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Need an small amp circuit for my car

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Long story short : my radio died in my Sorento and it's really expensive to replace, and it's a big molded thing.

Anyway, I tapped a stereo 1/8 jack to my two front speakers and what we do is plug a Walkman MP3 Player (like a usb key but with an output).
Needless to say, it is not very loud !

I'd like to make a small circuit to put between the 1/8 jack (well, let's just say the speakers..) and the MP3 player. If it could run on USB it'll be great, otherwise I can always use the cigarette 12V thing. But the USB stops as soon as I turn off the car so it'd be a big plus.

Now, I know I can use a small LM386 with a cap and stuff, but they're crappy. I want to have good audio. If I could double the volume that's be great...

I made one in school a long time ago, but it's mono. I guess I'd have to make the circuit double or change the chip for a "dual" one or something, but it'd then differ from my small reference. It's also 9v instead of 5, which is a bummer cause I'd have to use the cigarette thing.
The circuit is also made for 8ohm but I think the speakers are 32 (would have to verify).

So I guess, if I do that, could you recommend good parts ?
Or even, if it already exists, I'd buy it right away ! Does it ?

Thanks !
 

dr pepper

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I've used a couple of these before, dont expect audiophile sound or vehicle shaking bass, but if you want something just enough to listen to music at lower power these might be ok, they are 6 x the power of an lm386, but that still isnt much.
You'd need more juice than a usb port can provide to make as much racket as the original radio did.
http://www.ebay.co.uk/itm/PAM8403-5...066596?hash=item360733c264:g:QY8AAOSwGvhT7HY0
Car speakers as a rule are 4 ohm, meter them with a meter, if you havent got one, get one.
 

crutschow

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I recommend going with 12V for decent output power.
If you want the amp to be controlled by the ignition power, just tap into the auxiliary power from the fuse box.
 
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Great suggestions. I do have a meter, but I'm a bit lazy with the car these days (it's around -30 over here). I need a quick fix until I replace the whole unit around summer and don't require much power, the mp3 player alone is "almost" enough but it's made to drive small headphones lol. Twice the power would be more than enough tho, it works fine until we hit the highway right now
 

cowboybob

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Have you tried the current LM386 amp with one side (L/R) of the output from the MP3 player?

The 386 circuit should run off 12VDC just fine. Only problem would be the max DC voltage ratings of the electrolytic caps. If they're rated for, say 15-25VDC or better, then 12VDC source will be fine. I understand your preference for the 5VDC as a power source, but to get the sound level you'll need, I think adding a switch for control of the cigarette lighter source would be a small price to pay, if this little test works out OK.

As for the impedance of the speakers, you might have to "tinker" with the output cap's value a bit, but try adding a low R (guessing, about 10-15Ω, 1W, if you're correct about the car speaker's being 32Ω) resistor across the speaker leads to sort of even things up (audioguru will argue mightily about this, but for a simple test it'll do...).
 

audioguru

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Car speakers are 4 ohms or 2 ohms. Headphones are 32 ohms. Most car radios have bridged amplifiers that produce 14 real Watts into 4 ohm speakers per channel but they are advertised at 20Whats to 50 Whats with very high clipping distortion.
The LM386 overheats when it has a 12V supply and a 4 ohms (or 2 ohms) car speaker. The datasheet shows that any power supply voltage higher than 9V produces extra heat and hardly any more power. With a 5V USB supply its output at clipping is only 0.2W into 4 ohms.
 

crutschow

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Have you checked to see if the cigarette lighter is controlled by the ignition switch?
Mine is.
 

Tony Stewart

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To make an Audio amp, the theory is that the output impedance should not attenuate the AC voltage and the output should not clip or become distorted. To do this the ideal driver is a voltage source which is defined by zero source impedance. In practice , zero is impossible so the quality factor is defined by this ratio of load to source impedance. The output impedance of an bipolar Op Amp is normally quite high around 300 ohms can be reduced by the negative feedback gain of 10k at 100 Hz thus becomes Zout becomes 300/10k= 30 mOhms but if the current exceeds Vout/300 ohms or (12-2Vsat)/300ohms ~= 33mA which means the speaker if 8 ohms gets only 33m*8ohm= peak voltage, which is not a lot.

So the practice of impedance ratio must also include current drive limits which is where emitter followers come to be included in all Audio IC's to increase the current gain.
So linear audio amps have low Zout to give high impedance ratio which we also call dampening ratio . Normally due to saturation drop in Car Amps you lose +/-2V so even with 14.2V you only have 10V²pp/8Ohms or 12.5 Watts approx and with differential outputs 24W. ( sometimes rated as 11W and 22W per speaker for single out and diff out.

We know from impedance matching theory maximal power transfer occurs when load matches source impedance, but in audio cases, the internal current distortion can be too high or clipping. So we expect the Op amp maximal power to be the same as the open loop resistance of 300 Ohms. Adding an emitter follower with hFE of 100 reduces that to 3 Ohms and a Darlington reduces that by hFE² at the expense of reduced voltage swing but significant more power with low impedance speakers.

Next changing Darlingtons to complementary MOS switches and using a LC filter to remove PWM modulated Audio IC's now you have a switched mode audio power source. Which you can make for a few bucks or buy even better designs. Note that the feedback R is after the complementary FET driver below to make the output linear. The same is done with complementary Darlingtons.


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tcmtech

Banned
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If it was me I would just go buy a cheap car amplifier with 2 or 4 channels and a cheap headphone plug to RCA converter cored and be done with it.

I see no point inf spending more money and time doing a DIY mini audio amplifier build and trying to integrate that into a vehicle when there are millions of small plug and play units that could be used for far less and will do the job far better. :rolleyes:
 

audioguru

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Normally due to saturation drop in Car Amps you lose +/-2V so even with 14.2V you only have 10V²pp/8Ohms or 12.5 Watts approx and with differential outputs 24W. ( sometimes rated as 11W and 22W per speaker for single out and diff out.
What kind of Watts use peak to peak voltages? Whats?
10V p-p is 3.54V RMS. The power in an 8 ohms speaker is only (3.54V squared)/8 ohms= only 1.6W, not more than 12W. Into a normal 4 ohm car speaker it is only 3W. Bridged (an amp driving each speaker wire with opposite phase) you get about 11 Watts.
 

dr pepper

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If you look in the existing radio at the output ic's for the speakers, then search for the datasheet, you might be able to tap into the input to them, and route it to a headphone socket as an input.
You might also need to figure out how to get the amp to power up, usually a logic i/p.
Assuming the audio side of the radio is ok.
 

audioguru

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Beware of amplifier datasheets that state the power at 10% horrible clipping distortion. Only acid rock "music" has that much distortion but it makes the power number look higher. If the output is grossly clipping so that all sounds are squarewaves then the power number is doubled.
 
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