--- In [email protected], "Roy J. Tellason" <[EMAIL PROTECTED]>
wrote:
>
>
> Are they? I have a guitar amp here that uses one at the input, and
it's very
> noisy. I thought I might be able to improve that by replacing the
FET but it
> didn't help it much.
>
> --
> Member of the toughest, meanest, deadliest, most unrelenting -- and
> ablest -- form of life in this section of space, a critter that can
> be killed but can't be tamed. --Robert A. Heinlein, "The Puppet
Masters"
> -
> Information is more dangerous than cannon to a society ruled by
lies. --James
> M Dakin
>
The reason it didn't help is most of the noise is from the guitar
pickups, most of which are poorly wired (not single point grounded)
and use unshielded cavities and a good portion is also due to the
thermal noise of the 1M input impedance, usually set by a 1Meg
resistor ... at high impedances a FET will naturally be quieter (if
properly designed) than a bi-polar because a FET has lower current
noise than a bipolar ... respectively at a low impedance a bipolar
with win out because it has a lower voltage noise than a FET ... At a
high impedance the current noise is the dominating noise source, at a
low impedance voltage noise is the dominating noise source ... the
same holds true for bipolar vs FET input opamps, the general rules of
thumb is for impedances of 10k or less use a bipolar input opamp and
for impedances of 100K or more use a FET input opamp ... 10k-100K is a
'gray area' where both work about as well ... of course everything
changes at RF frequencies and these rules of thumb for audio won't apply
I currently have 5 guitar amps, 4 of which are designed and built from
scratch, the 5th is actually an amp simulator (Berhinger V-Amp Pro),
all my electric guitars ('68 SG, '74 Les Paul and a '04 Telecaster)
have been completely rewired and have full cavity shielding
This may seem off topic to most but the above is quite relevent to the
design of SDR backends and should give people ideas on ways (or at
least reasons why) to improve over the mediocre performance of the INA
series of mic preamps which are only found in low end recording/sound
mixers and some mid-range DJ mixers ... yes I have an INA series mic
preamp in my SDR testbed but only as use as a baseline to compare with
(hopefully) better designs
JR
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