dummy load

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EWorkshop1708

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Joined 2004
Do any of you listen to the amp while it's pushing the dummy load?

For me, ears are the best for testing the amp.

On my dummy load, I use a 4 ohm coil from an old subwoofer, with a 25W, 150 ohm resistor in series with a speaker, paralleled to the 4 ohm coil, to listen to the amp as I test. They way the speaker isn't too loud when driving the amp at full power.


I also have a resistor 3.3 ohm 15W dummy load as well for lower power testing. Or if I want to test high current, I parallel the resistor with 4 ohm speaker to get under 2 ohms. :D
 
Tarzan

Tarzan

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Joined 2004
To Brett:
If you can use a dummy load that keeps it's resistance value over time, then you can do all your power measurements under the same load conditions. So you can compare the measured results against each other and that even after a period of time or a different amp.

That can not be said if you start to add coils, caps etc...
Unless you use very stable and high quality components.
But then again you are leaving the DIY erea and are earning a lot of money with your repairs or designs.

A good and stable resistive and non inductive power load can be made for not so much money and an accuracy great enough for domestic use. Even for semi pro use...

Tarzan
 
Brett

Brett

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Joined 2002
Tarzan said:
To Brett:
If you can use a dummy load that keeps it's resistance value over time, then you can do all your power measurements under the same load conditions. So you can compare the measured results against each other and that even after a period of time or a different amp.
Click to expand...
Decent quality WW resistors don't change over time, nor do they have a lot of inductance, and if you cool them adequately, the resistance values should not vary much.
Tarzan said:
A good and stable resistive and non inductive power load can be made for not so much money and an accuracy great enough for domestic use. Even for semi pro use...
Click to expand...
Exactly my point. Nothing fancy is required for non-lab use.
Cheap, free ideas such as secondhand heater elements etc are perfect for the average hobbyist. I don't want people without a need for reference grade measurements to spend money on a test rig when the cheap options will be more than adequate.
 
Tarzan

Tarzan

Member
Joined 2004
Originally posted by Brett:
Decent quality WW resistors don't change over time, nor do they have a lot of inductance, and if you cool them adequately, the resistance values should not vary much.

Response:
That's what I'me using right now for over a decade...


Originally posted by Brett:
Exactly my point. Nothing fancy is required for non-lab use.
Cheap, free ideas such as secondhand heater elements etc are perfect for the average hobbyist. I don't want people without a need for reference grade measurements to spend money on a test rig when the cheap options will be more than adequate.

Response:
Allthough I would suggest to use a "standard" value of 4 or 8 ohms.
Not that it's absolutely nesecary; 6.35 Ohms will do the trick to.
Only the calculations are a bit more cumbersom.
OK, some can suggest to use 10 Ohms; that make's it even easier.
Well; I have been tought to use a resistor of 8 ohms, long time ago, at school. (1968...)
Kept myself at that value. Only the power went up; from 20 to 1000 Watt's Wirewound and air cooled.
I love to repair these big pa amps in the winter.
It keeps my workshop warm...
Cheers,
Tarzan
 
Nico Ras

Nico Ras

Member
Joined 2005
I stripped an old toaster and marked the point for 8 ohms and 4 ohms. It is wound around a sheet of mica and I can put a fan on it if I don't want to make toast. It works well, resistance would change with temperature though, but you can put it in a bucket of water. Costs virtually nothing.
 
Zero Cool

Zero Cool

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Joined 2004
Brett said:

In a lab, this is important. But how relevant is it to a DIYer or someone repairing amps? Hint: none.
Click to expand...


One of the test's i need to do when repairing QSC power amps is to do a full power into a 2 ohm load and check for symmetrical clipping!

Some of these amps are in the 2-3Kw range per channel at 2 ohms.

I have a large heatsink with 100 watt Dale Aluminum house resistors on it with binding post's that allow me to configure various resistances and power levels. But at 2 ohms, the resistors only total 800 watts.

I have pumped close to 2.5KW into them for a few seconds at a time while quickly making this adjustment without problems. BUT i do worry about the gross overload of those resistors. Each time i check the temp and while they do get warm, they have never been hot to the touch. never even as warm as say a mug of coffee on the outside.

So the temp is not an issue. i worry more about the current through each resistor and that i might one day damage those resistors.

I need to build a much larger bank for such test's.
 
Brett

Brett

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Joined 2002
No disagreement with everything else I edited from your post (just to keep it brief). My contention has simply been that NI resistors are not neccessary for the hobbyist. Use them if you can get then cheap, but don't fret it. Unless it's a requirement of QSC to use them, I don't see what difference it would make in your testing and calibration process either.

Zero Cool said:
One of the test's i need to do when repairing QSC power amps is to do a full power into a 2 ohm load and check for symmetrical clipping!

Some of these amps are in the 2-3Kw range per channel at 2 ohms.
Click to expand...
I like QSC amps a lot, but they're too expensive down here.

Cheers
 
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