Connect with us

Audio impedance question

Discussion in 'Audio' started by tjnic, Sep 17, 2011.

Scroll to continue with content
  1. tjnic

    tjnic

    10
    0
    Jul 13, 2011
    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
     
  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    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.
     
  3. poor mystic

    poor mystic

    1,067
    31
    Apr 8, 2011
    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.
     
  4. tjnic

    tjnic

    10
    0
    Jul 13, 2011

    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.
     
  5. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    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.
     
  6. poor mystic

    poor mystic

    1,067
    31
    Apr 8, 2011
    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
     
    Last edited: Sep 28, 2011
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-