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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


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8 Ohm Dummy Load.jpg
8 Ohm Dummy Load.jpg [ 80.7 KiB | Viewed 1055 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 ?
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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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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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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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"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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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 966 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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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 913 times ]


Attachment:
Ported Speaker Impedance Graph.jpg
Ported Speaker Impedance Graph.jpg [ 54.3 KiB | Viewed 913 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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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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 Post subject: Re: Dummy load for audio amp, non inductive resistor necessa
PostPosted: Feb Mon 24, 2020 8:54 pm 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
The effects of power were investigated about 65 years ago for ported
box systems.
If you could find a copy of the book stuff would be found.
Interlibrary.
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However the importance hinges on transistor development.

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 Post subject: Re: Dummy load for audio amp, non inductive resistor necessa
PostPosted: Feb Tue 25, 2020 1:57 am 
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Fascinating stuff, speaker design. I had a brush with it once upon a time, a long long time ago, when I was thinking about building my own. Fortunately, sanity set in, and all I had to decide was ported vs sealed. I settled for AR2AX, later AR3. I wish I'd kept them. Again.

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 Post subject: Re: Dummy load for audio amp, non inductive resistor necessa
PostPosted: Feb Wed 26, 2020 11:05 pm 
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Joined: Mar Thu 29, 2012 8:51 pm
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Location: upper midwest
Quote:
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.


Yes they do!!

About 14-15yrs ago at work I was in the lab working on a prototype 3 phase motor drive. There was a current spike being drawn from the 50v/150A DC supply I was trying to figure out. I put one of those gold resistors in series with the supply to limit inrush current as part of my troubleshooting.

Forgot to take it out when I moved on and it was hanging in the harness coming off the rack mounted power supply and was behind me while I was sitting on a stool working on the proto. I started up the motor and of course that caused the drive to pull a lot of current. Resistor went off like a gun shot right behind me and about caused my heart to leap out of my chest!

The terminal on one end had come popping out of the housing and was hanging off a couple feet of unspooled wire. Those are wirewound around a ceramic rod and the terminal ends with plastic bushings are pressed in. I can only assume it was so hot that air inside built up enough pressure to blow out the end cap.


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 Post subject: Re: Dummy load for audio amp, non inductive resistor necessa
PostPosted: Mar Sun 01, 2020 1:29 am 
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Joined: Jan Sat 16, 2016 4:15 pm
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This topic raised a couple questions for me.

I have a few of those Dale high power resistors - 250 Watt 9 Ohm. I thought they would be perfect for amplifier testing with some parallel resistance to result in 8 Ohms, but they are the inductive RH series. The NH non-inductive ones would be best. I wondered just how much inductance they actually have so I tested one with my Extech LCR200 meter which has test frequencies up to 100 kHz.

The one I tested measured 4.18 micro-Henrys which equates to 2.6 Ohms at 100 kHz or 0.525 Ohms at 20 kHz which is the top of the range usually used for testing amplifiers. They would probably be OK for non-critical work.

The other thing I wondered about is the speaker impedance discussion. qmavam commented on the graph posted by khutch and I agree that it looks strange. The ones qmavam posted look like the ones you usually see.

"Speaker" actually seems to have two meanings: the sort of thing one buys for a stereo or a part such as the speaker in a radio. Maybe the technically correct term is "driver", but I think the common usage is just "speaker".

The curves qmavam posted are for the former type of speaker - an enclosure with one or more drivers. The web site for the speaker khutch mentioned also has a similar looking curve.

I was curious about the impedance of the sort of speaker used in old radios vs. newer ones so I dug around and found a few to look at.

The first one I tested is a classic looking 3.5" unit labeled "Zenith 49-1052" It's also labeled 3.2 Ohms. It has a DC resistance of 3.2 Ohms and the impedance curve is pretty flat with no apparent resonance peak.

The second one I tested is a newer style 3" driver out of a computer speaker. It has a foam surround and a large magnet. It's labeled "Altec Lansing 44549 - 4 Ohms" and measures 4 Ohms DC resistance. It's impedance curve is not flat but does show a resonance peak.

I guess that would suggest replacing the speaker in an old tube type radio with a newer driver might not be a good idea.


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Zenith 49-1052 800.png
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Altec Lansing 44549 800.png
Altec Lansing 44549 800.png [ 33.28 KiB | Viewed 848 times ]
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