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

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Testing Amplifiers - Speaker or dummy load
Testing Amplifiers - Speaker or dummy load
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 27th September 2010, 04:44 PM   #31
porkchop61 is offline porkchop61  United States
diyAudio Member
porkchop61's Avatar
Join Date: Aug 2004
Location: Mansfield, Connecticut
Originally Posted by Wavebourn View Post

A dummy load as an attenuator for a speaker.

I have a lot of 20 Ohm resistors. If to connect 5 of them in parallel it is 4 Ohm in total, for higher power. If to add computer fans for forced cooling they will dissipate even more power without a problem.


Thanks for those pictures! I was just going to recycle an old small form factor PC. Now I have a use for it!
"Imagination is more important than knowledge"
  Reply With Quote
Old 27th September 2010, 08:17 PM   #32
Globulator is offline Globulator  United Kingdom
diyAudio Member
Globulator's Avatar
Join Date: Aug 2004
Location: Cambridge, England.
If you use NFB over more than one stage this is an interesting experiment to do:

1) Create an A/B switch so you can switch between speaker and resistive load.
2) Connect a scope probe to a signal within the feedback loop
3) Setup a square wave (500Hz etc - to suit)

Then look at the wave shape change as you switch between the resistor and speaker!
  Reply With Quote
Old 28th September 2010, 03:17 AM   #33
boywonder is offline boywonder  United States
diyAudio Member
boywonder's Avatar
Join Date: Jan 2008
Location: So.Cal.
Originally Posted by chrish View Post
Along the lines of this discussion, I have tried to source some reasonably priced high wattage 8R resistors, but my usual sources are always out of stock. I do have a pair of 6R 50W that I have been using. Is this going to induce big errors in my testing?

Chris: At the risk of stating the obvious, if your output transformers have 4 ohm or 8 ohm taps, then a 6R load will not reflect the proper load to the output tubes, it'll either be a little high or a little low, depending on the tap. Pretty much like a loudspeaker...

So your theoretical load line/bias point and what you measure on the scope are going to be a little different.....and you've got to divide by 6 instead of 4 or 8 to determine output power.
  Reply With Quote
Old 28th September 2010, 12:46 PM   #34
chrish is offline chrish  Australia
diyAudio Member
chrish's Avatar
Join Date: Oct 2003
Location: Sydney
Since that post I managed to get some 8R 50 Watt resistors. I mounted each on a piece of heatsink I parallel them for 4R.
  Reply With Quote
Old 28th October 2015, 06:00 PM   #35
ghg is offline ghg  Austria
diyAudio Member
ghg's Avatar
Join Date: Apr 2002
Location: Vienna
Well, have to restart this thread ...

Did one actually measure the inductance of these so called " non inductive resistors" ?

I put four of the common yellow 50W Dale's in series to get 8 Ohm and my cheap LCR meter ( ELC-131D )
measures 0.3uH @ 1kHz; near the measurement limit.

Any good ?

TX Gary
I hate "sounding amps", except these are Marshalls, driven by Eric C. or Jeff B., period.
  Reply With Quote
Old 28th October 2015, 06:04 PM   #36
rayma is offline rayma  United States
diyAudio Member
Join Date: Apr 2011
Originally Posted by ghg View Post
Did one actually measure the inductance of these so called " non inductive resistors" ?
Roughly speaking, the least inductance you will get in standard components
is around 10nH per cm of overall length. The 75nH Dales should be fine, though.

Last edited by rayma; 28th October 2015 at 06:10 PM.
  Reply With Quote
Old 28th October 2015, 06:16 PM   #37
artosalo is offline artosalo  Finland
diyAudio Member
artosalo's Avatar
Join Date: Feb 2010
Originally Posted by ghg View Post
...measures 0.3uH @ 1kHz; near the measurement limit.
Any good ?
0.3 ÁH represents 0.02 ohms inductive reactance at 10 kHz.
It is not much compared to 8 ohms.
For me it would be good.
  Reply With Quote
Old 28th October 2015, 06:56 PM   #38
Johan Potgieter is offline Johan Potgieter  South Africa
diyAudio Member
Join Date: May 2005
Location: Pretoria, South Africa
As Artosalo said. That could be even less than the inductance of some leads/connecting-wires in a build.

For newbies: I generally find an overrated concern in audio with inductance etc., qualitatively mentioned instead of putting some figure to the same. In decades of practical experience I never had a problem with inductance, di-electric material, hookup wire resistance and such in audio circuits. Yes, these could have an effect in critically designed circuits, but such effects come in only at frequencies quite outside the audio spectrum and should have been taken care of by the designer to begin with.
  Reply With Quote
Old 28th October 2015, 08:31 PM   #39
DF96 is offline DF96  England
diyAudio Member
Join Date: May 2007
Speaker voice coils have some inductance too!

As you say, people get far too excited about resistor inductance in audio circuits.
  Reply With Quote
Old 29th October 2015, 06:54 PM   #40
Miles Prower is offline Miles Prower  United States
diyAudio Member
Miles Prower's Avatar
Join Date: May 2005
Location: USA
Originally Posted by Rob11966 View Post
Specifically, will the trace be identical for the same test signal sine wave?
No, it won't. A resistive dummy load is a pure resistance at audio frequencies for all practical purposes. AC into a pure resistance introduces no phase shift, but reactive loads do. Being a synchronous AC motor, a speaker won't always have a 100% power factor. Another difference is that an AC motor is designed for, and operates at, one specific frequency. Add in that your speaker needs to operate across a wide frequency band, and impedance can wary widely in both magnitude and phase.

You should always power up a new project for the first time -- especially if you include gNFB -- into a resistor. If Mr Murphy has wired your feedback positive rater than negative, you've just built yourself a high powered Royer oscillator. I had two projects that took off oscillating at ~20Hz because of that which could bring a 30W, 8R test resistor to red heat in under a minute if I let it run that long. That would blow your speeks, not to mention your ears, if you didn't use a dummy load.

Even if you get it under control, your neighbors are likely to complain if you run full power, 1.0KHz tests into your speeks. You can still learn quite a lot with a nice, resistive test load. If you're not stable into a test resistor, then you certainly will have major problems when you connect the speeks.
There are no foxes in atheistholes
  Reply With Quote


Testing Amplifiers - Speaker or dummy loadHide 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
Power load, dummy load (pic) luka Power Supplies 43 9th February 2012 02:50 PM
Tri Dummy Load raudio1969 Parts 0 25th February 2009 01:14 AM
A simple active DC dummy load for PSU testing Bootstrapper Power Supplies 42 17th August 2008 08:57 PM
dummy load reddish75 Solid State 72 3rd April 2008 01:15 AM
Dummy - load JensRasmussen Solid State 24 23rd February 2007 01:04 PM

New To Site? Need Help?

All times are GMT. The time now is 11:16 AM.

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