dice45 said:
Remco,
how dod your option 3 turn out to be sonically? I know 2 completely homebrewn DACs which use a tube output stage with a step down output transformer ... both have truly amazing sonics.
The CD enhancers are after a year and a half and more than 20 produced (and some by others as well) still the best I can do. All I have been able to better is details and components. I have a test version running from the output of an SACD player with 6BX7's helped by transistor CCS'es feeding into an amorphous core OPT. Output impedance is 8 ohms (!). Distortion below that of the dac. All this is is a super-duper version of the same design, nothing new. The sound of the output stage I would describe as transparent. You retain the 'signature' of the dac used (philips=warm, bassy, pioneer=relaxed, neutral, burr-brown=analytic, holographic, sony=dynamic, transparent, etc).
What fascinates me about the sigma/delta dacs, is that most of them are voltage output types, with only one bit weight. This avoids the necessity of I/V conversion, with the inherent jitter susceptibility in that. Jitter susceptibility is greatly reduced i any case by the randomizing nature of the encoder used. If the dac utilizes a good way of smearing its asymmetry, as e.g. Pioneer and Sony do, you are virtually jitter immune (Guido's klok did nothing vastly different in my Pioneer player).
Furthermore, the in-band noise-freeness and (often available) differential nature allow for a simple analog stage with all functionality included. IMO simpler is only good if it still has all functionality.
Since I've built so many of them, I've also done a lot of measurements. A lot. A huge lot... Although I'm an audio nerd, I am also a science nerd and while I like it when something is better, I can only truly appreciate it if I know why (this is IMO the only way to progress). Correlating audible results with measurements is very satisfying. My preliminary conclusion is that it's the absence of (T)IM distortion that gives a great advantage over opamps. Especially with 1 bit outputs (every bit is a full-scale transient), opamps cannot handle the steepness of the transients and run without fb half of the time, thus making for a two-faced circuit: slewing while without feedback, and being highly linear when the feedback kicks in. The distortion created while slewing is a Fourier product of both audio as well as sampling frequency as well as the duraton of the slewing period, which in turn leads to unpredictable products folding back into the baseband. According to propability theory, this produces a normally distributed layer of noise over the audio (with amplitude increasing as frequency increases (!)), none of which is measured when doing the normal sine wave testing. I have done some modelling and come up with a way to actually measure the consequences of IM distortion with arbitrary input signals (and not just sines, sines say next to nothing). The differences are dramatic. Although the measuring method does not provide much insight into the distortion mechanism, it clearly shows the magnitude (and I already know the cause
). I can't describe this method into much detail yet, since I've made it part of my university research in order to combine work and pleasure. Hope to have the theory complete in a few months (along with some less expensive solutions than amorphous core OPTs...)
Regards,
Remco
