Switchable Hi-Z input impedance, how ? - Page 51 - 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 18th April 2013, 10:24 AM   #501
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
att gain control pot mounted directly on jfet output
Attached Images
File Type: jpg bass pre jfet BC547 buffer.jpg (31.4 KB, 25 views)
  Reply With Quote
Old 18th April 2013, 12:20 PM   #502
shanx is offline shanx  Canada
diyAudio Member
 
Join Date: Dec 2010
Location: Quebec
Can try it like that, in that position the 2 additional resistors should be relatively high value. Then I would suggest trying to move the attenuator to after 4 uf at the output side. Maybe a 10K or 50 K pot.. the output impedance is related to the emitter resistance in parallel with the pot. So it is still low. Are you getting about a volt of signal swing at the jfet output? You will probably want all of that going into the BJT buffer , instead of attenuating it. Will keep noise lower. My humble suggestions only..keep playing around with it and hope your having fun!!
  Reply With Quote
Old 18th April 2013, 12:54 PM   #503
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
Quote:
Originally Posted by shanx View Post
.... output impedance is related to the emitter resistance in parallel with the pot. So it is still low.
ahhh, ofcourse....simple emitter follower

I understand you better now
good info

thanks
  Reply With Quote
Old 18th April 2013, 03:37 PM   #504
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Tin,
you should add a DC blocking cap between the first stage output and the input to the attenuator.
Do not pass DC through the pot.
__________________
regards Andrew T.
  Reply With Quote
Old 18th April 2013, 04:15 PM   #505
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
Quote:
Originally Posted by AndrewT View Post
Do not pass DC through the pot.
sounds like the pot is in the wrong position, with the simple bjt emitter follower

and since the simple emitter follower will work well having a pot on its output, like shanx suggested, that is what Im going to try
Attached Images
File Type: jpg bass pre jfet BC547 buffer output pot.jpg (27.1 KB, 21 views)
  Reply With Quote
Old 18th April 2013, 04:37 PM   #506
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
The arrangement of the buffer after the pot is correct.
Now that you have a Cap between the two, you can set the bias properly.

Just stop DC flowing through the pot.
__________________
regards Andrew T.
  Reply With Quote
Old 18th April 2013, 05:15 PM   #507
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
att pot on input instead
Attached Images
File Type: jpg bass pre jfet BC547 buffer pot first.jpg (28.8 KB, 20 views)
  Reply With Quote
Old 18th April 2013, 07:04 PM   #508
shanx is offline shanx  Canada
diyAudio Member
 
Join Date: Dec 2010
Location: Quebec
Hi Tinitus, if your bass has a volume control (which I assume you do) you don't need the pot at the input. Adding another attenuator (parallel with you bass volume control) at input will defeat the purpose of the Hi Z input. Your previous schem will do what you need and you can drive your circuitry down the line to a level that suits them.
  Reply With Quote
Old 18th April 2013, 08:37 PM   #509
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
ahh....confusing, I know

right now I have a Hi Z preamp driving this BJT buffer
and its got two jfet gain stages, with att/volume control

and its a single input jfet, split into two channels

so basicly it would look like this
Attached Images
File Type: jpg bass pre jfet BC547 buffer pot first.jpg (63.2 KB, 18 views)
  Reply With Quote
Old 18th April 2013, 08:41 PM   #510
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
but maybe I dont need the BJT buffer at all

it was just to see if it would work
and I read its good driver with low output impedance

I guess ideally, the Hi Z jfet should be integrated on-board with the instrument
and in such case, the preamp with att at input would be fine
  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 12:32 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