diy pedal acting like an antenna

A quick question - I recently built a pedal based on a schematic for the interfax harmonic percolator, a type of fuzz pedal  with mostly second order harmonics. While it works perfectly, and sounds like it should, with guitar, when hooked up to the theremin (etherwave standard) weird behavior happens. It does at modest settings I think improves the etherwaves tone, making it a bit more string-like, i think it, or the general idea, might have potential, but -- the theremin becomes un-tunable. and moving near the pedal changes the theremins pitch drastically, its like the etherwave has a second pitch antenna. I connected the grounds through the metal case, if that matters.

What may be causing this? Is there a solution? When building pedals for theremin use what else does one have to consider in general (except changing the values of high/low pass filters)

If it matters the schematic I based it was off of http://www.madbeanpedals.com/projects/PepperSpray/docs/PepperSpray.pdf  I used slightly different values depending what was on hand, and russian ge transistors/diodes cause they were cheaper, [it sounds like what the pedal is supposed to sound like on guitar] but the circuit was basically the same.

Hello Scott,

I looked at the schematic and thought the design was just clipping peaks which I think in the end results mostly as odd harmonics. It may add a little noise. Guitars develop such a harmonically rich and complex signal it allows for good sound shaping.

Theremin audio signals on the other hand are starving for harmonic enrichment and excitement as if they have been recently neutered. There is a better way to add even harmonics to a theremin. That would be knowledge found in the Inner Sanctum, you know this place. (-‘

The misbehaving theremin response you mention would first guide me to check for a faulty house ground. You need to check this at the wall. Good Earth Ground is so important to theremin operation that I have begun recommending to theremin enthusiast to use a power strip that has a built in green LED ground indicator. In older houses outside ground rods do corrode away. A bad ground will change your sound and usually not for the better.

I have been running really long audio cables and I find ground loop hum to be an interesting challenge. There are several variables with no single ideal solution. Trial and error works best.

Christopher

The design clips asymmetrically. There is a resistor in series with one of the diodes. Also look at the odd way the transistors are connected. The emitters are connected together to ground via the 47uf cap. It seems, the PNP  sends a copy of the input signal to the emitter but the npn sends an amplified and inverted signal. Really, I was making one for a friend, for guitar, and figured Id make a few, as the cost in parts to make one, vs 5, is not linear, and try it with the theremin. 

Regardless, the house I live in is old, and a rental. The ground is via the cold water pipe, there is no grounding rod. to be found. Certain outlets have seemingly non-working grounds, but those are taped over for safety. Ill see if I can find a power strip with a ground indicator, is there a way to test ground resistance, otherwise? 

Ive never had a problem with other pedals, would adding a buffer likely to help at all? Does the etherwave need to ground via the amp?  

Hello nieradka,

I think first you need to verify your earth ground; this is so darn important for theremins. Here is a budget tester which is a good start for everybody in the states that owns a theremin. This indicates the presence of ground but not how good the conductance actually is. )-:

A water pipe may not always be a good earth ground, my water pipes are good because they are copper out to the street. I think pure water is an insulator unless it has minerals in it. Your pipe in the ground may be pvc but most likely metal.

"Ive never had a problem with other pedals, would adding a buffer likely to help at all? Does the EtherWave need to ground via the amp?" -nieradka

The EtherWave and amplifier are both grounded at the wall and to each other, that is if there is a ground at the wall? I would imagine other pedals don't rely on high frequencies.

I am by far no one to give circuit analysis opinions but for me I just think the left side of the schematic amplifies the signal using a circuit designed before we had Op Amps and the right side clips. I did extensive experimenting using germanium diodes & germ transistors hoping magic would be found. I even built my own tube theremin front end looking for the golden sound.

Today I think I have my answer, not for everyone, but I do have one, solid state no less.

The subject of what a theremin should sound like has been one of the more popular topics at TW over the years. To me a theremin should not imitate rather express what only a theremin can express and as musically as possible, then add in the Thereminist.

I spent years evolving from a squeaky mouse sound to get to something that sounds mature. I like sound samples and displayed wave shape samples because they allow for analysis and can give an "idea" of a sound visually, yet perfect waveform characteristics do not mean it is going to sound good musically.

I use TW as my second opinion to a sound; everyone has a different idea of what they expect from a theremin so let me ask:
 
What quality did you hope the circuit in question would give you?  An answer to this helps me learn.

Christopher

I didnt expect much, It was more I was making it for a friend with a guitar, so I figured, lets plug it it and see what it does with a theremin. Despite its unstability, it had an interesting effect, of turning the etherwaves brightness (or waveform I forget which) knob, it was sort of additive  the combination of the knob on the pedal plus the setting on the etherwave, but the sound was a bit "smoother" and string-like without being buzzy at modest settings. That and then one could switch with ones feet between a flute-like and string like tone, seemed interesting.

The more important question, was though, was the un-stability due to the particular circuit design, or something else i did, so if i make delay, or chorus, or other pedals better suited for the theremin, would I have the same problem. And beyond attenuating the theremins signal, and adjusting any high-pass or low-pass parts of a circuit, is there anything else I should consider.  

Scott

Hi Scott.

I do not see anything in the circuit which could cause the problems you are having "the theremin becomes un-tunable. and moving near the pedal changes the theremins pitch drastically, its like the etherwave has a second pitch antenna."

To me, this must be related to the grounding of your theremin.. "its like the etherwave has a second pitch antenna" is in fact what it has, if the theremins "ground" is floating.

Think about whats happening - The theremin works by completing a capacitive circuit "through" the player - Every capacitor has 2 "plates" - one "plate" in the theremin is usually (actually, always) connected to the theremins 0V or "internal ground" - either directly, or by on-board capacitors.. The other "plate" is connected to the "antenna.

When the "0V" "internal ground" is connected to "Real ground" then the "ground plate" becomes extremely large compared to the antenna "plate" - The ground "plate" includes ground and everything (House wiring, plumbing, metalwork, amplifiers, leads, lighting circuits etc) connected to ground.. So the players coupling to ground and the theremins ground becomes a large almost constant value..

BUT - If the theremins "0V" "internal ground" is not well connected to "real" ground, and is "floating", it effectively becomes an antenna - the capacitine circuit is "closed" (or at least the total capacitance "seen" between this floating ground "antenna" and the pitch "antenna") through the players proximity to either / both of these antennas..

Seems to me that your pedal is simply extending the length of your "ground" antenna - that your theremin is incorrectly grounded, and that if you sort this out the theremin (and pedal) stand a good chance of working together in harmony - and you will probably (most likely after a good re-tune) find your theremin much more playable.

Fred.

Thanks fred (and chris), that makes alot of sense. (also makes sense when i moved into a different room in the house, my theremin playing got noticeably worse for awhile) Ill look into the grounding this weekend. I hooked up a powerstrip with a ground indicator light, and it lit green, but flickered. Which cant be right. The wiring in this house is sketchy in general, its probably time I looked into it. 

Scott

Sorting out problems with your house wiring is a good idea anyway - poor / faulty wiring and/or grounding can be hazardous.

Another thing you can do is to deliberately create a large ground "antenna" - the easiest connection to the theremins internal ground is probably the screen connection to the audio jack plug / socket -

If you connect a wire to this and connect it to a conductive mat (a sheet of vynle or other flooring material with aluminium foil or other (metal - "space blanket" for example) conductor bonded to it, and connect the wire to this, you have a mat you can stand on which will greatly improve your coupling to the theremin.

I know there are some places where grounding is poor, even in a reasonably well wired house (I have been in email communication with some people in Russia who are bugged by these sort of problems) - The "dummy ground" mat goes a long way towards improving performance and linearity.

Fred.

One other thing - IMO it is a good idea to fit a ground terminal to all audio equipment (and racks / metalwark in the room - something you can get to and connect a wire to... One can then run a wire directly to the ground pin of a mains plug, or run ground wires from each piece of equipment to one central ground point which is connected to a good physical ground.

If one does then even if your common ground point is not well connected to "physical" ground, everything will be tied to the same potential, so many of the problems will be "fixed" or greatly reduced.

When I wire up a studio (or even a Hi-Fi) I will common up all the grounds to one central point, and usualy connect signals with leads which are balanced or where there is no screen connection on one side (just sometimes put a small capacitor for HF bypassing).. this ensures that grounding does not cause loops.. I did Pro studio installation / debugging for a few years in my youth, and its astounding how many (most?) problems are related to grounding.. Re-wiring the grounding, Cutting screens, and occasionally fitting bypass capacitors was almost always enough to cure every problem.....

Saying that though - my "studio" is a mess right now - I have multiple ground paths and all sorts of horrors, but not enough space to pull the kit out and re-wire it.... There comes a point where one needs to rip everything out and do the job again - I am well past that point! ;-)