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
mfiddles OP
10 posts
 

And just to answer the prior poster's question, no the connections are not balanced (XLR). They are RCA. Yes the low pass filter is entirely passive, and it was reversed in the fashion you describe. The filter has a male (output) and female (input) end, and the male (output) end was plugged into the output of the preamp, while an RCA cable going to the amp was plugged into the female (input) end of the filter. I would suspect you are right about the oscillation and that the preamp outputs are not independently wired. 

In my 20+ years of having separate audio components, I have made setup errors on occasion. I never expected that one as seemingly minor as this would cause such an extreme outcome (smoke, fire, etc.).

bigkidz
3,381 posts
 

There are things that can burn without causing harm.  For long term though, I would not use the amp.  Take the cover off and look around to see what has burned.  I am in New Jersey if I can assist in anyway.

 

 

mfiddles OP
10 posts
 

Here is a link to a photo of the inside of the amp. As should be evident, there is no visible damage to any component. I have looked at each component closely and see nothing out of the ordinary. 

https://drive.google.com/file/d/1rDaytti32GrukFYlsTD6B5lxwI6JQxdP/view?usp=drive_link

larryi
3,969 posts
 

It is hard to tell from the pictures, but, above the two big electrolytic caps, there appears to be two burned out resistors, one leaving a big white stain from what is left of it, the other a smaller resistor right next to it that also looks damaged.  
 

It is not worth the risk running this amp, and you have no assurance that the next failure won’t take out other components, like your speakers.

mfiddles OP
10 posts
 

Yes, upon further inspection, I found the offending component. It is a single resistor, labeled D785, in the location described by larryi. (The component adjacent to it is a diode and does not appear damaged.) It appears that the coating of the resistor burnt off, but it is otherwise still in tact. The resistor is carbon 10ohms 5% 1/2 watt. I can order a replacement for a dime or two (literally), and then pay someone to solder it in, I would suppose. And I would imagine that will take care of the problem. What a relief! :) :) :)

Here is a link to a closeup of the damaged resistor:

https://drive.google.com/file/d/1qsmkHSIehBGblFcgsevhQXNXhewcJ6Om/view?usp=drive_link

Did not want to get sappy in my earlier posts. But this amp, whatever its market value, was a gift from a favorite uncle who recently passed away. I like it's sound and being able to use it brings fond memories of time with him. He was known for playing music very loudly.

More to discover