FFT question with audiotester

[zigzagflux]

Originally Posted by SY

It's nice when your instruments help pinpoint a problem!

As frustrating and finicky the hardware/software is, you are so right- this $300 sound card has identified a number of problems with my builds over the months that my ears NEVER would have caught. In fact, I liked the sound of the AppleTV Trust both the ears and the instrumentation, and question when either one of them indicates a problem.

After messing around some more, I managed to get the laptop-optical direct method to match or better the sound card's spdif output, which is very comforting. Now I get the following results, which is what I was hoping for. -10dB on the graph is actually FS output on the DCX, 1.67Vrms.

Ultimately, my goal is to do some simple power supply upgrades to the DSP board, and take a look at before/after results. These are of course the before.

Hope by the end of the week to provide some results for those interested; I might be able to provide some aid instead of always asking for it....

[Pano]

Well done! Glad you figured it out. Was it something with the Toslink?
Your graphs look about like what I see with a transformer output on the DCX, H3 & H5, plus the odd noise up top.

Depending on the size of your transformer core, distortion will rise at lower frequencies.

[zigzagflux]

Okay, some pretty good results. I went about replacing the DSP board electrolytics with organic polymer caps. Removed IC7 and replaced IC8 with an On Semi MC7805ABTG, hand picked for 5.01V output.

Noise floor hasn't changed much, and the -20dB looks about the same, but there was decent improvement in the 0dB measurement. Lower 2nd, 3rd, and 5th. The 60Hz IMD around the fundamental is lower, too.

Still got some annoying performance with the toslink that I am trying to work out, and the power connector from PS to DSP board is garbage, but all in all a simple and effective improvement !

[zigzagflux]

Originally Posted by Pano

Depending on the size of your transformer core, distortion will rise at lower frequencies.

Right - I tried to squeeze the largest tranny I could inside the chassis.

[TheGimp]

Love that hump at 20KKHz. I chased a similar noise source for months. Do you have 48" florescent lights in the house anywhere?