Why Is My Boss Subwoofer Amplifier Distorting at High Volume?

[Tesladude]

I have a boss subwoofer amplifier which I recently discovered was causeing the distortion I have been hearing in my car.

this amplifier consists of a PWM powersupply turning the 13.5v from my on car to a +45 -45 power supply.

Then there are 8 mosfet transistors, 4 in parralel powering one phase of audio, then another 4 in parrelel powering the other phase of audio.

I am reading on my DMM that the amp when put to the max I can get a 40 ish volt output which is plenty more than I need and to be expected.

But when the amplifier is hooked to my 2 subwoofers
at 2 ohms (which the amp is rated for) I can get an absolute max of only 14-ish volts and a lot of distortion.

This sounds exactly like the power supply pukse width modulator is not working correctly. So I checked all of the power supply switching mosfets and they all work.
so then I replaced the modulator chip itself expecting that to fix the problem and yet it did not.

any ideas?

 

[Averagesupernova]

Have you measured the power supply rails to see if they are sagging? Also, what do they do at various loads? Can you see a change in the output to the switcher on a scope as the load changes? Try these things with your subs hooked in series or just one at a time to change the load. Also, is the 12 volt supply holding up feeding to the amp? If you figure the amp at 100% efficient (which it obviously is not) a 2 ohm load at 80 volts peak to peak will pull about 30 amps from the 12 volt source. Do you have a way to measure this current? Also, do you know if both phases are doing the same thing? If so, it indicates a power supply problem.

 

[Tesladude]

Have you measured the power supply rails to see if they are sagging? Also, what do they do at various loads? Can you see a change in the output to the switcher on a scope as the load changes? Try these things with your subs hooked in series or just one at a time to change the load. Also, is the 12 volt supply holding up feeding to the amp? If you figure the amp at 100% efficient (which it obviously is not) a 2 ohm load at 80 volts peak to peak will pull about 30 amps from the 12 volt source. Do you have a way to measure this current? Also, do you know if both phases are doing the same thing? If so, it indicates a power supply problem.

Why do I read 100V AC between each rail and ground?

I have no scope,
also the power supply is a running car useing 10 gauge coper wire, this is a little thinner than it should be but not nearely thin enough to make such a difference in output power.
At 8 ohms it does the exact same thing and starts dieing at 14v.

right now I am trying to figure of there is any transistor or anything damagable leading to the PWM chips current sensor.

 

[Averagesupernova]

I haven't worked without a scope on stuff like this probably ever. I was fortunate enough to own a scope before even having any formal training in electronics. I would not like doing this without a scope. You seem to do a lot of amplifier tinkering. I think you could justify the investment. Can you temporarily disable the current sense to determine if this is causing the problem?

 

[Tesladude]

Ya, i really need to get one.
but i don't have any money, that is why i need t fix this lol.

I am working on it but the datasheet is not a big help...
and it looks like the feedback pin is just grounded with a few caps and resistors.
it's real weird.another thing is that i have a homemade work bench power supply that can put out a few amps at 12v,
the entire amplifiers idle current is just on 800ma, i put a 4ohm dumby load on the amplifiers output and for some reason i was reading that the amp was drawing 2 amps while i only had about 1.5v output on a 50Hz sine wave!

1.5V/4Ω=0.375A

0.375A*1.5V=0.5625W

0.5625W/12V=0.046875A

0.046875A+0.8A=0.846875

The math shows that i should only be reading 0.845A, not 2.0A