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Old 7th January 2010, 06:31 PM   #1
Eric is offline Eric  United States
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Default Resistor for Amp output testing

Is this a good resistor to use with a completed Aleph-X amp to test power output and look for clipping behavior with a scope?

16 ohm 120w wire wound resistor

I was planning on getting 4 of them. I could put 2 of them in parallel for an 8ohm resistor and 4 of them in parallel for a 4 ohm resistor.

Thanks,
Eric
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Old 7th January 2010, 06:38 PM   #2
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Quote:
Originally Posted by Eric View Post
Is this a good resistor to use with a completed Aleph-X amp to test power output and look for clipping behavior with a scope?

16 ohm 120w wire wound resistor

I was planning on getting 4 of them. I could put 2 of them in parallel for an 8ohm resistor and 4 of them in parallel for a 4 ohm resistor.

Thanks,
Eric
Sounds like a good plan. I have some of these myself, they're quite rugged and still go on happily after you have fried your whole hand trying to move them under heavy load
They may be very slightly inductive but for testing purposes that isn't an issue. How much power can an Aleph-X deliver, max, steady-state?

jd
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Old 7th January 2010, 06:41 PM   #3
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Originally Posted by janneman View Post
How much power can an Aleph-X deliver, max, steady-state?
I'm building a set of them that will likely run 100wpc into an 8 ohm load and 150-160w into a 4 ohm load. Thus, putting multiple resistors in parallel will also boost their power handling capacity.

thanks!
Eric
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Old 7th January 2010, 07:37 PM   #4
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Quote:
Originally Posted by Eric View Post
Is this a good resistor to use with a completed Aleph-X amp to test power output and look for clipping behavior with a scope?

16 ohm 120w wire wound resistor
I have a bunch of those. They work great.

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Old 7th January 2010, 07:47 PM   #5
Eric is offline Eric  United States
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Excellent! Thank you, Nelson!
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Old 8th January 2010, 12:38 AM   #6
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Do not forget capacitance across Rload...However I have not done this looking at my amps on the O'scope I have read it makes a difference in the scope pattern and is more like the actual load.
That being said good luck and let us know your results.Oh Yeah be careful those resistors get really hot, burning fingers do not feel or smell good...


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Old 8th January 2010, 12:59 PM   #7
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Hmmm.. How much capacitance are you supposed to add? I've never heard this before...

After getting two warnings about burning my hands, I'll be sure to use oven mits or at least screw the resistors down to a board.
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Old 8th January 2010, 01:29 PM   #8
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Hmmm.. How much capacitance are you supposed to add? I've never heard this before...
[snip].
The idea is that a real speaker load often represent a complex network not of just a resistor but also capacitors and/or inductors, and that even varies with frequency....
Especially the capacitive part can sometimes provoke an amp into instability/oscilations.
So, to check the power output of your amp in various steady state loads, the resistors are fine.
If you also want to verify amp stability, try paralleling those resistors with capacitors varying from perhaps 10nF through 100nF, 1uF, 10uF. Especially the latter is hard on the amp with high frequencies because it represent a low impedance, so you probably don't want to do that longer than just a few secs. Always slowly turn up the signal and watch the scope for funny patterns, turn it off if you see that!

Let us know how it goes!

jd
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Old 8th January 2010, 01:36 PM   #9
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Mine are a bunch of Dale's that I bought at Electronic Surplus in Cleveland -- I use 5 50Watt, 40 Ohm -- mounted on a piece of spare pcb material. When I was testing the bridged LM4780 article for AX I accidentally placed the "tester" on a scotch tape dispenser -- and it melted the plastic onto my workbench.

Probably a good idea on settling on a connector scheme at the same time. While banana plugs are most frequent, you might also want to consider those times when you'll use pin plugs for old amps with those push-in speaker connectors.

Sometimes you'll see Heathkit dummy loads for audio on Ebay. I used a Heathkit "Cantenna" back in the ham radio days -- filled it with salad oil if I recall correctly. Oh, salad oil does go rancid after a while \m/
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Old 8th January 2010, 01:54 PM   #10
Eric is offline Eric  United States
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Thanks for the tip on caps and a hook-up scheme! Much appreciated!

I'm not ready to do testing just yet, I'm probably a few weeks away from that yet. PCBs are populated, Fets are matched, I'm now waiting for heat sinks to arrive so there is still some work to do.

Thanks!
Eric
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