System distortion and Single Ended Tube Amps - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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 15th November 2007, 07:08 PM   #1
diyAudio Member
 
Join Date: May 2007
Default System distortion and Single Ended Tube Amps

Some time back I came across a paper by Eduardo de Lima on the subject of harmonic distortion cancellation. The idea was that harmonic distortion components of single ended tube amps and loudspeakers can cancel each other out.
I have been able to confirm by measurements that the harmonic distortion for my single ended tube amp is lower when loaded with a loudspeaker then when loaded with a 8.2ohm resistor.

The results can be found on my System distortion page.

An example:
1173Hz sine wave:
SPL @1m on axis: 111.2 dB
THD @1m on axis: 1.366%
THD on amp while loaded with LS: 1.449%
THD on amp while loaded with 8.2ohm resistor: 2.366%

I also compared THD and IMD in room @1m for my tube amp and a NAD C 320. I used 4 two-tone pairs at 2 sound pressure levels. This yields 16 measurements. The score was 11-5 for the home team.

You are all invited to replicate/repudiate(with measurements please) these results.
If you can find any flaws in my procedure please let me know.
  Reply With Quote
Old 15th November 2007, 07:45 PM   #2
AKN is offline AKN  Sweden
No snake oil
diyAudio Member
 
AKN's Avatar
 
Join Date: Aug 2005
Location: In the middle of Sweden
Send a message via MSN to AKN
Hi Gerrit Boers,

If you arrange so that output impedance on both amplifiers being equal, what result will you get then?
__________________
/ Anders
  Reply With Quote
Old 15th November 2007, 08:00 PM   #3
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Thanks for posting this. There was obviously a lot of hard work here, and I'm delighted when people stop speculating and go and actually measure things.

My I have the temerity to make a couple of comments? It might be helpful to dig into the repeatability and reliability of the measurement. Here's something to try: do the same measurement you did on your web page, but repeat the measurement pair, i.e., instead of AB, do ABAB with full connecting-disconnecting, and see how well the repeat measurement corresponds with the first pair. It may be that everything is fine and consistent, which you will now have data to support. But it may be that there's a systematic error (order, for example) that needs to be tracked down.

The IMD chart is interesting. You state that the levels of the two tones are the same, yet the graph shows them as different.

The data need to account for differences in source impedance. A series resistor on the output of the NAD can do a great deal to equalize things. Mills and Hawksford have shown some interesting results for distortion with current source drive, and to a lesser extent, that could be the case with the high output Z amp you used here. Well, if output Z reduces this speaker's distortion, we have a simple and cheap mod to the NAD. (edit: I see that 4fun said the same thing while I was typing)

The last thing I'd caution about is something I think you already have an idea about- frequency choice and binning.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 15th November 2007, 11:53 PM   #4
arnoldc is offline arnoldc  Philippines
diyAudio Member
 
Join Date: Apr 2003
Location: Makati
Send a message via Yahoo to arnoldc
Thank you very much for opening up a discussion on this topic. While I have downloaded that and converted to PDF for easy reading, I have not really immersed myself into it due to my rather pathetic technical capability.

This thread could really enlighten the likes of me about the subject.

I'm not sure if related, but I've seen a "distortion cancellation" circuit from one of Mr. Borbely's creation or is this a totally different approach?
  Reply With Quote
Old 16th November 2007, 03:49 AM   #5
diyAudio Member
 
Join Date: Nov 2005
Very interesting!

I am more curious about the presence of the 5th and 7th harmonics in-room with the SS amp, where they are unmeasurable at the amplifier's output (is that the output while driving the speaker???). Those high-order components are not visible in the SET, suggesting they aren't intrinsic to the driver. The reduction of 2nd harmonic distortion in the room via cancellation makes sense to me -- but these 5 and 7 components in the SS plot suggests that attempts to reduce distortion at the speaker terminals via feedback may produce more (and higher order) nonlinear distortion from the driver into the room.

link
  Reply With Quote
Old 16th November 2007, 05:43 AM   #6
diyAudio Member
 
Join Date: May 2007
Quote:
If you arrange so that output impedance on both amplifiers being equal, what result will you get then?
Good question I'll have to try that.

Quote:
It might be helpful to dig into the repeatability and reliability
Definitely, it was my intention to repeat the series at different levels. Your remark on frequency choice and binning (the frequencies are FFT bin adjusted) is also spot on.
The level differences for the two-tone is the result of frequency response deviations of my system+room. I will add the pink noise measurement which will show the peaks and dips. It will also explain the choice of frequencies for the two-tones.

Quote:
I'm not sure if related, but I've seen a "distortion cancellation" circuit from one of Mr. Borbely's creation or is this a totally different approach?
No, they're not related. This is something that 'just happens' if you connect a single ended tube amp to a loudspeaker. Note: I used a single ended amp for these series, I do not know if this is a SET thing or just a result of the high output impedance of tube amps in general.

Quote:
I am more curious about the presence of the 5th and 7th harmonics in-room with the SS amp
You should not put too much weight in the first measurement, this particular result could not be reproduced.
Also keep in mind that the Fostex has some nasty whizzer peaks at 10KHz (working on that) which will probably skew the results.


The most striking result was the difference in distortion for the tube amp loaded with a resistive load and loaded with the loudspeaker. This reminds me of the 'iron in the spinach' story (the iron content was determined by chemical analisys, but Fe3+ is not absorbed by the body). Same thing here, we measure the distortion of a tube amp with a resistive load and get high levels of distortion. However, these high levels of distortion are not to be found in the room.

I also still have my trusty Fisher KX90 integrated tube amp (EL84 PP), will have to try that one to.
  Reply With Quote
Old 16th November 2007, 06:31 AM   #7
arnoldc is offline arnoldc  Philippines
diyAudio Member
 
Join Date: Apr 2003
Location: Makati
Send a message via Yahoo to arnoldc
That's a really interesting result.

Will the quality of the speaker determine the distortion results?
  Reply With Quote
Old 16th November 2007, 07:03 AM   #8
diyAudio Member
 
Join Date: May 2007
Quote:
Will the quality of the speaker determine the distortion results?
It certainly will affect the in-room distortion. In how much the loudspeaker affects the differences between amplifiers I wouldn't know, this is the subject of another investigation.
If you compare my results to those of others, pay special attention to the sound pressure level and mike distance. As an example, the measurements on Zaph's site were made at 50cm with a level for the fundamental of 92dBSPL.

One thing I will certainly investigate is the influence of a passive crossover. I'm contemplating the idea of adding a Raal ribbon to the system but first I want to be sure I do not loose the benificial effect on tube amp distortion by adding a crossover.
  Reply With Quote
Old 16th November 2007, 02:47 PM   #9
diyAudio Member
 
Join Date: Nov 2005
Quote:
Originally posted by Gerrit Boers

You should not put too much weight in the first measurement, this particular result could not be reproduced.
Also keep in mind that the Fostex has some nasty whizzer peaks at 10KHz (working on that) which will probably skew the results.

The most striking result was the difference in distortion for the tube amp loaded with a resistive load and loaded with the loudspeaker.
I'm glad we can ignore that first measurement -- increased high-order distortion would be a troubling finding.

So, that aside, it does appear that the 2nd harmonic distortion of the tube amp may cancel slightly with the 2nd HD of the driver. The total distortion of the Fostex driver is, of course, much higher than either amp. I'll be curious to see how the Fisher performs here. In any case -- very interesting!
  Reply With Quote
Old 16th November 2007, 05:34 PM   #10
Jeb-D. is offline Jeb-D.  United States
diyAudio Member
 
Jeb-D.'s Avatar
 
Join Date: Nov 2005
Location: SoCal
Excellent work!
I've always had my suspicions regarding this matter. "why does my amp that benches at 2%THD sound cleaner with these speakers than my amp with .01%THD?". But without an acoustical measurement setup there wasn't really any way to prove anything.

However I did build a line stage that would intentionally inject a variable ammount of 2nd harmonic. While listening I'd tweak it, there would be an noticable spot where the sound started to clear. Going with too little 2nd harmonic sounded ok, and going over would sound progressively worse. The sweet setting seemed to vary with betweens speakers, which makes sense.
  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
F* amp VS Single Ended DHT tube amp xecluded Pass Labs 15 11th August 2009 07:59 PM
Single Ended, Single driver: In-room distortion measurements Gerrit Boers Full Range 4 27th November 2007 08:12 AM
building Single Ended amps akimball442 Tubes / Valves 3 7th January 2007 07:07 AM
WTB: Single Ended tube amp bepatient Swap Meet 4 29th January 2006 08:11 AM
How to use single ended inputs with X power amps? Petter Pass Labs 4 27th February 2003 08:06 PM


New To Site? Need Help?

All times are GMT. The time now is 11:15 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