The anatomy of HISS

[Rich G.]

Anyone have some good info or a URL they can post pertaining to  the reduction/elimination of circuit hiss?  I did a search, but only found hiss as related to specific circuits-- no general precautions.

[toneman]

Hey RichG,
"hiss" ??
U might mean "noise".
google "noise reduction".
Ever heard of companding?
or "dbx"
As a general rule, the less U amplify,
the less U boost hiss/noise.
"Noise" comes from the electronics--
i.e. transistors/opamps.
afn
tone

[Rich G.]

"hiss" ?? U might mean "noise".

"Hiss" noise in the sense of the Sssssssssssssss sound heard from high gain distortion effects as opposed to 60 Hz hum noise due to ground loops.

A forum search on "hiss" yields quite a few results, but they are typically in reference to "my xxyyzz box makes a horrible hiss sound.  how do I fix it?" queries.

I'm looking for preventative measures to avoid getting hiss in the first place.

[petemoore]

Picking you're favorite build.
 Then rebuild it.
 Use 1% tolerance resistors for the larger or all R's.
 Use silver Mica or other fancy caps throughout.
 Use Low-Noise / High Performance Opamps.
 Try high end [that's where the hiss is] roloff caps.
 On OA feedback cct.s sample valued caps in conjunction with other rolloff caps [signal path to ground] in the circuit, sometimes one place in the circuit or the other will reduce hiss/where it cuts highs in a way you might like.
 I haven't noticed a huge difference, but bipolar transistors can be low noise, 2N5089 being an example of a high gain low noise choice.
 High gain and noise often go hand in hand. Separating the two is where the ceiling is found...how close can you get'.
 All the usual shielding stuff, tho I've found that with a reasonable layouts, it often makes no discernable difference. I put it on a long input wire sometimes, just for good measure, and there's a good amount in my 'longbox'...phaze90, Sweet Lava, Hot Silicon also. [The Hot Si uses 5089's]

[Mark Hammer]

Just about ALL components generate thermal noise that comes out as hiss.  Beyond observing those practices that Pete noted, regarding component choice, the traditional rule of thumb is to observe whatever practices permit maintaining the best signal-to-noise ratio.

The classic strategy is to have as hot an output from A as B can tolerate, within its headroom limitations.  That way, whatever hiss is accumulated en route or at stage B will be small in comparison to the contribution of stage A.  The prototypic case of this is with compressors.  The amount of gain reduction that compressors provide is contingent on how hot a signal they get.  If you feed a compressor a puny signal, it ends up boosting the input most of the time, rather than cutting back on gain, plus you end up having to keep the output level of the compressor up to achieve reasonable volume.  Feed the compressor a hotter signal and you end up making it reduce gain more of the time, plus you can keep the output volume a bit lower.  The end result is that your compressor spends less of its time cranking up the gain and amplifying the hiss during the otherwise silent parts.

Another strategy to adopt is to decide how much bandwidth you truly need and limit bandwidth appropriately wherever possible.  Most effects, unless something like a fuzz deliberately designed to keep fizz in retreat, will aim for full (20-20khz) bandwidth, simply because the designer has no idea what you're feeding it and what you want to do with the output signal.  If you're feeding a pedal a P-90 or humbucker output signal, the bandwidth on those things is fairly limited (well, let's say more like voice than cymbals, since I'm not trying to take aim at these pickups).  In that instance, it really makes little sense to allow any effects in line to pass signal above 10khz or so, and in many instances you can safely stick small value caps here and there to do just that.  This way, whatever hiss is coming down the pipes from resistors or semiconductors can be tamed a little bit.  Certainly in those instances where lots of gain is applied (i.e., distortion pedals) rolling off highs can make the difference between a pedal you can't wait to turn on and one you can't wait to turn off.

Try looking up information on some of the noise filter chips like the LM1894 or the SSM2000 (used in Rocktron HUSH systems).  Documents on signal-to-noise ratios will also be useful.  Do note that hiss and hum, while both forms of audible noise, come from different sources, and whatever you do to cure the one will have largely minimal effects on the other, unless it is a shotgun approach like a gate.

[petemoore]

from cap to cap, there may be some differences, haven't had time to document it throughly, but I have been known to change caps even of the same number, must be a color psych, but seems in certain critical areas, more cap swaps, even with the same value, is what I do, without a scope to tell for sure if this happens, it's all theoretical mojo voodoo.
 But yepp I'm PDsure I couuld hear a difference in the same values, could have been a faulty part. ...All by ear, none of my observations are based on perfect lab test conditions.

[bwanasonic]

The generalized tips for hiss reduction are helpful, but you may find you need a really specific remedy for your setup. Do you now have a hiss problem, or is this more of a theoretical question? If you are using really high gain pedals, most likely you are going to get hiss. Generally some form of noise gate, either manual (volume pedal, or turning off the effect between songs, etc.) or automatic is in order.

Kerry M