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 Post subject: Re: Dummy load for audio amp, non inductive resistor necessa
PostPosted: Feb Fri 14, 2020 2:12 pm 
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I use these as a 8 ohm dummy load when needed for high output amplifiers. They are rated 120 watts.

Sal


Attachments:
8 Ohm Dummy Load.jpg
8 Ohm Dummy Load.jpg [ 80.7 KiB | Viewed 148 times ]

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 Post subject: Re: Dummy load for audio amp, non inductive resistor necessa
PostPosted: Feb Fri 14, 2020 5:59 pm 
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khutch wrote:
..

Attachment:
SpeakerZ.jpg


..


Notice how flat everything is up to, and including 1kHz? That provides for some awesome test results at 1 kHz. Almost as if it was by design. :wink:

As for the impact of dummy load inductance (ignoring distributed capacitance), X_L = 2*pi*F*L. I see no real need for carborundum resistors at audio frequencies. :roll:

BTW: those aluminum-cased wire-wound resistors will come apart with extreme force if suddenly overheated. It's sort of like sitting next to a shotgun loaded with slugs. :shock:

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 Post subject: Re: Dummy load for audio amp, non inductive resistor necessa
PostPosted: Feb Fri 14, 2020 6:19 pm 
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Great stuff everyone. Nice to see my hunches confirmed by graphs for the most part.

I am still going to order a few non inductive loads to experiment with, but mainly because I need a second set anyway. I'll post results when and if I ever something scientifically evaluated.

real life keeps getting in the way

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 Post subject: Re: Dummy load for audio amp, non inductive resistor necessa
PostPosted: Feb Fri 14, 2020 9:17 pm 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
The dummy should have no inductance.

Consider 2 *pi * 15,000 * L

94,200 ohms per Henry .

4 ohms R, in series with 4 ohms Xl , = 5.7 ohms Z

At 15,000 Hz , 4 / 94,200 = 42.5 microHenry

So what ?


When an amplifier is rated in total harmonic distortion, (THD)
the quoted % value is determined over a frequency response at a
RATED power output level.

If it is from 50 to 15000 Hz, the load impedance must be 4 ohms
over the range. If it rises to 5.7 ohms at 15,000 Hz. , the result is crap.


Arguments? Did I blow the math ?
Attachment:
ni i.jpg
ni i.jpg [ 252.64 KiB | Viewed 129 times ]
Attachment:
ni ii.jpg
ni ii.jpg [ 88.11 KiB | Viewed 129 times ]

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 Post subject: Re: Dummy load for audio amp, non inductive resistor necessa
PostPosted: Feb Sat 15, 2020 7:24 pm 
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Location: Houston, TX
edit: quoted wrong user
mescalero wrote:
BTW: those aluminum-cased wire-wound resistors will come apart with extreme force if suddenly overheated. It's sort of like sitting next to a shotgun loaded with slugs.

As should be the case with any properly-designed load box, the aforementioned Sencore models also have shut down circuits to protect the loads from over temps, in addition to the fans. Some people like to rag on Sencore, but these units are nice.


Last edited by Vin Tageman on Feb Sat 15, 2020 8:33 pm, edited 1 time in total.

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 Post subject: Re: Dummy load for audio amp, non inductive resistor necessa
PostPosted: Feb Sat 15, 2020 8:03 pm 
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Posts: 13353
Location: Omak,wa,usa
Hello Vin,
yes they are nice units .
how I built mine non inductive load was to take a bunch of 8ohm 20watt non inductive loads and Parallel series a bunch these till got the wattage I wanted bought them at radio shack in the day miss having that store around thou .
Sincerely Rich


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 Post subject: Re: Dummy load for audio amp, non inductive resistor necessa
PostPosted: Feb Tue 18, 2020 1:31 pm 
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Location: Now outside Dallas, TX - with bobcats and coyotes and more! Oh my!
"The dummy should have no inductance."

I'm now working in picoseconds and GHz. Trust me, "no inductance" ain't never gonna happen. Perhaps a better approach would be to consider the impact of what inductance is there.

When testing audio amps for a little old $5B/yr company (now defunct), THD was rated at 1kHz. Where a few mH would become an issue for us might be in IMD and, certainly, in frequency response. We used "non-inductive" resistors because of the work that we did. If you're not doing lab-grade analysis, but just loading the amp to look for anomalies without making a ruckus, then don't get too concerned.

Remember, Alan measured ~38uH, not 38mH.

That's my opinion, anyway.

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 Post subject: Re: Dummy load for audio amp, non inductive resistor necessa
PostPosted: Feb Tue 18, 2020 4:43 pm 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
Here is a test on a 4 ohm power resistor. It could add 5.6 ohms Xl in series
with 4 ohms R . But at 250 kHz, the top freq spec for my Heath audio watt meter.

The Z would rise to 6.7 ohms

I burned out the 4 ohm non inductive resistor ages ago.
Attachment:
Four ohm resistor inductance.jpg
Four ohm resistor inductance.jpg [ 185.81 KiB | Viewed 59 times ]

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 Post subject: Re: Dummy load for audio amp, non inductive resistor necessa
PostPosted: Feb Mon 24, 2020 6:02 pm 
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Joined: Feb Sat 22, 2014 1:40 am
Posts: 273
khutch wrote:
Here's a typical speaker chosen at random from here:

https://www.eminence.com/pdf/Alpha_4_4.pdf

Attachment:
The attachment SpeakerZ.jpg is no longer available


A couple hundred uH doesn't seem to be of much concern.

So how is it that the impedance varies so much and therefore the current and power the amp can deliver to the speaker varies so much over frequency yet the sonic output is reasonably constant? The answer is that the speaker impedance is actually very low. Using this online calculator for the speaker above:

http://www.sengpielaudio.com/calculator-efficiency.htm

I get an efficiency prediction of 0.4%! If the efficiency were very high then the load the amplifier sees would have to be nearly pure resistive at any frequency in the speaker's flat range and it would represent the electrical effect of the acoustic load on the speaker. With this low an efficiency the amplifier barely sees the resistive component from the acoustic load on the speaker. All of the power is going into the voice coil!


To me those are some pretty strange speaker impedance curves.
Here are two one of a sealed box and one that has a tuned port, by tuning you
change the big bump into two smaller bumps to help even out the curve.
Note the change of the impedance values on the left.

Attachment:
Sealed Speaker Impedance Graph.jpg
Sealed Speaker Impedance Graph.jpg [ 29.27 KiB | Viewed 6 times ]


Attachment:
Ported Speaker Impedance Graph.jpg
Ported Speaker Impedance Graph.jpg [ 54.3 KiB | Viewed 6 times ]


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 Post subject: Re: Dummy load for audio amp, non inductive resistor necessa
PostPosted: Feb Mon 24, 2020 6:08 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5999
Location: Sunbury, Ohio 43074
Not that it makes much difference but I wonder what happens to those two speaker impedance graphs as you increase the power
Speakers present such a dynamic and complex impedance...

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