Any issues with mounting amp to sub box?

Just curious.

under normal circumstances, no ... I've done it before...

BCAE says...

Amplifier mounting:
DO NOT mount an amplifier on your subwoofer box. I know that there has been a great deal of discussion over mounting an amplifier to an enclosure and many people do it all of the time with no problems but those people probably build good enclosures from 3/4" (or thicker) MDF with extensive bracing. Most people (especially young impatient people) are too lazy to do that and build unbraced enclosures from 5/8 MDF. These enclosures will flex considerably more than a proper enclosure and will likely cause amplifier failure if the amp is mounted to the enclosure.
REASON:
When the woofer(s) moves in or out, the box flexes and therefore causes the sides of the box to vibrate. This vibration is transferred to the amplifier mounted to the box. All of the electrical components in the amplifier have mass. Inertia (an object in motion tends to stay in motion, an object at rest tends to stay at rest) tells them to stay at rest, the box vibration is trying to make them move. The energy from the box's vibration is transferred to the components through the electrical leads which are soldered into the circuit board. All of this will cause the components to break loose and therefore cause the amplifier to fail prematurely. Basically, the amplifier will commit suicide! :-) I'm not telling you this because someone told me it was bad. I've been repairing amplifiers since ~1985. Virtually every amplifier that's come into my shop with parts rattling around inside them have been mounted on the speaker box. It causes the legs of the semiconductors to break (which causes amplifier failure). It causes the capacitors to break off of the board (which can cause catastrophic amplifier failure). It causes solder joints to break on the semiconductors mounted to the heat sink. It causes transformer windings to grind into one another (which causes lots of smoke to pour out of your amplifier). People who repeatedly tell others to mount their amps on the speaker box because they've never had a problem remind me of people who drink and drive and say there's nothing wrong with it because they've never crashed their vehicle. Eventually, in both cases, problems will arise.


NOTE:
Mounting the amplifier on the enclosure also allows someone to steal BOTH your amps and speakers at the same time with no extra effort. It's bad enough to have one or the other stolen but losing amps and speakers (and anything else mounted to your speaker box) is really gonna suck.


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Now for those who absolutely have to mount the amplifier to the enclosure:

NOTE:
The top of the enclosure has been removed to show the bracing.

Absolute Worst Situation:
This diagram shows the amplifier mounted in the center of the largest wall of the enclosure. You will notice that there are no braces under the amplifier's mounting points (red arrows). Because this part of the enclosure will flex more than any other, this will cause more physical stress (from panel flex and vibration) to the amplifier than any mounting position. This would be made even worse because the enclosure has no center brace/divider.

Somewhat Better Position:
As you can see, this enclosure has a center brace which will help to reduce panel flex. You should also notice that the amplifier's mounting points are very close to the braces or on top of the wall of the enclosure. These points will have less vibration from panel flex (when compared to points farther from the dividers or walls).

As good as it gets (with amp on box):
This method of mounting will relieve most of the stress from the amplifier because the wood braces will connect the top of the enclosure to the bottom of the enclosure and won't allow the panel to flex. The braces should be glued (with a good wood glue) and screwed on both ends. The other mounting points are directly on top of the walls or dividers which will vibrate VERY little when compared to the an unbraced panel.

So how does a child remove a baby tooth? They wiggle it until the connecting tissue fatigues.
The heavier the component the more likely you will have a failure and transformers are a heavy mass that are suspended by only wires that are only soldered to a circuit board. That would cause me pause.

David
Earmark Marine

so at what point does this become different from having the amp mounted to a rigid structure in a stiffly sprung vehicle? Is there something special about the vibration from a sub that is different from the vibration transmitted though the body of a vehicle (or a boat pounding across a choppy lake)?

Seems to me the general pounding an amp takes in my car (which I drive like the General Lee, by the way) is worse than it could ever be from being mounted to a sub box.

And yes, I have mounted an amp to a sub box, and no it never failed, even after 5 years of abuse in the back of my old pathfinder (modest sealed ten in a sealed enclosure getting 350w... which used to be a lot back in the day!).

YMMV and all that.

Everyone has installed an amplifier or two on a sub box more than once. Shhhh....just don't tell anyone. So it does happen frequently and without issue. On the other hand I have seen heavier EI transformers in car amps come free for this exact reason even though it has been rare. I've also seen the board components become unsoldered and rattling around inside due to extreme abuse and heat. It would be easy for vibration to turn a solder joint cold and the added resistance would produce excessive heat especially in the power supply side.
There is a little difference between the normal shock and vibration transmitted from driving or running and the wear and tear from a sub box. A subwoofer is an inherent jackhammer that violently changes directions nearly 200 times a second. Kind of a mini paint-shaker of sorts. Thats an accumulative force that never rests. And, a poorly constructed box amplifies the vibration because the walls are less rigid. Certainly alot of output can be lost from a cheap box so rigidity is a factor. And, most off-the-shelk boxes have a 5/8" or 3/4" baffle while only having a 1/2" wall thickness on the other five surfaces.

David
Earmark Marine

"Don't do it, just don't do it." Shaun Murray, Detention.

This is just bad idea. As has been mentioned above the enclosure tends to rattle all component level circuitry. Typical failure points at the board level are the large input filter caps, ps windings and as things are getting smaller and small these days many designs use daughter boards to allow user adjustment out the top or side pannel of an amplifier or stand up crossover cards. These entire boards can quickly get fatigued and FAIL.

Internally, we replaced our shake table with a simple dual 12 sub box with the woofers out of phase with each other to elim the sound. We mount alpha level pcb's to box and let it rip over the weekend or through the night. Doing so gives a pretty good indication of an amplifiers failure points from a physical standpoint.

Simply said, if you go down this path, you are asking for trouble at some point in the future. I'd try to avoid it at all costs.

-Brian
Exile Audio

I have 10" Kickers under the rear seats of my Avalanche, and my Eclipse amp mounted on top of that (not much choice due to the limited space).

Have been rolling it that way since 2006, and still works just fine - and I don't exactly listen to my music quietly. This is not how I wanted to mount the amp, but sometimes your choices are limited. Just a pre-made internet-ordered box too - nothing special.