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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:

R87, R88, R89, R90

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:

http://www.newark.com/multicomp/mcprm07wjp751b00/metal-film-resistor-750-ohm-7w/dp/16R1987?exaMfpn=true&categoryId=&searchRef=SearchLookAhead&searchView=table&iscrfnonsku=false

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).

six white resistors

[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.

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  • \$\begingroup\$ all resistor choices meet the voltage requirements of 50V. Since 80/2=40 Why are you worried? \$\endgroup\$
    – D.A.S.
    Commented Feb 13, 2017 at 18:44
  • \$\begingroup\$ @Tony Steward: I'm not quite sure what power rating and voltage rating of resistors are. I thought I'd better ask knowledgeable people here. R87-R90 get hot and I'm looking to replace them with resistors of different rating. The related question is here: electronics.stackexchange.com/questions/285529/… \$\endgroup\$
    – zeron
    Commented Feb 13, 2017 at 18:54
  • \$\begingroup\$ @zeron Considering that one of the resistors desoldered itself and the close proximity of the resistors to each other (thus reducing their power dissipation capability), you might want to see if you can find metal-encased power resistors and attach them to something (e.g. a sheet of aluminium) to increase their power-dissipation capability. But it lasted for 20 years as it was, so maybe not worth bothering with. \$\endgroup\$
    – Andrew Morton
    Commented Feb 13, 2017 at 19:20
  • \$\begingroup\$ @Andrew Morton: Thank you for your suggestion, but as you can see in the picture I just linked, the space around them is really tight. The reason one of the resistors desoldered itself could have been caused by a short I created while touching up solder joints. I eliminated the short and with a temporary resistor in R88's place (just hanged from the through holes), the amp worked OK. So, my guess is that they get really hot, but not hot enough to desolder themselves under normal condition. \$\endgroup\$
    – zeron
    Commented Feb 13, 2017 at 20:47
  • \$\begingroup\$ 300-500V is not a power rating. You're asking about voltage rating but you're only quoting power rating and resistance. Please clarify what you're actually asking about. \$\endgroup\$
    – user207421
    Commented Feb 13, 2017 at 21:14

3 Answers 3

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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.

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  • \$\begingroup\$ Whoever downvoted this, what do you think is wrong? \$\endgroup\$
    – Olin Lathrop
    Commented Feb 14, 2017 at 11:35
  • \$\begingroup\$ I certainly wouldn't call it wrong nor downvoted but it does lack consideration for ambient of adjacent equal powered R's and de-rating by perhaps 50% for 1 side and more on both sides. \$\endgroup\$
    – D.A.S.
    Commented Feb 21, 2017 at 17:50
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The question now says, "minimum voltage rating". The answer is obvious: since the power supply is +-80V, the resistors could not be exposed to signals of more than 160V. This is the formal answer - 160V. Any resistor of this size would probably exceed this rating.

I am not sure though what the actual problem is that needs to be addressed. What has failed? Why to replace the resistors? Too high temperature of the resistors, so they de-solder and fall off?

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  • \$\begingroup\$ The amp started generating noises in January this year when it's warmed up. The four resistors definitely contribute to the amp getting hot. Something else in the amp could be causing overheating, but these resistors must be changed because one of them definitely failed after desoldering itself: it reads only 426 ohms or so now. One interesting thing is that the BOM for the amp lists the resistors as 5W 1.5K ohms. It's likely that the original resistors' values may not have been optimal, so I want to replace them with ones that are compatible and will help the amp run cooler. \$\endgroup\$
    – zeron
    Commented Feb 13, 2017 at 23:16
  • \$\begingroup\$ @zeron, all indications are that you have half of your amp seriously blown out, so replacing the simple resistor divider will NOT solve your problem. Reading of resistor's values in-circuit could be anything, because there are other paths for current that will contribute to measurements. You need to test ALL TRANSISTORS in your amp for functionality if you want to fix it. If it doesn't smoke right away with no signal, try to measure DC voltages at every point, and watch for Vbe to be less than 0.7V. \$\endgroup\$
    – Ale..chenski
    Commented Feb 14, 2017 at 0:39
  • \$\begingroup\$ "... all indications are that you have half of your amp seriously blown out...." Can half-blown amps drive woofers to make musically coherent sounds? Mine does. However, the output level may have dropped by 3dBor a little more. I don't want to increase the volume too much because the woofer is outside of its intended enclosure now. Is it possible that only like the positive side of the amp is amplifying the signal? But if that's what's happening, I'd expect much more severe distortion. Also, with the plastic cover off, noises usually don't show up for hours. \$\endgroup\$
    – zeron
    Commented Feb 14, 2017 at 2:01
  • \$\begingroup\$ Replacing the four resistors didn't solve the noise problem. Touching up the solder joints of the three pins of Q9 did. I wouldn't have reached the solution without learning a lot from posts here. Thanks. More detail is here: electronics.stackexchange.com/questions/289929/… \$\endgroup\$
    – zeron
    Commented Jul 28, 2018 at 19:52
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Each 750ohm part dissipates <2.5W or 50% of 5W suggested rating so with convection air flow and raised above the board should be ok with ventilation. Expect it to be 100'C or 80'C above ambient. Any smaller rating will need more air flow.

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  • \$\begingroup\$ Stewart: You wouldn't go with resistors with higher power rating or higher ohm value? I'll add a picture of the amplifier with its cover off to the original post soon. As you'll be able to see in the picture, the ambient temperature could keep rising. (I actually drilled some holes to the plastic case that originally had not holes for ventilation.) \$\endgroup\$
    – zeron
    Commented Feb 13, 2017 at 20:28
  • \$\begingroup\$ same value is best for bias regulation, higher power rating allows cooler operation. Value is a tradeoffs between current limit and Pd \$\endgroup\$
    – D.A.S.
    Commented Feb 13, 2017 at 21:08
  • \$\begingroup\$ Stewart: "same value is best for bias regulation, higher power rating allows cooler operation." So, would you say that the 7W 750 ohm one is a good choice? The distributor that carries it confirmed that its voltage rating is 500V. \$\endgroup\$
    – zeron
    Commented Feb 13, 2017 at 22:34
  • \$\begingroup\$ use what the original BOM says \$\endgroup\$
    – D.A.S.
    Commented Feb 14, 2017 at 15:25
  • \$\begingroup\$ The original BOM says 1.5K 5W. \$\endgroup\$
    – zeron
    Commented Feb 14, 2017 at 15:54

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