Audio Circuit Help

[Brandon]

I'm working on a project in my free time and I think I have a
generally good design but I there are two or three issues that I need
to address before I start building a prototype. I'm sure some of
these are basic for experienced electrical engineers and the likes but
hopefully everyone will cut me at least a little slack. I'm just
a hobbyist.

My first question has to do with grounding components in a circuit.
In the circuit I am working on, I am taking an audio signal and, among
other things, running the signal through some opamps (sometimes for
amplification, sometimes for buffering) that are powered by a 9V DC
power source. When I ground components, can they all be run directly
to the ground bus without affecting one another? What I am afraid of
is losing the buffering I am getting by using opamps by having them
share the ground bus directly.

The second question I have has to do with picking resistors for use
with an opamp. In an inverting configuration, an opamp's Vout =
Vin(Rfeedback/Rinput) so to achieve a gain of 10, your feedback
resistor would simply have to be 10x that of our input resistance.
But what other considerations are there in terms of power consumption/
efficiency, noise, etc.?

The final question I have has to do with reducing interference. In
audio applications, it's common to use high and low pass filters to
get rid of interference at "unwanted frequencies" like radio
stations. In cases where there are a lot of "stages" (separated by
opamps in this case,) is it common to filter those unwanted
frequencies at each stage or would it be acceptable to just filter
when the signal first comes in? Pristine audio is my goal in this
project.

Any help would be appreciated.

-Brandon R

 

[Bob Woodward]

Brandon wrote:

In the circuit I am working on, I am taking an audio signal and, among
other things, running the signal through some opamps (sometimes for
amplification, sometimes for buffering) that are powered by a 9V DC
power source. When I ground components, can they all be run directly
to the ground bus without affecting one another? What I am afraid of
is losing the buffering I am getting by using opamps by having them
share the ground bus directly.

As long as your GND is sufficiently low-Z there is no problem.
This is achieved by wide, short traces or a ground-plane on the pcb.
avoid creating a loop in the ground circuit.

When using ONE 9V power supply you have to create 1/2 supply as a
reference. Make sure this one is also low-Z, at least for audio.

When using just 9V power, your choice of op-amps is limited and
specially low-noise and low distortion don't match with low supply.

The second question I have has to do with picking resistors for use
with an opamp. In an inverting configuration, an opamp's Vout =
Vin(Rfeedback/Rinput) so to achieve a gain of 10, your feedback
resistor would simply have to be 10x that of our input resistance.
But what other considerations are there in terms of power consumption/
efficiency, noise, etc.?

To low values => not enough drive current
To high values => noise + limited bandwidth
2kOhm - 100kOhm is OK
op-amp outputs driving an interconnection-cable should have at least
some 100 Ohm series resistor to avoid oscillation.

The final question I have has to do with reducing interference. In
audio applications, it's common to use high and low pass filters to
get rid of interference at "unwanted frequencies" like radio
stations. In cases where there are a lot of "stages" (separated by
opamps in this case,) is it common to filter those unwanted
frequencies at each stage or would it be acceptable to just filter
when the signal first comes in? Pristine audio is my goal in this
project.

RF rejection of inputs by using inductors, ferrite-cores, small caps.
Op-amps with moderate to high gain or driving loads should have some
22pF compensation in their feedback.
Avoid multiple filtering ( low-pass or high-pass ) at the same frequency
If your project needs to pass official tests concerning EMC, also
apply RF filtering of outputs and supply. There is a lot of info
on the web about this.

Robert