New Lineup IDEA - Power Follower/Output stage - Page 25 - 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 10th December 2012, 05:56 PM   #241
Struth is offline Struth  Canada
diyAudio Member
 
Join Date: Aug 2012
Location: books at londonpower.com
Hi Guys

A ringing amplifier output is aberrant behaviour, so measuring THD during such an incident is counterintuitive; I don't know of anyone making such a test. As to audibility, that will depend on the frequency of the ringing.

In some situations ringing will sound like a chirp or glitch. I've heard this and the scope verifies it.

As far as I know, the 2uF in parallel with 8R was suggested by a magazine reviewer as a way to mimic the loading of an amp by an electrostatic panel. It would be interesting to know what capacitance the raw panel looks like - it would be reflected differently to the amp as most ESLs have a drive transformer with a high step-up ratio around 75x or so. Maybe this puts the raw panel at 27nF-ish?

Otherwise, most amplifier ringing corresponds to quite high ultrasonic frequencies according to most intell. And counterintuitively it is loading in the 100nF range that can cause more difficulty than the 1-2uF range for many amps.

Have fun
Kevin O'Connor
__________________
London Power / Power Press Publishing
www.londonpower.com
  Reply With Quote
Old 10th December 2012, 10:07 PM   #242
expert in tautology
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
Any time I have tested a solid state amp, excited with a square wave and loaded with a capacitive load, I see ringing on the square wave. That is, IF it has an output inductor. Beyond that, I know nothing.

I think the value for some ESLs have been stated in one of the ESL threads here. I can't recall at the moment. Undoubtedly the inductance of the transformer will have some effect, but perhaps not very much in the audio band, I am guessing.

I'm not talking about parasitics or other oscillations.

_-_-bear

PS. guess what i am aiming at is that an output inductor would appear to have an effect on the way a circuit deals with "fast" signals and things like impulses - like maybe a fast cymbal shot?
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  Reply With Quote
Old 10th December 2012, 10:41 PM   #243
Bonsai is offline Bonsai  Taiwan
diyAudio Member
 
Bonsai's Avatar
 
Join Date: Jul 2003
Location: Taipei, Taiwan
"I agree better vas transistors are available but at least they are much better than mje340/350 which are commonly still found even on commercial products."

These are terrible. Their parameters change a lot with Vce and in the VAS role they often lead to parasitic oscillation on the signal peaks - especially if driving mosfet outputs. They are load switching devices.
__________________
bonsai
Amplifier Design and Construction for MUSIC! http://hifisonix.com/
  Reply With Quote
Old 10th December 2012, 11:59 PM   #244
Struth is offline Struth  Canada
diyAudio Member
 
Join Date: Aug 2012
Location: books at londonpower.com
Hi Guys

Every study ever cited about how fast music signals are concur that there is nothing that even approaches the steepness of a squarewave. I assume this includes things which to human perception are very fast, such as a cymbal crash, as that is a "natural" instrument sound as compared to an electronically generated signal.

Where squarewave testing is good for assessing certain aspects about the circuit performance, it does not look like ringing through the tiny inductor into a capacitive load means anything at all. I believe if you look at the amplifier side of the choke you will see a good wave and at the output the ringing. The rise time of the step is more critical to how much overshoot occurs, with faster waves causing greater overshoot.

The typical 40kHz or so ringing is just the resonance of the output inductor with the load and does not reflect a problem with the amp itself. oscillations within the amp are much higher -100-500kHz - and are usually destructive. Low-feedback designs have greater immunity to capacitive loading causing oscillation than do high-feedback designs.

In the LU buffer, the gain of importance is in the diamond feedback stage. The transconductance there can be "compromised" as I did in my "slow" version with bootstrapped resistive current sources for the front end. This tends to make the circuit more stable without an output inductor.

Have fun
Kevin O'Connor
__________________
London Power / Power Press Publishing
www.londonpower.com
  Reply With Quote
Old 11th December 2012, 04:41 AM   #245
expert in tautology
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
Dunno about every study doing anything.

But my experience is that I can predict to some degree of certainty the general nature of the sound that an amplifier will produce when listened to by looking at the leading edge of a square wave output from the amp on a scope.

So, if you think that that little blip of overshoot is inaudible, or more precisely is not a predictor of the general sonic category of an amp, that's fine, it's not been my experience thus far. Of course, I may be delusional, and people have said worse, so feel free to join in should you be so moved.

_-_-bear
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  Reply With Quote
Old 11th December 2012, 09:47 AM   #246
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
yes that overshoot, or the opposite, overly rounded, will impact on the sound when the signal is not a simple single tone.
It is a much under utilised technique for setting up an amplifier. JLH described it in at least one of his amplifier builds.

It is also used oppositely. Some builders use the deliberate overshoot emphasis to give "enhanced" treble bite & detail. Quite wrong in my view.
The problem with this "by ear" technique is that the builder usually does not realise/know what they are doing.
__________________
regards Andrew T.
  Reply With Quote
Old 11th December 2012, 02:33 PM   #247
diyAudio Member
 
Join Date: Nov 2010
Location: The City, SanFrancisco
I think there are numerous reasons for ringing on an amp, and each may cause different effects. For example, some options:
Low damping on the LC output.
Slow recovery from slew rate limitng.
Low phase margin.
Single stage parasitic oscillation.

The first two would appear to only cause a problem if the excitation is fast enough, for example do the fastest audio transients actually cause slew rate limiting (or get reasonably close). In which case then yes the amp will show severe distortion (at least for insufficient slew rate).
I'm not sure how low phase margin would cause added distortion, I would expect it to simply be a reliability concern as aging or speaker substitution may cause it to become unstable.
Parasitic oscillation may or may not be heard (as it would depend on its envelope or the susecptibility of other stages within the amp to it) but should be measurable in some form (direct oscillation or increased offset, power etc).

THanks
-Antonio
  Reply With Quote
Old 11th December 2012, 04:23 PM   #248
Struth is offline Struth  Canada
diyAudio Member
 
Join Date: Aug 2012
Location: books at londonpower.com
Hi Guys

Unless the amplifier is poorly designed, oscillations from within the amplifier are usually high frequency, as above, so you won't hear them. if they do not destroy the amp, you will notice overheating of the amp, and if this is while listening to the amp, you might notice a graininess in the sound.

If the stability margin is eroding towards positive feedback, this sounds fatiguing and the sound will lack resolution.

The ringing referred to above is strictly the resonance of the output inductor with the reactive load. A properly designed amp will absorb reverse currents associated with this and not be damaged. As discussed in another thread, if you can hear what the output inductor does then its value is far too high.

The studies mentioned are the mass of independent reviews made by various investigators to determine how fast audio signals can be. These studies began even before the notion of TIM or slew rate limiting had such names. See external sources for links - Pass has a link to one study, for example. The rise times of music studied could be fast enough to warrant better OT design in tube days, but was never so fast as to cause a problem for properly designed BJT gear. First generation BJT amps were utter garbage; second generation was passable; third generation finally got it right.

Speed and bandwidth in an audio circuit has both benefits and perils. Every design is a compromise.

Have fun
Kevin O'Connor
__________________
London Power / Power Press Publishing
www.londonpower.com
  Reply With Quote
Old 11th December 2012, 05:08 PM   #249
diyAudio Member
 
Join Date: Nov 2010
Location: The City, SanFrancisco
Quote:
Originally Posted by Struth View Post
...
If the stability margin is eroding towards positive feedback, this sounds fatiguing and the sound will lack resolution.
...
Kevin

Interesting, I dont understand why this would be so (unless its actually oscillating some of the time). Can you supply some comments or links as to the rationale?

Thanks
-Antonio
  Reply With Quote
Old 11th December 2012, 05:20 PM   #250
Struth is offline Struth  Canada
diyAudio Member
 
Join Date: Aug 2012
Location: books at londonpower.com
Hi Guys

I'm sure all the Nyquist and control theory math predicts this and explains it if you understand any of it. I don't, so have to rely on my own experience.

This is an effect that is easier to hear in preamp circuitry, where the is no risk of devastation. Poor layout is a key cause to positive feedback in this situation, but so can be the circuit design.

An example is the Dynaco phono preamp. This uses a combination of positive and negative feedback. The former tends to increase gain where the latter reduces it. In the context of a phono EQ, high frequencies are being rolled off anyway, so the circuit is generally stable. However, if you use a similar circuit for straight voltage gain, as you increase positive feedback from a point of transparent effect to where you can hear it, the sound is dry, raspy and fatiguing.

As long as the circuit is not going to destroy itself, I would think the audible effects would be the same for a large-signal circuit. As positive feedback is increased, the stability margin is being eroded until the circuit breaks into oscillation. At that point, Nyquist has left the building but you now have a stable oscillator.

Have fun
Kevin O'Connor
__________________
London Power / Power Press Publishing
www.londonpower.com

Last edited by Struth; 11th December 2012 at 05:43 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
four transistor emitter follower (diamond buffer) as power output stage? capslock Solid State 113 12th August 2013 07:16 PM
mosfet source follower output stage. Printer2 Tubes / Valves 21 18th January 2013 05:43 AM
LINEUP Input-VAS, idea lineup Solid State 57 15th March 2011 12:33 AM
Do we always need an emitter follower to drive a LATFET output stage??? Leolabs Solid State 37 9th January 2011 05:16 PM
Cathode follower Class A output stage? Circlotron Tubes / Valves 14 16th May 2003 12:11 AM


New To Site? Need Help?

All times are GMT. The time now is 03:04 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