AT power supply and car audio

[dave m]

Ok,
I've got an old car power amplifier that I want to use to run some 50W rear effects speakers for my home theatre, it's a 300W amp so I'll be running it on tickover. The question is how can I power it? Any chance that I could use a leftover 230W AT power supply? What worries me is the 30amp fuse in the power amp, but will it be OK so long as I don't crank it up too high and maybe put a lower amp fuse in?
Thanks,
Dave

[Greebe]

It'll work fine and there is no real need to use a lower amperage fuse as most PS have over current/short circuit protection... just don't crank it!

[dave m]

Thanks Greebe. Strange result:
I gave it a go and the PSU kept cutting out even with input level on the amplifier on zero with no signal going in. Then I tried a 4A battery charger on the amplifier and it ran fine with the meter on the charger looking like it was pulling only about 1A.

So I tried to get the PSU to run a high pressure 12V tyre inflator, and it had no trouble with that at all, which surely must have been pulling a higher ampage. Must be one of those shy, sensitive types So I'll use the charger and keep the PSU for other 12V projects, at least I now know it can run some things.

Dave

[Greebe]

Alot of amplifiers require a considerable amount of idle current to run even at idle with no signal input. This is caused by the switching power supply within them being inefficient(<70% efficiency rating) and of older design. I know the amp I use for powering my sub (400 watt monster) requires some 5-6 amps of current just to start, but in my case I have a huge unregulated DC supply with a 50,000uF cap to absorb current surges that computer supplies lack.

Warning do not attach a large cap thinking it'll resolve this problem! It won't and will only do more harm than good.

It appears that your supply doesn't have the needed startup current to run the amplifier. But alas there is one more thing you can try.

I'm assuming that you have the amp directly connected to the computer supply (of AT type) with the power signal lead to the amp hardwired to the +12vdc output of the supply and are turning it all on via the PS ON/OFF switch

If so don't do this.

Instead powerup the supply and then connect the power signal lead to the amp or install a switch inline to enable you to do so remotely after a brief period of time (give it a few seconds to stabilize) then enable the power IN (enable) remote lead shortly there afer. This may overcome the inrush current problem you're experiencing and allow the amp to run normally.

What is your PS's +12vdc current rating?

If it's around say 12 amps max, then you must take ~5 amps (typical) and subtract it from the total available to detirmine how much the PS has in reserve to run the amp.
If the above example is true then you'll only have ~7 amps to play with which isn't very much... that figures to ~=<84watts total or roughly 42 watts per channel peak.

Hope this helps

[dave m]

Ok, I'll give it a go. I don't hold out much hope though, based on what you said. The power supply turned out to only be 200W, with a maximum output of 8A at 12V, so if it'll start at all I'll have virtually no currrent to run it.
Thanks again,
Dave