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drumguy48
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Technical Question

Hi all, If you required a 100 watt resistor can you substitute 5 x 20 watt resistors in parallel?

j_j
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Re: Technical Question


Quote:
Hi all, If you required a 100 watt resistor can you substitute 5 x 20 watt resistors in parallel?

Well, if each of the 20 watt resistors is 5x the intended result, and they all have the same resistance, yes.

Also, you have to allow space for each to cool appropriately.

But look for 100w dummy load resistors at parts express, if that's what you need.

drumguy48
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Re: Technical Question

Thanks jj - yes I need 6.8 Ohm ,100 watts, will use 5 x 33 Ohm, 20 watts.
It's for the Acoustic Reality Series Crossover

Jan Vigne
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Re: Technical Question

Then you should also consider the combined inductance of the circuit change. Adding reactance to a crossover isn't going to result in the same end product.

Welshsox
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Re: Technical Question

Check the resistors for tolerance, if you have say 20% tolerance it could easily unbalance the load sharing

drumguy48
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Re: Technical Question

Thanks for the comment Welsh, planning to use 2% Mundorf 20 watt units
Jan-I don't understand your question, are you saying that using 5 of something will introduce adverse affects that using the one of something would'nt?
Have found a site called Ohmite.com that sells 50 watt audio resistors will contact them about availability

Jan Vigne
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Re: Technical Question

One resistor is not necessarily the equivalent component to another resistor. Five resistors replacing one resistor is equivalent only in that the final number for resistance adds to be the same. Or, roughly the same. 20 watt resistors are probably either ceramic or wire wound types which can have a significant variation from spec. If the OEM resistors were at 10-20% tolerance, then tightening that spec to 2% should be acceptable. Not necessarily better than OEM, but acceptable. Depending on how the designer of your crossover went about their buisness, 20% tolerance resistors could have been ordered in bulk because they're quite inexpensive at that value. The lot of resistors could have then been culled down to only those resistors matching the designer's actual spec and the rest sold off again to another company which had less strict tolerances. Doing this could prove to be less expensive than ordering the entire production lot at the tighter tolerance. So, a 20% tolerance resistor won't always mean the actual component used isn't closer to a 5% tolerance in actual value. And "real world value" is what you need to shoot for when trying to upgrade a crossover. You really need to pull such resistors out of the circuit and measure their actual values before you begin ordering replacements. Changing from a 5% to a 2% tolerance isn't going to be an audible change in most crossovers. The quality of the resistor might change but considering where in the crossover most 20 watt resistors would be placed, I doubt you'll notice much improvement if the above scenario has taken place.

Most high wattage resistors are still likely to have an inductive value also. The inductance of a single resistor might have been acceptable to the designer of the crossover. The combined inductance of five resistors in parallel might change the total inductance of the circuit to create a new filter value.

Are you replacing the resistors to repair a crossover? Or, are you trying to tighten up the quality of parts used and hoping for better sound? If the latter, be careful. There are a few designers out there who are thinking of all the components in their designs in sufficient detail to include even the impedance/reactance value of their internal cabling. Change just the cables inside the speaker and you've altered the design to some extent. When trying to "upgrade" a speaker by rethinking what the designer did, you often end up degrading the entire speaker as a system rather than just a collection of parts.


Quote:
Wire resistors have an inductance, leading to a reduced conductivity at higher frequencies.
http://www.lautsprechershop.de/hifi/resistor_en.htm<br />
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drumguy48
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Re: Technical Question

Thanks for the explanation Jan - It's a pair of diy project speakers-I am already using the prototype xover as shown on Acoustic Reality Series Crossover website -the final step they strongly recommend is to replace the 5 watt Radio Shack resistor with a Hi Quality Hi power resistor of 50 watts or more -If I decide to use the Mundorf units, I'll use 3, giving 60 watts
To be honest just doing it cause I love fiddling with stuff, and they are just used as my 'puter speakers.

Jan Vigne
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Re: Technical Question
j_j
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Re: Technical Question


Quote:
Thanks for the comment Welsh, planning to use 2% Mundorf 20 watt units
Jan-I don't understand your question, are you saying that using 5 of something will introduce adverse affects that using the one of something would'nt?
Have found a site called Ohmite.com that sells 50 watt audio resistors will contact them about availability

Any time you parallel multiple units, you will tend to wash out, to some extent, the tolerances, so if anything, you'd wind up with a bit better tolerance than you started with.

As far as inductance, you'll have 5 in parallen, which reduces the inductance, which ought to be a good thing. The only thing that will increase will be the parallel capacitance of the resistor, and that is extremely likely to be no issue at all.

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