Noise Reduction/ Supressor Unit

[grrrrrant]

Anyone know if there is a kit or some info on a noise supressor/ reduction pedal?  I'm not looking for a gate.  Just something to clean up that analog noise and what not.

Thanks, Grant

[grrrrrant]

BUMP!

[Mark Hammer]

"Noise" comes in many flavours, but the big two are hum and hiss.  A gate will clearly eliminate both, but since they tend to arise out of different sources or points in the signal chain (e.g., hum tends to be at origin where hiss accumulates over the signal path), they tend to demand different optimal solutions when tackled in a more specific manner.

The noise type I imagine the majority of us here deal with is hiss, since we are prone to having many things on in series, some of them high gain that exaggerate any hiss from preceding devices.  While a gate will suppress that, often a better solution is to use a sliding lowpass filter that takes advantage of the masking principle: hiss goes unnoticed when and if the musical signal is robust enough, but becomes objectionable when the music disappears.  So-called "noise filters" operate on this principle and are essentially auto-wahs that target the hiss end of the spectrum and also tailor the envelope properties so that the filtering out of noise is as unnoticeable (make that undistracting) in its action as possible.  Central to that non-obvious action is a fast attack/onset and a slow-ish decay, such that all treble is seemingly preserved in the note attack, and the treble is not removed too fast as a sustained note fades out.  One of the advantages of this is that, with no overall gating, the "nose and tail" of what you play are never chopped off, and the worst that happens is that the fadeout is a little more dull-sounding than you might like.  Consensus is, however, that noise filtering is more natural sounding than a gate.

Another fundamental aspect of noise filters is the use of downward expansion.  That is exaggeration of the dynamics of small signals.  This expansion does not mean that anything sounds louder than it really is.  In fact, just the inverse; if it was already quiet, it will sound even quieter.  This achieves several things.  First, it sidesteps that nasty aspect of gates - chopping things off.  Second, it allows the filtering to act with even more specificity.

Now, I suspect there are single-chip solutions for this.  The LM1894 DNR chip (dynamic noise reduction) comes to mind, as does the SSM2000 "Hush" chip (Rocktron's patent).  In fact, I stumbled on an old issue of the now defunct Audio Electronics magazine the other day while cleaning up before company arrived, and there was an SSM2000-based noise reduction project in there.  Given that this is a DIY forum, however, we can be simpler and smarter than that.

As mentioned, a noise/hiss suppressor is essentially an envelope-controlled filter with slow decay and a gentle filtering action that targets the hiss range, while leaving the fundamentals and lower harmonics intact.  That second part sounds like a job for a simple 1-pole lowpass filter.  Happily, this is a resistor in the a signal path followed by a cap to ground.  As the resistance gets smaller, the corner frequency of the filter goes up.  Sounds perfect for an LED/LDR combo; play loud and the LED makes the LDR resistance get small such that the corner frequency of the filter is well above what your speakers can deliver.  Stop playing and the LDR resistance rises, resulting in a treble cut as the note decays.

Happily, since many LDRs have a sluggish response time anyways, they don't always require flawless rectifier circuits to feed them.  You can get away with a simple Dr Q / Nurse Quacky type single-diode envelope follower.  Because the filter only affects hiss and not the whiole signal, there is little need, if any, for gain in the signal path.  You could make one of these with a TL072 or NE5532 and a single FET.

Here's the gist (can't draw it out here).  Front end is simple JFET buffer.  Buffer feeds an envelope follower with variable gain, and envelope follower feeds an LED through some sort of current limiting resistor.  Rectifier has a large-ish averaging cap (say 47uf instead of the usual 10uf candidate one sees) and a parallel 500k pot to set the decay speed.

Buffer also feeds a fixed resistor in series with a 500k pot and output end of pot has selected value of cap to ground.  LDR goes in parallel with pot.  The pot and fixed resistor set the high and low points of the filter range, and also compensate for the LDR's range.  Junction of the LDR/pot/cap goes to the input of a unity-gain non-inverting op-amp, and output of that op-amp goes to the usual DC-blocking cap, terminating resistor and output jack.

Done.  In principle, what this should result in is a treble cut that fades in after you stop playing.  Three controls: sensitivity, decay time, adjustment amount/range.  Output should be the same volume as the input.