Becker "MB Sound System" amp analysis

[compu_85]

A good trip to the junkyard netted me a newish replacement amp for my 1991 350SDL's audio system (the label on top says it was produced in 1996, it came from an 89 car).



MB Claimed this setup was 25 watts per channel, for a total of 200w. Considering the physical size of the amps I was doubtful. So I took this one apart to see what makes it tick.





A fairly simple device. The 4 chips mounted flat on the board are preamp / compressors, ST tda7232.
http://www.datasheetarchive.com/pdf-...SA-634896.html

The 8 mosfet's are ST BUZ71FI.
http://www.datasheetarchive.com/pdf-...SA-561210.html
The datasheet says they are rated at 17 amps continuous, with a quick pulse of 68(!).

So I guess this thing really could put out that much power. The heatsinking really leaves a lot to be desired though, there's no thermal compound anywhere, you can see how tiny the internal heatsink is, and that just contacts the non-finned case to dissipate heat.

The construction of the cabling to the amps is quite good though, the line level inputs run in a cable with 2 layers of copper shielding

Overall the system preforms well in my car, but deep bass notes get really flabby. It's almost like it's overdriving the speakers.

-Jason

[electroking]

The power supply voltage has a lot to do with power capability... Do you know
how much it is?

[compu_85]

Being a car, it's 12 volts DC As I recall the speakers are 2 ohm.

-J

[electroking]

I meant the internal supply voltage. I trust there is some inverter in there to
generate a higher voltage. Does anyone know for sure?

On a 12 V supply, the most you can get into 2 ohms is (V^2)/(8*R), that
is 144/16 or 9 watts.

[trinescope]

Also remember this is a bridge amplifier, so theoretically the power number quadruples because the amplifier can push the 12V in one direction, then the other. In the real world, however, it's closer to double the power of a single amplifier. It's the equivalent of a single 24V supply in this case. So, ideally, the amplifier can deliver about 50W into a 2 ohm load. 12^2/2 = 72W. That's the peak to peak power rating. Normally you'd have to divide that number by 2 then multiply by .707 to get RMS, but since this is a bridge amplifier you'd simply multiply the 72W number by .707 which gives 50.9W. Of course there are losses in the MOSFETS and the filter circuits, so the power will be less than the calculated value.

This is a rather early example of a class-D amplifier so the sound quality difference doesn't surprise me.

[electroking]

Thanks for the explanation. I remember reading about bridge designs like this
in Popular Electronics in the late seventies, but I never sat down to analyze the
circuit. Regards.

[compu_85]

Thanks for the info guys!

-J

[coffee123]

Hah i remember those...in moms 420sel...i ditched the factory deck that had to be reset by the dealer every time the battery was disconnected for an alpine, and 4 channel amp. The system is 4 ohms, with the front door midbass and hi freq' drivers in the dash are wired in paralell, 8 ohms i think? been a while since i was in there...easy enough to upgrade, but could use some better midbass drivers. always did wonder how strong those little boxes were though

mark

[Gunslinger]

I think Becker is still in New Jersey.