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.

Amplifier Design for Fractional Ohm Loads
Amplifier Design for Fractional Ohm Loads
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
Thread Tools Search this Thread
Old 9th February 2010, 06:09 PM   #1
SlunkBoy is offline SlunkBoy  United States
diyAudio Member
Join Date: Jan 2007
Default Amplifier Design for Fractional Ohm Loads

I recently cobbled together a plain vanilla Class AB amplifier with an opamp front end and a push-pull Darlington output stage in order to drive a fractional ohm resistive load. The opamp is a TL082 and the output pair is comprised of a TIP120 and a TIP125. The supply rails are +/- 8 volts with about 20,000 uF of filtering on each rail.

I understand that circuit resistances that can be ignored under normal conditions become significant when driving very low resistance loads and I suspect I am losing a considerable amount of power in the various leads connecting the power supply to the transistors and subsequently to the load and back to ground.

Given these conditions, I am still curious as to why the amplifier can only manage output swings of about 1.5 volts before clipping. Very little heat is being dissipated by the amplifier and the Darlingtons should be capable of 8 amp pulses. While I am not able to accurately measure the resistance of the load since it is so low, the .1875 ohms suggested by dividing the 1.5 volt peak output swing by the 8 amp collector current limit of the output transistors seems reasonable.

While I have tried paralleling the output transistors, the necessary inclusion of current sharing resistors (.47 ohm was the only handy value) has made for a difficult comparison.

Does it stand to reason that the collector current limit of the output transistors is causing the clipping?

Are hardier transistors and thicker, more direct leads the solution?

Any insight would be much appreciated.

  Reply With Quote
Old 9th February 2010, 06:29 PM   #2
Mooly is offline Mooly  United Kingdom
diyAudio Moderator
Mooly's Avatar
Join Date: Sep 2007
Amplifier Design for Fractional Ohm Loads
Darlingtons suffer from a 2 vbe volt drop and at high current low supply voltage that's significant. So you could lose 1.5 v swing or more there.
Would have to see a circuit
Driving low impedance loads is fraught with difficulty... the transistors will exhibit severe gain droop at those levels you talk of. I suspect you need far higher drive currents than the TL082 an provide and multiple parallel pairs of outputs.
You need to scope along the wiring to see if you have power losses... or use a DVM and drive the amp at DC and measure volt drops along the wiring etc (ohms law) but make sure the heatsinking etc is up to the job.
  Reply With Quote
Old 9th February 2010, 06:31 PM   #3
event horizon is offline event horizon  England
diyAudio Member
event horizon's Avatar
Join Date: Feb 2008
Location: East Midlands, England
I think you need to seriously think about increasing the amount of storage capacitance

Here is a nice simple way of working out the ripple voltage.

From what i can see, that is likely to be the culprit. In other words the voltage is sagging so much that you end up with damn near no output.

You are also likely to blow those transistors if you expect each one to carry more than the maximum current rating. Be careful as if you increase the capacitance you'll have the capability of doing that.

This is all of course assuming your transformer can supply enough current.
"Never let your morals prevent you from doing what is right!" Salvor Hardin
  Reply With Quote
Old 9th February 2010, 07:01 PM   #4
SlunkBoy is offline SlunkBoy  United States
diyAudio Member
Join Date: Jan 2007
Mooly and event horizon,

Thank you both for your replies. I allowed for the opamp and biasing losses in the design and the same amplifier driving a 8 ohm load is capable of reasonable voltage swings without clipping.

From what I've been able to measure with a voltmeter, the power supply seems solid, though I suppose it could be suffering from instantaneous sag. I'll hook the scope up to the rails tonight and investigate.

The gain droop idea seems to be the most promising avenue to pursue. I had been assuming a gain of about 2,000 for the Darlingtons, which the TL082 should be able to drive, but I hadn't taken into account the fact that that gain drops at higher currents. My design currently has the opamp driving the output transistors directly, but I'll incorporate a drive transistor tonight and see if that improves things.

Thanks again for the great suggestions.

  Reply With Quote


Amplifier Design for Fractional Ohm LoadsHide 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
good ESL amplifier that works with extreme capacitive loads thadman Solid State 2 31st January 2007 11:20 PM
Wanted: DIY amplifier builder with experience. I need a 0.7 ohm stable amplifier Audiophilenoob Swap Meet 34 16th May 2005 11:58 PM
GainClones & 4 Ohm Loads SRMcGee Chip Amps 5 9th August 2004 11:56 PM
using 2 ohm loads on leach amp Wagener Solid State 4 1st April 2004 07:02 AM
Need something stable into <8 ohm loads Jean Solid State 7 25th September 2003 11:05 AM

New To Site? Need Help?

All times are GMT. The time now is 10:31 PM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio