ring modulator clone

Post by CZ Rider » Thu Aug 07, 2014 9:48 pm

The transformers I used were Edcor WMS10K-10K. Listed as $10.23 each but the shipping was almost as much as the two transformers. Well worth the price though. Did not want to loose any bass response ring modulating the Moog 901 oscillators.
Here is the link to the Edcor USA site:
https://www.edcorusa.com/wsm10k-10k

thealien666 wrote:P.S. BTW, I've got the mod wheel pot on my Minimoog D that is showing signs of wear (worn carbon wiper and resistive surface). As you probably know, it's a 50K Audio (taper ?) Allen-Bradley type J pot. Does anyone know where I could get a "new" one for replacement ? Thanks for any advice.

I did find those PEC brand 10K audio pots and they tested at almost 50 ohms fully CCW or CW. So those were not very usefull. Out of many pots I tried the Alpha type from Radio Shack had the lowest resistance with around 1 ohm. Some like the PEC were surprisingly high.
I started to study that whole "shunt to ground" the modulation buss has on the Mini. While experimenting with modular patching, it was difficult to really eliminate any modulation bleedthrough using that patching method. On a modular the patch would be sending the modulation source to a multiple and from there patching into the source (oscillator) to be modulated. You then patch in an attenuator output to the mulitple. The attenuator will shunt to ground at fully CCW. (Did see this same patch in an old 60's R.A.Moog patch diagram.) There was always a micro amount of the modulation present. A very slight ammount and I had to listen to a single oscillator and plug in/out the mod source to hear the difference. R.A.Moog attenuators are AB type J 25K linear. Audio pots would have a better curve for this.
Kevin Lightner wrote a nice page on the Mini modulation problem if you have not read it in a while. And it explains it very well and offers a few solutions.
http://www.synthfool.com/docs/Moog/mini ... heelfixes/

There was also the official Moog fix using a special 10K audio pot from the Micromoog on the same site:
http://www.synthfool.com/docs/Moog/mini ... ng_832.jpg


EDIT: Those 10K center tapped pots I used on the above reversable attenuators work great for this. My Prodigy is modded with one of those pots on the mod wheel. I get two different kinds of modulation with a center off. The real difficulty with the Mini mod wheel is getting the same exact response a stock Mini has with the 50K audio pots.
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Re: R.A.Moog old school ring modulator clone

Post by MC » Fri Aug 08, 2014 1:08 am

That solution fixes the symptom but not the disease.

The minimoog modwheel is a current shunt. Everyone else (including moog synths that succeeded the minimoog) rigs their modwheel as a voltage divider. THAT'S the difference.

You can rewire the minimoog mod wheel as a voltage divider. I did this and it cured the bleed problem without replacing the pot or using glass reed switches and magnets. However you don't get the wide modulation depth of the original current shunt, if you want to retain that then you have to modify the LFO circuit to pump out a heftier current.
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Re: R.A.Moog old school ring modulator clone

Post by CZ Rider » Sun Aug 10, 2014 2:31 pm

Made a modular patch diagram of the difference between the standard method and Minimoog method of modulation ammount.
Image

Image

Looking at the Minimoog type patch, it looks like it should not work, but that is how it's done. Fully CCW the attenuator grounds the signal at the multiple. With a standard 25K linear passive attenuator, the range is about 0 to the 1.5 tic mark for almost full modulation. With an audio taper pot a more controllable range can be dialed in. I tried a few different types and values, and with some there was always a slight ammount of bleedthrough. Have to remember too, the wheel only gave a 90 deg. travel to the modulation pot. I'm sure when they designed the Minimoog the small tradeoff of slight modulatin bleed in favor of controllable range was the reason for this odd method. And it is a very slight ammount of bleedthrough, have to really listen for it. With some potentiometer brands/values I was able to null the modulation almost completly.

Speaking of modulation ammounts, forgot to mention what the ring/mixer switch on that module does.
Image
Since I had a CP3 type mixer amplifier after the ring mod output, I figured it would be wasteful not to also be able to use it for mixing/amplifing too. By adding two jacks/sockets labled 3 and 4 with a DPDT switch, it can convert back to a CP3 mixer with four inputs and +/- outs. So the jacks labled X and Y can be switched from the transformer inputs to the 22K input resistors on the CP3 mixer/amp.
When dealing with modulation ammounts from an oscillator or noise source, both the Minimoog and Moog modular have low "line level" signals comming from the oscillators/noise. So a module that can boost these levels is usefull for modulation purposes and audio signal boosting. The typical CP3 mixer input has a gain of 2 and each input adds together. I normalled the inputs with Switchcraft 112 jacks that have an extra switched tab, where inserting a plug breaks this normalized connection. So input 4 is normalled to input 2(Y) and input 3, while input 3 is normalled to input 1(X). With this configuration a single input to 4 will output a signal amplified 8 times. Or two inputs 3 and 4 both mixed and amplified 4 times each, or any configuration to 4 seperate inputs gain times 2 all mixed together. This is handy anywhere in a Moog system where a gain is needed to overdrive an input or get deeper modulations.
Paired with the big 901, those attenuators on each waveform output can now be mixed together for audio or modulation. So with only 6 jacks/sockets and a switch worth of panel space, I was able to add a very usefull dual purpose mixer/amplifier along with a ring modulator.
Image
This makes those four waveform attenuators on the 901 much more usefull when patched that way, since the ring/mixer has no attenuators. When in ring mode the other two inputs 3 and 4 are still active and will give different mixed combinations of ring output with straight signal. Been getting some interesting modulation waveforms this way since the ring is DC coupled. Not your standard pulse/sine/tri/saw waves, but strange combinations with the ring added.
Very usefull 1U module in a smaller system like I have! :D
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