Re: gEDA-user: buzzing board

[DJ Delorie <dj@xxxxxxxxxxx>]

> 1) Have you put a "reasonable" 16 ohm load across the speaker pins of
> U302 and U303?

Done, no change.

> 2) The datasheet for TDA7056 shows a 5K resister between the input pin
> and ground.

It shows a 5k input impedance in the test circuit.  The other circuit
doesn't have the resistor.  My resistor and the input impedance act as
a divider to get the signal to the right amplitude.

> 3) from the dac to the speaker drivers you have an R C could you be
> getting a parallel inductance back through ground? Try shorting out C304
> and C305

Done, no change.

> 4) The gain control circuit is interesting to say the least.

Yup.  Normally, the pot sends a voltage from 0.4v to 1.0v to the
volume control pin, which is about it's useful range (given 4w
speakers).  The MCU pin normally monitors this via an adc, so it can
give some feedback on the display.  However, it can also force this
pin low to mute the amps (they go into standby when the input is under
0.3v).  The opamp is just a voltage follower because the pot is high
impedance and the amps aren't.

> You might isolate its output pin and ground the signal ground to
> ground.

If it's below 0.3v it doesn't matter, the amps are off.


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