Some observations on bias vs. distortion - 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 May 2013, 01:41 PM   #1
6L6 is offline 6L6  United States
diyAudio Member
 
6L6's Avatar
 
Join Date: Oct 2010
Location: Denver, Colorado
Default Some observations on bias vs. distortion

Some interesting measurements concerning bias levels -

Here are some quick and dirty measurements I did looking at various levels of bias on the F4. ( +/- 22.5V rails )

The conditions are, amplifier to normal temperature, one channel driven (because I cant read 2 at once), input to amplifier 3V 1.1kHz sinewave from 339A, distortion read across 4ohm 100W dummyload resistor, oscilloscope and FFT connected to HP 339A monitor, which was outputting the residual distortion waveform.

Bias value is mV across .47ohm

Click the image to open in full size.
Bias measured across a .47 source resistor. (This is 1/2 the normal level, for illustration's sake)

Click the image to open in full size.
Distortion (Meter is set to the .1 scale, look at the arc above the mirror, here showing .065%)

Click the image to open in full size.
Here is the shape of the distortion residual. (No distortion would look like a perfect sinewave.)

Click the image to open in full size.
FFT of the distortion residual. (please excuse the photo of the screen. The next time I do this I will get screenshots.) The 1st peak at 1.1k is the fundamental. (1.1k chosen as it aligns with the gridlines. ) Looking right, the 2nd peak is the 2nd harmonic, the 3rd peak the 3rd harmonic, and the 4th peak the 4th harmonic. Higher level harmonics are not visible, as they are very small and lost in the noise.


Now let's increase the bias to the recommended amount, 200mv measured across the source resistors.

Click the image to open in full size.

Click the image to open in full size.
Distortion has now decreased to .016%. All that has changed is the bias. What a marked difference!

Click the image to open in full size.
The distortion residual has lost it's humps and is looking very smooth.

Click the image to open in full size.
The 4th harmonic is no longer visible, and the 2nd and 3rd are greatly reduced.


Increasing further -

Click the image to open in full size.

Click the image to open in full size.
Distortion now .08%

Click the image to open in full size.
Residual very similar to above, but even more smooth. (A little bit, anyway…)
The noch on the one peak is just a probe artifact.
Click the image to open in full size.
FFT showing the 2nd and 3rd harmonic even lower. Neat!


So the logical thing is to just crank it all the way up, right? Well, yes and no. Here are the reasons the way I understand it. (and people with more experience please correct me if I'm wrong.)

1) Heatsink - at some point you are going to get too hot. With the 5U that wasn't an issue. A good rule of thumb is the transistors 65C max and heatsinks 55C max.

2) Bias current vs. VA of transformer. You don't want your total bias current (in watts) to be more than 1/2 or maybe 2/3 or your transformer's VA rating.

3) Transistor dissipation. Make sure you look at the datasheet for the output transistors, they all have an absolute maximum and a de-rating as they get hotter. For long life don't set more than 1/2 the max dissipation.

4) Point of diminishing returns. In this example the measured distortion continued to decrease (down to .045 or so) as bias was increased to 370mV, but the gains were very small and it would added lots of heat. The sweet spot was closer to 300mV bias.

Last edited by 6L6; 1st May 2013 at 06:49 PM. Reason: Fixed a couple typos
  Reply With Quote
Old 1st May 2013, 02:08 PM   #2
diyAudio Member
 
buzzforb's Avatar
 
Join Date: Oct 2010
Location: Burlington, NC
As you say, diminishing returns. In reality in takes time, listening, and measuring, to decide. My new naked f4 output will have 12 pairs on output. How many I use will depend on the aforementioned investigation. Consider Pass amps and number of pairs vs bias current and figure what each pair is biased at. I think .5A will be my max on big amp. In FW type implementations, where you are trying to yield maximum potential from single output pairs, high bias is ideal, but when more power is needed, things change a hit for a number of reasons.
__________________
...Shape the sound , Man!
  Reply With Quote
Old 1st May 2013, 03:35 PM   #3
6L6 is offline 6L6  United States
diyAudio Member
 
6L6's Avatar
 
Join Date: Oct 2010
Location: Denver, Colorado
After looking at this it seems (to my very inexperienced eye) that you would actually want the minimum amount of devices, and therefore the highest bias for your heatsink.

Talking to one of the Gurus sorta confirmed this, but he said to always look for what Nelson has described as the 'Sweet Spot'. In the case of this particular amp, that point is about 300mV, or about .6A per device. Anything past that required a bunch more bias for very little decrease in distortion.

Last edited by 6L6; 1st May 2013 at 04:00 PM.
  Reply With Quote
Old 1st May 2013, 03:46 PM   #4
diyAudio Member
 
buzzforb's Avatar
 
Join Date: Oct 2010
Location: Burlington, NC
Don't think that is in conflict with my statement. With smaller power needs, I agree. If looking for higher power, I think things change a bit. More devices give more current capability, more thermal capability, more DF, more gain. Consider your bias level of .6A. If you want 200W, you can either increase bias or add more fets at this same bias level. The answer is not necessarily a given. It depends in sinks, load, longevity. I am not saying I am right, just discussing.
__________________
...Shape the sound , Man!
  Reply With Quote
Old 1st May 2013, 03:51 PM   #5
diyAudio Member
 
Join Date: Mar 2007
Location: Canandaigua, NY USA
Great stuff! Smaller photos would be nice. I think it's very design dependent. Some designs will actually increase their distortion beyond a certain bias. A really good example is a quasi-comp circuit with no Baxandall diode. It just gets hot, it doesn't get better.

There's also thermal stability to consider- high bias can put you closer to thermal runaway. I like to run high power and get things heated up to the maximum I dare, then monitor the bias and see if it's increased and if the system cools down as expected. With good thermal feedback and the right Vbe multiplier you can go very high on bias, if needed.

I've seen some amps where making the decision about diminishing returns is difficult because things really do improve, but at the cost of ever increasing dissipation. I just worked on an old ARC amp and they've got the thing dissipating 200 watts at idle. Dang, not class A, but it shows what happens when you get carried away.
__________________
I may be barking up the wrong tree, but at least I'm barking!
  Reply With Quote
Old 1st May 2013, 04:03 PM   #6
6L6 is offline 6L6  United States
diyAudio Member
 
6L6's Avatar
 
Join Date: Oct 2010
Location: Denver, Colorado
Quote:
Originally Posted by buzzforb View Post
I am not saying I am right, just discussing.
Nor was I trying to correct you!

The point of this post was merely to get the discussion flowing, and it's doing exactly that.

I want to try a Burning Amp complementary output with 2 pair and 4 or 6 pair, with similar total bias and see what the subjective difference is. It would be an interesting project.
  Reply With Quote
Old 1st May 2013, 04:14 PM   #7
diyAudio Member
 
Join Date: Feb 2009
also one thing to think of. the fewer fets you use (at a given bias), the thicker the baseplate of the sink need to be.
  Reply With Quote
Old 1st May 2013, 05:24 PM   #8
diyAudio Member
 
Join Date: Jul 2012
Location: Arizona
Great thread. Thanks for posting this. I'd love to see what you find in a similar analysis of your F5 turbo.
  Reply With Quote
Old 1st May 2013, 06:50 PM   #9
6L6 is offline 6L6  United States
diyAudio Member
 
6L6's Avatar
 
Join Date: Oct 2010
Location: Denver, Colorado
Quote:
Originally Posted by toxic ingestion View Post
Great thread. Thanks for posting this. I'd love to see what you find in a similar analysis of your F5 turbo.
I will do the Standard F5 next, and the Turbo as well.
  Reply With Quote
Old 1st May 2013, 09:11 PM   #10
Eric is offline Eric  United States
diyAudio Member
 
Eric's Avatar
 
Join Date: Nov 2000
Location: Central PA, USA
Great stuff! Thanks for sharing these results!

Eric
__________________
My DIY Basement Theater, Aleph-X Amplifers and Avro Open Baffle Speaker construction pages.
  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
High bias and distortion in one channel of GFA 545 w schematic spice house Solid State 5 18th January 2012 05:37 PM
Observations tvrgeek Tubes / Valves 0 26th December 2009 10:08 AM
7th harmonic distortion from PP pentodes depends on bias? ray_moth Tubes / Valves 7 1st April 2009 10:04 PM
Bias and distortion 6EM7 mashaffer Tubes / Valves 0 6th June 2008 02:30 AM
class a bias current and distortion jerluwoo Solid State 1 2nd May 2007 08:48 PM


New To Site? Need Help?

All times are GMT. The time now is 05:10 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