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Master_Scythe

Car amp from laptop brick

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I have a 15v 5A laptop charger (cars can spike to 17v at redline, and normally charge around 16v so a car amp will handle that).

 

Am I right in calculating 15v x 5A, means i could power a 75W amp or is it not that simple?

 

(i only need stereo output) I'll try find an old 4 channel 'home amp' and use that, but I was hoping to just run 4 speakers @ about 20w RMS (4ohm), which i'd do with a 2 channel car amp.

 

Would a very old PSU be a better choice? Like a 300W PSU? I have both lying around. I just know the wattage is across all rails, not just the 12v.

 

 

Also considered is powering a standard old car head unit. It has a nice 'small' amp in it.

Edited by Master_Scythe

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im not too sure but i wouldnt try it, but thats just me sticking to rules and set ideas.

 

i know my 8 gauge wiring delivers approx 40 amps (varies on volume, dangerous shit to test aswell, sparks all over the place, see what lengths i go for you?)

but thats a monoblock running just one sub.

so might be different for a 2/4 channel amp

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Yeah, a sub will draw at least 100W by itself.

 

I want to provide about 40W into 2 channels, at 4ohm (20W per speaker, quadrophonic)

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A few issues you may need to consider.

 

1. Amplifier output power will be less than its input power.

 

No amplifier is 100% efficient. In particular, audio amplifiers are usually either linear, or they may be switching amplifiers (which may approach 90% or so). You really need to pay attention to the input current requirements, not try to deduce it from the output power. (Especially if that's measured in PMPO!)

 

2. Computer power supplies (AT, ATX, etc) are generally regulated by the 5V rail.

 

When demand on the 5V rail increases, all rails increase. The upshot is that you may find the voltage will suddenly drop on current peaks leading to current starvation, and distortion in the amplifier's output.

 

3. Computer power supplies do not like sudden loads.

 

Voices and music typically have lots of sudden peaks. Particularly voice, but also percussion. I found this out the hard way when I tried using a 600W computer PSU to power my Kenwood TS-120S transceiver.

 

The TS-120S requires 13.8V 20A to produce 100W (either SSB or CW). I noticed this power supply was capable of that, so I bought it and rigged it up. The remark was made along the lines of my audio being distorted. Indeed, if I tried winding the microphone gain up too high, I saw the ALC kick up and the backlighting dim to my voice.

 

The average consumption was probably in the order of 5A or so. However, in the space of maybe 2msec, it'd surge from maybe 2A up to 20A then back again. This was too quick for the power supply to react to, and would knock the stuffing out of it every time. Point 2 probably contributed to this too. The upshot is that the voltage would drop, and the "peak" would get clipped and distorted.

 

I wound up buying a 13.8V 20A linear power supply instead, which I've been using ever since.

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i ran my old kenwood head-unit off a 500w psu once

yeah it works ...but when that bass hits ~ shutdown

 

and a 'normal' car has a charge of 12.7 and while running charges at 14.7

if it spikes to 17v id be weary of it

Edited by st8smn

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couple of years back, I ran a pioneer cd head unit + soundstorm 2ch 800w (max) amp + 1 sony 12inch sub (250w norminal, 800w max) + 2x 60w 6inch something speakers

(i think they where made by fijitsu ten on it) all bridged together off the amp, was all running off a old superflower 450w power supply, had no problems, used it for like a month

till i got my car working again lol (broke the transmission + diff), the psu + pioneer head unit are still working till this day, although the PSU is now running my bro's pc,

the speakers + sub + amp arnt working anymore (thrashed them out too much over a 6year period :P)

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i ran my old kenwood head-unit off a 500w psu once

yeah it works ...but when that bass hits ~ shutdown

 

and a 'normal' car has a charge of 12.7 and while running charges at 14.7

if it spikes to 17v id be weary of it

13.6V~14V should be about right, or maybe ~14.5 or so when the car is running.

 

12.7 sounds like your battery is sick. Otherwise, your experience with the PSU pretty much mirrors what I had with the TS-120… only in my case it was the amplifier getting starved of power instead of the power supply shutting down.

 

Bass → sharp spike in current → voltage drop (because PSU is too slow) or shutdown (because inrush currents exceeded limits).

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hmm.. i was told a spike of 17v at max redline, especially on old vehicles was quite common (like my fathers 1962 pontiac) and it runs an amp quite well.

 

I'm looking for a cheap oldschool home audio amp, or one of those 'all in one' dvd players that have their own built in small amp. Some way to do this without spending hundreds.

Edited by Master_Scythe

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Well, you'll all be glad to know i bought a 'home amp' :P

 

DSE 8100, its 25W per channel 5.1, and 50W for the sub output.

 

Cost me $20 at cashies, with remote.

 

While most speakers are rated way higher than 25W (especially what I'm trying to power) All it needs to do is fill a small room, and anything will do that. I'm happy with my purchase.

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i dunno about earlier stuff , but iv got a 1972 HQ stato with a 350

and iv only ever produced 14.4 with 2 different alternator's

 

12.7 not running ,fully charged......

 

anyways good stuff , glad it worked out

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