Getting rid of hiss

[Super Locrian]

I'm experiencing some minor hiss noise when going through pedalboard. I expect that the my culprit is an inexpensive digital delay.

I want to see if a simple noise-reducing circuit after the delay could work. It's a low hiss that is not audible when playing.

I suppose a lot of you will want to say that I need to analyze the problem properly instead of adding something new to the chain, but I don't want to go on a long troubleshooting quest or replace the pedal that likely is the cause.

I'm a novice when it comes to building, but I figure that I will be able to put together a small noise-reduction circuit.

[antonis]

I suppose a lot of you will want to say that I need to analyze the problem properly instead of adding something new to the chain

I suppose exactly the same..

You want to eliminate "something" without knowing anything about it's nature and orientation..

Let's presume that it's an anoying tone(s) of a specific frequency(ies). If this frequency lies in your scale of interest you can't just downgrade it  - say via a filter - without affecting this specific and also it's adjacent frequencies..

[Rob Strand]

Add companding and/or drop the filter cut-offs of the filters before and after the delay element.

[Mark Hammer]

One of the common problems that cropped up during the last 10 years is the heterodyning of clock signals, riding as spikes on shared power lines.

If you're 50 or more, you'll remember a time when dad using a power drill, or mom using a sewing machine, anywhere else in the house, would screw up TV reception or the stereo or radio, because the spikes on the shared power line coming from AC motors would spread across the entire house.  As more appliances began using DC motors or at least better filtering power lines, this became less and less of a problem.

The high-frequency clock signals found in digital pedals, are SOOOO high that we can't hear them.  But if one DSP circuit is running at 1.999mhz and another pedal is running at 2.001mhz,  they can smash into each other on a shared power line and produce a difference signal, which we CAN hear.  The result is that pedals which were dead quiet on their own, suddenly introduce noise of various kinds when combined on the same pedalboard with other digital pedals sharing the same power. 

Of course, this quiet-on-its-own-but-noisy-when-combined thing mystifies many users, and misattributes blame.