Discrete R2R DAC (part 2)

My first test was 16bit using DIP and through-hole on vero board. It works well at its first fire up. In-fact, I read Sonic Illusions R2R DAC page for reference, and I found a lot of useful info from there, and many thanks to Mr. Tom Browne , the owner of Sonic Illusions web site, for his further info about COB / 2’s complement . All my experimental and current design are simple non-oversampling with passive RC or LC filter.

When I start to design a new board for 24 bit versions, I found some Japanese Diyer’s page that actually built nearly the same concept R2R DACs and schematics are virtually the same. My 24 bit versions are completed and I’m listening to it now. I’m planning to build another one and finalise its power supply, some filters, and also make it a switchable from unbalanced to balanced out, but with just maybe 20 bit only because resistors for R2R need to be very accurate which is virtually impossible to obtain such perfect resistors for higher bit. I will post another article about it.

Discrete R2R DAC schematic

From the schematic above, The BCK signal had been buffered using 2 inverters, and to give a little bit of delay, to ensure all timing are in order. The LRCK using inverter to split into L and R channel clock, and 74HC164 perform as a serial in to parallel out shift register, then feed to next D latch stage. The MSB bit also inverted by 74HC14, so that 2’s complement can be aligned into normal COB sequence.

I used 2 separate power supply here, one for digital ( 5VA ) and another for 74AC574 D Latch ( 5VB ) . My experimental board using LM340 +5V three terminal regulator IC, and after some experiments, I found that the power supply 5VB for 74AC574 need to be very low noise, I mod it into simple zener diode shunt regulator and sound quality improved a lot. the value and quality of capacitor for 5VB also very important for good sound, I need at least 4700uF or above to make it sounds right. with zener diode regulator and large capacitor, I get huge soundstage and very good bass, with darker background. NiMh battery works well as 5VB too.

I will cover 74AC574 D latch sections and R-2R resistors , and hopefully complete the whole in part 3, too many projects to share, too little time!

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