JFET based audio source selector - Page 2 - diyAudio
Go Back   Home > Forums > Source & Line > Analog Line Level

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

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 22nd November 2010, 08:30 PM   #11
Obe1 is offline Obe1  United States
diyAudio Member
 
Join Date: Jan 2010
Location: Stumptown
Quote:
The 8 volt can come from a zener... the current needed is essentially zero and the -0.3 from a germanium diode (as I used) although I can't see why just resistive dividers couldn't be used if the supplies to the opamp are stable. Tie the gates to the -0.3v via say 2.2meg resistors... that put's both FET's off. Then use the switch to overide that by feeding the 8 volts via say 220k
This looks very good and uses less parts than what I was thinking about.

I can't wait to try this out. Hope my local toy store has what I need to test this baby out.

Thanks again Mooly!

Last edited by Obe1; 22nd November 2010 at 08:34 PM. Reason: typo
  Reply With Quote
Old 22nd November 2010, 09:14 PM   #12
Obe1 is offline Obe1  United States
diyAudio Member
 
Join Date: Jan 2010
Location: Stumptown
Hi Mooly,

Please verify the voltage polarities for me. I thought an n-channel JFET would need 0.0 to say plus (+) 0.3 volts to turn on and a minus (-) 6.0 to (-) 8.0 volts to turn off.
  Reply With Quote
Old 22nd November 2010, 09:35 PM   #13
Banned
 
Join Date: Jan 2008
Blog Entries: 2
That's an N-channel jfet? That would make the control voltage polarities in the original diagram correct. If you take the gate of an N-channel jfet more than about 0.5V more positive than the source, then the gate-channel diode will conduct.

w
  Reply With Quote
Old 22nd November 2010, 10:26 PM   #14
Obe1 is offline Obe1  United States
diyAudio Member
 
Join Date: Jan 2010
Location: Stumptown
Thanks wakibaki,

I got twisted around when I looked at Mooly's original schematic that shows gate control voltages of +0.3 volts and -8.0 volts.
  Reply With Quote
Old 22nd November 2010, 11:08 PM   #15
diyAudio Member
 
abraxalito's Avatar
 
Join Date: Sep 2007
Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Blog Entries: 98
Send a message via MSN to abraxalito Send a message via Yahoo to abraxalito Send a message via Skype™ to abraxalito
Quote:
Originally Posted by Obe1 View Post
Hope my local toy store has what I need to test this baby out.
You might find the JFET a bit tricky to source, its a Sony part. Doug Self's book 'Small signal audio design' has a whole chapter dedicated to signal switching circuits and goes into some depth with JFET switching arrangements. He prefers to use the J112 which is very widely available.
__________________
It doesn't have to take the form of a conspiracy, rather a consensus... James H Kunstler
  Reply With Quote
Old 23rd November 2010, 12:08 AM   #16
Obe1 is offline Obe1  United States
diyAudio Member
 
Join Date: Jan 2010
Location: Stumptown
Thanks abraxalito,

It's nice to have choices!

I'll also say that the more I try to learn about JFET's the more I get confused. For example, here are specs for several popular n-channel JFET's that might be candidates in this application:


Datasheet for 2N3819: Vgs (off) = minimum not stated, maximum +8.0 Volts

Datasheet for BF245C: Vgs (off) = minimum -0.5 Volts, maximum -8.0 Volts

Datasheet for J112: Vgs (off) = minimum -1.0 Volts, maximum -5.0 Volts

So why do some of these have a positive Vgss (off) value and others are negative?
  Reply With Quote
Old 23rd November 2010, 12:17 AM   #17
diyAudio Member
 
abraxalito's Avatar
 
Join Date: Sep 2007
Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Blog Entries: 98
Send a message via MSN to abraxalito Send a message via Yahoo to abraxalito Send a message via Skype™ to abraxalito
There's a very simple answer to this question Datasheets sometimes contain errors - you've found one in the Fairchild datasheet for the 2N3819. Onsemi's is just as bad. The correct (and most comprehensive) data is found in Vishay 2N3819
__________________
It doesn't have to take the form of a conspiracy, rather a consensus... James H Kunstler
  Reply With Quote
Old 23rd November 2010, 12:32 AM   #18
Obe1 is offline Obe1  United States
diyAudio Member
 
Join Date: Jan 2010
Location: Stumptown
Thanks again abraxalito,

From now on I'll check the datasheet's from multiple manufacturers on any given part.
  Reply With Quote
Old 23rd November 2010, 12:38 AM   #19
diyAudio Member
 
abraxalito's Avatar
 
Join Date: Sep 2007
Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Blog Entries: 98
Send a message via MSN to abraxalito Send a message via Yahoo to abraxalito Send a message via Skype™ to abraxalito
Good move. My working assumption is this - the datasheet containing the most details is going to be the source datasheet, thus the least susceptible to errors. The others will be copies.
__________________
It doesn't have to take the form of a conspiracy, rather a consensus... James H Kunstler
  Reply With Quote
Old 23rd November 2010, 06:18 AM   #20
Calvin is offline Calvin  Germany
diyAudio Member
 
Calvin's Avatar
 
Join Date: Nov 2004
Location: close to Basel
Hi,

I wouldnŽt recommend JFETs as swiching devices, especially not those named here. The 2N3829 for example features an Rds(on) of 150Ohms, the J112 50Ohms. What You want is a as low and linear-over-signal-voltage as possible device. IŽd rather opt for a low Rds(on) MOSFET instead or a dedicated switch or multiplexer like the ADG- and DG-series of AnalogDevices or the 460x-series (and others) of Maxim.

jauu
Calvin
  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
Volume / Source selector - open source project ? AuroraB Analog Line Level 22 22nd September 2012 02:21 PM
Arduino based Input Selector for preamp _atari_ Analog Line Level 5 30th June 2010 02:42 PM
source selector mnr102 Multi-Way 0 24th February 2008 11:11 PM
how to make a audio source selector? no183 Analog Line Level 7 28th November 2007 11:36 AM
Relay based source selector. Help needed! MASantos Chip Amps 9 8th August 2007 11:28 PM


New To Site? Need Help?

All times are GMT. The time now is 01:39 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