Bob Cordell Interview: Negative Feedback - Page 61 - 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 19th March 2007, 04:14 PM   #601
ingrast is offline ingrast  Uruguay
diyAudio Member
 
Join Date: Sep 2004
Location: Montevideo
Quote:
Originally posted by Graham Maynard
......
This displays what I personally so dislike about quadrature phase shifted damping.
That is, at moments when dynamic loudspeaker generated back-EMF leads (musically altered) NFB loop controlled amplified input, an instantaneous error voltage is generated ......
Graham:

We are aware this is a issue that particularly worries you, yet it is not the crux of the matter.

Remember these plots depict the error signal, i.e. the amount of correction NFB thinks must be inserted to strighten things. The real issue is how the actual output voltage follows input and controls cone movement, for this is what actually gets listened.

This nonwithstanding, a much better high frequency response is readily achievable with modern devices, meaning negligible error signal phase shift can be attained up to and above 20 KHz.

Rodolfo
  Reply With Quote
Old 19th March 2007, 04:28 PM   #602
R.I.P.
 
Graham Maynard's Avatar
 
Join Date: Dec 2003
Location: UK
This is the kind of output test I would do, though I have never actually used a current source before to do it.

The error is greatest at maximum amplitude and is close to zero again at minimum - though not zero due to the fractional phase shift. Similar to the more efficient Symasym.

Now this error of 3mV at 2A = 16V @ 8R is greater than the steady sine measured THD for the circuit, BUT quite close to the figure for the amplifier distortion figure for first cycle distortion.

This back-EMF induced error is greater than THD with most high NFB amplifiers, but will not be demonstrated without either amplitude nulling the output when driving a dynamic loudspeaker load, or reverse driving an output stage.

Of course non feedback designs do not actively produce any damping and the voltage cannot keep from drifting under the influence of back-EMF beyond the passive bias loading of the loudspeaker, but at least the amplifier does not itself then generate a phase shifted error potential, which is likely why some of Nelson's designs have earned a good reputation.


PS Could someone at diyAudio PLEASE change the upper attachment limit to 1024. What a nuisance it is having to convert image sizes - the detail gets lost !.
Attached Images
File Type: gif gemtest.gif (39.3 KB, 400 views)
  Reply With Quote
Old 19th March 2007, 04:53 PM   #603
R.I.P.
 
Graham Maynard's Avatar
 
Join Date: Dec 2003
Location: UK
Hi Rodolfo,

I am freely passing on my viewpoint of entries presented here, and as back-up showing some of my own results, which I do not claim to be perfect, but which satisfy me.

Forward and reverse control phase shifts being different means that topology has the most significant effect, even with high speed devices.

If the error signal was being illustrated I was not aware of this, and it was NOT stated. I said I could understand how those those plots had been made and asked for illustration.

PMA are those plots output terminal potential error, or an internal voltage error ?

As far as controlling the loudspeaker - I suggested that forward amplifier testing be completed using a realistic load with fundamental nulling, for that includes the imaginary back-EMF current.

When an amplifier is virtually immune to back-EMF its forward measured response can be better relied upon, but I must agree and have written elsewhere that I do not assume that a perfect voltage amplifier is the best way to drive a loudspeaker.

Cheers ........... Graham.
  Reply With Quote
Old 19th March 2007, 05:38 PM   #604
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Default loop gain

Quote:
Originally posted by andy_c

...I'll put a loop gain probe in the inner loop...
Hi Andy,

As I'm not familiar with LTSpice (any longer), please tell me a little more about these 'loop gain probes', for example, are they based on the double injection technique of R.D. Middlebrook?

Cheers,
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
  Reply With Quote
Old 19th March 2007, 06:08 PM   #605
mikeks is offline mikeks  United Kingdom
Account Disabled
 
Join Date: Jun 2004
Location: Animal farm
http://www.diyaudio.com/forums/showt...317#post464317

http://www.diyaudio.com/forums/showt...406#post467406
  Reply With Quote
Old 19th March 2007, 06:09 PM   #606
andy_c is offline andy_c  United States
Banned
 
Join Date: Apr 2003
Default Re: loop gain

Quote:
Originally posted by estuart
As I'm not familiar with LTSpice (any longer), please tell me a little more about these 'loop gain probes', for example, are they based on the double injection technique of R.D. Middlebrook?
Yes, and also this paper. It is a subcircuit you can drop onto your schematic. The STEP command is used to turn the probe's voltage and current sources on and off, and a plot command which uses some odd-looking syntax to refer to voltages and currents in each state of the STEP is used to compute loop gain.

This circuit came from the Yahoo LTSpice users' group.
  Reply With Quote
Old 19th March 2007, 06:26 PM   #607
PMA is offline PMA  Europe
Design engineer, consultant
diyAudio Member
 
PMA's Avatar
 
Join Date: Apr 2002
Location: Prague
Graham,

I have already described it, you have not been patient enough

The amplifier is driven to its output with input grounded. The plots show output pin voltage vs. current forced into output. X axis is current to output, Y axis is voltage at the output pin created by forced current, as a voltage drop on output impedance. This output voltage should be ideally zero (for infinite damping factor, not dependent on frequency and amplitude). I create it in simulation with X axis current, Y axis voltage.

I am going to measure it by conventional scope as X-Y plot.
  Reply With Quote
Old 19th March 2007, 06:55 PM   #608
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Quote:
Originally posted by janneman
...Well, LTSpice seems to be the de facto standard here because everybody seems to have it; newbees soon discover that it helps immensely the conversations if you have a common baseline...
Jan Didden
Hi Jan and all other LTspice users,

OK LT guys, you win as I came down a peg or two (saves of lot of energy). I've reinstalled the S/W package from LTC.
Still, the data exchange isn't foolproof, because the definition models are not included in the asc files (as far as I know). Any suggestions how to tackle this source of annoyance.

And Mike,

Now that I can read your drawings (Stuart's_LG2.asc, upper diagram), I saw a loop gain probe put at a place, which I had never expected. If it was you intention to characterize the global feedback loop in this way, I'm sorry to say so, you'll get erroneous results. But maybe you had something else in mind. Anyhow, this too explains that we have been at cross purposes.

Cheers,
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
  Reply With Quote
Old 19th March 2007, 07:00 PM   #609
R.I.P.
 
Graham Maynard's Avatar
 
Join Date: Dec 2003
Location: UK
Thanks PMA.

I thought I had read that somewhere but, when I went back to check after Rodolfo suggested they were error signal, I could not find it.

Cheers ....... Graham.
  Reply With Quote
Old 19th March 2007, 07:00 PM   #610
mikeks is offline mikeks  United Kingdom
Account Disabled
 
Join Date: Jun 2004
Location: Animal farm
Quote:
Originally posted by estuart
........you'll get erroneous results.....
No.
  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



New To Site? Need Help?

All times are GMT. The time now is 01:34 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