Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

Simple Symetrical Amplifier
Simple Symetrical Amplifier
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 23rd February 2012, 10:31 PM   #1961
Jay is offline Jay  Indonesia
Banned
 
Join Date: Feb 2003
Location: Jakarta
Quote:
Originally Posted by Bigun View Post
From the very few projects I've completed, I find that the ear is the best instrument for tuning the compensation caps (phase lag especially but also phase lead) once you are within the envelope of good stability.
Thanks everyone for letting me know that the compensation cap can be tuned by ears. Initially I was skeptical even tho all the experts had confirmed the fact (). But after Bigun mentioning the phase lag thing suddenly I realized

Still, LC has not confirmed about the cap. Small one could be issue, but none couldn't be, as long as it is stable?
  Reply With Quote
Old 23rd February 2012, 11:40 PM   #1962
Sheldon is offline Sheldon  United States
diyAudio Member
 
Join Date: Dec 2003
Location: San Diego
Quote:
Originally Posted by Salas View Post
You were contemplating some Vbe mult. between the drivers and/or TO-92 DN2540s instead of trimmers, weren't you?
It may be that a Vbe multiplier is required. I can't see the current for the drivers being very stable, and it directly affects output bias in this version. It's not clear to me that a current source, instead of a resistor (plus trimmer) will have much affect on reaching a stable equilibrium. Cascoding the drivers might help by reducing the dissipation.

But I think something else is amiss also, as I see significant distortion in the waveform when driving an 8 Ohm load. With a load, it overshoots, but doesn't settles. With no load, it rings quite a bit. I built it up first without the Miller comp caps. I'm pretty sure it was oscillating. I replaced the drivers, when I added the compensation caps. I'm wondering if oscillation could have damaged the outputs too.

I'm not sure I want to chase this down for the TT version. Maybe Nico's design is the way to go for a simple version of the amp.

Sheldon
  Reply With Quote
Old 23rd February 2012, 11:56 PM   #1963
Salas is offline Salas  Greece
diyAudio Chief Moderator
 
Salas's Avatar
 
Join Date: Oct 2002
Location: Athens-Greece
Simple Symetrical Amplifier
The Vbe multiplier mod to test at least, I think. The main issue with the Sziklai and Latfets was the drivers current running and I put the Jfet CCS there for instance. Maybe you got to find another operating current for the drivers to see how it affects ringing?
  Reply With Quote
Old 23rd February 2012, 11:58 PM   #1964
Salas is offline Salas  Greece
diyAudio Chief Moderator
 
Salas's Avatar
 
Join Date: Oct 2002
Location: Athens-Greece
Simple Symetrical Amplifier
BTW, can you tell me how much current your input bjts run and if its near between pnp and npn? Drop in mv over their 10R emitter resistor will show.
  Reply With Quote
Old 24th February 2012, 02:05 AM   #1965
Bigun is offline Bigun  Canada
diyAudio Member
 
Bigun's Avatar
 
Join Date: Jan 2009
Location: Waterloo, ON or Herefordshire UK
Simple Symetrical Amplifier
Quote:
Originally Posted by Sheldon View Post
The VAS current is very sensitive to the VAS device temperature.
I realized this is a feature of all symmetrical BJT VAS designs - at least those I've seen. I noticed that the Bryston amp I have, which is also a symmetrical VAS, uses large emitter degeneration resistors on the VAS. So I also used large degeneration resistors on the VAS of my TGM5 amplifier. I believe it helps to reduce sensitivity of the VAS current to operating conditions.

Quote:
Originally Posted by Sheldon View Post
It may be that a Vbe multiplier is required. I can't see the current for the drivers being very stable, and it directly affects output bias in this version. It's not clear to me that a current source, instead of a resistor (plus trimmer) will have much affect on reaching a stable equilibrium.
I went through the same issues in TGM5, albeit in Spice simulations. I concluded that the VAS current would not be stable enough to guarantee good output bias without paying some attention to the Vbe multiplier. I ended up using a design from Hagerman. I don't find the output bias as stable as my TGM1 but it appears to do the job well enough. In fact, the amp has been more stable with respect to temperature than I had feared, both bias and dc offset.

I used small compensation caps on both the VAS and the pre-drivers of the Triple output stage.

This topology sounds good, it's worth persevering.
__________________
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

Last edited by Bigun; 24th February 2012 at 02:09 AM.
  Reply With Quote
Old 24th February 2012, 02:58 AM   #1966
AKSA is offline AKSA  Australia
diyAudio Member
 
Join Date: Sep 2001
Location: Melbourne, Australia
Gareth,

If you use three 1N4148 diodes in series with a 200R pot, you can ameliorate the bias current instability of a sensitive dual VAS configuration, particularly with the J1058/J162 latfets.

Hugh
__________________
Aspen Amplifiers P/L (Australia)
www.aksaonline.com
  Reply With Quote
Old 24th February 2012, 08:08 AM   #1967
Nico Ras is offline Nico Ras  South Africa
diyAudio Member
 
Nico Ras's Avatar
 
Join Date: Nov 2005
Location: East Coast of South Africa
Quote:
Originally Posted by AKSA View Post
Gareth,

If you use three 1N4148 diodes in series with a 200R pot, you can ameliorate the bias current instability of a sensitive dual VAS configuration, particularly with the J1058/J162 latfets.

Hugh
For some reason, 1N400X performs much better than 1N4148 in that situation.
__________________
Kindest regards
Nico
  Reply With Quote
Old 24th February 2012, 10:24 AM   #1968
MiiB is offline MiiB  Denmark
diyAudio Member
 
Join Date: May 2005
Location: Denmark
I believe that the reason for the temperature sensitive VAS is the fact that is also current vise dependent on the input-stage, so two thermal issues is playing a role, and as none of them is driven by a CCS, you will run into thermal problems...This circuit sounds fantastic, but needs better household...!! The Salas Jfet is good for the VAS, bur does not solve the input-stage current dependency
  Reply With Quote
Old 24th February 2012, 01:13 PM   #1969
Esperado is offline Esperado  France
Banned
 
Join Date: Jul 2011
Location: Portugal
Well, you can add two caps in serial with the CR at the mass reference points. It will reduce to 1 the gain of the amp at DC.
  Reply With Quote
Old 24th February 2012, 01:14 PM   #1970
Bigun is offline Bigun  Canada
diyAudio Member
 
Bigun's Avatar
 
Join Date: Jan 2009
Location: Waterloo, ON or Herefordshire UK
Simple Symetrical Amplifier
Quote:
Originally Posted by MiiB View Post
I believe that the reason for the temperature sensitive VAS is the fact that is also current vise dependent on the input-stage, so two thermal issues is playing a role, and as none of them is driven by a CCS, you will run into thermal problems...This circuit sounds fantastic, but needs better household...!! The Salas Jfet is good for the VAS, bur does not solve the input-stage current dependency
My sims also highlighted this. I actually found you can fix this by incorporating a thermistor in the circuit but that seems like a kludge. In the end, I didn't find thermal stability to be an issue. But there's an opportunity still to improve it - but achieving that without spoiling the topology doesn't look trivial.
__________________
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
  Reply With Quote

Reply


Simple Symetrical AmplifierHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Symetrical schematics are alike plague in Brazil, do you like them? destroyer X Solid State 151 1st July 2010 02:09 PM
Symetrical out low Zout karsten21 Tubes / Valves 5 1st February 2010 10:19 PM
going balanced/symetrical - what benefits? weissi Solid State 15 20th October 2007 08:06 AM
Symetrical field, is this? Raka Multi-Way 6 14th September 2003 01:21 PM
Non-symetrical SMPS output cm961 Parts 4 21st August 2003 10:27 PM


New To Site? Need Help?

All times are GMT. The time now is 01:14 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki