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Old 27th September 2010, 04:44 PM   #31
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Quote:
Originally Posted by Wavebourn View Post
Both.

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.

http://wavebourn.com/forum/download.php?id=437&f=7

http://wavebourn.com/forum/download.php?id=438&f=7
Thanks for those pictures! I was just going to recycle an old small form factor PC. Now I have a use for it!
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Old 27th September 2010, 08:17 PM   #32
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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!
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Old 28th September 2010, 03:17 AM   #33
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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
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.
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Old 28th September 2010, 12:46 PM   #34
chrish is offline chrish  Australia
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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.
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Old 28th October 2015, 06:00 PM   #35
ghg is offline ghg  Austria
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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
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Old 28th October 2015, 06:04 PM   #36
rayma is online now rayma  United States
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Quote:
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.
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Old 28th October 2015, 06:16 PM   #37
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Quote:
Originally Posted by ghg View Post
...measures 0.3uH @ 1kHz; near the measurement limit.
Any good ?
0.3 H represents 0.02 ohms inductive reactance at 10 kHz.
It is not much compared to 8 ohms.
For me it would be good.
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Old 28th October 2015, 06:56 PM   #38
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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.
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Old 28th October 2015, 08:31 PM   #39
DF96 is offline DF96  England
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Speaker voice coils have some inductance too!

As you say, people get far too excited about resistor inductance in audio circuits.
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Old 29th October 2015, 06:54 PM   #40
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Quote:
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.
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