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Old 30th October 2011, 10:19 AM   #1
Gates67 is offline Gates67  United States
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Default Need help building an amp for car

Ok so a little background.
I moved to a much bigger city, got my car stereo stolen on the first night.
Well that was a two months ago.Finally get a new stereo put in and last week I see my window is smashed out and its gone.

So instead of buying a new stereo to get it stolen again (and another window smashed) I want to build a chip amp to replace it.I will put it into a small enclosure and just drop it into the dash (into the hole where stereo was) so itll look like there is no stereo at all.

Now I only want a 3.5 jack in and volume control knob. I will only use a smartphone with mp3 and radio apps so its needs to just be an amp. I dont need huge volumes but be reasonable for a small car.

Is this possible? .Ive built a few guitar pedals with (bought the pcbs already made. didnt want to mess with chemicals). Ive seen the altoid tin headphone amps and its been on my to do list. There should be a souped up version that would handle my car (its a small vw).

A car puts out 12v dc so thats ok for chip amps correct?
Any ideas?

Thanks
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Old 31st October 2011, 02:26 AM   #2
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Philips TDA1554Q, Philips TDA8561Q

Suited to car speakers
Enough power to avoid clipping, despite 12.5vdc~14.8vdc car battery, car alternator range
Can be made point-to-point low parts count, no board
Runs cool enough to hide out

You can use two of TDA1554Q to power 4 car speakers

With all car purpose bridge chips, its necessary to use an input load, such as a dual gang 10k stereo potentiometer for the front channels and a pair of ordinary 10k resistors for the rear channels. There are many other possibilities as well.

To block DC from coming out of the chip and frying your input load and mp3 player, use an input capacitor for each channel of the amp. Size this capacitor to make a slight reduction on frequencies lower than speaker capacity. Its an especially important concept for little front speakers.

With an Iphone, it may be useful to add a little RF filter cap (or RC) at the amplifier's input, so as to avoid phone blips and other non-audio signals.
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Old 31st October 2011, 12:58 PM   #3
AndrewT is offline AndrewT  Scotland
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Don't use an lm3886. This is designed for higher impedance speakers than commonly used for In Car Audio. It is also designed for higher supply voltage than available to In Car Audio.

Select a chipamp designed for In Car use, i.e. to operate properly from ~11Vdc to ~16Vdc
Designed to drive lower impedance speakers, eg. 4ohm

Usually these chips use some form of bridged output to roughly triple the power they can deliver compared to a non bridged version.

The maximum output voltage that a non-bridged chip can deliver when powered from a 13.8Vdc supply is ~6Vpk, this is equivalent to 2W into 8r0.
Using a 4r0 load, this maximum power output increases to ~4.5W
A bridged amp could possibly give ~13W into 4r0 if designed well.

Beware manufacturers' specification quoting 25W into 4ohms @ 10% distortion from >=14.5Vdc supply. They are hiding bad performance behind useless specifications.
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Old 31st October 2011, 01:20 PM   #4
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by Gates67 View Post

Now I only want a 3.5 jack in and volume control knob. I will only use a smartphone with mp3 and radio apps so its needs to just be an amp. I dont need huge volumes but be reasonable for a small car.

Is this possible?
wouldn't you also need a mp3 receiver/converter
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Old 31st October 2011, 05:00 PM   #5
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Quote:
Originally Posted by tinitus View Post
wouldn't you also need a mp3 receiver/converter
The amplifier's input cable has a headphone plug, and the MP3 player's headphone jack is used as audio source.
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Old 31st October 2011, 07:11 PM   #6
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The closest match is this:

TDA7850 Car Audio Amplifier Kit with BA3121 Denoiser NW | eBay <---link
QC: The item was reviewed by a couple of owners as well working

You'll need a stereo (dual gang) potentiometer for the front channel. <---link

If you're up for it, you might want to try replacing the input filter caps for the front channel to something that decreases bass at pitches lower than speaker capacity. This *might* sound better than the smd caps, but it will for sure sound better than x-max. A little polyester from Wima or maybe even from Radio Shack, is workable to arrange a little bandwidth trim. The concern is tiny speakers, fairly powerful amplifier and an iPhone that may have its bass booster switched on.

The rear channel will also need an input load, but that can be simply resistors from input+ to input ground.

Screw terminals of that sort are unwise in actual use--solder your connections under-board instead of using the screw terminals.

The chip will need a heatsink.

You will also need an iPhone compatible cable, which would have the 3.5mm plug's outer dimension small enough to fit the headphone jack. I've seen 3 foot long iPhone headphone extension cables that could probably be re-purposed for amplifier input cable.

Although this module is pre-assembled, there are plenty of opportunities for soldering practice, such as making all those speaker connections and adding the input loads and input cable.
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Old 1st November 2011, 07:26 AM   #7
Gates67 is offline Gates67  United States
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I like that idea of that pre assembled board,Ill make some other more substantial projects later.For now housing and wiring something like that up will be more than enough.

I have a few male-male 3.5 cables so I will just get a small jack for the enclosure.

Now a few questions. Where would I wire a power switch? Would it just interrupt the 12v + lead going to the board?


I see there are 4 inputs.I assume input A has output at A, channel B at B and so forth. That means I must use all 4 inputs for wiring the jack.
Tip/left ch. would lead to 2 of the inputs (one for left front and one for left back,say A and B) and ring/right channel to the other 2 amp inputs C and D right front and right back. Sleeve ground would go to all 4 ground.
Is this correct

The pot is confusing me, Its for the volume control correct?
If the pot is the load for the front L/R, and the loads for the back channels are resistors then that means its not wired to the pot. How does that work, wouldnt they be on full blast all the time?

Im obviously missing something here

Thanks for all the help
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