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Author Topic: Using car HiFi amps  (Read 3192 times)

syndromet

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Using car HiFi amps
« on: June 18, 2007, 10:00:05 AM »
Would it be possible to use something like this with guitar?


Specs
Distortion:                            <0,08 % THD at 2ohm
                                          <0,1 % THD at 4ohm
Frequenzies                           20 Hz -20kHz +/-0,5dB
Dampening                            >95dB
bass boost                            +8dB, centered around 45hZ
Input impedance                     20kOhm
Input sensitivity                      150mV - 8V
Output impedance                   Stereo: 2-8 ohm
                                           Bridged: 4-8 ohm
Dc input                                10-16 V


RMS                                     2 x 50W at 4 ohm and 14,4 V
                                           2 x 75W at 2 ohm and 14,4 V
                                           150W bridged at 4 ohm
Max power                             200w bridged at 4 ohm
Fuse                                     20A


Protected aginst shorts, to high dc and overheating.

Looks like a typical poweramp to me, and the chassis is made of tinned aluminium. Would it be suiteable with guitar ,bridged for mono and high power with the right preamp?

teemuk

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Re: Using car HiFi amps
« Reply #1 on: June 18, 2007, 03:41:56 PM »
I don't see why not - with the right preamp of course. Like you say, it's just a typical power amp - although a HiFi one.

However, I would assume there will be a few problems: You need a big power supply for it, something like a car battery or a huge power transformer: Have you calculated how much current that amp will draw. A 12V transformer with suitable VA will be colossal. And think about rectifying that current. In the latter case (or perhaps both) you also need some huge caps. That thing is going to get quite heavy. Since preamp is about mandatory you have to wonder how to power it up as well. If you decide to use the same 12V supply it will be somewhat a limit. (Possibly you might find a low voltage dual rail from the amp if you probe around).

Secondly, for those power ratings the amp must likely use a switched-mode power supply (or maybe it's digital). Personally I don't like the idea of high frequency switching in linear amps because noise from it might leak to output signal. The SMPS circuit is also quite hard to fix if it decides to blow.

Short circuit-, output DC- and overheat protection is nothing fancy. You find most of those features even from a chip amp. Besides, there are numerous ways how to implement them - some better and some worse than the others. Truthfully, I think that amp wouldn't be anything remarkable if used as guitar power amp. I would rather invest the money that "project" would need to a decent 150W, SS guitar amp. Maybe buy one second hand. Likely even a typical consumer-quality HiFi amp would be a better choice than a car amp (plus it already has the power supply). Both might be a bit more expensive (assuming you have to build the power supply then likely even cheaper) but likely would prove to be a lot more versatile in the long run. Last time I checked a second hand stereo 75W hifi amp was being sold at about 5 eur in a local fleamarket. Anyway, that's just my opinion.
« Last Edit: June 18, 2007, 03:45:29 PM by teemuk »

joecool85

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Re: Using car HiFi amps
« Reply #2 on: June 18, 2007, 08:52:48 PM »
I've done it before.  When I was in highschool I hooked my brother's bass up to my amp/subwoofer in my car.  It worked quite well actually.  Like TeemUK said however, it will be a chore finding a power supply for that beast.

However, if done with a car amplifier, you would have an extremely unique guitar amp that could quite possibly sound very impressive.  What I would do is try it with the amp installed in a car, if you can make it sound killer, worry about a PSU at that point in time.
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syndromet

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Re: Using car HiFi amps
« Reply #3 on: June 20, 2007, 03:13:00 AM »
Well, a friend of mine is selling me the amp for 100NOk, aprox $17, brand new in box. Sounds like a good project to me. Now I have to build myself a suiteable power supply. calculating from the watts

150w/14,4V = 10,41A

 10,41A to runit bridged with a 4 ohm load on 14,4V. Since the VA rating and W is equal, the absolout mininmum transformer I could run it on is 150va



20A*14,4V = 288VA
The amp has a 20 A fuse, wich tell me that the amp will never use more than 288VA before blowing the fuse, bridged at 14.4V.


12V*20A = 240VA

14,42 / 12,02 = 207,36 / 144,00 = 1,44.
This gives that the amp puts out 1,44 times the power at 14,4volts. It probably only use about 60% of the PS to amplify, wich means 390Va.......

150W *0,56= 84W

 I'm planning on running it bridged on a regulated 12v supply, wich gives me a max consumption of 390VA, and a wattage of 84W RMS.

So what I should be looking for is a transformer with 240V primaries, 9-15V secondaries (it will be rectified and regulated at 12V) and a VA rating abowe 390Va, right? Thats kind of hard tol find....
« Last Edit: June 21, 2007, 01:47:08 AM by syndromet »

joecool85

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Re: Using car HiFi amps
« Reply #4 on: June 23, 2007, 07:09:10 AM »
To make sure you have enough "room" to regulate the circuit down to 12v (most of the time you need to start with 15v) you will want 11v or more on the secondaries.  390VA seems about right.  And yeah, it will be hard to find.  I'm pretty sure you could use multiple transformers though, IE - 2 x 11v @ 195VA tied together in parallel.  Of course, you could tie as many as you want together that way and you might be able to get like 5 or 6 small ones for super cheap.

**edit**
The hardest part is going to be regulating it btw.  I would almost just set it up so that it has 15v with no current draw and let it stay unregulated.  Car amps are made to take anywhere from 10.5v - 15v (sometimes all the way to 18v, yours specifically says 10-16v, so you're good there) due to the charging characteritics of the car's electrical system.  Granted, most amps don't put out their best power ratings until you are at 14v or more.
« Last Edit: June 23, 2007, 07:13:35 AM by joecool85 »
Life is what you make it.
Still rockin' the Dean Markley K-20X
"New" amp: Fender Frontman 25 DSP (FM25DSP)
thatraymond.com