Bridging a car stereo amp

[worik]

Friends

I have a APX2120 power amp (designed for car stereos) and I want to use it as a power amp in bridged configuration. 

I do not know what to do wit the inputs.  Should I provide identical inputs to both channels or should I invert one channel?

cheers
Worik

[morgoth2006]

Earlier this week I thought about buying a cheap car amp and bridge it, so please tell me how it goes.
You can use this project by Rod Elliot http://sound.westhost.com/project14.htm
Also the October issue from EPE Magazine has a bridging circuit.
Just remember, the speaker impedance will be halved since each channel will see only half. The speaker choice will be critical.
The APX2120 says it gives 90W @ 2 ohms and 60W @ 4 ohms. Im not sure about the speaker level Input, you may need a good preamp?

200 Watts of Maximum Music Power
60W × 2 Into 4 Ohms, 20Hz-20kHz @ < 0.1%THD
90W × 2 Into 2 Ohms, 20Hz-20kHz @ < 0.1%THD
Selectable Bass Boost; 0dB/6dB/12dB @ 50Hz
Adjustable -12db/ Oct. High/Low Pass Crossover, 50Hz-300Hz
Gold Plated Connectors; RCA/Speaker/Power
Speaker Level Input
Dimensions 10 5/8" × 10 7/8" × 2 3/4"

Good luck!

[J M Fahey]

Hi Morgoth.
I have already posted here a bridger circuit, including VeroBoard layout and a practical layout suggestion, which can be used to drive your commercial car amplifier, and is powered from the same 12V supply, in:
http://www.ssguitar.com/index.php?topic=1442.15
Good luck.

[morgoth2006]

Well, like you said, "Now you have no excuse to avoid building it " eheh

This should give a nice weekend project.
Thanks!

[Enzo]

Are not most car stereo amplifiers already a bridged circuit?  (Neither side of the load is grounded)   And as such are they bridgable in the same sense as pro audio amplifiers?

[J M Fahey]

Hi Enzo.
AFAIK, most real cheap ones already are, being powered straight from the battery, those almost 20W per channel stretched magically up to 120W (and even 200W PMPO) by publicity..
These can't be re-bridged, as you state.
Now, the better ones have switching converters, can have any rail voltage they want (I have seen a couple hairy +/-70V ones) and are essentially repackaged regular HiFi/PA amplifiers.
There are even some advanced Class H or D out there.

[Enzo]

I knew anything over a few watts was SMPS powered, but I was under the impression even those 400 watt - or whatever - models were of a bridged output design.  Not?   I don;t work on car stereo at all.  The guy next door does those.

[worik]

Are not most car stereo amplifiers already a bridged circuit?  (Neither side of the load is grounded)   And as such are they bridgable in the same sense as pro audio amplifiers?

I am using an apx2120.  It is labelled as bridgeable.    Mostly, it seems from googling, done for bass boasts (where stereo looses its importance and power matters most.

But nowhere on the interweb can I find instructions on how to configure the inputs.  If it wants the same signal to both inputs or inverted.

cheers
Worik

[J M Fahey]

Badly written manual.
I checked it, it speaks and shows regular stereo and bridged mono output connectors, but it does not have a bridge(or Mono)/stereo switch or explain how to do it.
I had thought that maybe activating the crossover in LPF position *also* activated an internal bridger, but not, the 2120 example shows it driving *two* subwoofers, each with its pair of terminals.
Poor/confusing/incomplete manual.
Write them for detailed instructions, stating their manual is poor (so they do not refer you to it), and telling that otherwise you won't buy their product.
State that you want to use it for bridged full range audio, not subwoofer duty.
Good luck.