Morley PFV Hiss Reduction?

[Dave Curtis, dB AudioTech]

Has anyone tried to reduce the hiss these things put out? Got any suggestions? Here's the circuit:

http://www.morleypedals.com/pfapfves.pdf

TIA,
Dave

[cjlectronics]

Hiss can be caused by many things. I've seen transistors cause hiss, dirty pots, and even resistors cause hiss. See if you can isolate the hiss to a certain section of the circuit.

CJ

[Mark Hammer]

Hiss in phasers is a recogized issue, particularly with respect to phasers that have more than 4 stages and especially when the regeneration is turned up.

A common strategy is to apply just a touch of lowpass filtering once or twice in the chain to keep hiss at bay. So, for instance, if you added a 1500pf cap in parallel with the 10k feedback resistor at the 3rd and 6th phase-shift stage, that would tame cumulative hiss, by providing a rolloff around 10khz. If the intent is to use it only for guitar, you could go up as high as 1800pf. You'll notice the difference when you crank the regen/emphasis.

This particular phaser uses LDRs instead of FETs, so that's good. LDRs are quieter than FETs. But it also uses 741 opamps, which could be improved upon by more contemporary devices. An NE5534 could be used to replace the 741 chips. It uses/draws more current, but the pedal is mains-powered anyways, so you don't have to worry about battery life as a result of changing chips.

[Dave Curtis, dB AudioTech]

Good post, Mark. Thanks. You've validated what I was thinking about small value caps in the FB loops and the 741s not being the greatest.

[Mark Hammer]

My pleasure to serve, sir.