Charles Hansen on DAC design
HOW DOES A PERFECT DAC ANALOG SIGNAL LOOK DIFFERENT THAN A CHEAP DAC6 hours ago, mansr said:
What makes the Pono Player so special? I gather it uses your interpolation filters. Anything else?
The thing that I see over and over and over in this thread is an irrational belief in the importance of the DAC chip itself. Just about everything affect the sound of an audio product, but when it comes to DACs, I would rank (in order or sonic importance the general categories as follows:
1) The analog circuitry - 99.9% of all DACs are designed by digital engineers who don't know enough about analog. They just follow the app note. The specs on the op-amps are fabulous and digital engineers are inherently seduced by the beauty of the math story. There are minor differences in the sound quality between various op-amps, but it's kind of like the difference between a Duncan-Heinz cake mix and a Betty Crocker cake mix. 99.8% of the op-amps are used a current-to-voltage converters with the inverting input operating as a virtual ground. This is probably the worst way to use an op-amp as the input signal will cause the internal circuitry to go into slewing-limited distortion. http://www.edn.com/electronics-blogs/anablog/4311648/Op-amp-myths-ndash-by-Barrie-Gilbert
With discrete circuitry, the only limit is your imagination. You are free to adjust the topology of the circuit, the brands of the parts, the active devices, the bias current in each stage - anything you can think of. Think of this as going to a world-class patisserie in Paris and seeing all the different things that can be made.
2) The power supplies - 99.9% of all DACs use "3-pin" power supply regulators, which are pretty much op-amps connected to a series pass transistor. Everything in #1 applies here.
3) The master clock - jitter is a single number assigned to measure the phase noise of an oscillator over a fixed bandwidth. It is far more i important to know the spectral distribution of the timing variations and how they correlate to audible problems. 99.9% of all DACs use a strip-cut AT crystal in a Pierce gate oscillator circuit. It's pretty good for the money but the results will depend heavily on the implementation, particularly in the PCB layout and the power supplies (#2).
It's hard to rank the rest of these so I will give them a tie score.
4) The digital filter - 99.9% of all DACs use the digital filter built into the DAC chip. About a dozen companies know how to make a custom digital filter based on either FPGAs or DSP chips.
4) PCB layout - grounding and shielding, impedance-controlled traces, return currents, and return current paths are all critical. For a complex digital PCB, 8 layers is the minimum for good results.
4) The DAC chip - almost everything these days is delta sigma with a built-in digital filter. Differences between different chips is one of the less important aspects of D/A converter designs. Both ESS and AKM have some special tricks to reduce out-of-band noise, which can be helpful, but not dramatic.
4) Passive parts - the quality of these can make a large difference in overall performance, especially for analog. Not many digital engineers sit around listening to different brands of resistors to see what sounds best.
These are just a few of the things that make differences in the way that a DAC will sound.
Hope this helps,