mosfet buffer for power stage - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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 20th January 2007, 04:59 PM   #1
hacknet is offline hacknet  Singapore
diyAudio Member
 
Join Date: Nov 2002
Location: sg
Default mosfet buffer for power stage

my interest in such stages was suddently perked after browzing through another thread which someone mensioned about mosfet buffers.

i am hoping to use this buffer to push power tubes over the edge, to class A2. i must say i am not extremely well versed in electronics and i may need alittle help from you guys to tidy my design.

i intend to use something like a irf820 or 830. bias it up to maybe 5ma by tweaking the source resistor.

the thing that interests me most is the front of the mosfet. i was thinking of using the pot to bias the power tube connected to the source of the mosfet.

as you can see, the tube with a 'high' output impedance drives the gate via a voltage divider bypassed with a capacitor. the voltage divider lowers the DC voltage, enough to bias the tube, this way i dont need another circuit to bias the tube and the mosfet, it also takes away the need for a coupling capacitor of some sort to connect the source to the power tube.

the capacitor bypass R1 so the AC signal doesnt get divided. that way, gain isnt affected greatly. i did some workings and i realised a 500k pot and a R1 with a value of around 200k would work fine with a bypass capacitor of only 0.22uf. the 2 resistors values can be further optimized to get the right voltage at the source to bias the power tube.

does this circuit make any sense?

thanks man!
Attached Images
File Type: jpg mosbuff.jpg (15.6 KB, 308 views)
  Reply With Quote
Old 20th January 2007, 05:33 PM   #2
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Default Re: mosfet buffer for power stage

Quote:
Originally posted by hacknet
does this circuit make any sense?

thanks man!
One big problem. That RC coupling circuit has a zero at: 1/CR1, which doesn't present much of a problem. However, it also has a pole at: [1/C][R1 || R2]^-1, and this does cause problems since it will change when you alter the setting of the potentiometer. The lower you set that, the higher that turn-over frequency will go. You could potentially be looking at a considerable loss of low frequencies, and that will make for poor bass performance. It's best not to get too fancy here, and just couple to the MOSFET conventionally. That will also make what the MOSFET is doing independent of what the VT is doing there.
Attached Images
File Type: jpg coupling-circuit.jpg (3.3 KB, 247 views)
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 20th January 2007, 05:33 PM   #3
Giaime is offline Giaime  Italy
diyAudio Member
 
Giaime's Avatar
 
Join Date: Oct 2004
Send a message via MSN to Giaime
Hello,

you could get inspiration from the various threads about TubeLab's PowerDrive, try to search the forum.

I do similar things, maybe you can get inspiration also from these:
http://giaime.altervista.org/hivolt_eng.html
  Reply With Quote
Old 20th January 2007, 05:34 PM   #4
hacknet is offline hacknet  Singapore
diyAudio Member
 
Join Date: Nov 2002
Location: sg
thanks for the great reading material man!
  Reply With Quote
Old 20th January 2007, 05:39 PM   #5
hacknet is offline hacknet  Singapore
diyAudio Member
 
Join Date: Nov 2002
Location: sg
i noticed another problem with my design. tubes need a negative bias on the grids, i have positive bias. i noticed the problem after studying giaime's schematics.

i probably would need a -ve voltage rail to do any biasing with that mechanism.

miles:

i see your point man, but the way i see it, a larger capacitor and tweaking the resistor values to keep the loading low will preserve much of the signal. do you see it the way i see it?
  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
four transistor emitter follower (diamond buffer) as power output stage? capslock Solid State 113 12th August 2013 06:16 PM
Lateral mosfet w Normal or Diamond buffer? fab Solid State 22 8th June 2007 04:23 PM
what about diamond buffer MOSFET? padamiecki Solid State 26 5th October 2005 03:55 PM
Fet buffer (?) for I/V stage ? Bernhard Digital Source 44 17th November 2004 09:38 AM
x-bosoz as a buffer stage jjacks4x Pass Labs 1 5th December 2002 02:28 PM


New To Site? Need Help?

All times are GMT. The time now is 04:12 AM.


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