Switchable Hi-Z input impedance, how ? - Page 33 - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps

Instruments and Amps Everything that makes music, Especially including instrument amps.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 21st September 2012, 09:42 PM   #321
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
Quote:
Originally Posted by tinitus View Post
and why the noise almost goes away with the stereo hifi preamp driving the output
must be something to do with going from 0/+ supply to symmetric +/- supply

looking at Nelson's B1 buffer, could this be the reason why he 'constructed' a pseudo midpoint for the jfet's grid to ground resistors
  Reply With Quote
Old 22nd September 2012, 11:49 AM   #322
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
tried adjusting drain and source resistors
15ohm source res stays in, no doubt
not sure what kind it is
old and beige colored, so I suspect its a carbon film, 1/2W or something
tried a metal film, but didnt sound so nice
(signal resistors are metal films)

Drain resistor is lowered a bit, from 1K5 to about 1K2 or 1K3

measured +7.18V over drain res, and -82mv over the source res
dont know if those are the best values, but sounds good
  Reply With Quote
Old 23rd September 2012, 11:20 AM   #323
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
Quote:
Originally Posted by tinitus View Post
15ohm source res stays in

Drain resistor is lowered a bit, from 1K5 to about 1K2 or 1K3
nope, drain res is still 1K5
well, actually 1K6, since its 2 paralel 3K2
I removed series resistors from the power supply
and thats it
at input, instead of 1M to ground I have 850K
will try increasing input/gate series resistor
  Reply With Quote
Old 24th September 2012, 12:49 PM   #324
shanx is offline shanx  Canada
diyAudio Member
 
Join Date: Dec 2010
Location: Quebec
Is it a hum? or more like white noise? I have a couple of suggestions, but depends on the type of problem. You may have a bit of ground loop on the input. Do you have an insulated input jack?
  Reply With Quote
Old 28th September 2012, 06:59 AM   #325
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
ups, been busy

yes its snerring hum
my present setup have variable attenuation on final output, and takes away most of it(its a stereo preamp buffer)
at 12 o'clock nothing, but above that it begins be audible, and gets really aggressive when turned up fully
almost no hiss, and in general very silent right now
weird setup btw

bass guitar--compressor--jfet boz
then devides into two channels
one channel, jfet boz--headamp--stereo pre--poweramp--mid/tweeter
second channel, jfet boz--stereo pre--poweramp--woofer

inserting the headamp gives me a lot more control, and a picture of how it ought to work
I think it will work very well when Im able to control every gainstage in both channels
its getting closer
  Reply With Quote
Old 28th September 2012, 07:07 AM   #326
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
btw
I have tried almost all possible configurations with the effect units I have, and some sounded pretty weird
one acted like a synth, another like pure distortion, and others sounded just muffled and phased
tells it not so simple at all with effect 'pedals
I only use compressor pedal unit right now
would like to do without, but right now its not possible
but maybe later
  Reply With Quote
Old 28th September 2012, 12:44 PM   #327
shanx is offline shanx  Canada
diyAudio Member
 
Join Date: Dec 2010
Location: Quebec
For the hum situation, I would try an isolated input jack, input signal wire shielded from jack to jfet input, but shield terminated only at the signal ground directly at your jfet's source resistor ground. That is the most direct route for return path to your instrument. Adding a bypass cap across the source resistor may help. Check all other signal grounding points in your preamp are star grounded or bussed onto a heavy gauge copper grounding wire in your cabinet. You may need to add shielding metal panels in future if you are picking up ambient EMF. Your outputs are splitting to two different stereo preamps? You may have a ground loop issue there too. let me know if they are separate preamps powered on AC, DC etc. one output may need to be isolated to break ground loops on outputs.
  Reply With Quote
Old 28th September 2012, 12:49 PM   #328
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
hey, will try bypass cap on source res

note, its not the channel splitting
hums also with only one channel
but its a proto built on a piece of plywood

the other evening I could hear faint spanish radio
but was gone again just by a simple off/on switching
seems to be a one time only

but I might have a faulty plug on my bass
its gets noisy when not touching the strings
  Reply With Quote
Old 29th September 2012, 09:15 AM   #329
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
Quote:
Originally Posted by tinitus View Post
hey, will try bypass cap on source res
tried 22uf el cap
sounded ok, but a bit muffled, or maybe more like phased

then a 1.0uf film cap
sounded more open, but nervous

then 2.5uf
bang, sounds just right, very smooth
great 'egalite', responsive and tuneful, tight bass
I would say spot on
  Reply With Quote
Old 29th September 2012, 09:50 PM   #330
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
2.2uf works ok too
actually, in some way even better
compressor sounds ok again

for some reason my compressor didnt work so well with 2.5uf
could be because it was a different type
also tried a 3.3uf orange colored cap
didnt last one minute

should I have a selector option, it would switch between 2.2uf and 2.5uf
worth the trouble ? yeah, its enough to make that little important difference
might be it actually affects the tuning pitch just a bit, but enough for 'certain occations'
anyway, a switch would at least make it easier to test different caps

well, enough of that
working on a simple regulated supply for the jfet
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Switchable balanced / unbalanced input buffer nigeljking Analog Line Level 4 12th September 2011 12:03 PM
How to design input stage with specific input impedance/pot value mahleriana Tubes / Valves 0 16th November 2009 09:37 PM
Input Impedance Bengali Analog Line Level 7 15th August 2007 10:24 PM
Input impedance nhuwar Tubes / Valves 24 23rd July 2007 11:11 PM
Input pot and input impedance Bricolo Parts 1 19th March 2003 01:59 PM


New To Site? Need Help?

All times are GMT. The time now is 11:41 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2