Why did smoke come out of my amp?

Hi, I have a problem and I am not sure where to post it. Hopefully this isn't too wrong a place.

We just built a new house and I installed some JBL speakers in the ceiling and a JBL HTI88 subwoofer in the wall. I puchased an Advent 300 receiver to power the ceiling speakers. To power the sub, I bought a new AudioSource AMP100. Upon receiving the amp, I discovered that in order to use it in mono mode I had to have a mono input. To get this signal, I took a pair of RCA cables from the 300's preamp outputs, connected each channel to a Y cord, routed one side of the Y back to the amp inputs on the 300, and then ran the other half to another Y connector, which I then plugged into the amp. The power amp was set to mono. I think the amp is rated for a minimum of 8 ohms when so set.

The HTI88 has two speakers in it and two crossovers. It has provision for hooking up each speaker separately to an amp, but since I only ran one wire to that location, that's not an option. It also has shorting straps to connect the two speakers so that they can both be run from one input.

So last night, I was listening to some lame disco crap from the seventies and I turned it up and smoke-tested the power amp. Literally. There was a crack and a bunch of smoke came out of the amp; now that I let all the smoke out I can't get it back in and the house stinks. Ideally, I'd rather not repeat this performance. So I am trying to figure out what caused the problem. Was it

a) the woofer was operating at less than 8 ohms because I was using the grounding straps;

b) my combining two line level inputs was a big no-no;

c) my amp was simply defective; or

d) I got what I deserved for listening to lame disco crap?

FWIW, here is the manual for the speaker:

http://manuals.harman.com/JBL/HOM/Owner's Manual/HTI88 OM(web)4-19-05.pdf

I checked, and it doesn't appear to say that when the grounding straps are used, impedance goes to 4 ohms.

I looked at the manual for the amp, and it appears to be rated only for 8 ohm loads when bridged.

Assuming that (a) (or (a) and (d)) was the correct answer, any suggestions for an inexpensive amp to replace this smoke-tested unit? We ain't listening to classical here!

Thanks.

Norm Witte

The correct answer is D,disco sucks! & let that be a lesson to ya:nono:

I dont think you had a problem the way you hooked the amp into the system,ive done exactly as you described many times over the years,i think its an impendance issue,shorting straps will have nothing to do with lowering the speakers impendance.

See if you cant find out what your sub's lowest impendance is,even though its rated at 8 ohms thats a nominal rating & it could very well have an impendance dip that goes to 4 ohms or below.

Another thing is the amp itself,the little Audiosource amps are decent amps but not for subwoofer duty,for running a sub you need an amp with real balls & the Audiosource does not have any balls.

If you dont care about looks try a pro amp that can be bridged to mono,look at some amps from Crown,Alesis & Behringer & dont skimp on the wattage,a minumum of 300 watts in mono is what you'll need,preferably 400 watts.

definitely D :thumbsdn:

I'd say Option D or possibly you dropped outside of the impedance range.

panhead said:

The correct answer is D,disco sucks! & let that be a lesson to ya:nono:

I dont think you had a problem the way you hooked the amp into the system,ive done exactly as you described many times over the years,i think its an impendance issue,shorting straps will have nothing to do with lowering the speakers impendance.

See if you cant find out what your sub's lowest impendance is,even though its rated at 8 ohms thats a nominal rating & it could very well have an impendance dip that goes to 4 ohms or below.

Another thing is the amp itself,the little Audiosource amps are decent amps but not for subwoofer duty,for running a sub you need an amp with real balls & the Audiosource does not have any balls.

If you dont care about looks try a pro amp that can be bridged to mono,look at some amps from Crown,Alesis & Behringer & dont skimp on the wattage,a minumum of 300 watts in mono is what you'll need,preferably 400 watts.

Click to expand...

I kinda agree with your idea.. the resistance probably was too little, and the genie decided it was just too much work in there for him...

Another amp to take a look at.. Tapco has an 800W amp (bridged)... I've had a two ohm load on my 1400w, and it didn't seem to mind it.. I don't think I would purposelly do that again though... I didn't realize what I had done.. Anyway.. they've got great prices on them.. and they are a great clean sounding amp..

Here's what's inside of them...

Maybe I'm missing something here, but... what I use to drive my subs are plate amps. Designed specifically for that purpose. Include line level and speaker wire type inputs, adjustable filters & phase switching. Last year I put a Dayton (Parts Express) plate amp on my old Visonic passive sub. Works great. 240 watts, only $99 (don't know what they are now tho.)

Cheers, Jim

Jim Shearer said:

Maybe I'm missing something here, but... what I use to drive my subs are plate amps. Designed specifically for that purpose. Include line level and speaker wire type inputs, adjustable filters & phase switching. Last year I put a Dayton (Parts Express) plate amp on my old Visonic passive sub. Works great. 240 watts, only $99 (don't know what they are now tho.)

Cheers, Jim

Click to expand...

I dont think your missing anything,the 2 different kind of amps are entirely different designs,most plate amps ive seen are class d amps,basically a car audio amplifier,designed to run high wattage at low ohmages,the amp the OP used was a class a/b amp that can only be ran in mono with an 8 ohm load.

Under normal speaker duty the lil amp might have been fine running a low ohm load but subwoofer duty puts a hell of a strain on an amp,especially when its underpowered & over driven,i suspect that the OP over drove the Audiosource till the magick smoke appeared.

Never connect outputs together, whether preamp or power amp levels. What you did was to short the outputs of the preamp together, without any current limitation between the 2 outputs. You can expect to take out the preamp, then, when the preamp goes, it will take out the the power amp. I suggest that if you check the preamp, it is toast.

Mixing of signals requires a mixer circuit, which consists of, at a very minimum, a resistor network, if not some kind of active mixer stage.

Sorry to give you the bad news...

Enjoy,
Rich P

pustelniakr said:

Never connect outputs together, whether preamp or power amp levels. What you did was to short the outputs of the preamp together, without any current limitation between the 2 outputs. You can expect to take out the preamp, then, when the preamp goes, it will take out the the power amp. I suggest that if you check the preamp, it is toast.

Mixing of signals requires a mixer circuit, which consists of, at a very minimum, a resistor network, if not some kind of active mixer stage.

Sorry to give you the bad news...

Enjoy,
Rich P

Click to expand...

He didnt hook 2 outputs together,from his post i gather that he went from output to input in the correct manner,the volume control on the Audiosource would be nothing more than a gain control.

ncwitte said:

To get this signal, I took a pair of RCA cables from the 300's preamp outputs, connected each channel to a Y cord, routed one side of the Y back to the amp inputs on the 300, and then ran the other half to another Y connector, which I then plugged into the amp. The power amp was set to mono. I think the amp is rated for a minimum of 8 ohms when so set.

Click to expand...

OK.. trying to understand this... took both outputs from the pre, and hooked them into a Y... ok.. I understand that.. BUT.. I would never hook two outputs to a Y... one output to a Y is good.. but two outputs into one Y is a :nono: You are effectively shorting the channels together.

Now lets look at option two you are trying to explain.. you took each output and connected each output to a seperate Y... that is OK.. then you split each output seperatly... Effectively giving you two outs of channel 1 and two outs of channel 2. Good for plugging into 4 different mono amps...

This is where you really lose me.... You routed one side of the Y from the pre output back into the pre???? that's a super big :nono: ???

Then you took what was left of the Y's and went into the main amp...

WOW.. I'm surprised you have any gear left if I am understanding what you did...

I think a few guys may have misread the first post,the OP did not hook 2 outputs together,he states in his 1st post that he used preamp output to amplifier input, which is a standard connection,his receiver has a preamp out & running it to an external amp is its intended use.

As for hooking up a Y adapter to the signal comming from the preamp there will be no issues with that connection,no wattage is being sent from that line level signal,all he did was to split the signal comming from the preamp & send it to two seperate amplifiers,one amplifier being in the receiver & the 2nd amp being the audiosource.

The way he has the Y splitters hooked up is no different that taking a pair of Y's from the preamp & hooking a set of interconnects into the left channel,hooking a set of interconnects into the right channel,then running 2 seperate amps from the single preamp,the only difference is that one amp is built into his receiver.

Option B is wrong,he did not hook 2 line level's together,what he did was to split the line level signal in half which is not dangerous.

Sorry to have missed out on all these great comments, but we have been in the process of moving. So, first, let's be clear about what I actually did, and the wisdom or lack thereof of my actions, leaving aside the disco as to which there seems to be universal agreement on my fundamental lack of good taste.

On the Y cords, I have a single set of speaker wires to the sub. The amp is bridgable to mono, but it needs a monaural input. So what I did, and I am getting the impression that I did something wrong here, is to try to make a monaural signal from my stereo preamp outputs. I took a left and right pair from the preamp outputs; plug a Y splitter in to each side. One side of each Y went back to the Advent 300, plugged into the respective inputs for their channels, not combined. Ok so far. BUT...the other side was to go to the Audiosource proletarian amp. In order to get a mono signal, I took those two outputs and combined them with another Y cord. Mistake? Sounds like a resounding yes to me from the comments. The Advent's not fried yet (at least it is still working) but the Audiosource is dumpster food.

I did look at some plate amps and I am going to be building a built-in desk which would probably work marvelously with a plate amp. Most seem to be able to accomodate a stereo signal so that would solve another of my problems.

I don't much care if the plate amp was a car stereo in a former life. I have a nice collection of sweet vintage Sansui gear in another system so I can be a card carrying audio snob if forced (though certainly only a mid level audio snob, I'm no priest of esoterica to be sure) but my main goal is to make my wife's Christmas CDs sound decent for company. Given the set of compromises built into this project, if I thought a Radio Shack PA amp would carry the water, I would be game.

Wow, that was verbose.

Anyway, thanks for your collective wisdom.

You can't just "Y" together L/R stereo output to get a mono - that's a no-no.

However, I don't believe that's what caused your amp to smoke. If anything, that "Y" connection shorting L/R channels could have (but very unlikely), damaged your receiver and you say that's OK.

I'm guessin' there was some type of connection error in the bridging setup and or the wiring/connection to the speaker.

The other possibility, although I think very unlikely, is that the amp was just bad out of the box and only took a little load to pop.

The input signal wiring to the amp was incorrect, but again, I don't believe that's what caused the amp to blow. Something on the output side of the amp sounds like it got shorted.

Hmmmmmmmm. Well, I just eBay-Buy-It-Now-instant-gratificationed me a plate amp that is rated to go down to 4 ohms and takes stereo inputs--all for just $79 including shipping!! Now if only I could get the same deal on a Class C motor home. As for the bridging, I had it set to the bridged-mono setting, and wired the speakers per the manual. I don't think it was possible to screw up the subwoofer wiring (though, with no false modesty, I can be pretty creative at times) and I think that there's a reasonable chance that the impedance of the speaker was just too low, despite the nominal 8 ohm rating. If I am wrong, it's dumpster power for another 79 bucks.

Thanks for your input (pardon the pun).

Ususally, if the impedance is too low (without being a short circuit) the amp will get hot and shut down. Something literally going pop and smoking suggests something other than impedance just a bit on the low side - at least to me.

Anyway, good luck.