Real Load THD - a somewhat philosophical question... - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 12th April 2012, 10:33 AM   #1
coluke is offline coluke  Italy
diyAudio Member
 
Join Date: Mar 2009
Default Real Load THD - a somewhat philosophical question...

Consider the two multi opamp buffers in the first image: they have - more or less - the same current output capability on low Z loads, but very different output impedances - the first one sits at about 2.5 ohm, while the second one is in the tens-of-milliohms range (up to reasonably high frequencies, at least). THD on resistive loads is the same too (~0.0008%-0.0009% @ 1Vrms out on 30 ohms), but when switching to * real * loads the high-Zout one shows a dramatical (~ 2 orders of magnitude at low frequencies) increase in THD - see images 2 & 3.

Well, if you drive a non-linear load through an * ideal * distortionless sinusoidal voltage source, the voltage on the load is undistorted even if the current flowing in it is far from being sinusoidal; if you switch to a * real * distortionless sinusoidal source (ie a distortionless source with finite Zout), the higher its output impedance, the more the voltage on the load is distorted * by the load itself *; sounds ok, doesn't? It seems only a matter of output impedance, and opamps, feedback and so on have no responsibility at all - the same holds for a no GFB ampifier I'm playing with: it is actually a discrete CF architecture which seems to work very well open-loop - I'm managing to build a HP amplifier out of it with the option to run it in both ways. Its open-loop output impedance is ~2.5 ohms, and when driving real loads it shows the same THD increase that I see with the opamp buffer; closing the loop lowers the Z out down to ~50milliohm, and there's no THD raise at all when switching from resistive load to real one.

Given that headphones and louspeakers * do are * nonlinear loads, my question is: the THD I see when driving real load through high Z buffers is simply the THD of the load itself? Or a low damping factor worsens overall LF THD too? I know the question may look pretty naive, but maybe I finally got why noone manages to perform real load tests on audio stuff...

Ciao,

L.
Attached Images
File Type: png OpAmpBuff.png (61.2 KB, 187 views)
File Type: png 4x5532highZoutTHD.png (37.5 KB, 183 views)
File Type: png 4x5532lowZoutTHD.png (34.8 KB, 178 views)

Last edited by coluke; 12th April 2012 at 01:58 PM.
  Reply With Quote
Old 12th April 2012, 12:03 PM   #2
PChi is offline PChi  United Kingdom
diyAudio Member
 
Join Date: Jan 2012
Location: Staines, UK
Hello,
I think that the THD seen when driving a real load through high Z buffers is likely to be the THD generated by the non linearity of the load itself.

It would be interesting to know what load you used to generate the plot.

Crossover inductors using a magnetic material aren't linear. I haven't seen a graph of inductance versus current for commercially available inductors making it difficult to select one for a particular power level. Possibly insufficiently rated inductors could be the reason behind stories like some loudspeakers are difficult to drive.
The capacitors are often electrolytic. Again non linear and it's probably not too difficult to exceed the current ratings at high levels. I guess that film capacitors are likely to be much less of a problem. I used polypropylene but they were expensive. Polyester are reputed to have some non linearity issues.
The actual transducers are also a non linear load. The inductance isn't linear. The cone movement hence back emf isn't linear either.
I guess that for the lowest acoustic THD the loudspeaker should be driven from a low impedance. But I guess that any electrical non linearity will be low compared to any mechanical / electromechanical non linearity.
I guess that any loudspeaker manufacturer would be happy with THD < -70 dB. Not many specify it.
I think that it's difficult / impossible to test an amplifier at the maximum power output using a real load because few loudspeakers would survive for long. Also as you imply the amplifier THD would be increased by the real load. If the loudspeaker survived the voice coil would heat up and the resistance increase changing the distortion measurement as well.
  Reply With Quote
Old 12th April 2012, 12:27 PM   #3
diyAudio Member
 
Join Date: Apr 2010
Location: Bath, UK
PChi
you might find this thread on Amplifier Dynamic Damping factor testing of interest in your search for answers?
http://forum.speakerplans.com/amplif...777_page1.html

Mik
  Reply With Quote
Old 12th April 2012, 01:20 PM   #4
coluke is offline coluke  Italy
diyAudio Member
 
Join Date: Mar 2009
Quote:
Originally Posted by PChi View Post
It would be interesting to know what load you used to generate the plot.
Hi PChi, the 'real load' was Denon AH-D310 headphones (the cheapest ones in the AH-D Denon's line), whose nominal impedance is 32 ohm. Given their sensitvity (105dB/mW), at 0dbV (~31mW) input the sound pressure is fair high and quite hard to sustain for a long time (with pure tones in the kHz range, at least) - the test was quite an extreme one. Reducing the drive lowers the THD, and the same holds if you try with higher Z headphones.

Ciao,

L.
  Reply With Quote
Old 12th April 2012, 02:41 PM   #5
PChi is offline PChi  United Kingdom
diyAudio Member
 
Join Date: Jan 2012
Location: Staines, UK
Quote:
Originally Posted by coluke View Post
Hi PChi, the 'real load' was Denon AH-D310 headphones (the cheapest ones in the AH-D Denon's line), whose nominal impedance is 32 ohm. Given their sensitvity (105dB/mW), at 0dbV (~31mW) input the sound pressure is fair high and quite hard to sustain for a long time (with pure tones in the kHz range, at least) - the test was quite an extreme one. Reducing the drive lowers the THD, and the same holds if you try with higher Z headphones.

Ciao,

L.
Hello L,
Thanks for the answer about the real load. At least that removes the crossover from the circuit. A quick look at 'Testing Loudspeakers' by Joseph D'Appolito section 7.3.4 describes drivers as weakly non linear devices. Section 2.6.1 mentions that parameters measured under large excursions will differ from their "small signal" value.
I have an ancient Hi-Fi Choice booklet on Cartridges and Headphones that doesn't appear to have any data on distortion and the 'Loudspeaker and Headphone Handbook' also ignores the issue!
  Reply With Quote
Old 12th April 2012, 03:52 PM   #6
diyAudio Member
 
Join Date: Mar 2007
Location: California
Looking at your schematic, it seems the main difference between your two circuits is that the "low output Z" circuit has the last op amp directly powering the load, while the others are connected via their own 10 ohm resistor, and then the combined signal goes through another 10 ohm resistor. Is that a mistake, or intended?

-Charlie

Last edited by CharlieLaub; 12th April 2012 at 03:56 PM.
  Reply With Quote
Old 12th April 2012, 04:58 PM   #7
coluke is offline coluke  Italy
diyAudio Member
 
Join Date: Mar 2009
Quote:
Originally Posted by CharlieLaub View Post
Looking at your schematic, it seems the main difference between your two circuits is that the "low output Z" circuit has the last op amp directly powering the load, while the others are connected via their own 10 ohm resistor, and then the combined signal goes through another 10 ohm resistor. Is that a mistake, or intended?

-Charlie
Hi Charlie, you're right - the schematic of the second buffer is wrong: its output actually is the upper common node of the 10 ohm resistors; I'm sorry, but drawed it in a few minutes, and something went wrong with the cut'n'paste as usual...

Anyway, nothing really new - this load sharing technique is described in an old NS Linear Brief written by Bob Pease - just a clever method to get more power out of opamps without increasing output impedance (with the parallel buffer a-la Doug Self you need far more opamps, or pretty low balancing resistors, to keep output Z low) - actually it works very well, although with some opamps it may need a damping RC network in order to prevent instability (it seems unnecessary for the 5532s).

L.
  Reply With Quote
Old 12th April 2012, 05:03 PM   #8
coluke is offline coluke  Italy
diyAudio Member
 
Join Date: Mar 2009
Quote:
Originally Posted by PChi View Post
(...) A quick look at 'Testing Loudspeakers' by Joseph D'Appolito section 7.3.4 describes drivers as weakly non linear devices. Section 2.6.1 mentions that parameters measured under large excursions will differ from their "small signal" value.
I have an ancient Hi-Fi Choice booklet on Cartridges and Headphones that doesn't appear to have any data on distortion and the 'Loudspeaker and Headphone Handbook' also ignores the issue!
It seems it's a very neglected topic - I've spent a few hours googlin', but found nothing really interesting or useful (except a couple of AES papers that aren't freely downloadable...).

Ciao,

L.
  Reply With Quote
Old 12th April 2012, 05:34 PM   #9
PChi is offline PChi  United Kingdom
diyAudio Member
 
Join Date: Jan 2012
Location: Staines, UK
I agree that Loudspeaker distortion both acoustic and load impedance is neglected but very important.

Douglas Self mentions in passing the effect of loudspeaker load non linearity in Chapter 8 of the Audio Power Amplifier Design Handbook.

I have only briefly measured acoustic harmonic distortion of poor quality speakers used in Car Handsfree units. It was possible to measure many 10s of percent at certain frequncies.

High Performance Loudspeakers by Martin Colloms includes a copy of a graph for a LS5/5 loudspeaker showing acoustic harmonic distortion levels of around -60 dB at 1 kHz and considerably more at lower frequencies. It also has a graph for a low distortion Yamaha Monitor Loudspeaker.

I guess that the measurements are difficult and don't look good so aren't available. The only hope is that an independent reviewer takes a scientific approach to testing loudspeakers which would encourage an improvement in THD performance both acoustic and load with better linearity.
  Reply With Quote
Old 12th April 2012, 10:20 PM   #10
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
Benchmark Media's John Siau discussed this a while back.

It is interesting that an output impedance of just 2.5 ohms (DF of ~12..13, which would normally be considered ample for headphones) would provoke such a marked increase in distortion.

What one should do is compare this to acoustic distortion. If this is much bigger than on the electrical side for each harmonic (at least 10 dB), further discussion is probably moot. Then you're basically seeing the driver's reciprocity in action, i.e. it feeds a distorted signal back which partly drops over output resistance.

Ultimately the decisive question should be whether output impedance has any effect on distortion on the acoustic side. (Which, as you'll hopefully agree, is where it ultimately counts.) If it doesn't, one can concentrate on its well-documented linear effects. Those are likely to be kicking in well before any degradation in nonlinear distortion are to be observed, though I guess the details would depend on the headphones in question (e.g. stiffly suspended AKGs vs. Sennheisers on the other end of the spectrum).

Last edited by sgrossklass; 12th April 2012 at 10:29 PM.
  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
OT: how can I measure THD of my amplifier if my sound card is 0,02% THD ? ygg-it Solid State 1 13th June 2010 07:54 AM
Question - THD Measrement/Spectrum Analyzis LinuksGuru Solid State 1 30th March 2009 12:57 AM
Question about THD boxedin Solid State 5 2nd January 2006 04:21 PM
Zen V4 - square wave with real load wish Pass Labs 4 21st February 2005 07:54 PM
A quick THD+N question SamM Analogue Source 4 9th October 2004 01:34 PM


New To Site? Need Help?

All times are GMT. The time now is 07:03 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