F5 Turbo Builders Thread - Page 154 - 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 10th March 2013, 12:13 AM   #1531
PKI is offline PKI  United States
diyAudio Member
 
PKI's Avatar
 
Join Date: Aug 2011
And will benefit from more homogenous heat sources placement on the sink :-). The question is how much bandwidth you need? :-)
  Reply With Quote
Old 10th March 2013, 06:34 AM   #1532
diyAudio Member
 
Join Date: Feb 2009
another reason for more fet's can be to drive low impedance in class A.
same bias pr device, dobble up the devices and run it at a lower voltage.
or as a X'ed/balanced amp.
  Reply With Quote
Old 10th March 2013, 03:59 PM   #1533
diyAudio Member
 
Join Date: Jun 2011
Default F5T v2 up and running

First of all I need to thank Nelson pass, DIYAUDIO pass club forum, Omishra who formulated Group buy

After a long preparation, research I am done with F5T v2. Currently both the channels are biased at 320mv. I have tested with my old take 5 speaker they are sounding nice. Need to test it on my paradigm signature s4. I am seeing dc offset varying between -30 to +30 mv. What are the limits of dc offset. what dc offset is bad for a speaker.

let me know

Thanks
pandu
  Reply With Quote
Old 10th March 2013, 04:05 PM   #1534
6L6 is offline 6L6  United States
diyAudio Member
 
6L6's Avatar
 
Join Date: Oct 2010
Location: Denver, Colorado
Is the offset varying with no signal and the inputs shorted? Is the amp up to temperature?

The offset does seem to vary a bit with a signal and speakers (or a load resistor) playing.
  Reply With Quote
Old 10th March 2013, 04:15 PM   #1535
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Id of the upper and lower FETs is completely dependant on the Vgs of the relevant devices.
Low Output Offset (OO) requires the upper and lower current to almost exactly match.

This close matching of currents is very easily unbalanced by very small changes in Vgs for either, or both, of the upper and lower devices.

It would take a small wind/draught blowing over the heatsink for these critical Vgs values to be varied differentially.

This is why the OO varies, small changes in heatsink temperatures. The Thermistors are too slow to respond.
__________________
regards Andrew T.
  Reply With Quote
Old 10th March 2013, 04:25 PM   #1536
diyAudio Member
 
buzzforb's Avatar
 
Join Date: Oct 2010
Location: Burlington, NC
IF you have avrying bias, you may have some oscillation. Try larger gate resistors. If that doesnt stop it, try bypass caps across feedback resistors, like in article.
__________________
...Shape the sound , Man!
  Reply With Quote
Old 11th March 2013, 04:40 AM   #1537
diyAudio Member
 
Join Date: Jun 2011
Quote:
Originally Posted by buzzforb View Post
IF you have avrying bias, you may have some oscillation. Try larger gate resistors. If that doesnt stop it, try bypass caps across feedback resistors, like in article.
Bias is pretty much stable with 10 mv variation after attaining temperature.
  Reply With Quote
Old 11th March 2013, 04:45 AM   #1538
diyAudio Member
 
Join Date: Jun 2011
Default dc offset

Quote:
Originally Posted by 6L6 View Post
Is the offset varying with no signal and the inputs shorted? Is the amp up to temperature?

The offset does seem to vary a bit with a signal and speakers (or a load resistor) playing.
Yes the dc offset measurement is with no signal.
When I switched on the amp early this morning, when amp was dead cold its dc offsets measured 90mv. Over a period of half hour to one hour one of the channels dc offset came down to 10mv But the other one came down to 50mv and stayed there.

For now I readjusted the bias on the second channel made the dc offset to less than 10mv.

Is it normal to have higher dc offset when the amp is cold. If that is the case is it ok to run the speakers when the amp is cold.

Thanks
Pandu
  Reply With Quote
Old 11th March 2013, 05:42 AM   #1539
sangram is online now sangram  India
diyAudio Member
 
Join Date: Sep 2002
Location: India
You should have a shorted input before measuring offset.

The way this topology works means the offset varies slightly with operating temperature. There are some pointers on the original F5 thread. The offset is caused by slight differences in the operating point between the top and bottom halves of the circuit, as Andrew as has already answered to you.

Basically the output devices conduct and warm up, then the thermistor tracks them and reduces the bias. It is sort of thermal feedback, and goes on till thermal equilibrium is reached. As this happens on each tracked transistor, the two halves are always conducting slightly differently. This means the offset will vary a bit as the amp reaches operating point. How much depends on the build and how well the thermistors are coupled to the outputs, and how much heatsink you have.

It is possible to have a far more stable offset by removing the thermistors completely (at least, it was in the F5) but the penalty for that is the amp needs to be rebiased as the ambient temps change (for example, summer vs. winter) since there is no external reference. I did that for my first F5 because it was always meant to be a winter amp. Nelson also notes in the F5 manual that the amp takes longer to warm up and reach operating point with no thermistors. I see no issues with that given that we run in a very hot climate compared to a lot of guys on the forum.
  Reply With Quote
Old 11th March 2013, 05:57 AM   #1540
diyAudio Member
 
Join Date: Jun 2011
Quote:
Originally Posted by sangram View Post
You should have a shorted input before measuring offset.


Basically the output devices conduct and warm up, then the thermistor tracks them and reduces the bias. It is sort of thermal feedback, and goes on till thermal equilibrium is reached. As this happens on each tracked transistor, the two halves are always conducting slightly differently. This means the offset will vary a bit as the amp reaches operating point. How much depends on the build and how well the thermistors are coupled to the outputs, and how much heatsink you have.

It is possible to have a far more stable offset by removing the thermistors completely (at least, it was in the F5) but the penalty for that is the amp needs to be rebiased as the ambient temps change (for example, summer vs. winter) since there is no external reference. I did that for my first F5 because it was always meant to be a winter amp. Nelson also notes in the F5 manual that the amp takes longer to warm up and reach operating point with no thermistors. I see no issues with that given that we run in a very hot climate compared to a lot of guys on the forum.
what do you mean shorted input.

the ntcs are close to the transistors not attached. does it matter here. because a ntc can be close to the transistor and other can be little farther.

Few mm difference in the ntcs spacing with teh mosfet can make the temperature difference significant.
  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
F5 Turbo Circuit Boards chchyong89 Pass Labs 2347 14th July 2014 02:19 PM
B1 builders thread Bas Horneman Pass Labs 696 14th July 2014 12:57 PM
F5 Turbo is posted Nelson Pass Pass Labs 635 6th April 2014 10:51 AM
F5 turbo problems Rob Dingen Pass Labs 17 11th January 2013 08:38 PM
F5 Turbo ............. if only !!!!!! a.wayne Pass Labs 23 21st January 2012 02:24 AM


New To Site? Need Help?

All times are GMT. The time now is 07:51 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