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One more stupid electronic question, sound this time. — Parallax Forums

One more stupid electronic question, sound this time.

JohnGayJohnGay Posts: 57
edited 2012-03-01 04:31 in Propeller 1
I've got a Quickstart board and am making an adaptor for TV out. Thank you for the answer regarding the Video circuit.
Now for the sound. Since I'm going to be using the audio inputs on my TV, can I remove the amp chip from the demo board schematic and just use the 10k resistors and caps straight to the audio inputs on the TV? This point should be at line level, correct?

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-02-28 16:37
    Line inputs typically have an impedance of 600 ohms, so you may not get enough volume from a circuit providing a 10K impedance. But just decrease the resistance by a factor of 10 and increase the capacitance in the lowpass filter by the same factor, and you should be okay.

    -Phil
  • cavelambcavelamb Posts: 720
    edited 2012-02-28 16:43
    These ARE the outside audio inputs to the TV, right?
    Not a connection inside the case?
  • JohnGayJohnGay Posts: 57
    edited 2012-02-28 16:58
    Yes, the RCA connectors for left/right audio in. I'm planning to just use a long A/V cable soldered to my adaptor board.
    PhiPi:

    OK, I know you mean drop the 10k resistors to 1K, but for the caps, are you talking the 1uF in series, or the 10nF caps to ground?
  • CircuitsoftCircuitsoft Posts: 1,166
    edited 2012-02-28 17:11
    Line level, in pro audio, may be 600 ohms, but RCA audio inputs are rarely <100K. They should be fine straight off the demo board, but the amp chip doesn't hurt anything. May as well leave it.
  • JohnGayJohnGay Posts: 57
    edited 2012-02-28 17:19
    Except I'm building a simple plug-in board to attach to my Quickstart board from scrounged parts, not removing parts from a demo board.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-02-28 17:41
    The 10 nF caps to ground are part of the low-pass filter, so those should be increased to 0.1uF. You may also have to increase the series capacitance if the input impedance of your TV's audio input really is 600 ohms. If you don't, your low-frequency response will be diminished.

    -Phil
  • JohnGayJohnGay Posts: 57
    edited 2012-03-01 04:31
    Found another schematic which uses a 220 Ohm resister, 1uF cap in series and 0.1uF cap to ground for RCA connectors. Think I'll go with this one instead.
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