Got some nifty PCBs for my DAC today. This project is moving at at a snail's pace since I am working on three or four things at once, but hey, progress!
So what the hell do these do? This necessitates some technical explanation on digital front-end design for these old R2R DAC chips...can skip to TL;DR if it's too much mumbo jumbo.
Popular chips for these NOS R2R designs (e.g., TDA1541A, AD1865, PCM63, PCM56, etc.) are typically designed to operate with one or both of two different digital data stream protocols, PCM or I2S. For PCM input chips, the right and left channel data streams are separated prior to the DAC input. Later, I2S protocol was implemented, which places both left and right channels on the same serial data steam at double the clock frequency (two channels of data, twice the frequency). The data for two channels is then divided (de-multiplexed) in the DAC integrated circuitry.
The problem comes up when you want to use a PCM input DAC (say PCM56, like my SW1X DAC II) with a digital front end that outputs I2S - the data streams for the two channels MUST be split before they hit the DAC input. A common approach when using PCM chips with an I2S digital front end (e.g., CS8414 SPDIF receiver, like in my DAC II), is to "split" the channels using a hex-inverter, then re-align them using a shift register, otherwise there will be a very small inter-channel timing delay (although some designs just live with the delay rather than re-aligning the channels, given the delay is in the micro-second timeframe and is of questionable audibility). Again, this is the approach in the DAC II.
Moving from the TDA1540 to the TDA1541A, Philips introduced the ability to use either PCM or I2S protocol. To get the absolute best sound out of the TDA1541A, all reading, learning, and discussion has pointed toward operating it with PCM inputs, aka simultaneous mode. This cuts the clock frequency in half, reduces jitter artifacts, and leads to the best subjective listening experience.
The boards above were designed for a USB digital front end solution that can be configured to output PCM, an all-in-one solution to interface with PCM input chips, and it came highly recommended.
TL;DR - basically these boards will let me design a USB input DAC and operate the TDA1541A in simultaneous mode for the best sound from the chip.
Next thing for this project is to start to assess the specific needs for a low-noise voltage-regulated power supply, that is going to take some time to design.
Okay, that's enough of that...
On the OTL amplifier front, the sound is excellent and I am very happy with where it is from a sonic standpoint. However, I am designing some ancillary circuitry before moving onto finalizing the chassis: new voltage-regulator PCBs, time-delay-switched relay boards, fine-tuning the more functional aspects of the circuit. But the good news is the amp sounds very good, just need to finalize exactly what is going inside the box before we build one
more to come.
the 421As will be in the mail, or I can forward direct to the junk yard if that would be easier.
