PSU for an auto amp?

[Tom Smit]

I hooked up a power supply unit to my auto amp that I use for my AT....did this in the house. To supply the amp with signal, I tapped into the speaker wires that feed the ghetto blaster size stereo. The highest the voltage goes on that ghetto speaker is 1.3v. The PSU was plugged into the same power bar as the house stereo and put out 11.8v. When I hooked the first house stereo wire to amp input, all was OK. When I hooked up the other speaker wire to the remaining input there was I believe a 60hz hum. It didn't matter if I put them into the speaker input or the RCA input.

That is the setup. Now the question...why the hum?

A week later I brought a 12v car battery in, installed in place of the PSU and everything worked well.

[Splatmaster527]

Is it a PSU or a battery charger??
If its a PSU its most likely very underpowered since a 500Watt car amp would pull easily over 75A of DC current.
Anyway to the question. You have ground loop. In a car the amp and the source (head unit) share a common ground (car frame/body) in this situation you are sending a signal from a source with a different ground.
http://www.radioshack.com/product/index ... Id=2062214
Something like that should help your problem. Or maybe even just ground the amp to the earth ground somehow...

[CoronaOperator]

Sounds like ground loop to me. Take a DMM and measure the voltage across the negative of the psu and the negative of the stereo speaker wire that your using for signal. If there is a measurable voltage you will get ground loop noise. Try using an ipod/mp3/cell phone as your source signal along with the psu. If the hum dissapears you have a ground loop, if not its not ground loop noise and most likely your psu has major ripple which some automotive battery chargers tend to do. If your dead set on using the psu, and you have determined that the noise is from a ground loop, then a ground loop isolator will eliminate the problem.

http://www.thesource.ca/estore/product. ... ct=2700054

Before I would purchase one of these I would first try grounding your amplifier chassis directly to the stereo receiver chassis, just some left over speaker cable should suffice

[Tom Smit]

Thanks guys!

If its a PSU its most likely very underpowered since a 500Watt car amp would pull easily over 75A of DC current.
Anyway to the question. You have ground loop. In a car the amp and the source (head unit) share a common ground (car frame/body) in this situation you are sending a signal from a source with a different ground.
http://www.radioshack.com/product/index ... Id=2062214
Something like that should help your problem. Or maybe even just ground the amp to the earth ground somehow...

It's a PSU, and I have two more which I thought I'd try work in trio, perhaps using a diode trio if need be (I have a spare one available). Interesting that you said that it can pull 75A because another amp that I had in the car was protected by only a 20A fuse and it never blew(this amp has a 60A). Granted, it was an Audiovox 60 x2.

Sounds like ground loop to me. Take a DMM and measure the voltage across the negative of the psu and the negative of the stereo speaker wire that your using for signal. If there is a measurable voltage you will get ground loop noise. Try using an ipod/mp3/cell phone as your source signal along with the psu. If the hum dissapears you have a ground loop, if not its not ground loop noise and most likely your psu has major ripple which some automotive battery chargers tend to do. If your dead set on using the psu, and you have determined that the noise is from a ground loop, then a ground loop isolator will eliminate the problem.

http://www.thesource.ca/estore/product. ... ct=2700054

Before I would purchase one of these I would first try grounding your amplifier chassis directly to the stereo receiver chassis, just some left over speaker cable should suffice

I'll try a different sound source.
I'll check for a voltage difference between the amp and .....other things.
I was wondering about the potential of a ripple after mentioning the hum in a conversation with a buddy. If I used several PSUs in parallel, would a capacitor help with the ripple? and what size would be good?

[Bill Fitzmaurice]

That is the setup. Now the question...why the hum?

Ground loop.
http://www.rane.com/note110.html

[Israel]

which power supply are u using?? you can make one out of an old desktop power supply

[Tom Smit]

I think I used a 200 watt PSU but I'm not sure of the brand. I also have 2 or 3 other 2-3-400 watt PSUs, all of which were cannibalized from junk CPUs. I'm going to have another try in the new week using various methods.

Thanks.

[petegt]

which power supply are u using?? you can make one out of an old desktop power supply

+1 on this.

I have been using a 650W pc power supply for around a year for running car amps and it works very well. They are safe, well regulated, have over current cut out and thermo cut out. Its easy to do too and all you need it a resistor too provide a dummy load on the 5 v line. On the newer supplies which have voltage sensor lines they must simply be connected to the relevant outputs, mine has a sensor input for 5v and 3.3v.

You may not need one this powerful though just for home listening, I have had to use my car amp as an emergency back up to power small 8" tops before which it has done well. Just make sure you have a nice efficient amp with a low idle current

[Tom Smit]

Well, I tried the same PSU for a teen band-led Youth service at our church, and it worked OK. I did not have a hum. I did have another problem -buzz- which was solved by careful adjustment of gain and output volume of the powered mixer. Here is the chain ....200 watt PSU ->> car audio sub amp lowpassed at 80 hz ->> AT. I ran all the XLR inputs from up front of the sanctuary back to the main board, mixed back there and sent the mix back up one XLR to the front again. From there it went into one input of a Yorkville powered mixer and on to the two O10.5s on stands. Using the "monitor out" I fed the car sub amp. It sounded good.

I have one CD, which has five different European pipe organs built by Rudolph Garrels, and our organist heard it while I was trying to get the sound set up right. He actually thought someone was playing our pipe organ....but then realized that there was too much lows compared to ours (only 8' I think).
Before the service started I played the "Tonight" CD by Tobymac which has some good deep end.

About the 200 watt PSU, when the amp was off the PSU voltage was 11.8v, but when the amp was on, and when it was running the voltage was 10.5V. It still sounded OK, granted, I did not run it hard at all with the most being 5v to the AT. (I have to be careful with the "senior" members).