Audio impedance question

[tjnic]

Hi,

I am looking to build a pre-amp to drive a car audio amplifier from the DAC output of a DSP Chip.

The input of the car audio amplifier is balanced RCA and I want to drive it with a differential signal at around 3-4 Volts RMS.

My input is a single ended source from the DAC (approx 0.9 Volts RMS) which I am planning to couple with a 10uF Cap and a 560 Ohm resistor (I also intend have a 40 Khz lowpass RC filter on the input line)

I have been looking at using the Philips tda8542 audio chip for the preamp

http://ics.nxp.com/products/tda/datasheet/tda8542.pdf

This amplifier chip is obviously normally intended to drive loud speakers directly, so the datasheet only refers to output impedances of 8 and 16 ohms. Is there any reason why I cannot use it to drive my higher impedance balanced RCA amplifier input?

Can I simply connect the differential outputs from this chip via coupling capacitors into the RCA balanced input of the amplifier, or would I need to do something more than that to deal with the impedance?

Sorry if this seems a dumb question, but I am more used to dealing with digital electronics, and all this impedance business is a bit of a mystery to me! I would appreciate any assistance or alternative solutions, I am keen to provide a strong differential input to the amp as the input leads will be in an electrically noisy environment.

Regards


Tim

 

[Resqueline]

The impedance means nothing in these applications, as long as the load impedance is equal to or higher than the intended load impedance of the amp.
Be aware however that a high-gain amp may introduce noise (hiss), and other bad performance numbers.

But you're using the words balanced and RCA in the same sentence, and that's where I get really sceptical. What kind of solutions/compromises has been made?
Have a look at this application note. It's from a transformer maker so it has some transformer based solutions but there are op-amp circuits too.

 

[poor mystic]

I wonder whether the audio amplifier will be able to make use of such a large signal? 4Vrms as a sin signal is nearly 12Vp-p. I'm surprised the signal you already have is not large enough.

 

[tjnic]

The impedance means nothing in these applications, as long as the load impedance is equal to or higher than the intended load impedance of the amp.
Be aware however that a high-gain amp may introduce noise (hiss), and other bad performance numbers.

But you're using the words balanced and RCA in the same sentence, and that's where I get really sceptical. What kind of solutions/compromises has been made?
Have a look at this application note. It's from a transformer maker so it has some transformer based solutions but there are op-amp circuits too.

Thanks for that,

Differential-balanced RCA jack inputs are actually quite common on high end car audio amplifiers, I guess it allows a standard single ended input or a differential input to be used via the same socket. If you're interested, the amp in question is a JL Audio XD400/4

http://mobile.jlaudio.com/pdfs/18703.pdf

Thanks for the link, that is an interesting read.

 

[Resqueline]

Huh, I guess you're right, never heard of it before..
It's certainly way better than the usual unbalanced RCA systems, but it's not perfectly ideal either. I guess it's due to hassle-free compatibility as you say.
The XLR contacts are too big to be practical for car audio use, and DIN contacts were history decades ago.
Here is one example I found of a universal balanced/unbalanced converter btw.. It seems to have better spec's than the TDA circuit.

 

[poor mystic]

wow
if I had been asked to convert a balanced signal to unbalanced I would have earthed one leg but now I know there is a box which does it for me