Sabtu, 13 Agustus 2011

TDA1541A DAC, Part 2, " measurements "

Now I have finished my DAC, also have done many tweaks (and still continuing...) It's time to do some "measurement", why the "...."? well because it is not very serious measurement, I have no mega hertz oscilloscope, no proper signal generator, but anyway there are plenty soundcard based software, and here is my favorite : Visual Analyser 2011, it is free and in my opinion, a must for DIY audio.

Why measuring ?, I think it is a proper step to at least measuring the frequency response of our DIY audio projects. We audiophile likes to have low distortion and flat freq response and how do you know that without measuring? no way, maybe possible with a very trained ears and a reference audio gears. Me for sure likes to see  what happened with my music signal, starting by putting simple sinus, square and pink-noise signals.

How do we start? first we must have test signals, some test CD are available, and you can make your own if you're lazy like me ;), I have my signals made with Audacity, loop recorded with EAC then burned to CD. The Pro's among us must be laughing when they see the 16 bit/ 44.1kHz digital sinusoidal waves, at 10kHz the signals looks like a triangle wave, at 20kHz even worst :). So I limit the signal up to 1kHz, a proper sinusoidal and square form still very good recognizable. With this frequency we can at least compare distortion and harmonics of our DIY gears, no more and no less.

I have 3 gears to test, my new DAC, my old tubed Sony CDP950 player and battery powered bit stream Sony DiscMan. I had a 8xTDA1543 DAC at home, listened to it, too bad have not measure it. Here is the setup when measuring CDP950.

It is very important to have any software like Visual Analyser calibrated  correctly or you'll get garbage results. Be careful with your measurement voltages, it can easily destroy your sound card. A simple voltage divider (variable resistor) or a full blown measurement jig like mine (SW Jig II) is a must. Measure the signal with your multimeter before connecting to soundcard.

What else? after calibration I measured the sound-card it self, I connected my laptop headphone out back to line input. I measure all signals including the 'silence' track to see the noise floor of my sound-card. Here are they, signal is my favorite 500Hz/0dB

the 'Silence' track :
 500Hz/0dB :
I know, I should have set the X axis to Log scale..hey...I'm just a beginner remember? What do we see here? the noise floor is below -114dB, and the 500Hz gives quite a lot of low magnitude harmonic spikes that will come back when you measure other gears. Ok, now my old CDP950, silence track and 500Hz see? that 500Hz thing looks way different than the sound card, notice the high (and nice sounded) even harmonics...well, it sounds muddier than my DAC alright, but veery nice..not enough for me now haha,  I want transparent and nice sound here, so the CDP needs tweaking for sure. How about the noise? well, it is just below -108dB, no worries for this AC heated tubed out CDP. If I have log X axis you can see the 50Hz and 100Hz peaks there quite high. And now my new DAC with very low noise Salas shunt regulator (big thank Salas...)

Now the noise? way below my CDP950, maybe a lot cleaner than the sound-card, and noise floor bit above at -114dB. And the 500Hz is way cleaner than CDP950, bet this that gave me better transparency :).

And how the discman measure? This is interesting, TDA1541 on my CDP and DAC are both multibit non oversampling, while the discman has 1bit deltasigma and battery powered, here is the 500Hz, forgot to measure the silence here, but you can see here as well, close to sound card noise floor. So battery does very good job indeed.

And this Philips Bitstream DAC measure very well, very transparent, much less harmonic spikes and in reality sounds smooth but have less soul in the music.Attack and decay in the music seems constrained, high is relaxed but bit feels synthetic / canny..I still prefer my NOS multibit TDA1541 here ;). I did try to measure the interesting square 1kHz signal, but its for next time :), thanks for reading and get this wonderful Visual Analyser right away. (below 1kHz square on sound-card). Happy measuring...