Highly evolved converters - the next generation

Its been over a year since I blogged about DACs, so, long overdue, here's an update. I've abandoned my considerable development on the AD1955 because I found something that's more interesting - multibit. In the first instance - NOS.

A while back I bought a TDA1543 NOS DAC to play with, just to see what all the fuss was about with NOS. Plenty of people have waxed lyrical about the sound. At first listen, I wasn't impressed although it had a certain tonal richness in portraying orchestral instruments that was alluring. Bottom line - its soundstage was compressed front to back. This made it sound a little 'forward' - foreground instruments and voices were more highlighted compared to acoustic spaces ('background').

A second aspect which plagues pretty much all NOS DACs is their frequency response can hardly be termed 'accurate' - owing to the sinc function of first-order hold, they exhibit a significant HF roll-off which begins around 5kHz and reaches 3.2dB at 20kHz. So HF energy is missing.

Given that I'm an acoustic space addict and I at least aim for flat frequency response I put the NOS DAC back on the shelf, to use only as reference for deciding if a particular dac was 'sibilant' or not. Given that it has only passive I/V, its not subject to the electronic (opamp) glare of other designs with active I/V and active filters. So it's been just my benchmark against which I measured a DAC for sibilance but not used for serious listening.

Earlier this year I was browsing DDS solutions on Taobao and found a DDS vendor also selling these. AD603 was a part I'd read about in Analog Dialogue when I was working in vibration monitoring and had been fascinated by the inventiveness of the gain controlling element. But I'd forgotten the part number long ago so had to look this one up. On downloading the datasheet I figured this part might make a good back-end as volume control for a DAC. Then a friend suggested 'why not use it in I/V?'. Up until that point I'd not been a big fan of passive I/V, but here was a part which provided sufficient gain AND a volume control element. Two birds killed with one stone seemed so attractive I had to try it.

One of the reasons for wanting to try this combined amp and volume control is a drawback of passive I/V I haven't so far mentioned - the one that kept me devoted to active I/V for so long. That is the spectre of DAC output compliance. In short, current-out DACs are designed for very low impedance loads as any voltage variation on their outputs gives current variations. For this reason, low value I/V resistors are preferred, to keep the output voltage in the 10's of mV range. That's not enough to drive an amp though, so designers of stand-alone DACs like the TDA1543 one I tried use bigger resistors to get a bigger voltage. This leads to greater distortion. Incorporating the AD603's not insignificant gain (34dB minimum) allows the use of sub-100R resistors for I/V duty.

The pic shows my prototype lash-up - I went for shunt regulation, two TL431s per rail. The 2-pin connectors to the DAC I/V resistors are on the far side of the chips. To be continued...

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