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

Equipment & Tools From test equipment to hand tools

Amplifier Dummy Load for THD measurements / non-inductive?
Amplifier Dummy Load for THD measurements / non-inductive?
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 15th October 2017, 07:32 AM   #21
trobbins is offline trobbins  Australia
diyAudio Member
 
trobbins's Avatar
 
Join Date: Feb 2009
Location: Melbourne, Oz
Quote:
Originally Posted by Zymorg View Post
The overshoot is not caused by the Pico, just tried it with a metal film resistor as you proposed. No ringing at all. But the Visaton resistor shows ringing, so not complety free of inductivity.
Ta, that makes sense.
  Reply With Quote
Old 15th October 2017, 08:02 AM   #22
1audio is offline 1audio  United States
diyAudio Member
 
Join Date: Mar 2004
Location: SF Bay Area
Amplifier Dummy Load for THD measurements / non-inductive?
I have experimented with various film type power resistors. My experience is that you need to derate them. They do not handle transients well. It was an expensive mistake.
I used a precision ESI videobridge to quantify the cheap wirewound resistors.

If you want to buffer the Pico output the question is linear or a fast switch. Two powerfets make a great very fast power switch for fast transients. Otherwise I would look at a tpa6120a2 since it has power and bandwidth beyond what you need. You can get the demo board to make implementation quick.
__________________
Demian Martin
Product Design Services
  Reply With Quote
Old 15th October 2017, 09:16 AM   #23
Zymorg is offline Zymorg  Germany
diyAudio Member
 
Join Date: May 2016
Location: Hesse
Amplifier Dummy Load for THD measurements / non-inductive?
For the buffer my current favorite is this one: BUF634

Should do a good job, what do you think. Performance is better than tpa6120a2 for this use case I would say.
  Reply With Quote
Old 15th October 2017, 10:37 AM   #24
1audio is offline 1audio  United States
diyAudio Member
 
Join Date: Mar 2004
Location: SF Bay Area
Amplifier Dummy Load for THD measurements / non-inductive?
The buf634 is very different from the tpa6120a2. The buf is meant to be inside the feedback loop of another opamp. The tpa is actually a current feedback amp for driving dsl lines. The tpa can deliver more current at lower distortion to very high frequencies. It can be hard to use. That's why the demo board is nice. Remember you aren't listening to it. The buf would need an opamp to do similar and you would need to deal with stability issues getting the upper bandwidth from it.
__________________
Demian Martin
Product Design Services
  Reply With Quote
Old 15th October 2017, 10:58 AM   #25
Zymorg is offline Zymorg  Germany
diyAudio Member
 
Join Date: May 2016
Location: Hesse
Amplifier Dummy Load for THD measurements / non-inductive?
Iven if the tpa6120a2 would be the mor suitable device, the package would require a PCB and the demo board is too expensive.

The BUF634 I can just put on perfboard and the TO220 has the best thermal performance.

BUF634 can be used open loop according to the application node. The open loop gain is 1 (almost). So following all the design rules there, that should do it, don't you think? Lowest THD is not a requirement, at least I don't see a use case for that.
  Reply With Quote
Old 15th October 2017, 11:17 AM   #26
1audio is offline 1audio  United States
diyAudio Member
 
Join Date: Mar 2004
Location: SF Bay Area
Amplifier Dummy Load for THD measurements / non-inductive?
If you are using the buf open loop its pretty simple. You may need a ground plane and good bypassing to get the performance. I would airplane over some copperclad.

I don't know what the performance of the Picoscopes arb is. Its probably better than 1% but in many applications that's not critical.

Breadboarding the tpa is not an option. However I found an assembled board for 20 euros on eBay. You are right he demo board is too expensive to buy.
__________________
Demian Martin
Product Design Services
  Reply With Quote
Old 15th October 2017, 11:21 AM   #27
Burnedfingers is offline Burnedfingers  United States
diyAudio Member
 
Join Date: Nov 2002
UXP600-8RK - 8-Ohm 600W Ultra High Power Resistor (EBG) at The Electrostore.com - Electronic Surplus Parts & Equipment

I used this type for my dummy load. Mine are 4 ohm instead of 8 ohm and require a heat sink in which the resistor can sit flat on with 100% contact.
I have used a bank of these in a series /parallel config to test large power amps.

I was able to purchase some surplus UXP/600/9 4RJ the 4 ohm version.

My large load consists of 8 of these resistors mounted to an old JBL heat sink which is cooled by 3) 5 1/4" fans.
  Reply With Quote
Old 15th October 2017, 11:27 AM   #28
Burnedfingers is offline Burnedfingers  United States
diyAudio Member
 
Join Date: Nov 2002
Several dusty pictures of my dummy load.
Attached Images
File Type: jpg DSC04681.JPG (157.8 KB, 98 views)
File Type: jpg DSC04682.JPG (140.7 KB, 100 views)
  Reply With Quote
Old 15th October 2017, 11:58 AM   #29
Zymorg is offline Zymorg  Germany
diyAudio Member
 
Join Date: May 2016
Location: Hesse
Amplifier Dummy Load for THD measurements / non-inductive?
Good hint looking for assembled boards on ebay, there is one with even two BUF634 for around 15$. A single device from germany would cost almost 15$ alone. Shipping from China, but for this price I am willing to wait a few weeks.

Link
  Reply With Quote
Old 15th October 2017, 12:18 PM   #30
BlacK_Chicken is offline BlacK_Chicken  Germany
diyAudio Member
 
Join Date: Nov 2005
I use a self-made, dc-coupled tpa6120a2 amplifier with +20dB gain for my PicoScope AWG.

Attached, the open-load frequency response (at1Vpp) of the device. Phaseresponse limits to ~100-200kHz, depending what you are up to.
Attached Images
File Type: png 20170512_freq_open.png (53.5 KB, 89 views)
  Reply With Quote

Reply


Amplifier Dummy Load for THD measurements / non-inductive?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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Adding a dummy load to prevent amp from being damaged by having no load? Joku Tubes / Valves 6 12th May 2015 07:27 AM
DDDumy Load, a versatile power dummy load tool dddac Equipment & Tools 6 14th January 2014 12:36 AM
Testing PKN XD2500 ClassD Amplifier On DUMMY Load. DjLeco Class D 9 22nd January 2013 05:07 PM
Amplifier Dummy Load Nichrome Wire Brycebw Solid State 13 25th March 2012 05:15 PM
Power load, dummy load (pic) luka Power Supplies 43 9th February 2012 02:50 PM


New To Site? Need Help?

All times are GMT. The time now is 02:52 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