Sound and Car Amplifier problem

[maxim]

I am having a problem with my MAME sound set-up using my car audio amplifier.

SYSTEM SPECIFICATIONS:

PC Computer with Sound Blaster 16 PCI soundcard
Sony 2x50 Watt Car Amplifier
PC Power Supply to power amplifier (12VDC)

Upon plugging in the computer audio output to the car audio amplifier, there is a huge THUMP and then the amplifier faults and shuts down.

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Now at home, I think i may have thought about what was wrong.  The computer was plugged into a separate power cord which did not have a ground pin (broken plug) while the MAME cabinet including the amplifier etc... was connected properly to a power outlet and grounded.

I assume that since the computer was essentially "floating", that the sound card outputs may have been "floating" and referenced possibly to the 120VAC line which would then overload the grounded input on the amplifier (I'm assuming the line-level inputs on the amplifier are not true differential inputs)

Any thought?  I'm hoping this is what the problem is.  I'll try powering up my cabinet again once I hardwire my power / ground to the computer inside my MAME cabinet.

THanks

[n0pe]

the setup should be:

Soundcard output -> jack to RCA -> RCA into amp (keep gain down, then raise up slowly)

Power supply -> Yellow (12v+) and black (12v-)

red wire is 5v+ and won't work.

hookup yellow wire to REM (remote) and POWER (+)

then the black wire to 12v (-)

i would recommend hooking up a 15a fuse before the amp to protect computer. the fuse on the amp is to protect the amp not the computer.

hope that helps.

[StephenH]

Consider a Separate Power Supply for that Amp!


The PC Supply probably does not put out enough amps at +12V to drive that big of an amp!     50W x 2 = 100 Watts.   If the amplifier were 100% efficent, then that would need about 8 Amps at +12V.     Since most amps are nowhere near that effienent, you probably need at least 16 Amps or more at +12V.   Note: the wattage of the power supply in general is not always what its outputs are.   Look at the supply's rating for +12V output.

What you need to get is a high current power transformer, designed for running Car Radios on house current.    I am not sure if Radio Shack even sells ones that output this kind of power.    

What you may want to do is ask Circuit City, Dow, or another Car Stereo store, what they use to power the car stereos on display, especially the higher powered ones.   A search of Google, HotBot, or other search engines for this type of power supply would be worth trying, too.

As an alternative, you could consider an AC line powered amplifier.

[liche]

Both of the replies are correct but not the solution to the problem.  The original poster was correct in the last statement he made.  The problem is a grounding problem, not power supply problem.

Since the computer was not properly grounded and the MAME cabinet was (where the amplifier was located) you have a huge voltage differential between the output of the soundblaster card and the input of the amplifier.  In a best case scenario, the computer is now referenced to the power line NEUTRAL wire which although connects to the GND at the electrical panel, there is enough impedance in the wire to drop at least 5-10 VAC on this wire.  So in effect, your soundcard output is not swinging at this 5-10VAC reference.  Sure, plug this into the amplifier which is properly grounded and you are effectively plugging in a 5-10VAC, 60Hz signal right into the amplifier.  You'll find that once you ground everything properly, it will work fine.


Here are some errors made in the above statements:

1.  A 100W amplifier will require an 12V, 8A.  This correct provided the amplifier is operating at full load conditions and is a Class D amplifier as most mainstream amplifiers are.
During normal gaming conditions (unless you want to be completely deaf), I would guess youd only be operating at may 10 watts per channel average.

2.  What you mean by high-powered transformer supply is actually referred as a linear supply.  These are not switching supplies are the benefit is a much cleaner output (much less output ripple).  However, for large linear supplies, they are extremely expensive, very heavy, and very bulky.  Circuit City, Best Buy, and other audio houses almost always use large switching supplies similar to those used in computer supplies.  However, there is much more additional filtering in this case.

[liche]

At lunch today, I managed to duplicate the problem as the original poster had.

I opened up my MAME cabinet, and disconnected the GROUND wire to my computer.

Upon power up, the amplifier immediately entered an overload fault condition.  

Measuring the line-level outputs from the un-grounded computer sound card in reference to the AC GROUND of the cabinet on my oscilloscope yielded a hefty 60Hz signal almost 12 VAC.  Definitely not good for the amplifier.

I think if you connect a ground to your PC, you'll find it'll work just okay!

[n0pe]

This is what I did.....

386/486 -- 250 watt PS, Switched PS - Leave On, will turn off from power strip - depends on setup.

Run 12v+(POWER) and 12v-(GND) to the amp.

Mame Computer -- Run 12v+ to the REM.


This is to turn on the AMP with you turn on your Mame computer. This will allow you to use your power from your computer without using any large amount of power.

If you know anything about amps then you know that all REMs run off the CDPlayer/Tape and doesnt use anything for power. all of it comes off the Main battery.

Hope that makes sence...