Rocktek Pedal voltage wierdness

[Circuitbenders]

Heres a bit of a wierd one.

I've been working on a few Rocktek effects pedals recently and have found that on both the phasers and the flangers if you are using an external power supply you actually have to supply more than 9.7v in order to get the LFO's to oscillate properly. If you supply anything lower the LFO sweep is just really slow with virtually zero depth. Get past 9.7v and it suddenly starts to run at the right speed and anything above that up to about 12.5v means the LFO runs faster the more voltage you supply.

It isn't really that surprising that it runs faster at higher voltages, but i find it a bit odd that due to the fact that these pedals just don't work on external power under 9.7v it seems that they are actually relying on the fact that most external power supplies people will be using will be shit, and probably be supplying a lot more than 9v anyway.  Theres actually a 330R resistor in the positive power rail for the external input which isn't on the internal battery supply. I just replaced it with a 220R resisitor and got far better results with a decent 9v power supply.

Surely a voltage regulator would have sorted this out in the first place.

[deathbender]

And what happens if you use a fresh 9V battery? Do they work as expected then?

[Circuitbenders]

yes, but thats because the power jack is switching and the 330R resistor is only switched into the circuit when you plug in a power jack and the battery connector is disconnected.

[Gordonjcp]

You could probably fix that behaviour by increasing the value of the feedback resistor in the LFO integrator stage (assuming it's a boring old two-opamp oscillator)

[Circuitbenders]

Really i'm just wondering if theres any reason why it would have been built like that in the first place, unless its just that they didn't stick in a voltage regulator because they're cheap.

It just seems a bit odd that they would be counting on the fact that nobody would be using a power supply that actually delivered an accurate 9v.