2. amps-preamps
  3. 186313-184988

? about Smoking Power Amp

I made an error in connecting my amps last night, and one of them started smoking. It now appears to be working fine, as do my other stereo components. But I am trying to assess whether there is likely to be any lasting damage to the amp that should give me pause before using it again, or damage to other components. If someone with more technical knowledge than me could please help me understand what took place to cause the smoke, I would appreciate it.

Let me explain in more detail the setup that caused the problem. I have two power amps being fed by two sets of line-level outputs from a single preamp. One of the amps then powers stereo speakers, while the other powers a passive subwoofer. Last night, I placed a passive low pass filter (100 Hz) into the signal path going from the preamp to the amp that powers the subwoofer. This passive filter is female on one end, which is supposed to receive the input, and male on the other, which is supposed to provide the output. I instead plugged the male (output) end of the filter into the output jack of the preamp, and then I plugged a cable leading to my power amp into the filter's input jack.

With this erroneous setup, I turned on all of my components in the normal fashion. Very soon after (before I played any music) white smoke, smelling of burning plastic, started to come out of the power amp connected to the stereo speakers (which produced a moderately-loud buzzing sound). An orange tinge was visible through the grates of the amp producing the smoke. I turned off and disconnected everything and realized my error in the setup. The smoke stopped right away when I powered down the amp.

As I said above, I tested everything, and all components now appear to work as they should. I tested the smoking amp by connecting it to some budget speakers, just in case it had an issue that could cause damage further downstream. Even this amp appears to be working normally.

So I am not sure, in any precise way, what caused this. To the best of my layman's understanding, the reversed low pass filter blocked the signal coming out of the preamp output, probably causing something such as voltage to get messed up in the signal that was coming out of the other set of preamp outputs. This "messed up" signal was then transmitted to the power amp that started smoking.

I feel certain the speakers are fine. I am wondering if there is any reason to be concerned about latent damage to the preamp, which is a valuable unit. I am less concerned about the amp that was smoking, which is an inexpensive older unit (market value <$150), which I had been using for convenience. But I am still wondering if I can safely use that amp in the future, and if it is possible that something extraneous (such as a plastic coating or paint) was smoking and nothing more than that. I am not too worried if that amp burns out from continued use, but I don't want to: 1) start a housefire; or 2) cause damage to my speakers.

Any guidance you could offer would be appreciated. I already know I need to be more careful when setting up my components, needless to say. Thanks and have a great day.

mfiddles
allenf1963
334 posts
 

@larryi and his eyes beat me to the post.  That resistor below the Diode labeled "R762" and above the Diode labeled "D753" got toasted....and not in a good "hitting the bong" way!

 

I understand the sentimental value of gear.  I inherited "Klipschorn Clone" corner speakers my late Step-Dad built around 1962 -- and they mean the World to me.  Other speakers out there would run circles around these speakers -- but not by much!  He was a talented Chemical Engineer (actually the top dog at Tennessee Eastman's Chemical Division during the 1970's and early 1980's), but had amazing knowledge of Electrical Engineering and acoustics.  I wouldn't trade or sell these speakers for any amount of money.  It means something knowing his hands crafted the Cherry and Oak wood for the cabinets, and built the crossovers and did the wiring.

 

Before putting this amp back in my system, however, I would spend the money to have a professional technician go through the amp and do the needed repairs.  Even though it looks like a simple job, for insurance purposes I would want a paper trail showing a "Professional" worked on the amp.  That way, if something does go wrong when you hook it back up and it does cause a fire, you have a greater standing with your insurance company or with the Technicians Business Insurance coverage when it comes to filing a claim.  It's sad that we now live in a World where you have to think along these lines...but it is what it is.

 

Good luck!

wolf_garcia
6,695 posts
 

It's been a couple of days, has the house burned down yet?

mfiddles OP
10 posts
 

Still standing, last I checked. :)

atmasphere's avatar
atmasphere
11,785 posts
 

@mfiddles D753 is a diode in series with the 10 Ohm resistor, R783.

The screen for the resistor might be visible once you clean the board.

This resistor is in the protection circuit and is not part of the amplifier circuit itself. So the amplifier circuit was undamaged in any way. However, given the state of the resistor its a good bet the protection circuit for that channel is damaged. It may fail to give protection if an event occurs in the future.

I would start by replacing the resistor and testing the associated diode for a short.

mrdecibel
3,063 posts
 

The cost of a tech’s repair might be more than the value of the amp, so that must be considered. I have several amps in my collection that would sell below $200., and they all work great. But if 1 of them failed and I could not do the repair myself, it would become a recycled item and imo not worth the investment. I predict that mcfiddles will continue to use the amp, as he sounds like he will. My best to all, MrD.

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