Reducing Hiss From Amplified Speakers

[PrIsMaTiC]

I remember when my dad brought home a small stereo system and
set it up in our living room. He ran the speaker wires aroung the
baseboard molding. As long as you stayed a few feet from the speakers
you never heard that low level hiss. If you bought a stereo system in
the last few decades that hiss was always there, no matter how much
you paid for your system.

There were all sorts of remedies for the problem. Bypassing the input
or output of the power suppy with one or more capacitors. You could
also try capacitors across the speaker terminals. There were a million
other cures I played around with when I was a teenager. More often than
not the hissing stayed the same or got worse. Casting a spell or saying
a prayer may have been just as effective as the technical fixes!

The problem these days is a little different. If you plug a pair of amplified
speakers into a laptop your head is usually relatively close to the speakers.
If your hearing good, that constant low level hiss can be a little annoying.
Ironically, if you buy a pair of good quality speakers for your laptop the
hiss is more faithfully reproduced! You can pick-up a pair of junk amplified
speakers for under ten dollars. The sound quality is awful, but the hiss
is usually less noticeable!

You would think the EMI/RFI suppressing circuitry built into the better spike
boxes might reduce the problem. I've got a good spike box, with all the
bells and whistles. The darn speakers still hiss too much.

The only thing I've tried so far is bypassing the power supply jack to
the amplified speakers with a tantalum and monolithic ceramic capacitor.
The hiss stayed the same. If you've tried something that worked for you
please clue me in.

Thanks guys!

[Chris Smith]

Have you checked to see if your ground wire in your house actually works?

A short run from the receptacle that you are using, directly to an grounding rod often solves many problems.

House wiring is often very long on its way back to earth, and induced signals in the grounds are not uncommon.

[Robert Reed]

With all the power bypasing you have been doing over the years and with no success, the problem lies in the signal train. The hiss you hear is of a white noise source and not neccesarily related to the 60 Hz power system. electronic amplifiers all produce a source of noise and this becomes evident with increasing gain of the system.Turn the volume contol to zero-if the hiss persists, it is emanating from that point on to the speaker. If not, then it is further upstream. What is the actual sound source-try unplugging it and see if the hiss dissapears. If the sound source is from a very low level device, then it takes much gain to reach a useable value. This is where noise makes its audible appearance and separates the good quality amplifiers from the run of the mill 0nes.

[MrAl]

Hello there,

If this is the kind of noise i think it is then it's probably white noise
and it's not possible to 'filter' that kind of noise out. The reason for
this is because it contains lots of frequency components, not just
one or two.
The source of the noise is in the first or second stage of amplification,
where either the transistors and/or the bias resistors produce noise.

The best way to reduce this kind of thing is to use an FET front end.
Can you reduce it to almost nothing? The ear is very sensitive
when there is no other noise (or music) so it's not likely.

You can experiment with this too. Use an NPN transistor as a preamp
and then use an FET as a preamp and notice the difference in hiss.
I actually did this many years ago when i was experimenting with
different kinds of audio amps. Compared to the NPN the FET is
really quiet.

Further reading:
Do a search on "Shot Noise"



Take care,
Al

[PrIsMaTiC]

I purchased an old house a few years ago. All the plumbing and
electrical was redone, mostly by me. When the inspectors came to
make sure I hadn't screwed anything up, they usually asked me if
I made my living as a plumber or electrician! I follow what the pros
sometimes call "Code Plus." This means I always exceed whatever
the local building codes are by a significant margin. I don't see how
you can run a short wire to an outside grounding rod if your speakers
are on the second floor of your home.

I think you pretty much got it right, Al. All those extinct hobby and trade
magazines that used to publish endless articles on the "perfect" stereo
setup never really resolved the problem. I hadn't given it much thought
in many years. It was only when I started using amplified speakers with
laptops that the problem of speaker hiss or white noise became noticeable
again.

PrIsMaTiC

[Mike]

Frequently hiss is caused by one of the following things:

- Output impedence of source and input impedence of amp mismatch (should be the same)

- Cheap wire

- Cheap amp

- Ground loop or non-grounded equipment

- Bad power supply caps/not enough caps

- Low-quality input source (cheap sound card in computer, cheap cd player, etc.)

- Low-quality audio (badly encoded MP3, CDs of old music transfered from record)

A lot of times you should be able to fix the hiss with either a ground loop isolater or a buffer circuit. For example, build an isolator with unity gain using a unity gain compatable opamp such as the Burr Brown OPA2134.

First try a new source, though. Say you were using your laptop and getting the hiss, connect the speakers to an MP3 player or a CD player and see if it happens. Also connect the laptop or whatever the source with the hiss to a different set of speakers and see if it happens. Let us know the results of this.