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.

Instability with totem pole output stage
Instability with totem pole output stage
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 April 2018, 01:41 PM   #1
alex5612 is offline alex5612  Austria
diyAudio Member
 
Join Date: Jan 2009
Location: Wien
Default Instability with totem pole output stage

Hello everybody,

I hope I did get the name of the topology right.
The story is as follows: I managed to get an excellent deal on two used 1600va 2x115V secondary toroidal transformers and right now I'm contemplating building a very large amplifier around these two.

Unfortunately there are no additional secondary taps available and thus the amplifier will have to be class B with no sliding rails.

There are complementary 350V bipolar transistors available from On-Semi, but the problem with these is that they're almost useless at these voltages due to SOAR limitations. I thought the next best thing would be to connect the output transistors in series just like in the "low-end zesilovac 1kW" from Alan Kraus. Czech magazine "amatorske radio" published this design back in 2000. I don't think it is worth the effort to build this design as both input stage and VAS are very simple and I assume that the performance won't be very good. In any case, I have a pdf available if anyone is interested.

I did like the output stage however and I tried designing something similar using more modern plastic-cased transistors. However the problem with this type of design is that I cannot get it running stable. I've run many simulations over the past weeks and I think the instability issues can be really narrowed down to the output stage itself.

I've attached a stripped-down schematic which shows just the output stage with 'ideal' VAS and bias generator. The circuit appears to be very sensitive to changes in R1 and there is significant gain above 1MHz or so. This could explain why even minor changes in the VAS stage make a mess of everything.

I've read critical comments about power cascades or totem pole arrangement output stages and I'm not so sure what to do with this information. I haven't seen this kind of arrangement anywhere except with the Alan Kraus design and the Leach Superamp and those two were using older and slower power transistors.

Right now I'm not sure if a complementary bipolar power stage is possible for such high supply voltages. I would appreciate any input or perhaps schematics for similar designs.
Attached Images
File Type: png screenshot endstufenbeschaltung.png (44.2 KB, 291 views)
File Type: png screenshot endstufenbeschaltung2.png (35.1 KB, 289 views)
Attached Files
File Type: asc 130v test-53-ausgangsstufe alleine-export.asc (23.6 KB, 6 views)
  Reply With Quote
Old 23rd April 2018, 02:00 PM   #2
nigelwright7557 is offline nigelwright7557  United Kingdom
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
I have found two areas that can cause instability in these kind of amps.
1/ The VAS stage capacitor needs to be just right. Usually 100pf
2/ The resistor between driver and output transistors needs to be right too. I put 10R in series with each output transistor base.
__________________
PCBCAD51 pcb design software. 2018 version out now with lower prices >> http://www.murtonpikesystems.co.uk
  Reply With Quote
Old 23rd April 2018, 02:45 PM   #3
alex5612 is offline alex5612  Austria
diyAudio Member
 
Join Date: Jan 2009
Location: Wien
Quote:
Originally Posted by nigelwright7557 View Post
2/ The resistor between driver and output transistors needs to be right too. I put 10R in series with each output transistor base.
You do have a point there. Stepping the output transistor base resistors does make a significant difference here. Didn't expect the difference to be that large to be honest.
Attached Images
File Type: png screenshot endstufenbeschaltung3.png (37.0 KB, 268 views)
Attached Files
File Type: asc 130v test-53-ausgangsstufe alleine-export - Kopie.asc (23.7 KB, 4 views)
  Reply With Quote
Old 23rd April 2018, 03:02 PM   #4
vzaichenko is offline vzaichenko  Russian Federation
diyAudio Member
 
vzaichenko's Avatar
 
Join Date: Jan 2010
Location: Moscow, Russia
Instability with totem pole output stage
One more consideration, if I may - with such a big number of the output pairs (10 pairs in each "barrel"), considering the sum of the base currents and total capacitance - I would either use something like njw0281/0302 (or even 3281/1302) for the drivers, or split the output pairs into two groups, 5 pairs each, having the separate drivers for each group.
This may also increase the "speed" of the OPS, positively influencing the overall stability.
__________________
If you can't explain it simply, you don't understand it well enough (c) Albert Einstein
http://vzaudio.com/
  Reply With Quote
Old 23rd April 2018, 03:42 PM   #5
radtech is offline radtech  United States
diyAudio Member
 
Join Date: Dec 2006
Something is off with the biasing.... getting a lot of DC on the output.
Try using two 1.2V sources in series instead of the 2.4V, and connect your signal to the center of them.
The signal source acts like a DC short and is keeping the top of the 2.4V source near ground potential.
  Reply With Quote
Old 23rd April 2018, 04:15 PM   #6
alex5612 is offline alex5612  Austria
diyAudio Member
 
Join Date: Jan 2009
Location: Wien
Corrected that.

@vzaichenko:

True. I'll consider that later on. Right now the drivers are very likely outside the SOAR for Tc=100
Attached Files
File Type: asc 130v test-53-ausgangsstufe alleine-export2.asc (23.8 KB, 8 views)
  Reply With Quote
Old 23rd April 2018, 04:19 PM   #7
vzaichenko is offline vzaichenko  Russian Federation
diyAudio Member
 
vzaichenko's Avatar
 
Join Date: Jan 2010
Location: Moscow, Russia
Instability with totem pole output stage
Quote:
Originally Posted by alex5612 View Post
True. I'll consider that later on. Right now the drivers are very likely outside the SOAR for Tc=100
Exactly
__________________
If you can't explain it simply, you don't understand it well enough (c) Albert Einstein
http://vzaudio.com/
  Reply With Quote
Old 24th April 2018, 02:57 AM   #8
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: Maine USA
115V+115V makes 322V total so over 100Vrms.

100V in 8 Ohms is 1,250 Watts. That appears to be a heavy load on 1,600VA transformers. We usually like 2X or 2500VA. Granted that there is no speaker which will comfortably take all of a 1250W amp long enough to warm-up the transformer, so this may be OK.

My rule of comfort for plastic transistors is at-most 50W output per pair. 40W may be safer. 1250/50 suggests 25 pair, 32 pair may be safer. You have 20 pair. I'd want more, but it should probably survive.

If changing the base resistor *radically* changes the frequency response (as it seems to do), then the amp is VERY load-sensitive and will probably go crazy on a real loudspeaker load.

Your peak output current is 20 Amps. While a triple Darlington has huge current gain, with base shunt resistors and all it may need 2mA peak into the array. This is a heavy load on any VAS stage.

The less-insane path may be to make single +160V DC to power cap-coupled amplifiers, four channels of 300 Watts. Or combine to a Bridge for 1200W (though with +80V DC on the speaker terminals!).
  Reply With Quote
Old 25th April 2018, 03:58 AM   #9
wg_ski is offline wg_ski  United States
diyAudio Member
 
Join Date: Oct 2007
Even if you manage to get simulations stable, the real thing will be difficult to stabilize. Parasitic capacitances and inductance which do not show up in your model start to get significant when you parallel that many devices. Don't try to get a crazy amount of open loop gain. Use local feedback in each stage like Leach did, then the overall loop has a chance of being stable. Local instabilities in the OPS will have to be dealt with separately. I only built a 5 pair per barrel and found that base stoppers on everything and zobels for the upper banks were necessary. As well as tinkering with the values of the base to base resistors on the outputs and drivers. Any capacitance to ground at the node between master and slave banks creates a parasitic colpitts oscillator (negative R at the base). You have to de-Q it. Output stages are much more unstable at high vce, regardless of whether stacked on not. When testing on a dim bulb or variac it may be perfectly stable, but fly when given full voltage. If you could do class H it would mitigate that - because vce is never crazy high if the transistor is conducting. Would it be possible to add a pair of 55-0-55 secondaries at half the VA rating? If the window is large enough you can get more copper in. Another possibility is to use the Yamaha EEEngine output stage. It has sliding rails but doesn't require extra taps because it is switch mode (and relatively simple with no critical dead time requirements). Of course you could just buy a 1000 va 55-0-55 and use it with the 115 (two different transformers). Then you could build a pair of amps which would be easier to design/build. You could clone the amp circuit of the Crest Pro 9200 which is proven and runs off +/- 80 and 160.
  Reply With Quote
Old 25th April 2018, 09:55 PM   #10
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
Reached this point in your personal line of investigation of amplification techniques, I recommend learning switching regulators, it can change your view about class AB output stages.

The high transformer voltage is a good starting point for the usage of switching regulators in the rails. This can simplify output stage considerably, just a few pairs of power bipolars, for same power as a few dozens without switching regulator. You can build more amplifiers with same amount of parts.

The switching regulators themselves are halves of a class D amplifier, but these do not need to be very accurate, so this is a platform for learning class D and SMPS.

There are known brands and products using this approach, but the point of DIY is doing it in a different flavor, maybe a more nutritive one, and documenting/sharing the process.
__________________
I use to feel like the small child in The Emperor's New Clothes tale
  Reply With Quote

Reply


Instability with totem pole output stageHide 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
totem pole substitution mosfet54 Class D 1 26th February 2016 02:46 AM
Another Approach to Totem Pole OTL Amplifier ballpencil Tubes / Valves 218 7th September 2015 05:42 AM
Problem with Totem Pole Amp michalakis Solid State 4 25th January 2013 09:16 AM
URL collection of Totem Pole and CSPP (Single Ended related) Solid State Output Stage tiefbassuebertr Solid State 7 10th May 2012 05:02 PM
Output stage instability Katapum Solid State 35 27th December 2005 04:31 PM


New To Site? Need Help?

All times are GMT. The time now is 07:50 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