thats only partly correct! FETs CAN have 1/f noise (shot noise) that sounds like frying eggs if its not designed for audio work. So it should be checked if its really an audio FET.
Alex 4Z5KS _____ From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of jr_dakota Sent: Thursday, September 14, 2006 2:54 AM To: [email protected] Subject: [soft_radio] Re: New file uploaded to soft_radio --- In [EMAIL PROTECTED] <mailto:soft_radio%40yahoogroups.com> ups.com, "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 [Non-text portions of this message have been removed] Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/soft_radio/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/soft_radio/join (Yahoo! ID required) <*> To change settings via email: mailto:[EMAIL PROTECTED] mailto:[EMAIL PROTECTED] <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
