Wein Bridge Volume Control.

Posted: 3/18/2011 8:01:35 AM

From: Northwest Arkansas

Joined: 3/18/2011

Perhaps this belongs in the misc. category, as this isn't a specifically theremin-related question, but rather about the vaguely related tannerin (electro-theremin, slide theremin, whatever), but here I go anyway:

I've been trying to make a tannerin using a wein bridge kit as the guts (rainbow kits). The problem I'm having is that the output level of these is in that annoying range of being too low to drive a speaker (not really what I wanted to do anyhow) by itself at a useful volume and far too high to use with an amp (uncomfortably loud with the volume knob at 0).

I've tried 100k pots to 1meg pots, linear and audio taper. Bridging the sleeve and tip of the output jack with a 10k resistor (yes, i tried higher resistors than that) helps, but does not get it to the point of completely blocking the signal. The idea was to have a volume control similar to that of the key on an ondes martenot. I could just use a momentary push button switch, but this would remove the possibility of having a "nuanceful" control over the volume...or any at all.

I have not tried putting a pot in one of the battery leads, but I'd think this would change the pitch and volume at the same time, which is not what I'm trying to do.

Clearly I'm no electronics expert, otherwise I wouldn't be resorting to the lazy method of using kits as the base for a project, and wouldn't be bothering you fine people with my questions.
Posted: 3/18/2011 12:10:59 PM
Chainsaw Willie!

From: Just a short walk away from Nike Missile Site S-13/14

Joined: 1/28/2011

Typical passive volume controls are just dumb old voltag dividers.

An oversimplified example: You solder a 90K resistor and a 10K resistor togeter. You tie the open end of the 10K to the low side of the signal, and tie the open end of the 90K to the hi side of the signal. Now you have 100K ohms across the output of your device. Your signal generator always sees 100K ohms on its 2V P-P output.

To connect the input to your amplifier you tie the low side of the amp to the low side of your signal generator output where the 10K resistor attaches.

The hi side of the amp input attaches to the other side of the 10K resistor, at the junction between the 10K and the 90K.

The 90K and the 10K have formed a voltage divider that divides by 10. The 2V signal in is attenuated to 0.2V out.

Passive volume controls with a pot do the same thing only they are variable voltage dividers, with the wiper changing the ratio of resistance.

So grab yourself a 500K (or a 1Meg) pot.
Attach the output of your signal generator to the outer legs of the pot.
Attach the low side of your amp to the outer leg of the pot that is the low side of the signal generator.
Atttach the hi side of the amp input to the pot wiper.

The output of your signal generator will always be stuffing 2V into 500K ohms.
The input to the amp will see some voltage between 2V and 0V depending on whether the wiper is near the high side, or the low side.
Posted: 3/18/2011 12:37:43 PM

From: Tucson, AZ USA

Joined: 2/26/2011

I highly recommend kit building because a pre-made circuit board is like gold.

Chainsaw Willie has the right idea!

This arrangement in the diagram below is variable and will stop your Wien Bridge from overdriving the Amp input. This simple circuit can also be used when a theremin overdrives the input of a guitar amp or microphone input. An isolation transformer is not always necessary. Once you find the values across each side of the Pot you could substitute two ¼ watt fixed resistors to keep it compact. You could fit it all inside the male side of a ¼” plug enclosure.

Posted: 3/19/2011 3:55:40 AM

From: Northwest Arkansas

Joined: 3/18/2011


Thanks a ton, guys. I was about to chuck that rat's nest out the window.

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