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My Way Over-Engineered 8 Ohm Dummy Load!
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Smoky:
During my last stereo repair project I needed an 8 Ohm dummy load for testing the output. Since I plan to work on tube amplifiers and vintage receivers in the 20-50 Watt output range, I bought two Riedon 50 Watt 8 Ohm chassis mount wirewounds.

I come to find out that non-inductive resistors should be used for audio, but what I bought, are not the non-inductive type.

Since I had already mounted the wirewound resistors, I connected them to my LC102 meter, and they measure 1.02 and 1.04 microhenries respectively.

Would it be best to find a non-inductive replacement or will these Riedon resistors work Ok? They appear to but I'm not really sure what the added inductance affects.

I did find these Caddock MP9100 non-inductive TO-247 resistors that I think might work since I already drilled and tapped several #6-32 threads into the base of this four pound Apple Computer heat sink :)

https://www.digikey.com/product-detail/en/caddock-electronics-inc/MP9100-8-00-1/MP9100-8-00F-ND/3013579





I did continue with the dummy load project...
Smoky:
I started with the base and a piece of aluminum angle to mount a few speaker terminals and the resistors...





The terminals work well since the banana to BNC cables to the oscilloscope and the speaker wires can be mounted simultaneously...





I added a 12VDC plug too so to power two small Sunon fans :)   btw, they're actually running in this picture!





They have what Sunon calls "Vapo" bearings. The fans were mounted using small SS pin nails cut and polished and pressed into holes drilled in the aluminum:





I made the small cable clamp from an aluminum gutter outlet:





I found out that my old security camera wall-wart fits the plug too:





Here's a shot through the fins. I do feel a good breeze on the opposite side:





And since there is still plenty of space on the base of the heat sink, and I have an extra pair of speaker binding posts, I may add one 4 ohm 100 Watt Caddock Thick-film TO-247 resistor and hang it off of the back?

Anyway, I haven't much time since I work construction this time of year, I wanted to fit this in and be ready for the Fall electronics projects!



oPossum:

--- Quote from: Smoky on May 22, 2020, 01:25:07 am ---Would it be best to find a non-inductive replacement or will these Riedon resistors work Ok? They appear to but I'm not really sure what the added inductance affects.

--- End quote ---

It depends what your use case is. If you are going to be looking at the amp output on a 'scope to see clipping and estimate power then ordinary power resistors are adequate. If you are doing measurements with something like a AP or QA box then you would want as close to a pure resistive load as you can get.
Electro Fan:

--- Quote from: Smoky on May 22, 2020, 01:25:07 am ---During my last stereo repair project I needed an 8 Ohm dummy load for testing the output. Since I plan to work on tube amplifiers and vintage receivers in the 20-50 Watt output range, I bought two Riedon 50 Watt 8 Ohm chassis mount wirewounds.

I come to find out that non-inductive resistors should be used for audio, but what I bought, are not the non-inductive type.

Since I had already mounted the wirewound resistors, I connected them to my LC102 meter, and they measure 1.02 and 1.04 microhenries respectively.

Would it be best to find a non-inductive replacement or will these Riedon resistors work Ok? They appear to but I'm not really sure what the added inductance affects.

I did find these Caddock MP9100 non-inductive TO-247 resistors that I think might work since I already drilled and tapped several #6-32 threads into the base of this four pound Apple Computer heat sink :)

https://www.digikey.com/product-detail/en/caddock-electronics-inc/MP9100-8-00-1/MP9100-8-00F-ND/3013579


(Attachment Link)


I did continue with the dummy load project...

--- End quote ---


Very pretty - you do very nice work!
richnormand:
Nice work :)

This is what I have been using for years on power amps to up to 1000W.
It is a 10 ohm load but you can make an 8, 6, 4, 2 ohm if you like using a jumper with alligator clips. All resistive old type.
Fill the plate with DI water or oil as needed. Just plain air is good to 100W with fan. Price performance ratio....

Real speakers will be resistive, inductive and phase reactive with enclosure air load and resonators and crossovers networks as viewed from the amp.
Just walking in front of the enclosure will change parameters.

All depend on what you are trying to measure with its internal feedback loop (including the wires to the speakers)



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