DACs and bass response?

The J-test uses a known digital signal and analyzes the spectrum of the analog final output of the DAC for spurious frequencies that should not be there. It is actually a complete approach to testing the entire audio chain from digital signal passed through any cable into the digital input of the DAC and a detailed examination of the final analog output. Any jitter at all anywhere that reaches the D to A chip will result in quite obvious spurious analog signals that do not match the known input. Any source jitter, cable jitter, interface jitter, LIM jitter and intrinsic jitter (in fact the sum of all and any jitter) is tested for.

I don't believe the DAC output is a good indicator.  It will change with every DAC you use in the test, so testing for instance a reclocker or a USB or Ethernet interface would be insufficient.  What if the particular DAC used introduces spurious signals unrelated to the jitter of the incoming signal?  It is a result of the DAC, not the USB converter driving the DAC for instance. I also don't like using a fixed signal.  I would rather see actual music data.

I have seen lots of J-test plots in Stereophile over the years.  None of them seem to correlate well with sound quality.  I think this direct digital measurement I am making is superior to the analog measurement for correlating to SQ. So far, its helping me make decisions on future products.

Steve N.

Empirical Audio

Hi Steve. I took a cursory look at the overdrive info on your website http://www.empiricalaudio.com/products/overdrive-dac and user manual. I couldn't find anything about the type/kind of D/A chip(s) or whether or not your technology is considered delta sigma, multibit r2r or something else. Please advise.

I decided to use a Analog Devices delta-sigma, but only because I can select the digital filters for it rather than it selecting the filter for me.  I cannot seem to get the HF transient response from older ladder chips that I can from newer delta-sigma chips.  It also supports 24/192 PCM, as well as allowing for reference voltage-based volume control.

I previously offered a tube DAC design using a ladder chip called the "Spoiler". It was very musical, but ultimately did not deliver on the accurate HF sonics that I'm after.  It also required hard-to-get expensive NOS tubes to sound really good.

The only other option is to design a ladder DAC from scratch or in an FPGA.  I don't have the bandwidth for that unfortunately.

BTW, audiostream will be reviewing my new SX DAC soon.

Steve N.

Empirical Audio