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Old 26th September 2005, 10:17 AM   #1
leander is offline leander  Malta
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Default replicating a 4 ohm load

I want to test my amp at full power before connecting a subwoofer to it

How can I make a 400w 4 ohm load instead of using a speaker?

Can I coil a piece of tick wire long enough to give a 4ohm reading onto a cylinder ?

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Old 26th September 2005, 01:48 PM   #2
DRC is offline DRC  United Kingdom
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What is TICK wire ?

I assume this is some sort of resistance wire. If so, then you will need to take care over power / temperature rise to dissipate 400W. Maybe a fan or liquid cooling is needed.

I used very low cost / surplus 15ohm/5w WW resistors (64 in total) to make a ~ 300w test load. This needs a large fan to hold a continuous load of 300w
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Old 26th September 2005, 01:54 PM   #3
CBS240 is offline CBS240  United States
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I have used a resistor(s) placed in water for this also. Even boiled some. The water doesn't seem to affect the resistance at 4 Ohms. Just don't add any salt You should also try to match the reactance of the speaker you are "simulating" as this is an important variable to the amp's operation.
All the trouble I've ever been in started out as fun......
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Old 26th September 2005, 02:58 PM   #4
djk is online now djk
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HeathKit used to sell a 1KW load that was 25pcs of a 2W resistor (25pcs of 100 ohm would give you a 4 ohm load) bussed together in a 5X5 grid and immersed in a 1 gallon can of vegetable oil.

When the can reaches 100*C (this takes a very long time, longer than you would be testing the amplifier) you need to let it cool off (which will also take a long time).
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Old 26th September 2005, 03:30 PM   #5
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Default use oscilloscope

if u have access to a power oscilloscope then use it

it will give u a true image of amp o/p as well as serve your purpose
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Old 26th September 2005, 03:54 PM   #6
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Try iron parallel with a kettle.

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Old 26th September 2005, 06:50 PM   #7
Tim__x is offline Tim__x  Canada
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That's actually not a bad suggestion! 3600W of low temp co appliances in parallel.
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Old 26th September 2005, 07:03 PM   #8
tlf9999 is offline tlf9999  United States
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get a heating element from your home depot (or the equivalent of it). a 110v 3kw one will do fine.

tempco could be a problem but should be minor.
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Old 26th September 2005, 08:46 PM   #9
anatech is offline anatech  Canada
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I don't have any imagination. I use the 250W 8R Dale resistors on a big heatsink. Two in parallel for 4R loads. As I recall, they made a 225W 4R resistor in a tubular format. I have a set of those as well. Much cheaper than the 250W jobs.
I remember it cost me $50.00 each to have heatsinks milled flat to mount them on in the late eighties.
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Old 26th September 2005, 08:47 PM   #10
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Tempco is non-trivial. Deriving the cold resistance of a heating element from its power consumption when hot could lead you to guess quite wrong. I strongly suggest measuring the resistance with a DVM when the element is cold, and when it's warm.

In other news, a class-AB amplifier is under most thermal stress at about 40% power, and not at full output, since at 40% the output devices are standing off more of the supply voltage than at max out. If you want to test that, be VERY sure to keep an eye on the amp at all times: I came close to burning up a Carver power amp after a 20 minute run at 40%. It sure did smell funny.

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