Calliope is the version of the Reference Tube DAC built on the Philips TDA1540, the converter of the first Philips CD players. Dmitriy Gubchenko voices it as natural and organic.
The Philips TDA1540
The TDA1540 dates from the early 1980s, the years in which the compact disc reached the market. It is a 14-bit mono multibit chip: binary-weighted current sources, one per bit, kept accurate by dynamic element matching, which continuously swaps the most significant sources so that their small errors average out.
In the first Philips players it worked with four-times oversampling and noise shaping, which let a 14-bit chip reach 16-bit performance. Like its successor, the TDA1541, it delivers a current rather than a voltage, and a stereo converter needs two chips, one per channel.
Sixteen bits on a 14-bit chip
Fed 16-bit words directly, a 14-bit chip cannot resolve the two lowest bits on its own, so how a non-oversampling design handles them is a design decision. In Calliope the chip converts the 16-bit / 44.1 kHz signal from coaxial S/PDIF without an interpolation filter or upsampling, and how the two lowest bits are treated is part of Dmitriy Gubchenko’s design. Each unit ships with its own measurement report.
As in every non-oversampling converter, the treble rolls off gently, by about 3 dB at 20 kHz, and image frequencies above 22.05 kHz reach the output stage. Our guide to NOS multibit DACs explains both.
Old stock
The TDA1540 has been out of production for decades and is the oldest of the five chips. Every chip used today is old stock; remarked and tired parts circulate, so each one is checked before it is fitted.
Calliope among the five
Listeners often describe the TDA1540 as natural and unforced, and Calliope is the natural, organic voice of the five. Persephone on the TDA1541 is fast and articulate; Aphrodite on the TDA1547 is dark, velvety and melodic. All five share the same chassis, input, output options and price. The stock output stage is solid-state; a tube stage on 6Zh4, EF14, EL8, AL4 or ML6 with a 5Ts4M rectifier is optional.