F5 gate stopper value question. - 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 17th September 2014, 06:50 PM   #1
diyAudio Member
 
Join Date: Dec 2011
Default F5 gate stopper value question.

I'm thinking about having another go at my F5. I've seen various schematics floating around. The “official” one in the F5T manual shows to use 100Ohm gatestopper before Q3 and Q4. However others show gatestoppers of 49Ohm. The use of gatestoppers are said to degrade the sound quality, so presumably a lower value is therefor preferred. But I suspect the 100Ohm value is chosen to be on the safe side of most eventualities.

My questions are, is it worth changing these out to 49Ohm (compared to 100Ohm) and what’s the risk of oscillation becoming a problem? Will the gain be higher with 49Ohm’s?
  Reply With Quote
Old 18th September 2014, 04:59 AM   #2
6L6 is offline 6L6  United States
diyAudio Member
 
6L6's Avatar
 
Join Date: Oct 2010
Location: Denver, Colorado
Quote:
Originally Posted by stijn001 View Post
The use of gatestoppers are said to degrade the sound quality,


Not anywhere as much as you might think.

Quote:
But I suspect the 100Ohm value is chosen to be on the safe side of most eventualities.
Possibly.

Quote:
My questions are, is it worth changing these out to 49Ohm (compared to 100Ohm) and what’s the risk of oscillation becoming a problem? Will the gain be higher with 49Ohm’s?
No change in gain.

Gatestoppers on a mosfet amp can be anywhere from 47 ohm to about 680, where a higher value will do more to damp ultrasonic ringing (I.E., oscillation). Use whatever you happen to have. I like about 150ohm personally.
  Reply With Quote
Old 18th September 2014, 08:20 AM   #3
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
The F5 has low gain and no stability compensation.
The gate resistor may well be a mild form of stability compensation.
Since no other compensation is used, I would consider the PASS value to be optimum and am wary of changing that.

100ohm is easier to read than 100Ohm.
But I prefer 100r for resistors. Very easy to read and understand.

ohm can be used for resistive, or reactive impedance and can be confusing in some contexts, so when referring to resistive, I always use the r, k, M, m as in 4r7, 2M2, 100r, etc.
__________________
regards Andrew T.

Last edited by AndrewT; 18th September 2014 at 08:23 AM.
  Reply With Quote
Old 18th September 2014, 11:20 AM   #4
diyAudio Member
 
Join Date: Dec 2011
Thanks. How would ultrasonic ringing manifest itself sonically, ie what would it sound like, how to tell it’s happening, besides possible heating up of the amp?
  Reply With Quote
Old 18th September 2014, 05:23 PM   #5
cviller is offline cviller  Denmark
diyAudio Member
 
cviller's Avatar
 
Join Date: Dec 2005
Location: Copenhagen
Blog Entries: 2
In principle, you cannot hear ultrasonic sounds - that is why they are named so. However, for audio equipment, you could risk e.g. heating up the tweeters or cause some other unwanted secondary effect, so it is beneficial to avoid it.
  Reply With Quote
Old 18th September 2014, 05:36 PM   #6
6L6 is offline 6L6  United States
diyAudio Member
 
6L6's Avatar
 
Join Date: Oct 2010
Location: Denver, Colorado
Ultrasonic oscillation will manifest itself as a set of destroyed MOSFET and burned gatestopper resistors. Especially on a F5T... Trust me with this information, I have multiple instances of first-hand knowledge.
  Reply With Quote
Old 18th September 2014, 06:24 PM   #7
cviller is offline cviller  Denmark
diyAudio Member
 
cviller's Avatar
 
Join Date: Dec 2005
Location: Copenhagen
Blog Entries: 2
So more of a smokey smell than a ringing noise...
  Reply With Quote
Old 18th September 2014, 06:43 PM   #8
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
There is a simple rule:

Always use Gate resistors to prevent local parasitic oscillation with Mosfets.

Everytime I have tried to get away without that, there has been trouble.

The value is usually not critical - 47 to maybe 220 ohms will only make a
marginal impact on the sound.

  Reply With Quote
Old 18th September 2014, 07:26 PM   #9
bob53 is offline bob53  Serbia
diyAudio Member
 
Join Date: Sep 2013
For test,I put 200r(had this in my shelf).Stable amp,easy to fix I thrugh mosfets,nice sound.
For final version,I put 47r and got unstable amp,high DC offset and smoke.
Thanks stijn.
  Reply With Quote
Old 18th September 2014, 08:25 PM   #10
diyAudio Member
 
jackinnj's Avatar
 
Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
You can try this experiment with gate stoppers -- as demonstrated in one of the TI (was Nat Semi) application notes -- it think the author was SpittinLama -- use 100R on the N-channel, and a 500R pot on the P-Channel. Inject a square wave into the amplifier and observe the output. adjust the trimpot until the poz and neg waveforms are symmetrical.

Square wave will tax the amplifier so 70% is probably as high as you want to go.
  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
Better way of installing gate stopper resistors? ridikas Solid State 3 9th April 2013 05:26 PM
Specific grid stopper question optimationman Tubes / Valves 22 10th March 2013 08:22 PM
Gate-stopper values ? sam9 Solid State 29 20th November 2012 10:16 AM
grid stopper question - 300B TubelabSE discomonkey Tubelab 6 28th April 2011 04:32 AM


New To Site? Need Help?

All times are GMT. The time now is 05:22 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