Acceptable Hum / Noise in Amp
I have built a couple of amps and Iam only able to hear the hum/noise in the amp when I stick my ear to the speaker. How quite can an amp be? I would like to know their opinion on how to achieve this from the more experienced members. Also are there special wiring and placement required in order to achieve a quite amp?
A quiet power amplifier is usually dominated by noise from the pre-amplifier,
thats why there isn't a school of ultra low noise power amplifier design.
Try disconnecting the pre-amp and shorting the inputs with 50ohm resistors.
For low hum careful layout is essential.
For the lowest noise the input stage uses low noise transistors
and the input resistors and feedback network use low value
resistors, input bootstrapping is needed to get an input
impedance of around 10K.
See Doug Self's 'Amplifier Institute' webpage.
IMO its not really worth the trouble - as the pre-amplifier dominates.
For low noise direct your attention to the pre-amplifier.
> able to hear the hum/noise in the amp when I stick my ear to the speaker.
That's a tough test. The level at the speaker is about 20dB higher than the level in the room, for typical rooms and speakers.
Just FYI: I have an antique Crown D-75 power amp on my office PC. With power amp pots full-up, but preamp VOLume at a low level (so Windows beeps don't shout), I hear nothing in the speaker. (This office isn't perfectly quiet though.) With preamp (Crown IC 150A (don't choke: it was free)) VOL all the way up (I'd never go there in practice) I hear hiss (including garbage that is probably my sound card) but no sure hum (it may hum but far below the hiss). This is with my ear smack against the grille over 6" and 1" drivers.
No magic here, except I do run the rig from a 2-pin (no ground) power cable (it is also improperly grounded through the PC, and around here I don't worry about safety.)
It should be possible to make hum "vanish", but rarely worth getting it so low you can't hear it ear-in-speaker.
Noise is always an issue but classically is limited by the original microphone or the recording channel. The D-75 and IC-150A are hardly the world's lowest-noise boxes (they use LM301-era inputs) but were always quieter than analog sources.
Obviously the lower noise, the better. We spec anywhere
from 700 uV (unweighted) in our noisiest amps to as low
as 300 uV or so. Usually we can get a lot better than that,
which is better than about -100 dB broadband.
We also spec random noise floor to peak output, which is
an astronomically higher figure, usually another 30-40 dB
Personally, I try to have stuff at or below 100 uV output
unweighted. If I can't hear it....
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