Reading Leaving Class A . . . - 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 1st August 2009, 12:32 AM   #1
Formerly "jh6you". R.I.P.
 
Babowana's Avatar
 
Join Date: Jul 2006
Default Reading Leaving Class A . . .

"We apply a single Class A bias at about 10% of the push-pull Class A bias."

Any diyer has ever built as shown in Fig.5 . . . ? Or, similar . . . ?

  Reply With Quote
Old 1st August 2009, 04:13 AM   #2
fab is offline fab  Canada
diyAudio Member
 
fab's Avatar
 
Join Date: Mar 2004
Location: Canada
Hi babo

See this post: http://www.diyaudio.com/forums/showt...06#post1572606

I am to see other peoples interested finally...
  Reply With Quote
Old 2nd August 2009, 09:59 PM   #3
fab is offline fab  Canada
diyAudio Member
 
fab's Avatar
 
Join Date: Mar 2004
Location: Canada
See attached picture of the pcb with the "current displacer" within the blue line area
Attached Images
File Type: png fab-f4_current_displacer_pic_1b.png (93.5 KB, 714 views)
  Reply With Quote
Old 3rd August 2009, 11:19 PM   #4
Formerly "jh6you". R.I.P.
 
Babowana's Avatar
 
Join Date: Jul 2006
Hi Fab,

Your project and results are very interesting
And, thanks for the info about Class XD and Displacer.

BTW, in connection to Fig.5, the additional SE bias, I'm reading Aleph 0s output and its SE CCS bias . . . (for me to try for my F4 hopefully soon) . . . And, I'm wondering where the "ISD" is going to? Do you (or anyone) know about this?

  Reply With Quote
Old 4th August 2009, 12:43 AM   #5
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Fig 5 doesn't have an Isd, but referring to that figure, at idle the
current which passes through the constant current source comes from
the current going from the positive supply through the N channel
output, through the current source to the negative supply.

For positive output current, the current source continues to get its
current through the positive output transistor. For negative output
current, it will start to also draw from the load, and at some
sufficiently high negative output current, it will draw entirely from
the load as the positive output shuts off.

  Reply With Quote
Old 5th August 2009, 02:10 AM   #6
Formerly "jh6you". R.I.P.
 
Babowana's Avatar
 
Join Date: Jul 2006
Papa,

Many say that one picture tells more than 1000words. For me, however, your words are easier than the picture. Thanks for the so kind explanation.

I'm going to try to modify (improve?) my F4 with two things:[list=1][*]Power supply (change in small detail as shown in the attached sketch)[*]Installation of the additional SE bias ccs[/list=1]

Thanks to your explanation, I would keep my confidence with the SE ccs which to be arranged between the output and the negative supply, and just try to decide what type of ccs.

Hoping I could do these soon . . .

Attached Images
File Type: jpg psuforf4-01.jpg (24.7 KB, 492 views)
  Reply With Quote
Old 2nd October 2009, 10:47 PM   #7
CBRworm is offline CBRworm  United States
diyAudio Member
 
Join Date: Jan 2007
Location: S. Florida
Choosing bias - some questions. I have searched, googled, and read lot's of stuff - but so much info on the net (even on this site) is contradictory.

If you can keep the MOSFETs cool, it seems like the current could be cranked way up and still be within the SOA of the outputs, the source resistors may need to be beefed up. Is this a correct statement? where is the limit? Is that limit a point that destruction occurs or does sound quality start to suffer beyond some point?

I built an F5 as designed and an F5 with 2 pairs of outputs. I biased them both higher and lower with forced air cooling for testing. I used the Fairchild devices.

I did this experiment primarily because I am driving speakers which are 2 ohms across some of their operating range and would like to stay in class A as long as possible - I also want to figure out the relationship between bias and sound quality at low output levels. For this experiment heat and current draw are not concerns.

I biased the 2 pair F5 at a slightly higher 1.5 amps per device for a total of 6 amps - it will drive my speakers almost to the voltage limits of the power supply in class A. The mounting tab of the mosfets stay around 52c

to my ears both amplifiers sounded best with the recommended bias current or more. I have a hard time removing volume from the equation though.

I understand that lowering the bias seems to lower sound quality even while still in class A mode - I am not sure if this is true, but seems to be the case, or maybe it is some kind of placebo effect (it's not getting super hot - it must not sound good!)

Originally I was thinking that if I only needed a few watts (say for active tweeter operation) I could lower the bias to lower heat and consumption, but keep the amp in class A operation for the few watts I need - it seems like there is a penalty for that and I would be better to lower the rail voltage to get my desired output for an intended impedance. Is this the case?
  Reply With Quote
Old 3rd October 2009, 12:09 AM   #8
Blues is offline Blues  United States
Lightning In A Bottle
diyAudio Member
 
Blues's Avatar
 
Join Date: Sep 2003
Location: The Milky Way
Quote:
Originally Posted by Babowana View Post
..Hoping I could do these soon . . .

I was wondering the other day how the Babo is doing?...been very quiet for a long while. Good to see you back!
__________________
Life simplified! Leap into the Warpspeed wormhole... http://www.6moons.com/audioreviews/warpspeed/1.html
  Reply With Quote
Old 3rd October 2009, 04:33 AM   #9
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Quote:
Originally Posted by CBRworm View Post
If you can keep the MOSFETs
cool, it seems like the current could be cranked way up
and still be within the SOA of the outputs, the source
resistors may need to be beefed up. Is this a correct
statement? where is the limit?
No, with higher current, the Source resistors can remain the
same, or if cooling is good, even be reduced. At higher
current, their larger percentage of the voltage drop adds
additional stability to the bias.

The limits are generally the temperature of the junction,
which you can infer from the known wattage and the
temperature of the case.

Typically the limit is 150 deg C, and the thermal resistance
from junction to case is maybe 1 deg C/watt, so the limit
for a device with a case temperature at 100 deg C is
about 50 watts.

  Reply With Quote
Old 3rd October 2009, 07:01 PM   #10
CBRworm is offline CBRworm  United States
diyAudio Member
 
Join Date: Jan 2007
Location: S. Florida
Ah - now it makes sense. I didn't realize you could figure out the junction temp from case temp, thermal resistance, and power. I thought it was just some nebulous higher than case temp number. Now I have a whole new spec to get my arms around.

Thanks Again!
  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
Leaving D.I.Y everything must go sale...... exurbia Swap Meet 36 15th October 2008 03:42 AM
Leaving Class A Nelson Pass Pass Labs 31 28th August 2008 11:19 PM
interesting reading -T.A.S. class D group review IVX Class D 2 13th January 2007 10:09 PM
Leaving X-former powered up Wynand Power Supplies 4 16th May 2006 01:07 AM
Leaving. fdegrove Everything Else 23 2nd December 2002 08:36 PM


New To Site? Need Help?

All times are GMT. The time now is 07:10 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