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Need help with Capacitor choice — Parallax Forums

Need help with Capacitor choice

CannibalRoboticsCannibalRobotics Posts: 535
edited 2010-03-21 21:41 in Propeller 1
This may not be the best forum location for this but·I'm building a prop based audio/video control. Part of it is driving IR, some serial and 2x 1 input 6 output line level volume controllers. This section takes a single stereo line level input pair and distributes·them accross 6 independently controlled outputs to amplifiers.
I've settled on the TDA7448 6x6 volume control chip for cost and power supply considerations.
Now the questions.
1) The data sheet application circuit (page 9 of http://www.st.com/stonline/books/pdf/docs/9227.pdf) shows 0.47uF caps on the input and 22uF caps on the output. There is no text in the data sheet on capacitor choices.
For the more expirenced analog guys, what would be the best choice for these capacitors?
2) If I am tying ALL of the inputs to the same source do I need 1 cap or all 6 as shown in the datasheet. My feeling is all six but am I wrong since channel sep is not an issue?

Many thanks,
Jim-



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Comments

  • LeonLeon Posts: 7,620
    edited 2010-03-21 14:58
    I'd use metallised polyester or polypropylene on the inputs, and electrolytics on the outputs (they are shown as electrolytics).

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    Leon Heller
    Amateur radio callsign: G1HSM

    Post Edited (Leon) : 3/21/2010 3:03:22 PM GMT
  • WolfbrotherWolfbrother Posts: 129
    edited 2010-03-21 15:00
    Hi Jim,

    Quick look at the datasheet and it looks like those caps are just blocking DC. You could probably get away with one cap in your idea but without looking at how the inputs are handled WRT to muted outputs, you might not be able to use this as a 1 to 6 line decoder.
  • CannibalRoboticsCannibalRobotics Posts: 535
    edited 2010-03-21 15:12
    The primary purpose of the volume control is to adjust/tune/ballance inputs to multple high power amplifiers for two or three zone configurations. Honestly, I had never planned on using the muting function via the chip. It would be handled at the pre-amp upstream of the two inputs. Still one?
    J-

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  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-03-21 15:50
    The input caps are to block DC, both ways. The input bias points of each channel might not be identical. So unless the cost/space is a problem, I would leave them in. Most comercial designs just copy the data sheet ccts.

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  • CannibalRoboticsCannibalRobotics Posts: 535
    edited 2010-03-21 16:00
    OK!
    Thanks All!
    Jim-

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  • Tracy AllenTracy Allen Posts: 6,666
    edited 2010-03-21 21:41
    The inputs are pulled to ground by a 100 kOhm resistors (fig. 13), so if you tie all the inputs together those resistors will be in parallel (=16.7 k&#937[noparse];)[/noparse]. Less than that, accounting for the impedance looking into the input transistorss, from table 5 the nominal input impedance is 50 kOhms. The input impedance probably changes when the selection switch for the channel closes. The impedance of a 0.47µF capacitor at 1000 Hz is about 340 Ohms, which is small compared to 50k or 100k, but less small with respect to 16.7kΩ or 8.3kΩ that you would get with all channels in parallel, and the switch closing on one channel might affect the level on the other channel. Use 6 separate capacitors! Film capacitor are best. You want a capacitor that does not change capacitance as a function of the voltage across it--don't use a high K ceramic.

    The DC output voltage level is 4.5 volts (table 5), and the output capacitors block that from the following stage. Usually the following stage will be biased at zero volts, so the polarity of the electrolytic capacitors is ok. The size of the capacitors can be adjusted if you know the input impedance of the stages that follow. A 22µF capacitor has an impedance of 7 Ω at 1000 Hz. The impedance of the capacitor has to be low with respect to the input impedance that follows, at all frequencies that you want to pass. Is it driving a 600Ω line? The capacitor forms a voltage divider with the input impedance of the following stage. Generally in audio electrolytics are used in preference to tantalums, as electrolytics are more forgiving if they happen to be slightly reversed biased, but you really don't want that to happen!

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    Tracy Allen
    www.emesystems.com
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