Tremolo Effects Pedal

[8163jb]

Just thought I'd show the project I'm currently working on: it's a tremolo effect pedal for a guitar. So it's just basic amplitude modulation of the signal. Everything is detailed in the blog post: http://doayee.co.uk/tremolo-effects-pedal/ any additions and comments are welcome I am still learning so feel free to point stuff out and correct things if they are wrong! Cheers!

[fcb]

Build an actual prototype and try it.

I think you might need to (at least) add a resistor from the synthetic ground to the C6/R9 junction (perhaps 270K) to bias the input, there might be some interesting asymmetric effects of the modulating FET also, but that's cool perhaps.  Also, your PSO might not have quite the range you want.

[8163jb]

Cheers very much! I'll try that when I'm testing it

Also, your PSO might not have quite the range you want.

What do you mean by PSO? I've not heard that term before? Is is what I call the sine wave generator? Thanks again

[fcb]

PSO - phase shift oscillator

[8163jb]

Ahhh right makes sense now! Never heard it called one of them before. Cheers!

[t0m]

I think tremelo circuits used a vactrol back in the day.

[8163jb]

I think tremelo circuits used a vactrol back in the day.

A vactrol? What is one of those? I've never heard of that before?

[T3sl4co1l]

CdS photocell coupled to a source of illumination (an LED these days, but might've been an incandescent or neon light back in the day!).

There are opto-FETs still available today (same thing, in silicon), though uncommon.  Let's see... H11F1 and variants appears to be the only family on Digikey.

The main downside is the low CTR (fractional mA output for 20mA input) and voltage range (under 0.1V; for low distortion, maybe only 10mV), since like JFETs, they are constant current devices for most voltages.

The main downside of CdS is -- besides the low availability of prepackaged vactrols -- the poor repeatability, drift, and long time constants make precision isolation difficult at best (but, not something you'd notice for a tremolo circuit).

Neat fact: lots of old circuits used the neon light type vactrols as isolated switches for gating chopper amplifiers.  This was used in very sensitive and low drift circuits, as far back as the vacuum tube days, where an average tube had microamperes of grid leakage (how much, and which direction, depends on where it's biased -- obviously a bit of a problem!).

Tim

[t0m]

It's like a current controlled resistor. They were also commonly used in compressors. You can get them from surplus electronics places.

[kaindub]

I'll try and be delicate
Your circuit will not work as intended.
The fet you are using only conducts linearly in one direction. In the reverse direction there is the body diode. Your circuit will work as a sort of fuzz box.
Secondly, where the fet is placed, the source is tied to the op amp output. The drive for the fet, (ie the gate to source voltage) will be modulated by the audio signal, in some cases with a high enough audio input, the fet will be cut off.
A better way to do this is to use some enhancement mode fet ie J201.
Connect your audio signal to a resistor in series with the fet. Fet to ground. Take the output from the junction of the resistor and fet. This is then just a voltage divider with a controlled resistance.
You won't be able to get much dynamic range using fets.
Also consider adding a depth ( of modulation) control.
Hope this helps
Robert

[fcb]

I think tremelo circuits used a vactrol back in the day.

If your making a one off, then a vactrol is a great way to do it - and simple to.

If your making a production unit, then forget it.  A vactrol (which as someone already said) is just a CdS photocell (google ORP12, cadmium sulphide, etc..) being illuminated by an LED.  CdS photocells are not, or ever will be RoHS compliant.

[t0m]

Absolutely true.

Doesn't stop them being used in kid's toys in the USA - even expensive ones. Check out this video - the CdS photoresistors are clearly visible under the puzzle pieces.

[8163jb]

Hey everyone!

Just thought I'd update you on the project I built it! And it works! video here:
 

[dentaku]

Hey everyone!

Just thought I'd update you on the project I built it! And it works! video here:
 

Nice. I recently built a VCA with an LM13700 OTA and it can do roughly the same thing. You control the amplitude of the audio going into it with an LFO and you get a tremolo.

I wonder what's the first effect Fran Blanche ever built?

[8163jb]

I wonder what's the first effect Fran Blanche ever built?

I wouldn't be surprised if it was a tremolo! These things are really easy to build

I might try a distortion next, they don't look too bad

[8163jb]

I wrote a second blog post about it, this one explains more about how it works and breaks it up into sections also I've started using KiCad instead of Eagle, much prefer it!

http://doayee.co.uk/tremolo-effects-pedal-part-2/