Six output pairs with standard F5T front end? - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

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 29th October 2013, 09:32 PM   #1
dhenryp is offline dhenryp  United States
diyAudio Member
 
Join Date: Aug 2003
Location: Mass.
Default Six output pairs with standard F5T front end?

The standard F5T V3 drives four pairs of parallel output transistors. Could I use the standard front end to drive six output pairs?
  Reply With Quote
Old 29th October 2013, 09:42 PM   #2
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
you can always double input Jfets
__________________
my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa... by Mighty ZM
  Reply With Quote
Old 29th October 2013, 09:53 PM   #3
dhenryp is offline dhenryp  United States
diyAudio Member
 
Join Date: Aug 2003
Location: Mass.
I could but do I need too?
  Reply With Quote
Old 29th October 2013, 09:55 PM   #4
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
need ?

no
__________________
my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa... by Mighty ZM
  Reply With Quote
Old 30th October 2013, 01:01 AM   #5
diyAudio Member
 
Join Date: Feb 2009
well. i would not use a single pair of 8mA fets to drive 6 output pairs as far as i remember, 8mA is about minimum for 4 output pairs.
  Reply With Quote
Old 30th October 2013, 11:40 AM   #6
dhenryp is offline dhenryp  United States
diyAudio Member
 
Join Date: Aug 2003
Location: Mass.
Well, I have one yes and one no. Anybody else have an opinion?

If I do have to go for parallel driver FETS, can somebody point me to an explanation of how to do it? I assume it would need some kind of source resistors to account for imperfect matching.
  Reply With Quote
Old 30th October 2013, 12:06 PM   #7
fpitas is offline fpitas  United States
diyAudio Member
 
Join Date: Apr 2011
Location: Leesburg, VA
Quote:
Originally Posted by dhenryp View Post
Well, I have one yes and one no. Anybody else have an opinion?
Since you're adding capacity by paralleling more output devices, the output slew rate will suffer unless more drive is available. Not a show stopper, but undesirable for best fidelity.
  Reply With Quote
Old 30th October 2013, 01:32 PM   #8
dhenryp is offline dhenryp  United States
diyAudio Member
 
Join Date: Aug 2003
Location: Mass.
The following is the brief discussion about adding parallel output stages by NP from the F5Turbo paper:

“That said, the capacitance of the output stage devices starts to pile up and the bandwidth starts to go down. If I can get 800 Khz bandwidth from this amplifier with 2 pair of devices, then I might expect something on the order of only 80 Khz from 20 pair.”

I assume that this is the slew rate issue that fpitas talked about. It seems that with 6 sets of output devices I could expect a bandwidth of 800 Khz/3 = 266Khz. That seems to me to be more than adequate for my use.


“Even this is not much of a barrier – we simply parallel input Jfets to raise the current available for driving these devices and decrease the impedance of the feedback loops. After about 4 sets in parallel you need to give them Gate resistors to avoid parasitic oscillation and you are good to go - probably to 500 watts or so into 8 ohms and more into lower impedances.”

Is the “4 sets in parallel” NP is talking about four sets of input JFETs in parallel or four sets of output FETs in parallel? I’m guessing he means input JFETs so this wouldn’t apply to me if I used 2 sets of parallel input JFETs.

Is the “decrease the impedance of the feedback loops” NP talks about mean changes that I need to make to feedback resistors? Which resistors and how would I go about calculating new values?

So back to my original questions;

1. If I can live with the ~266Khz bandwidth (I can), can a single pair of input JFETs drive sufficient current for six pairs of parallel output devices?
2. If a single pair of input JFETs can’t handle six output pairs, how do I go about modifying the schematic to add parallel input JFETs? Do I literally just put the additional JFETs in parallel with the current devices or are there additional components I need to add or change value?
  Reply With Quote
Old 30th October 2013, 01:52 PM   #9
fpitas is offline fpitas  United States
diyAudio Member
 
Join Date: Apr 2011
Location: Leesburg, VA
If you match the input JFETs reasonably closely, you can probably get away with just paralleling them; but it probably wouldn't hurt to add individual source resistors of a few ohms (say, 3.3 ohms).
  Reply With Quote
Old 30th October 2013, 03:44 PM   #10
dhenryp is offline dhenryp  United States
diyAudio Member
 
Join Date: Aug 2003
Location: Mass.
Quote:
Originally Posted by fpitas View Post
If you match the input JFETs reasonably closely, you can probably get away with just paralleling them; but it probably wouldn't hurt to add individual source resistors of a few ohms (say, 3.3 ohms).
fpitas - Thanks for the response. Can I infer from your post that you don't think a single JFET pair could drive 6 output pairs?
  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
Latest project SRPP valve front end with class D MOSFET output stage. nigelwright7557 Tubes / Valves 1 3rd May 2013 06:15 PM
HK 'Deuce front end on G2 drive output stage? GordonW Tubes / Valves 0 9th June 2008 02:22 PM
A75 front-end unwanted output signal ckwong99 Pass Labs 1 29th December 2007 09:28 PM
Elektor op-amp front-end with mosfet CF output stage Dusk Elektor 1 5th April 2006 04:09 AM
Hybrid tube front end and transistor output stage Original Burnedfingers Solid State 6 4th April 2004 11:28 PM


New To Site? Need Help?

All times are GMT. The time now is 12:21 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