DIY Tremolo effect

[phantompowers]

I was inspired by the 'Stutter effect' pedal on the Beavis Audio research site -

http://www.beavisaudio.com/projects/StutterPedal/index.htm  and I came up with this idea.

If, instead of cutting out the audio every time you pressed the pedal, you had it cut in and out automatically, at a fast speed, you would create a crude tremolo effect. So, I made a 555 timer circuit to flash a couple of LED's, made one into a 'vactrol' and attached the LDR's legs to the audio live and ground output. It worked great, a very lo-fi noisy tremolo effect! I can adjust the speed, I added a depth control and a ramp wave effect by adding a capacitor to the LED in parallel.
Only thing is and I can't figure this out is it works great on my guitar, works great on my 555 tone generators but it doesn't do a damn thing on my keyboards!! This really confused me and I thought my 'pedal' was somehow faulty so I dismantled every bloody solder connection 'til there was nothing left but a mass of crocodile clips only to discover that there was nothing wrong all along!!!
What the funk?!!!

[Circuitbenders]

Probably some grounding issue somewhere, it usually is when you can't work out why something works with a guitar but nothing else.

[Gordonjcp]

How have you got the LDR wired?  If it's just wired across the input and output, it will do bugger all to a high-impedance signal like a synth output.  Wire it as a potential divider insteadl, that'll work much better.

[phantompowers]

Cheers for your replies,

I wired the LDR stright to the output jack. One leg to ground, one leg to earth.

So,(I've just researched this), a potential divider will cut the signal out by decreasing the input voltage?