Burr-Brown PCM63P
Two 19-bit converters stacked per channel, a matched R-2R ladder, and a distortion figure that made this the multibit chip builders reached for when one-bit designs weren't good enough.
Overview
Burr-Brown's PCM63P is a 20-bit monolithic audio DAC. Inside the chip sit two 19-bit converters - one handling the positive half of the signal, one the negative half - sharing the same R-2R ladder network and the same reference voltage. The reason is bipolar zero distortion, the glitch conventional DACs generate when they switch the most significant bit across zero. Splitting the waveform between two converters and synchronizing them exactly avoids the non-linear artifacts that show up right where the ear is most sensitive.
The part came in three grades. The unsuffixed PCM63P is the cheapest, the PCM63P-J sits in the middle, and the PCM63P-K is the dearest. What separates them is accuracy without calibration. At 1 kHz and full drive, the -K version is specified at a THD+noise figure of -96 dB, the -J at -92 dB, and the base part at -88 dB. Elektor's construction write-up noted that the spread between the cheapest version and the -K could run £10-£15 depending on market. That was real money at the time, and the difference between an -88 dB part and a -96 dB part is not cosmetic. The -K is the one the high-end builders specified by name.
When Elektor built a full 20-bit converter around this chip, they paired it with Burr-Brown's DF1700 digital filter, whose pass-band ripple they measured at under 0.00005 dB. The filter's interpolating steps raise the resolving power at its output beyond the 16-bit input, and a converter accurate to more than 16 bits preserves that extra resolution through conversion. The PCM63P was, in Elektor's assessment, Burr-Brown's top multibit converter at the time. The magazine was blunt about why they chose it: one-bit converters were in fashion, but in their view they didn't deliver satisfactory results for top-of-the-range equipment.
The chip also turns up inside finished products, not only in DIY builds. Parasound's advertising for its D/AC-1500 leaned hard on the part, billing the converter as carrying four K-series Burr-Brown PCM63P DACs - "the highest grade available" - alongside dual mono, dual differential balanced circuitry, three power supplies, and premium Vishay resistors. That's marketing copy, not a lab verdict. Audio (September 1994, p.201) ran that Parasound copy in its pages, and the same pitch appeared the month before in the August 1994 issue.
For the DIY crowd, the PCM63P was easy to work with. It's a 28-pin device with a current output, so the analog stage is yours to design. Elektor's own output section used a fast current-to-voltage converter followed by a third-order low-pass filter built around a Generalized Impedance Converter rather than opamps, deliberately keeping the audio signal out of an opamp on its way to the buffer. Their parts list called for AD844 opamps in the I/V positions - chosen for 60 MHz bandwidth at unity gain, a 2000 V/µs slew rate, and 100 ns settling to 0.1% - with OP27s specified for the servo control because of offset voltage and low noise. Builders who preferred the NE5534A, LT1115, TLE2027, or OPA627 had latitude in the remaining positions. Elektor's listening tests found the OPA627 preferred in the output section by some listeners, though they were careful to say the differences were very small and only audible on first-class systems.
The tweaker crowd went further. One builder, writing about a tube output mod for a CD player, tracked down two Glass Audio articles and one from Positive Feedback that dealt with Burr-Brown 28-pin DACs - the PCM58P and the PCM63P - and used their I/V resistor approach as the basis for tapping the chip's current output and running it straight to a tube grid. That meant anything from a full Elektor motherboard to a weekend hack on a cheap player.
The PCM63P is not a plug-and-play part. The grade matters, the supporting filter matters, and the analog stage matters more than the chip's headline number. A base-grade PCM63P behind a mediocre I/V stage will not embarrass a -K part in a well-executed design.
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