output voltage range for audio projects

colorado_hick

Well-known member
Hey folks I am working on a mixer for a modular-esque synth that uses a combo of Teensy audio library and a 40106 based oscillator.

The chip oscillator comes in pretty hot (im using a 5v power supply) compared to the teensy, which is fine but in playing around with resistors and potentiometers I realize that I do not know what the target signal strength should be for consumer-grade PA line level signal. The research I have done keeps coming back to impendence. Which still does not make any sense to me, if impedance is the AC equivalent of resistance then how can there be such thing as an output impedance? That seems to me like saying how many ohms a battery is. What makes more sense is peak-to-peak voltage range (which I can see on the O-scope).
Can anyone provide some guidance on what sort of voltage ranges I should be targeting? Anything that would help me relate that to impedance would be great to just so I am smarter tomorrow then I am today.
Thanks!!
 
A CD40106 CMOS inverter should not be getting even remotely warm if used in an oscillator at audio frequencies, so I'd be worried about the
temperature you mention.

Audio signals are normally low impedance out (< 1k), high impedance in (10k+), so that voltage drops due to loading are minimal.

For instance 100 ohm output driving a 47k input will droop only 0.2% due to the divider effect between source/load impedances.

Circa 100 ohm outputs are quite common in opamp circuitry as opamp outputs are unstable direct into a capacitive load, but a
small resistance like 100 ohms typically protect them from this.

Impedance and level are independent issues, BTW.
 
Don't compute impedance, measure it

by "hot" I meant high voltage. Bad word selection on my part!
How do a compute my impedance out?

You can't really compute the impedance because you don't know the internals of the chips inside. If it's important, you can measure it. The simplest way is to put various loads (e.g. a 1K potentiometer) across the output and measure how much the signal drops. When it drops to half of the no-load value, then you output (the audio board) and the load (your potentiometer) are matched; just measure the actual resistance across the potentiometer.

However ... it's more complex than that, because that only gives a static load. The audio board has a capacitor inline with the signal, so at low frequencies the impedance goes up; at zero Hz the impedance is effectively infinite.

On top of that, the impedance probably doesn't matter. As long as the audio board's impedance is lower than the load you put on it, it will drive the device with no problems.

And to make it even more complicated, the device may have distortion with low-impedance loads. You have to test for this.

I looked at the PJRC web site, and the output impedance isn't specified, or at least I couldn't find it.
 
That seems to me like saying how many ohms a battery is.
That really is a thing... Batteries have internal resistance (which changes with state of charge, temperature, battery age) - but often it is in the range of milliohms, so you can often ignore it, but you'll see it in datasheets for those batteries with datasheets. You can think of it as a measure of how good a voltage source something is - less internal resistance (a.k.a. output resistance) means a better voltage source.
 
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