[sdiy] Analyzing a Waveform

[Glen]

At 09:07 AM 5/31/03 , René Schmitz wrote:

>Most likely this is a differentiation that takes place on a highpass 
>somewhere in your system. Either a coupling cap or a transformer.

Since my microphone is a dynamic mic, and the system is DC-coupled from the
microphone to the output of the preamp, I would suspect that this anomaly
is the fault of the sound card. Would you tend to agree?

I assume that the input capacitors on my sound card aren't large enough, or
possibly are just horrible-quality capacitors. I wonder if trying to tack
some good caps onto my sound card is a good idea? Has anyone tried
something like this?



>It could  be that during that loud peaks the power supply momentarily 
>breaks down. (If the PSU is "soft" i.e. high internal resistance. Or 
>power decoupling networks make the PSU soft either.)

Well, the power supply for the preamp under test uses standard 3-terminal
fixed regulators capable of 1 amp output. (7812 + 7912 regulators) The
transformer that supplies these regulators is capable of 3 amps output. The
rectifiers are (full-wave) bridge rectifiers. There was no other load on
the power supply, and there are no decoupling resistors in series with the
op-amp's power pins. (I didn't mention it, but my preamp is a simple 5534
op-amp preamp.) I should have had plenty of current available for a single
op-amp.

I'm inclined to believe that this aberration might also be a characteristic
of my sound card. I have no earthly idea what sort of decoupling they have
used, but I do know that decoupling a computer power supply for audio
purposes is a real challenge.

I think I'll start testing the behavior of my sound card with other signal
sources. I really need to pin down what the acceptable maximum input level
is, and what happens when that level is approached or exceeded.

Thanks for your help René.


later,
Glen