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Adjust volume after TI PCM1742 DAC?

 
 
Boki
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      09-29-2005, 02:59 AM

Hi All,
A box audio output is TI PCM1742 DAC, I want to connect to low
power speaker ( thin speake ) and I also want the volume is adjustable.


Do I have to add chip ?


or can I only add variable resistor to function that?


1. I have two channels.
2. The resistor can only decrease volume, right? our digital domain
audio signal to DAC already set to maximum, but I feel it is not loud
enoguh.


Could you please advice the method to adjust two channels volume
directly, or advice a chip that is suitable to connect after 1742?


Best regards,
Boki.


 
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Jan Wagner
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      09-30-2005, 05:05 PM
Boki wrote:
> Hi All,
> A box audio output is TI PCM1742 DAC, I want to connect to low
> power speaker ( thin speake ) and I also want the volume is adjustable.
>


Not sure how much is "embedded" here... ;-) Anyway, what do you mean
with "low power"? The PCM1742 drives very high impedance loads, so
something like a 8-ohm speaker likely won't do, and overloads the
PCM1742.

> Do I have to add chip ?
> or can I only add variable resistor to function that?


Have a look at the datasheet:
http://focus.ti.com/lit/ds/symlink/pcm1742.pdf
page 22 and onwards. Page 23 filter and buffer circuit (a) is one
that you can use. TI is using an OPA2353 opamp there, which can
drive 75ohm loads. Depending on your load, this could be sufficient,
or then you need to look for a different opamp.

> 1. I have two channels.
> 2. The resistor can only decrease volume, right? our digital domain
> audio signal to DAC already set to maximum, but I feel it is not loud
> enoguh.


It's probably not "loud enough" right now because you're overloading
the PMC1742. At least if you have the speaker directly connected to
it (via a series DC blocking capacitor, of course - you do have one,
right?).

And after adding the opamp: the PCM1742 has "digital attenuation",
which you could experiment with if you do not feel comfortable with
analog audio electronics design.

- Jan
 
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