Class B w/o crossover distortion (1975) - Page 13 - 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 13th February 2010, 08:49 PM   #121
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
I think you do not quite understand how your own circuit works:

a) There is a reaction delay, because the hold capacitor has to charge to adapt to the new dynamic regime when it changes, and this takes time, because only a fraction of the 15mA main bias can be diverted for this purpose. I insist it is a hold capacitor; what it stores is an information: the actual Iq setting, unlike a bootstrap capacitor whose only role can be likened to an energy storage device, where the quantity itself is irrelevant provided it is sufficient (a battery would be adequate).

b) The time constant of this process is longer than the primary thermal time constants of the semiconductor devices (the dice), and anyway it adds a large amount of dissipation to that caused by the class B operation. But at the same time, it remains servoed, and doesn't run the risk of going into the usual thermal runaway, say like a simple class B amplifier without emitter resistors.

IMHO, the main drawback of this amplifier is its inability to reach the full power without adapting the operating point.
If it could cover the whole dynamic range with the initial Iq setting, it would be near-ideal, as the current servoing scheme seems to work well, but does not cover the whole span.
This might be corrected, perhaps by substituting MOS output devices.
  Reply With Quote
Old 13th February 2010, 09:16 PM   #122
hahfran is offline hahfran  Germany
diyAudio Member
 
Join Date: Dec 2007
No there is only at the startup the initial load of the cap that is why one has to delay the sine source for 1 sec. It needs not to the operating point adapt thereafter. LTSpice does not provide a burst source so one cannot simulate. Further no, a change of iq due to junction temp is not servoed without emitter resistors. But anyway it should be very simple to built with modern devices.
  Reply With Quote
Old 14th February 2010, 09:38 AM   #123
hahfran is offline hahfran  Germany
diyAudio Member
 
Join Date: Dec 2007
The current throu the cap is largely independent of signal amplitude except when clipping.
This rises an interesting question , recovery from clipping, that is valid for any kind of amps
but has not been discussed yet. Thus I post it in a new thread.
  Reply With Quote
Old 16th February 2010, 11:15 AM   #124
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
LTspice can provide tone bursts in a number of ways:
Here is one.

Picture 2 shows the detail, and picture 3 shows also the voltage across the capacitor.

Quote:
Further no, a change of iq due to junction temp is not servoed without emitter resistors
It is, because the level of charge of the capacitor (which sets the quiescent current) relies on the splitting factor of the switching modulator formed by D1, Q2 and Q7.
When the average level of conduction of Q2 increases, the duty cycle of D1's conduction also increases, diverting the 15mA bias away from Q7 more of the time.
This decreases the charge going into C1, and corrects the Iq through Q2 (and Q1).
Attached Images
File Type: gif VischBurst1.GIF (55.2 KB, 382 views)
File Type: gif VischBurst2.GIF (58.7 KB, 358 views)
File Type: gif VischBurst3.GIF (45.6 KB, 349 views)
  Reply With Quote
Old 16th February 2010, 11:37 AM   #125
hahfran is offline hahfran  Germany
diyAudio Member
 
Join Date: Dec 2007
I have now built one all free wired on heatsink ( all trans are mounted to heatsink)
in that real model replacing D6 that sets the bias of the CS with a red LED Vf=1.36 Volts
improved the performance incredibly. With unregulated supply of 36 volts the amp puts out
30 v peak to peak into an 8 Ohms res at an incredibly low thd ( possibly below the accuracy of the soundcard based spectrum analyser ) . At any frequency between 20/s and 10000/s. Without the added emitter resistors and feedback from these thermal stability is EXTREMELY poor - not usable. After 20 seconds at full load the iq was at 300mA and rapidly increasing. It could be the thermal coupling of the rush transistors would bring iq down after a while but I turned power off. With the additional r's and feedback iq is reasonably controlled.
BUT: when it drives a real 8 Ohms speaker in a closed enclosure above res freq when it is mainly inductive at 20 volts pp the thd is 47% !! It is all odd harmonics.
Even worse with a 2 1/2 way speaker with some hefty crossover design. Here it ends up at about 15 volts pp after that thd increases steeply. There is no oscillation, however.
It appears this topology realised as a real circuitry and driving a real complex load is of little value it is worse than any simple emitter follower cascade.
  Reply With Quote
Old 16th February 2010, 08:34 PM   #126
hahfran is offline hahfran  Germany
diyAudio Member
 
Join Date: Dec 2007
Amazingly enough it works perfectly when driving a capacitive load...6.2 Ohms paralleled 220nF ... it just cannot stand inductive loads. In the case of capacitive the source impedance has neglegible influence ( I use a tube Wien bridge that can output 40 volts pp but has a high real part of impendance about 10 kOhms and some 200 pF parallel) in case of inductive buffering the source with an emitter follower appears better. Strange stuff.
  Reply With Quote
Old 16th February 2010, 10:11 PM   #127
diyAudio Member
 
Join Date: Nov 2007
Location: Dallas
Bootstrapped concertina?

I slightly disagree the placement of your Schottky. That would be more effective
to combine the collector current of Q2 with the emitter current of Q4 and run
both thru the same Schottky (and maybe 0.1R) to the output. Giving this Sziklai
pair an overall more square law curve and less dependence upon temperature.

You should compliment with a second Schottky+.1R from Q1+Q3 to the top rail.

Furthermore, the base of Q7 could connect direct to the -35 rail, and ditch R5
and D2 voltage reference???

Last edited by kenpeter; 16th February 2010 at 10:27 PM.
  Reply With Quote
Old 16th February 2010, 10:13 PM   #128
hahfran is offline hahfran  Germany
diyAudio Member
 
Join Date: Dec 2007
Ok got it fixed. The original circuit by Vish appears to suffer from the direct feedback to the base of one Rush cascode. That will in practice hardly ever work as its sensitivity to
load impedance is too high. It appears that with the old technology for ex. the single diffused 2N3055 this had not been a big issue.
Anyway it looks good now.
It is only a kind of natural evolution to redesign the circuit as such that it becomes symmetrical. The Rush is now driven at the common emitters and the feedback goes symmetrically to both bases.
  Reply With Quote
Old 16th February 2010, 10:56 PM   #129
diyAudio Member
 
Join Date: Nov 2007
Location: Dallas
Input between the emitters sounds like Allison.
  Reply With Quote
Old 16th February 2010, 11:08 PM   #130
diyAudio Member
 
Join Date: Nov 2007
Location: Dallas
This is what I meant (post #127) about repositioning Schottkys for square law effect.
Try trick with Iq about 600~800mA and observe how smooth the crossing can be.
Attached Images
File Type: gif JLH_Fold.gif (32.2 KB, 346 views)

Last edited by kenpeter; 16th February 2010 at 11:17 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
low crossover-distortion class AB output stage topologies capslock Solid State 72 10th May 2013 07:48 PM
Exactly titles of old transistor data books from NEC, SONY and Fujitsu (1975 - 1985) tiefbassuebertr Solid State 5 12th December 2011 05:31 PM
Crossover distortion - be gone! Circlotron Solid State 4 24th August 2009 12:09 PM
Crossover distortion lumanauw Solid State 100 14th January 2005 12:58 AM
Crossover Distortion? Killjoy99 Multi-Way 8 30th March 2004 05:10 AM


New To Site? Need Help?

All times are GMT. The time now is 03:19 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