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Chip Amps Amplifiers based on integrated circuits

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Old 11th September 2008, 05:21 PM   #1
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Default Chip based car pre-amp

Hi!

I am about to purchase a car audio system. The usual way were to buy a head unit, front speakers, amplifiers and a sub. I have however a problem with head units. As a source I use an iPod, all my music ripped an available whenever I want. So, I need a head unit with iPod connectivity. What do I get? If not for Alpine IDA series I get a CD drive I would not use. I get a radio I would use once in a month. I get inbuilt amplifiers I don't need with external amplifiers. And I get an iPod interface which is never as good as the original structure of the iPod. What I would use is a volume control and pre-outs.

Well, I could do that...Or at least I have built several pre-amps for home use. My problem is, that I have just discrete, class A preamps built with symmetrical power rails and now I have to make a chip based preamp around the single power supply of my car.

I would like to have suggestion as to which IC I should use for these application.

Thanks in advance.
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Old 11th September 2008, 05:49 PM   #2
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When Maxim purchased Dallas Semiconductor they inherited some nice single supply digital potentiometers -- push up/down -- Maxim's website is www.maxim-ic.com You could couple this with a single supply IC from Maxim or TI -- TI has a good working paper on making single supply opamps work.

TI has a series of digital pots with gain -- but you need to interface with a PIC, Stamp AVR etc. I have found that simple bit-bang routines work, but if you're not into micro-controller programming go with the Maxim products.
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Old 11th September 2008, 06:23 PM   #3
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Thanks. Actually I am planning to use a simple analog ALPS potmeter for volume

So question is just the IC, thanks for the advice with the TI.
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Old 7th January 2009, 01:32 AM   #4
shabaz is offline shabaz  United Kingdom
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Hi Banfi,

I'm working on such a circuit at present, for my own needs.
I'm using SSM2135, which is very good sound quality for a car. It can run off a single supply, which in my case will be a linear regulated supply of 8 or 10V from the 12V car supply.
As you say, an analogue potentiometer should be used here, since you just need to trim the level just once to fine-tune the
ipod volume level to the head unit. My circuit will be designed for an adjustable gain of 1-11, which is more than adequate.
I'm documenting it at http://360.yahoo.com/shabaz_yousaf
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Old 7th January 2009, 11:19 AM   #5
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Did you try to use the headphone jack as preamplifier output? It should have sufficient output swing and you could use the iPod's own volume control. Adapters should be easily available.

If you absolutely need a preamplifier, you can use nearly any op amp (NE5534A, OPA134, ...). All you need is a virtual ground, which you create with a rail splitter. That can be as simple as two resistors or more advanced with an additional transistors (e. g. see single supply application in datasheet for LM3886). Rail-splitters are also available as IC, e. g. TLE2426. With a rail splitter you always need DC blocking capacitors at in- and output.

Or you use the battery-based dual power supply option like on portable headpone amps.
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Old 20th January 2009, 03:12 PM   #6
rwaudio is offline rwaudio  Canada
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You could use a dc/dc converter, to get your +/- 15v rails from the 12v supply available. I've used the minmax mau119 (less than $5US) with good results and the slightly larger regulated version with even better results. They are all fairly inexpensive and available here http://www.cdistore.com/minmax/ then you can use any traditional split rail design that you want. Just be careful to get the right input/output filtering on the dc/dc.

The biggest advantage of that over a single supply system is less chance of noise since you have a new isolated ground, just incase your ipod is referenced to the cars chassis ground if your also charging it while you drive.
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Old 21st January 2009, 10:07 PM   #7
shabaz is offline shabaz  United Kingdom
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Hi rwaudio,

Absolutely. However, it is possible to go further:
Basically, I used a single supply rail (not +/- rails) to minimise
converter noise for my first stage. This stage increases the
iPod level from its low level (maybe 0.5v) to the desired level.
To ensure no ground noise, we need to ensure that the cable
from this amp to the iPod has very low resistance (easy to
control, because this is in our hands, it is not existing car
wiring). We can achieve it using substantial cable, and/or keeping
the cable length short. We can consider it a single ground point.

However, as you know, this does not prevent noise appearing
on that single ground, because it needs to be connected to
the car ground somewhere, and there is bound to be plenty
of noise as a result. So, my second stage uses a +/-15V supply
as you say.
Since this supply is at the same ground as my first stage,
my supplies will track the 0V+noise by +/-15V. At this stage,
I'm less concerned about converter noise, because my iPod
audio has already been amplified by a factor (I needed very high
voltage levels).
My second stage gives a balanced output.
Its not a perfect solution, but I think its the best compromise
so far, at reasonable cost.
I think this is a good idea, just waiting to prototype it and test it out. If it works, I'll share the diagrams.
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Old 21st January 2009, 10:07 PM   #8
shabaz is offline shabaz  United Kingdom
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Forgot to mention - the charging circuit is taken from the same point. Basically the single PCB is responsible for charging and for audio amplification. That means being careful with the PCB layout, but
these are things we can control, whereas existing car wiring to
cigarette lighter jacks, etc are hard to control.
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Old 22nd January 2009, 03:01 PM   #9
rwaudio is offline rwaudio  Canada
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Hi shabaz, I checked out your site, that's a pretty cool project you have going. What is the overall goal? And why balanced? (I know it's good, I've designed a few balanced line drivers, but those were to integrate with amps using balanced inputs)
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Old 22nd January 2009, 05:42 PM   #10
shabaz is offline shabaz  United Kingdom
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Hi rwaudio,

Just thought it would be nice to integrate ipod into BMWs, since they have balanced amp inputs by default, at around 11V p-p.
But the circuit contains both a normal and balanced output, so that balanced components only have to be populated on the board if this is needed.

Also, the microcontroller can be used for integrating external controls (e.g. buttons or steering-wheel controls) but again doesn't have to be populated if the ipod touch-screen is sufficient.

The board arrives tomorrow so will prototype it next week hopefully.

I'll keep you updated.
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