Help with Small Stone modded for Ring Mod...

[mabnotes]

Hey folks -

So I modded my Rev.J vintage EHX Small Stone Phaser to have a higher speed rate to get it up into some ring mod territory tonight. I wired a DPDT switch with the original 33uf cap on one end and a 1uf cap on the other. So when I switch out of the regular cap I get a higher selection of rate speeds. It works great, BUT - when I'm in the ring mod zone the "carrier" sound is just too loud (the hum/pitch of the phasor whirring along). Is there a way to get rid of that signal noise or is that just the nature of what I've done with the mod? Maybe I need to ground something...?

Thanks!

[knutolai]

Oscillator bleedthrough is a common problem in simple Ring Modulator designs. I imagine its probably why there are so few commercial "clean" RMs out there.
Also a note on terminology:
Carrier: Your audio signal (vocal, guitar, bass, whatever)
Modulator: The signal you modulate your carrier signal with. A oscillator of some sort.

As for solving or bettering the issue. If I'm not mistaken the small stone uses OTAs. You could check out some OTA (lm13700) based Ring Modulator designs. Many of these have a "null" or "offset" trimpot for betting rid of modulator bleedthrough.

[mabnotes]

^^^ Thanks very much for the input! Sorry about the confusion of terms, you are correct... Also, yes it does use OTA's so I am off to check out if I can find see some kind of null or offset schematic...

Can anyone else add anything to this? Would love to get it useable!

[mabnotes]

OK - i did some very brief reading and I'm seeing a lot of references to things that are waaaay over my head! Basically, I would like someone to tell me what I need to solder to where to try and fix this bleed through. I would love to know why, but I am not very good with knowing what part does what and why... I can solder well, and know some very basic things, but I would have no idea which components need to be connected by a trimpot and why it might correct the oscillator sound.

Here is a schematic of my SS - the 33uf cap I changed for this is the one pictured coming off of pin 5 of the OTA all the way at the bottom of the schematic. I changed this to 1uf (on a DPDT switch):

http://www.montagar.com/~patj/smlstone.gif

[Mark Hammer]

Although virtually any LFO-modulated effect can produce something that sounds like a ring modulator, the "nulling" of the modulator is not always something that can be done the same way it can be done with a true balanced modulator.

On the other hand, the Small Stone does share some attributes of a ring modulator, in that we have an audio signal being processed by a cascade of 4 OTAs, whose gain is varied by an external modulating current.  That varying gain is used to produce phase-shift in each stage, but it is still a current-controlled OTA.

Unfortunately, where chips like the 1495/1496 or 533/633 provide pins explicitly FOR the purpose of nulling, the CA3094 either does not, or is used in a manner here that precludes an easy mod.

One possibility to consider, on the other hand, is that maybe what you hear is essentially LFO "ticking" in the audio range, in which case a search for "ticking" and "Small Stone" here might lead you in the right direction.

[mabnotes]

Thanks Mark! That's a good point about the "ticking"... there is no ticking until I hit a certain speed on the knob and from there on up it increases in pitch and volume. I will do a search here and report back!

[mabnotes]

The latest:

After some searches here suggesting this, I tried sticking a 1uf cap across different pins of the OTA that I originally changed the 33uf cap on to increase the speed. I found some success connecting the top 2nd pin and the bottom 1st pin (from the left looking down at the front of the chip), but really this just lowered the speed a little, thereby lowering the pitch and the volume. I also tried this with a 100k resistor off the cap to ground, which resulted in slightly more of the same. Compromises the "ring mod" quality too much...

...clearly I'm just hacking away here though... any other suggestions?