Gonkulator Modulator question

[Zero the hero]

Hi,
I breadboarded a Gonkulator Modulator clone. I used a schematic which has been traced by some forum members while ago, based on guts shot by Moosapotamus. The schematic was not 100% accurate, there were some bad connections (one for all, no groung on pin 14 of 1496) around the 1496 chip and some component ratings were wrong.
I plan to post a layout soon, as long as I learn to use Eagle (shame on me, no more Photoshop!).

I only have a question: IS IT NORMAL that the carrier BLEEDS LIKE CRAZY? No matter how I twist the trimpot, it's still there. I have no multi-turn trimpots at home, so I used a standard trimpot. I am sure that a thousand-turn trimpot would be handy, but stock units used standard timmers, so is this a normal behaviour?

I don't know if it's very worth building, anyhow...

[Paul Perry (Frostwave)]

I have no experience with the Gonk, but it is usual that if there is a trimmer in this type of thing, there should be a point somewhere inside the range where the bleed is at a minimum.
How does the bleed sound with no input at all? Is it affected at all by the trim? Does it decrease toward one end of the trimpots travel but not get to zero?
Looking foward to the fixed up circuit!

edited to say, just looked at the 'gonkschemaybeV2' schem, and there is no trim on the oscillator side, only on the audio input side (unless I am misunderstanding things). Normally in a ring modulator, you have TWO trims, so that when EITHER of the inputs (signal or modulator) is zero, there is zero out. For an effect though, you might not want this, you might want to always have some of the unaffected signal coming through. Maybe George Gilbert can shine a light on this.

edited again to say, just looked at reviews, & several people say the carrier bleeds through (but is low enough to remove with a noise gate). Also battery level makes a difference. And the carrier is fixed frequency??? my aunt, what earthly use is THAT?? shame on you Dod! That is the lamest thing I ever heard of. Fortunately, we are DIYers..

[Zero the hero]

Hi Paul, thanks for replying!
http://experimentalistsanonymous.com/diy/Schematics/Ring%20Modulators%20and%20Frequency%20Shifters/DOD%20Gonkulator%20Schematic%20maybe.gif  here's the schematic I took as reference.
The oscillator looks good and works fine: the 33k resistor set the frequency of operation, it would be good to have it as a variable.
There were no errors in the input section, in the differential amplifier which follows the 1496 (U4/a), the distortion op-amp (U4/b) and Q6 buffer. By looking at the board pics, I noticed that after Q6, the signal goes into a J-Fet, I assume it's bypassing circuitry related, so I decided to end my version of the GonkMod here, right after the 10uF condenser connected to Q6. By the way, after U2/b the signal got lost...
Some components around U5 are a bit messed up: as previously stated, there's a missing connection to ground (pin 14 must be connected to ground), and the 1M resistor on pin 5 made a little sense to me. By looking at the board, the only resistor connected is the 9k1.
The 1M resistor between pin 1 and 8 is actually rated 150 (or 750, I don't distinguish its colour), and the 10uF on pin 8 is conneted to ground and not to Vb (so there are 2 caps in parallel to ground, correct?).

About the trimpot, I found that its rating is not 1M. I only can read 50k on it, but the first figure could be hidden, so it can actually be 150k or 250k... Both resistors connected to the trimpot are actually rated 20k.
With this ratings the trimpot response was ineffective, and the carrier was clearly audible at the output.
I changed both resistors to 4k7 and the trimpot to 220k. Moreover I connected the central pin of the trimpot to V+ (not ground), according to MC1496 reference manual.
With these settings, by acting the trimmer it's possible to minimize the carrier, but it never shuts up! The whole unit is more quiet than before, anyway. I set the trimmer with guitar input on ground and the sweet spot is around 3 o'clock.

I had some research by the time, and people said that having a bit of carrier bleedthough is normal in stock unit, I believe that it could be due to the lack of a modulation nulling trimpot.

[Zero the hero]

Paul, we both edited at the same time! 

[Paul Perry (Frostwave)]

We certainly did!
http://www.synthdiy.com/files/2006/ringmod.png
is a fairly clear ringmod, it is for +-15V synth rails, but it would convert to 9v reasonably easily.
It is tragic that a great name like Gonkulator has been wasted on this fundamentally flawed product!

(though, in all fairness, I found it difficult to sleep the other night after I realised - years late - that I could have easily put another knob on my
Blue Ringer so that the local oscillator could be varied from triangle to sine to smoothed square. True, there is nowhere to put the knob... but it still a missed riff. )

[Zero the hero]

I'm a bit scared by boxes with dozens of controls , maybe another knob for sliding from square to sine was not very necessary, after all...

Paul I fixed this thing! Thanks for the pic, by the way.
As a first attempt, I fed pin 8 with a variable voltage (a trimpot) for setting the nulling off-set, but the improvement was minimal. I suspect that the whole bias resistor network of the Gilbert cell is a bit quirky.
Designing the whole section from start was the only solution. I took both your pic and my BlueBoob ringmod as reference.
Now it's usable. The carrier is still faintly present but it's way better than the previous design.
By watching the output on the scope, I notice that the waveform not really symmetric (both sine wave as carrier and modulator), and some distortion is present. I don't know if this is due to bad biasing or insufficient power supply (is 9Vdc enough for the 1496?); anyway, the same asymmetry was already present and more noticeable on the original design...

In the schematic you posted there's an extra "output dc-offset" control. Is it necessary since the output will be coupled by a capacitor on our GonkMod?

[Dan N]

Hi Zero!

Doing that trace from photos was a real pain. I don't know where brain freeze comes from, but clearly my brain was frozen on the value of that trim. It says 5K as clear as a bell. That 1M resistor looks like it should be 750 ohms. I have to stand by that 10uf cap going to vb.

I now see that I had a toptrace connecting the wrong parts. Yes, ditch that 1M resistor on pin 5. Now we have to change the location of the 1K/10uf bit between pins 1 and 8. Here's how I see it now:

http://users.rio.com/senorris/junk/gonkchanges.gif

Are you drawing a new schematic? I hope you will add whatever mods you used to get your working!

[Paul Perry (Frostwave)]

Designing the whole section from start was the only solution. I took both your pic and my BlueBoob ringmod as reference.
Now it's usable. The carrier is still faintly present but it's way better than the previous design.
By watching the output on the scope, I notice that the waveform not really symmetric (both sine wave as carrier and modulator), and some distortion is present. I don't know if this is due to bad biasing or insufficient power supply (is 9Vdc enough for the 1496?); anyway, the same asymmetry was already present and more noticeable on the original design...
In the schematic you posted there's an extra "output dc-offset" control. Is it necessary since the output will be coupled by a capacitor on our GonkMod?

Don't worry about the DC offset control, I think that's just for the diy-synth crowd.
I think 9v is just about enough for the 1496, but firstly it isn't going to be running at its best, and secondly, 9v doesn't mean a FLAT 9v battery
Also when something like this is stuffed in a box (or just prototyped) it can be difficult to decide whether the bleedthrough is coming from imperfect multiplying in the chip, or if it is leakage via capacitive coupling or common power supply rail or what.
Unfortunately the fact that the ear can hear over a range of a million to one or so, means that if you don't want to hear something, it has to be DAMMNED quiet.
I find with the AD633, which really is just at heart a 1496 core with improvements of all kinds, that when the carrier is tuned out as best as possible, what is left looks like a harmonic. Certainly it is not a perfect sine.Once you get to that, you can't do much better.
Incidentally the AD633 has slightly better bleedthrough charaxteristics on one pair of inputs compared to the oteh, I don't know whether the 1496 is like this. Likely so!

[Zero the hero]

Dan, no need to feel sorry for brain freezes, your work on this was AWESOME!!!
Thanks for pointing out so much on this, Paul. I'm looking forward to start drawing this board!

[Zero the hero]

Just a quick update, this circuit is driving me insane!
It's embarassing, I never had so many problems with a layout, and all for such a sh1t of effect like the gonkmod.
The circuit works fine, and the rign mod part is quiet and I managed to get rid of carrier noise. ALMOST.
Unfortunately, the oscillator is heterodyning like crazy and, no matter what I do, it keep on injecting its wave everywhere in the rest of the circuit. It is still present in the output signal, even when the effect is bypassed!!!!
I believe that this is the source of noise I already had on the breadboard version.
I'm going to smash it with an hammer, that's what it deserves...

[jmasciswannabe]

Hang in there ZERO!!

[Paul Perry (Frostwave)]

I know EXACTLY how you feel, Zero. the Ring modulator is the only effect that ever literally reduced me to tears (OK, I had been working hella long hours, had spent hundreds on prototype PCBs with bleedthrough..... and maybe I hadn't been eating right, i don't know.)
When you say the "oscillator is hetrodyning" what does that mean? It has to hetrodyne AGAINST something.....
note that if there is any hum at all on the power rail, it's never going to be OK. How is it with a regulated suppy? (yeah, I used a regulated supply - then redesigned THAT, to fix a ground path badness, and get the last  few mV of suply rail hum out..... I was pretty close to the hammer, as well.)

[Zero the hero]

Hi Paul, thanks again for all the suggestions so far!!!
I misused the term heterodyne...    Here's an except from the small article I wrote for my site which hasn't been posted yet:

Many people claimed that this pedal is noisy and the carrier frequency is audible when not playing (some say even when bypassed). Carrier rejection has been improved by balancing BOTH inputs of the MC1496, but carrier frequency is still audible because the internal oscillator is DAMN loud, and I cannot found a way of routing signal for keeping it quiet. I don't have the knowledge to solve this issue at present time, so if you have a suggestion, drop me a line.
Internal oscillator injects its wave everywhere in the circuit, and it could be still audible even when bypassed, cause i noticed that the output jack catches this noise, and the easy solution for this issue is to desolder the jack and move it away from this position (a couple of cm are enough).
Using a probe, I noticed that the greates part of the noise comes from the components connected to pin 7 of the oscillator IC (any hint?).
Stock units are more quiet than this one because the component layer of the board is a ground plane which catches much of the noise coming from the oscillator (that's cheating!!!).
I don't know if a simple re-routing of the power supply and ground traces would suffice for getting rid of the noise, but, AGAIN, I don't have enough knowledge to judge this situation.

Here's the schematic and the layout I'm using:
http://topopiccione.atspace.com/pjimages/DODGonkMod.lay.gif
http://topopiccione.atspace.com/pjimages/DODGonkMod.lay.prn.gif (printer friendly)
http://topopiccione.atspace.com/pjimages/DODGonkMod.sch.gif

That's what I tried so far:
- with a probe I noticed the greatest part of hum is coming from pin 7 of IC1 (upper right corner of the layout) and its connected components (R3 and R4 are swapped on the board). C6 value has been increased from 47n of the factory units to 22u. Noise is reduced but still present, though.
- with an oscilloscope, I checked if the oscillation was present on the supply rails and there were no oscillations on Vdd rail (+4.5V) and on +9 EXCEPT on the supply of IC1, on the track connected to pin 8 of this IC. By incident, it happens that this track is laid under R3, one of the component which is responsible of radiating noise. My guess is that the small amount of noise on pin 8 is catched by R3 which is then amplified by IC1. Could this be right?
I'll try to remove R3 and hard-wire this component to R4, avoiding crossing the supply track. I'll try the voltage regulator, too!
- When bypassed, oscillator noise is still audible since it's caught by output jack. I suggested that moving the jack away solve this issue, but in my mind output jack must stay there, since it's the right position for holding the board too.
- When not bypassed noise is highly present on pin 5, 6 and 7 (expecially 7) of IC3A, which by incident is the closest audio component to the oscillator section. Could this be due to the high impedance of op-amps, so these are the part which are most susceptible to catching noise? I don't know, correct me if this is not the right explaination.
- Grounding is highly wrong: I should have kept oscillator ground separated from audio ground, right? But, NO, it's a mess. So I divided the ground traces of oscillator components and joined them toghether with a single piece of wire to the main ground entrance (this is not clear, I just hard-divided the grounds). No overall improvements has been noticed, however.
- Vcc looks good since it has been designed with a star-supply approach. No noise there.
- I'll put a 47u capacitor right after the power jack on the left wing, and the signal trace on the right wing will be surrounded by ground flood. Is increasing the ground track on the lower side of the board a good idea?
- Input and output tracks are close to each other on the bypass daughter-board connector. Not so clever, right? Actually bypassign has been implemented with a rough dpdt, since I had no time to etch Millennium C boards.

I've spent the last 2 nights reading this forum looking for suggestions and looking for answers, and I realised how many trivial errors I've done while routing this and many other boards. Looking to my previous projects on my site I'm now able to spot lots of improvements I could have done if I only had used more brain and knowledge.
It's a shame, cause now this project seemed more quiet than the original.
It's a shame cause in the last year I wasted a lot of time struggling and winning against personal battles (expecially against drugs, shame on me again), instead of spending more time trying to improve layout drawing, studying electronics and math, or doing something really useful for me and for the other people.
Time to be back on the right path, however.

[Paul Perry (Frostwave)]

Couldn't load the pics & gotta run to the veg market ... but somethng that I found crucial in ring mods, is to isolate the oscillator section by  heavily bypassed  330 ohm resistors in the power rail or rails. Also in one proto a bit of brass foil (grounded by a wire) helped to shield part of the circuit. Grounding the case is essential too.

[Antuan]

Hi, it's my first post here. I hope it will not be a bother to reopen this thread that has been abandoned for some years.
I am learning electronics and recently recovered my 1st generation Gonkulator, FX-13.

I've seen that DoD, since 2015 makes a new version that includes a frequency control.

My questions to you, who know a lot about the matter, are:

1 Is it possible to incorporate this type of control to the FX-13? My idea would be to connect a Wah (Volume, CV) type pedal to handle it with the foot, which the new Gonkulator can not (but I think with a mod I could)

2 Is it possible to see a scheme of the new Gonkulator to compare it with the FX-13 and see the different components?

Thank you very much for everything.

[Antuan]

Hello again, a question: the messages written in old post nobody reads them? thank you.

[Mark Hammer]

If you look here - http://topopiccione.atspace.com/PJ23DODGonkMod.html - the answer is YES, you can add a frequency control.  The site shows how.

[Feral Feline]

Topopiccione popped. I think he even had his pages pulled from the WayBack Machine.
I had archived some stuff, but not sure if this Gonk info is gone on my old computer or not.

Anyone have the gunk on the Gonk?

[moosapotamus]

http://experimentalistsanonymous.com/diy/Schematics/Ring%20Modulators%20and%20Frequency%20Shifters/DOD%20Gonkulator%20Schematic%20maybe.gif  here's the schematic I took as reference.

I rewired the HEAVE control as a variable frequency control. There are already separate controls for distortion volume (SUCK) and ring mod volume (SMEAR), so the HEAVE control for overall volume is redundant. I can't seem to find my modified schematic and it's been quite a while, so take this with a couple grains of salt... I recall completely disconnecting the HEAVE control and replacing it with a single 100K to Vb (or, it might have been a divider pair). Then, I rewired the HEAVE control as a variable resistor in place of (IIRC) the 33K between U3/a pin2 and U3/b pin7. If it wasn't that 33K, then it must have been either the 100K or the 47K between U3/a and U3/b. It was definitely one of the resistors in that oscillator. Hopefully, that at least helps narrow down the possibilities.

~ Charlie