Bob Cordell Interview: Negative Feedback - Page 50 - 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 16th March 2007, 06:11 PM   #491
mikeks is offline mikeks  United Kingdom
Account Disabled
 
Join Date: Jun 2004
Location: Animal farm
Default Re: Re: Re: Class-XD

Quote:
Originally posted by estuart


Hi Mike,

Nope. Neither of the two. Iíll give you one more change to figure it out.
As an expert on this subject, Iím convinced you will find the answer.

Cheers,

PS: Now, I'm going to my local pub, see you later.
I'll defer to you: enlighten us.
  Reply With Quote
Old 16th March 2007, 06:14 PM   #492
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Quote:
Originally posted by Bob Cordell
First, 0.04% of XO distortion at only 10 kHz is pretty bad, especially when one realises the crest factor of XO distortion. Secondly, this is at only 1 watt! If the XD circuit is working, one should not even be entering the crossover region at only one watt. Something is terribly wrong here
It does seem awfully odd. While I never assume a priori
that .04% is going to sound bad, I don't expect the circuit as
described to have that "feature", certainly no amplifier with a
1 Amp bias.

  Reply With Quote
Old 16th March 2007, 06:37 PM   #493
PMA is offline PMA  Europe
diyAudio Member
 
PMA's Avatar
 
Join Date: Apr 2002
Location: Prague
In case that 0.04% is crossover distortion in nature, it is pretty audible.
And yes, 0.04% is quite high distortion. It would not if it was only 2nd harmonic.
  Reply With Quote
Old 16th March 2007, 08:20 PM   #494
Tim__x is offline Tim__x  Canada
diyAudio Member
 
Join Date: Jun 2004
Location: Edmonton area, Alberta
I think Estuart is referring to a Cherry-esque second miller loop containing the output stage, with an R in series so that the traditional miller loop becomes dominant at high frequencies to maintain the original stability.
  Reply With Quote
Old 16th March 2007, 08:28 PM   #495
mikeks is offline mikeks  United Kingdom
Account Disabled
 
Join Date: Jun 2004
Location: Animal farm
Quote:
Originally posted by Tim__x
I think Estuart is referring to a Cherry-esque second miller loop containing the output stage, with an R in series so that the traditional miller loop becomes dominant at high frequencies to maintain the original stability.
Enclosing a power BJT output stage provokes local instability even with a series resistor.
  Reply With Quote
Old 16th March 2007, 08:36 PM   #496
diyAudio Member
 
Graham Maynard's Avatar
 
Join Date: Dec 2003
Location: UK
Hi PMA,

Without the circuit you cannot expect me to guess what might be going on ! I wonder if I am expected to think that plot is satisfactory ?

There is an error in #474 at higher audio frequencies, and this is of greater magnitude than the Symasym for most of the time.

Of course we get greater error voltages with tube and non-feedback designs, and this error voltage variation in time (which is generally far greater than resistively measured THD figures) contributes towards making amplifiers sound different.

Cheers .......... Graham.
  Reply With Quote
Old 16th March 2007, 09:04 PM   #497
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Quote:
Originally posted by Tim__x
I think Estuart is referring to a Cherry-esque second miller loop containing the output stage, with an R in series so that the traditional miller loop becomes dominant at high frequencies to maintain the original stability.

Exactly, thatís the crux. Chapeaux!
See below for this simple but valuable trick.
Note: It doesnít work without a preceding beta enhancement transistor in the VAS stage. Probably, thatís why D. Self misunderstood the potential of this subtle modification.

Cheers,
Attached Images
File Type: gif splitcomp.gif (8.7 KB, 1034 views)
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het lichtÖ(H.M. van Randwijk)
  Reply With Quote
Old 16th March 2007, 10:03 PM   #498
diyAudio Member
 
Graham Maynard's Avatar
 
Join Date: Dec 2003
Location: UK
Using a resistor from output to Darlington VAS base can also help to reduce output stage crossover distortion and improve NFB response by flattening the open loop bandwidth.
Using an equal value resistor from other half of mirror to ground prevents start-up thump and preserves differential input balance.

This is a Sim demo cct only.

Cheers .......... Graham.
Attached Images
File Type: gif 0504ab.gif (8.0 KB, 992 views)
  Reply With Quote
Old 16th March 2007, 10:05 PM   #499
mikeks is offline mikeks  United Kingdom
Account Disabled
 
Join Date: Jun 2004
Location: Animal farm
Quote:
Originally posted by estuart
Exactly, thatís the crux. Chapeaux!
See below for this simple but valuable trick.
Note: It doesnít work without a preceding beta enhancement transistor in the VAS stage. Probably, thatís why D. Self misunderstood the potential of this subtle modification.
Cheers,
This accomplishes nothing as far as linearity is concerned: loop transmission at 20KHz is typically 25dB~30dB, while DPC can be persuaded to deliver ~60dB at the same frequency.
  Reply With Quote
Old 17th March 2007, 12:14 AM   #500
diyAudio Member
 
Join Date: Sep 2006
Quote:
Originally posted by PMA
In case that 0.04% is crossover distortion in nature, it is pretty audible.
And yes, 0.04% is quite high distortion. It would not if it was only 2nd harmonic.

Exactly right.

Bob
  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 01:40 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