Low pass filter on DAC output

[Tenson]

Hi,

I have made some modifications to my Behringer DCX crossover. I bypassed the output stage and went direct from the DAC chip to a pair of S&B TX102 attenuation transformers. Now, I did this with a DEQ before and a friend suggested I put an RC filter on the DAC's output (I presume to kill noise from the digital filter?).

The component values I used in the filter were a 100R resistor in serial and a 4n7 cap in parallel across the two phases.

Now it always worked fine from my DEQ into my Bryston amps, but I have now changed to using the DCX with these same mods into a different amp and I am getting some strange results...

When I tried an LC Audio amp, it blew up! With my Alesis RA500 amps it works for a long time, then every now and again it just looses all the treble. A reboot of the amp brings it back.

So.. I am thinking that there must be some high frequency noise on the line from my modded outputs which is upsetting both amps. Does that seem reasonable?

Having looked up the formula for a passive line level filter, I find that (I think) I should have used a 100R resistor with a 47nf cap rather than a 4n7. Is that right? I just want to be sure I use the right values.

So what value cap would I need for a low pass at, say, 40KHz? Am I right in thinking that the source and load impedance don't matter in this context?

When I connect my oscilloscope to the outputs it is nice and clean when the DAC is off but when turned on, the line is all fuzzy. However, it does that even when not playing anything…. So I am not sure if that is HF noise it is showing or just my scope being funny because it is very old and flaky.

Help for this noob much appreciated!

[Tenson]

Okay, I think I got it figured out now, found the time constant formula which makes more sense.