working out the bugs in a tremolo pedal

[C Bradley]

I'm working on my first tremolo pedal design, and I've hit a few snags. First problem is that my tremolo ticks. I'm using a 555 oscilator and a 741 opamp for the circuit, with an LED/LDR arrangement. I've got a problem with the depth control; the control only works over part of it's range. It's a 50k control, and I'm wondering if the value is too large?

Other than that, I like it!  

Chris B

[gez]

Any LFO which produces a square wave as part of it’s design (even if the output is something other than a square) usually leaves current spikes on the rails, resulting in a clicking sound.  Filtering can help someway to reducing the click, as can layout - spikes can often be coupled into the audio chain by sharing a common ground connection, capacitance of wires etc.

Pay good attention to layout.  Try to keep the LFO circuitry and it’s associated wires separate to the audio’s.  Use separate ground connections for audio and the LFO and join them at a common point (usually the power supply).  Try a large value cap across the rails.  

Often you have to use a RC lowpass filter to reduce spikes on the rails - a resistor connects between the positive rail and the positive connections of the audio part of the circuit, and a large value cap connects between the junction of this connection and ground.  This is far from ideal as it leaves less headroom for the audio circuit, but, in my experience, it’s often the only way to reduce the clicking to an acceptable level (it’s sometimes not possible to TOTALLY eliminate it).   Running LFOs at lower supply voltages can (depends on the circuit) reduce the size of spikes, making them easier to filter.

If you can link the schematic, or give a little more detail, others might chime in over the depth problem.

[smallbearelec]

Quoting R .G. Keen: Design the layout so that the oscillator and Modulator have separate grounds, and bring them together only where the negative rail enters the board.

See my notes on the Tremulous Bear at www.smallbearelec.com, also the tremulus lune at commonsound.

You are not alone; others have dealt with this. It always takes sme careful tweaking of the circuit and attention to layout to work it out.

Regards
SD

[C Bradley]

Hmmm...

I'm using one of those boards from Radio Shack that has two power supply rails right down the middle of the board, and my circuit is built using those two rails as the supply busses.  

I've tried a 100uF cap between Vcc and ground, but it hasn't really helped.  I also can't post a schematic until I draw one. :roll: I've got the chips on opposite ends of the board, with the LED/LDR between them. Could the LED be causing the tick? It's a yellow LED, and it draws about 15mA of current when on.  Could the supply rail voltage be dropping when the LED lights?

I've come up with an alternate scheme if the LED/LDR is the problem; bias vary on the input of the 741 opamp. The idea is to have two voltage dividers putting out 4.5V. One comes straight off the 9V battery, the other off of the 555's output. A 1M linear pot would be between the two voltage dividers, wired in at the 4.5V points of the two dividers. The wiper goes to the non-inverting input of the 741 (wired as a non-inverting amplifier) and functions as the depth control. It supplies 4.5V when the output of the 555 is at it's maximum value, and grounded when it's at it's minimum output.

Will that work? I'm willing to test it, but if the supply rail is the problem, this won't fix it. It does however, prevent the need for a LED/LDR, which could be considered a plus.

Chris B

[gez]

I usually find that LED/LDRs cure ticking, not create it.  

My advice (probably not worth the silicon it's typed on), would be to measure your current draw, work out what you can get away with in terms of headroom, then select a resistor to connect between the positive rail and the positive supply connections of the audio circuit.  Once done, stick a LARGE value (bigger than 100u) cap from the junction of this connection to earth.  

I've done this in the past and it usually does the trick of REDUCING the tick to an acceptable level.

[C Bradley]

I think I'm just going to eliminate the depth control; it really doesn't adjust the depth all that well, and the tremolo sounds best with the control all the way up anyway.

The clean sound of the tremolo is acceptable, and the ticking isn't a problem unless I use a distortion or a high-gain preamp after the pedal.

Chris B