VSR series foil resistors
Description of Case Study
To build the best-sounding audio multi-channel digital-to-analog converter (DAC), we tested DAC ICs from various suppliers before settling on a solution using an FPGA and 96 VSR series resistors per channel in a 3-channel configuration.
Our goal was to create a no-compromises DAC and digital crossover with 69-bit processing. The DAC is configured as a an R-2R ladder. It is a 24-bit network although 0.01% resistors can not easily give a resolution higher than 14 bits.
The User Explains
At TOTALDAC, we build a DAC and digital crossover for high-end audio systems based on an FPGA and discrete resistors. The TOTALDAC board is assembled in an ISO 9001-certified factory in France and each board is inspected and its sound quality tested before shipping.
As I wanted to build the best sounding audio DAC, I tested the sound quality delivered by a number of mass-produced DAC chips from several well-known companies, including Analog Devices, TI, Philips, Cirrus Logic, Wolfson, and AKM.
Some of them were new old stock (NOS) chips known for their nice sound such as the TDA1541, TDA1545 (used in CD723), TDA1543TDA1543, PCM56, AD1852 (used in Teac VRDS 25x), CS4328 (used in Helios Stargate), PCM1702 (used in many high end CD players in the past), and PCM1704U-KPCM1704U-K.
Some were really nice, but never very accurate. I listened to them with and without oversampling whenever possible. I used a Pacific Microsonic PDM100 HDCD digital filter, one of the best digital filters available. I also listened to many new DAC chips from TI and Analog Device in particular. These usually required microcontroller programming. The sound was often a bit metallic. Lastly I listened to the sound quality from a discrete R2R DAC made of simple 1% resistors. The musicality was there but some accuracy was missing. This was the starting point of my ultimate DAC solution.
After two years, five PCB revisions, and other improvements to the FPGA, I finally settled on the VSRJ 0.01% resistor from VFR. Although 96 discrete resistors per channel are used, the signal path is actually very short actually, going through the 0.01% resistors, into a C-L-C filter, through a capacitor, and then into the potentiometer.
Fig. 1: Each channel uses 96 pieces of 0.01% VFR foil resistors
Once the prototype worked as expected I contacted VFR Europe to choose the most appropriate foil resistors. A less expensive through-hole version was available.
To my knowledge, no other DAC for the high-end audio market uses a 24-bit foil ladder configuration, which would be even more unique using VAR resistors. The VAR configuration however might be too cost intensive to preserve the high margin expectations for the manufacturer, the importer and shop that is typical for the conventional high-end audio market. But as its inventor, I sell directly on the internet the board only directly on the internet and leave it to the customer to supply the metal case, the power supply, and the triode output buffer. As a result the total margin remains very reasonable. But the foil components are still the key to its success.
Fig. 2: The 3 way TOTALDAC board uses 288 of these resistors.
Figure 3 shows the quality of the DAC for an extremely low amplitude signal such as concert hall long reverberation. The noise floor is as low as -132 dB, although 0 dB is really the maximum DAC level (1.6 Vrms). The measurements were done by a Rohde & Schwarz UPL audio analyzer. Only the second order harmonic distortion is visible, and is only at -126dB. The distortion would be lower with an operational amplifier, but I preferred the sound created by discrete transistors without global feedback. Some noise is visible at low frequencies, it is due to the mains power supply. A power supply regulator would remove this noise but I prefer the sound without global feedback in the power supply for the analog section.
Fig. 3: Spectrum of a 1 kHz sinewave at -80dBFs with the power supply and buffer board