I'm looking for four power resistors of the same type to replace R87, R88, R89, and R90 in the POWER AMP section of the amplifier of Klipsch SW-12 subwoofer shown below:

What should the minimum voltage rating of the following new resistors be?

Choice 1:

5W 1.5K ohm

Choice 2:

5W 1K ohm

Choice 3:

7W 1k ohm

Choice 4:

7W 750 ohm

Choices 1-3 all have their voltage rating listed (350V – 500V), but I can't find the voltage rating for Choice 4:

Datasheet:

http://www.farnell.com/datasheets/1679919.pdf?_ga=1.219978905.1878132867.1486504283

If I can't find the voltage rating for Choice 4, should I drop it from my list of choices for the replacement resistors?

[A picture showing R88 and R90 with four other white resistors: Added 2/13/17]

There's a similar-looking row of six white resistors on the other side of the amp, too (bottom in the picture).

[Added 2/13/17]

I received a reply from a US distributor of the Choice 4 resistor:

Received: Mon, Feb 13, 2017 9:33 am

Subject: MCPRM07WJP751B00

Hi

The max voltage rating is 500v. I don’t have a data sheet to send but the product manager was able to contact the mfg to get the voltage.

Have a nice day.

. . . . . . . . . . .

[Added 2/14/17]

This is the resistor's listing that prompted me to ask a question about voltage rating for resistors:

http://www.newark.com/cgs-te-connectivity/sbche61k0j/wirewound-resistor-1kohm-7w-5/dp/16R6555

I hadn't been able to find it until this morning.

## Best Answer

I wouldn't worry about this too much. The implicit minimum voltage rating of a resistor is

V = sqrt(W Ω)

where V is the voltage, W the power in Watts, and Ω the resistance in Ohms. If it were anything less, then the resistor couldn't dissipate the rated power.

The lowest of your choices is the 5 W 1 kΩ resistor, which must be able to handle at least 71 V to dissipate its rated 5 W.

This circuit has ±81 V supplies, with four of these resistors in series from one supply to the other. Assume worst case that the center point can be driven to either rail, so 162 V across two resistors in series. It looks like the resistors will divide that fairly evenly, so a bit over 80 V across any one of them worst case.

Any resistor you find that can dissipate 5 W or more is going to be physically big enough to handle well over 100 V. You're not going to be able to find a resistor that meets the power and resistance requirements that can't handle whatever voltage this circuit can possibly throw at it.