"Sound" of IRF240 bias point setting - Page 2 - 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 22nd January 2004, 12:41 AM   #11
diyAudio Member
 
Join Date: Feb 2001
Location: Columbia, SC
Decide how far you want to push each device thermally and go from there. The bias current per device x rail voltage will give you the wattage dissipated for each output device. At that point you simply use enough devices to get your target bias, which you determined beforehand by either looking at the lowest load you intended to drive or how crazy high you wanted to get.

Grey
  Reply With Quote
Old 22nd January 2004, 06:54 AM   #12
Klaus is offline Klaus  Germany
diyAudio Member
 
Join Date: May 2002
Location: near Frankfurt/Main
Default cooler...

Thanks for all the answers!

Well, I regret a little bit to have not bought laaaaarger coolers… Now I have somewhat like 30K warm up delta. In summer 60°C at the coolers. I will test the easy R19-tweak and test the sound change to maybe 0,47A per FET, equals +35K.

Regards

Klaus
  Reply With Quote
Old 22nd January 2004, 03:30 PM   #13
diyAudio Member
 
Join Date: Oct 2007
Default WOW Grey too!!

Thanks to the gurus for your bias answers!!
  Reply With Quote
Old 22nd January 2004, 07:16 PM   #14
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Default Re: So at what point....

Quote:
Originally posted by GringoAudio
...does more bias just become wasteful
At some arbitrary point, yes.
  Reply With Quote
Old 22nd January 2004, 07:33 PM   #15
Bakmeel is offline Bakmeel  Netherlands
diyAudio Member
 
Bakmeel's Avatar
 
Join Date: Jan 2002
Location: The Netherlands
I am wondering... would there be a physical explanation for the improvement of the sound quality? Could you estimate "sound" if you had some data of a fet used and its bias setting?

It would surely be interesting to know, it would be an aid to selecting fets and bjts...

Bouke
__________________
More Power Igor! More Power!
  Reply With Quote
Old 23rd January 2004, 03:26 AM   #16
grataku is offline grataku  United States
diyAudio Member
 
Join Date: Dec 2000
Location: -
I thought there was a limit to how much bias/how many dev in parallel. Besides the SOA there is the input capacity of the mosfet. Too many devices in parallel and the amp will start to sound "dark". NP mentioned something to that effect starting to happen in the biggest aleph.
  Reply With Quote
Old 23rd January 2004, 04:21 AM   #17
diyAudio Member
 
Join Date: Feb 2001
Location: Columbia, SC
Aye, but that will vary with the choice of device. Want a heavier, darker sound, perhaps for subwoofers...go for higher Gate capacitance. Or go for less capacitance for a tweeter amp.
I'd place that in the "voicing" category rather than the "heat" category when it came time to make decisions.

Grey
  Reply With Quote
Old 23rd January 2004, 05:28 AM   #18
moe29 is offline moe29  United States
diyAudio Member
 
moe29's Avatar
 
Join Date: Dec 2002
Location: Florida
Send a message via AIM to moe29 Send a message via Yahoo to moe29
is there a relationship between number of parallel devices and
rail voltage? Higher voltage spread over more devices is equivalent
to lower voltage over fewer (eg. 2 devices for Aleph 3, 6 for Aleph 2).
Equivalent in that more voltage in more devices overcomes increasing gate capacitence?

(i feel like a 1st grader in Algebra class again!)
  Reply With Quote
Old 23rd January 2004, 05:55 AM   #19
diyAudio Member
 
Brian Donaldson's Avatar
 
Join Date: Jan 2003
Location: Texas, Love it or leave it
All of the rail voltage is dropped across each fet. you use more fets on larger alephs to decrease current/fet so you don't exceed max wattage per fet. Aleph 2 is .5A at 45V per fet= 22.5Watt. Smaller Alephs have smaller voltage and maybe slightly more current over fewer fets while still not exceeding 25 watts/fet.

I had the bias cranked a little too much and had a fet and source resistor melt down. I think the faital blow was about 4 months ago when I was putting them through the ringers and enjoying too much bias. and they sounded great. I let the sink temp reach 175 deg before I knew it. It seemed to survive, but I bet that weakened the chips bond to the tab and let one of the current source (pos rail) fets short and the active set tried to fight it. I'm hell on fets. 80 volts at 25 amps and 160000 ufs can make a lot of magic smoke very quickly. The 8 source resistors looked like little halogen lights on a dimmer.
  Reply With Quote
Old 24th January 2004, 08:19 AM   #20
diyAudio Member
 
Join Date: May 2001
Location: Bangalore, India
Nelson says that it is the total bias and not the bias of each individual mosfet that matters. These may well be true for Vertical Mosfets running in Class A with a relatively high bias current!

One question in this regard is, what if one were using Lateral Mosfets of say, the Hitachi type; these devices require an optimum bias of 100mA. Hence, when parallelling these devices, would one multiply the number of output devices with 100mA or would TOTAL BIAS be set without taking into account the actual current flowing through each mosfet? In the latter case, would these devices still exhibit their negative TC characteristics?
__________________
Sam
  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
Development of a "reference" class D starting point johanps Class D 607 4th January 2006 08:10 AM


New To Site? Need Help?

All times are GMT. The time now is 04:19 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