JFETs as switch in 2-channel tube amp - Page 3 - 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 24th September 2012, 05:37 PM   #21
diyAudio Member
 
muadib25's Avatar
 
Join Date: Sep 2012
Location: Stockholm, Sweden
Send a message via Skype™ to muadib25
Quote:
Originally Posted by counter culture View Post
J111 & J112 are ideally suited to FET switching. If the source is treated as the input then it should be strapped to the gate using ~22k to ensure that Vgs remains @ 0V in the presence of an input signal. These FETs are apparently symmetrical.

Rather than try to reiterate the entire section from the book, I would suggest reading pages 408-421 of Small Signal Audio Design by Douglas Self which contain a comprehensive treatment of this subject.
Counterculture, thank you! It's added to my reading list, I am sure to cover a lot of things I am missing. As for the FETs I am using here, they are 2N5638, they belong to the "switches and choppers" class, same as J111, J112, J113 etc. Their ON resistance is at around 10-20 Ohm.

On the circuit I have the Drain facing the signal input and Source facing the signal output. But yes, these FETs are symmetrical.
Quote:
Originally Posted by Mooly
You must drive the FET's via a resistor. The diode leaves the gate floating which is no good. Make sure that -14 volts is within the G-S rating of your chosen FET.
Now I get it!
My misunderstanding here was that from 0V and upwards(+) the JFET's resistance would not change, hence my proposal of using a diode.

So, if I get it right, when the JFET is ON (Vc=0V) the resistor will be conducting the necessary offset to the gate.

When the JFET is OFF (Vc=-13) there will be some of the gate's negative voltage present... Won't I be needing some caps after each JFET for not having DC injected to the signal?

Here is the circuit with the 22k resistors added:
Attached Images
File Type: png PreampFETdetail2.png (47.9 KB, 163 views)
  Reply With Quote
Old 24th September 2012, 05:54 PM   #22
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
22k is too low resistance. Try something like 2.2 mega ohm or as high as possible, to lower loudness of Thump when switching. This resistor is needed to discharge gate charge, but unfortunately it provides parasitic path for control signal to output. T-configuration as I suggested before would be better in terms of Thumps.
__________________
The Devil is not so terrible as his math model is!
  Reply With Quote
Old 24th September 2012, 05:56 PM   #23
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Lets think about this... and I would recommend you try the design before building for real.

FET 1 and FET 2. Is the feed from V1B AC coupled such that this pont is at zero volts ? If so then both these FET's are tied to ground via the 2M2 and the unmarked resistor. The other side of each FET is AC coupled to the next stage and ground referenced again. So far so good. The gates can be pulled low (to a negative voltage) via the diodes.

FET 5 and 6. Not ground referenced on the output side as drawn ? Where does R19 go ? I'm guessing like the other FET's. That still leaves no ground reference. Add a high value resistor from the output of these FET's to ground.
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote
Old 24th September 2012, 05:59 PM   #24
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
Mooly, he has potentiometers that provide path to ground, but needs some grid leak resistor. But such rough switching on very sensitive input of guitar amp is something I would never consider.
__________________
The Devil is not so terrible as his math model is!
  Reply With Quote
Old 24th September 2012, 06:05 PM   #25
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Quote:
Originally Posted by Wavebourn View Post
Mooly, he has potentiometers that provide path to ground, but needs some grid leak resistor. But such rough switching on very sensitive input of guitar amp is something I would never consider.
Yes, but no grid leak. That is what I mean when I say add a high value resistor from output of these FETs to ground.

That is what you mean... yes
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote
Old 24th September 2012, 07:24 PM   #26
diyAudio Member
 
muadib25's Avatar
 
Join Date: Sep 2012
Location: Stockholm, Sweden
Send a message via Skype™ to muadib25
Quote:
Originally Posted by Wavebourn View Post
22k is too low resistance. Try something like 2.2 mega ohm or as high as possible, to lower loudness of Thump when switching. This resistor is needed to discharge gate charge, but unfortunately it provides parasitic path for control signal to output. T-configuration as I suggested before would be better in terms of Thumps.
I understand... Would a small cap in series with the signal path could isolate the control signal from the output?

Quote:
FET 1 and FET 2. Is the feed from V1B AC coupled such that this pont is at zero volts ?
Yep!
Quote:
If so then both these FET's are tied to ground via the 2M2 and the unmarked resistor.
Oops, sorry for that. The unmarked resistor is 2M2 too!
Quote:
FET 5 and 6. Not ground referenced on the output side as drawn ? Where does R19 go ? I'm guessing like the other FET's. That still leaves no ground reference. Add a high value resistor from the output of these FET's to ground.
Oops again. Yes, R19 is like the other resistors.

Ok, how about this?
Attached Images
File Type: png PreampFETdetail3.png (44.6 KB, 126 views)
  Reply With Quote
Old 24th September 2012, 07:27 PM   #27
diyAudio Member
 
muadib25's Avatar
 
Join Date: Sep 2012
Location: Stockholm, Sweden
Send a message via Skype™ to muadib25
By the way, I got in touch with Kevin O' Connor from London Power in Canada. He told me I should have a split-rail control voltage of both polarities, say +/-15 Volts. This would allow the audio signal to live within the control range window...
  Reply With Quote
Old 25th September 2012, 06:30 AM   #28
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Quote:
Originally Posted by muadib25 View Post
By the way, I got in touch with Kevin O' Connor from London Power in Canada. He told me I should have a split-rail control voltage of both polarities, say +/-15 Volts. This would allow the audio signal to live within the control range window...
If by control volts he means the gate drive voltage then I don't agree. Look back at this,
My MOSFET amplifier designed for music.

It is a similar arrangement to yours. The input is AC coupled and both sides of the FET are ground referenced. As soon as the gate voltage goes above around 0.2 volts (relative to ground) then the FET G-S or G-D junction (depends which way round you have the FET and for these types it really doesn't matter as they are symetrical) will begin to conduct and DC will appear on the output. Just what you don't want. So the positive "on" voltage has to be kept lower than this value. The negative "off" gate voltage has to be more than the peak voltage you are trying to block.

At some point you now have to build the circuit and confirm (or otherwise) that it works as expected .
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  Reply With Quote
Old 25th September 2012, 11:28 AM   #29
diyAudio Member
 
muadib25's Avatar
 
Join Date: Sep 2012
Location: Stockholm, Sweden
Send a message via Skype™ to muadib25
Yes, I see now. Perhaps there is something I misunderstood. I will build the circuit and get back to you.

By the way, on your schematic I see you have the gate supplied with +/-9 VDC. Isnt it the same thing Mr. O' Connor suggests?

Again, thank you all one more time for the invaluable help and your patience!
  Reply With Quote
Old 25th September 2012, 11:59 AM   #30
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Quote:
Originally Posted by muadib25 View Post
By the way, on your schematic I see you have the gate supplied with +/-9 VDC. Isnt it the same thing Mr. O' Connor suggests?
Yes, and I did it to highlight what happens when you increase the gate above around 0.2 volts.

The gate channel begins to conduct for real and to pass current. That gives a DC shift on the output which you don't want. Try it
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
  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
Guidelines for switching N-channel JFETs to P-channel JFETs (and vica versa)? Test Electrix Construction Tips 9 24th January 2011 06:22 PM
Tube Amp Channel Switching bubbab74 Tubes / Valves 1 19th November 2009 04:29 PM
Tube Preamp L&R Channel different current draw tube cadet Tubes / Valves 0 21st January 2009 12:37 AM
P-channel JFETs more linear? lazthegaz Solid State 27 29th June 2004 12:50 PM


New To Site? Need Help?

All times are GMT. The time now is 10:46 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