Do resistors fail under xtreme heat?

[Mike82]

I had a X-Over put together by someone who stacked all 5 of the mini statements resistors on top of each other. Today I smelt a burnt smell and popped out the X-Over. Turns out there was smoke in the cabinet and all the glue around the stacked resistors was burned black. The sound quality of the minis started going down hill I actually thought I blew the mids partially. I guess I am wondering when these get that hot do they start to become less accurate or does heat not affect them?

[Mike82]

Just pulled the resistors off the wood board and found that it got hot enough to literally burn that char black. Wow I am glad i caught this before it burnt my speakers or house down.

[ThomasW]

Tibor's stepson decided to turn the Aragon 8008 powering Tibor's M8a-MTMs up to "11".

When we removed the crossovers it was obvious the resistors had burst into flames. Fortunately the fire shelf extinguished due to a lack of O2, but not before melting the polyfill in the immediate vicinity of the XO into a charred mass of black plastic.

[Curt C]

Wow! And I was the one who told you that they probably wouldn’t overheat. ops: I guess you were exploring the superior regions of the SPL envelope…

Yes, in general, resistors will increase in value with temperature.

I’d suggest getting the resistors spaced off the board and separated by and inch or so to provide some airflow around them. This means don’t cover them with foam, or other stuffing materials.

The belt and suspender solution would be to use a higher wattage Arcol or Vishay resistors, but since this is the first I’ve heard of anyone having issues with the resistors in the Mini’s, I suspect good airflow is all that is required.

C


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[Mike82]

I was running around 60watts or so in them at my highest level. After ripping them apart I found some parts that were not what you called for in the BOM list. I know you are probably getting sick of me by now but I thought I would run them by you to see if they would impact the SQ that much. For the caps he used a Dayton 75uf instead of a 80uf and a Dayton 5.1uf instead of a 5.6uf. As for the resistors you called for two 10, 7, 4 and a 1. What I could make out was he had a 10 two 7's and a 4. I could not make out the other one to burnt. I am not sure what these different parts would do to it I guess.

[fjhuerta]

At that point, I'd be more worried about my hearing than some resistors!

[Curt C]

No problem at all Mike,

I'd suggest buying a couple of 4.7 ufd's to parallel with the 75 ufd, a couple of 0.47 ufd's to parallel with the 5.1's, and replacing all the resistors with Dale's, which are a bit beefier than the Daytons. -And their stout leads will make it easier to stand them off the board.

C

[Face]

What are you powering them with?

[Amphiprion]

Just remember with those aluminum encased power resistors that they need to be heat sink mounted w/grease to be capable of their rated power dissipation. Data sheets usually give guidelines for that stuff.

[digital desire]

I smoked one in my center channel too.

[Mike82]

I am using a Parasound HCA-855A power amp to power them. It runs 85 watts a channel at 8 Ohm.

[Curt C]

Assuming 100 watts applied, (at 4 ohms) I modeled the SPL to be about 110 dB @ 1 meter. This level would also represent the excursion limits of the woofers, and should be considered the maximum acceptable SPL for the design.

This is not to say that an amplifier rated at 100 watts is the maximum that should be used. Quite the contrary, transient peaks on the recorded material would easily require a 400 watt amp at that SPL level and still may clip the transients. A clipped signal from a marginal amplifier is far more likely to fry speakers, so it is always better to have an amplifier with lots of headroom.

C