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- Jan 15, 2018
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Hey ASR,
Showing another DIY project of mine, I said I wasn't going to post anymore but I am proud of my monstrosities
I designed a NOS DAC based on the vintage Burr-Brown PCM56. It's a 16-bit chip. This is my foray into digital design, so I thought simple is best. I know the NOS R2R thing isn't the most popular here, but I think there is something to the sound that goes beyond the measurements. I used to be a "by-the-measurements" guy when it comes to DACs, but I heard a NOS R2R design that really blew my mind and bought into it, so thought I'd build one.
It's a dual mono design with separate 4-layer PCBs, dedicated ground layers, separate power supply rails for the +/- analog and digital supplies of each chip. Power supplies are cascaded series regulators. To optimize for HF mains noise, using a 1:1 isolation transformer in balanced configuration for +/- 60VAC between mains and the DAC supply transformers. Digital front end is a XMOS-based USB module from a company called JLSounds configured for PCM56.
The output signal is taken from the Iout pin of the PCM56. I/V conversion is performed by a pair of Sowter SUTs wired in 1:5. There is a tube output stage to reach 2Vrms line level using the Russian 6E6P-DRY. This is a high gm tetrode strapped as a triode. It is extremely linear as a triode with a relatively low plate resistance of 1.2K. I am loading it with a IXTP08N100D2 / DN2540 cascode CCS. The tubes are battery biased with a 1.2VDC battery between the SUT secondary and ground. I am using a high-performing HV regulator for the B+ supply. Also using a DC heater regulator for the 6E6P-DRY to keep coupled AC noise as low as possible.
Here is a look at the I/V and output stage. Have some anti-parallel zeners on the output to limit DC transients on startup / shutdown as the output caps charge / discharge. As you can see, the output tubes / CCS are setup as a hybrid mu follower with the output taken from the source of the DN2540. This gets the midband output impedance of the stage down to around 135ohm.
Here is a look at the 6E6P-DRY triode curves from my curve tracer. Doesn't get much more linear than this for a triode.
So that's the design at a high level. Here are some pictures of the final build. Need to shorten and tidy the digital input wiring.
Here is a FFT of the left channel, 1kHz -6dB undithered sine wave. Did well to minimize LF mains noise, even a bit better in the right channel. Worth noting that the PCM56 is rated for THD at -94dB, the -87dB H2 is due to the output tube.
Here is the frequency response. The blip at 3.5kHz is an artifact of my soundcard measurement setup using the MOTU M4, along with the "waviness" of the FR from 10kHz-20kHz, seen in all of my measurements.
Would you believe me if I told you this DAC sounds incredible? Maybe not, but to me it does! Really happy with the sound
Showing another DIY project of mine, I said I wasn't going to post anymore but I am proud of my monstrosities
It's a dual mono design with separate 4-layer PCBs, dedicated ground layers, separate power supply rails for the +/- analog and digital supplies of each chip. Power supplies are cascaded series regulators. To optimize for HF mains noise, using a 1:1 isolation transformer in balanced configuration for +/- 60VAC between mains and the DAC supply transformers. Digital front end is a XMOS-based USB module from a company called JLSounds configured for PCM56.
The output signal is taken from the Iout pin of the PCM56. I/V conversion is performed by a pair of Sowter SUTs wired in 1:5. There is a tube output stage to reach 2Vrms line level using the Russian 6E6P-DRY. This is a high gm tetrode strapped as a triode. It is extremely linear as a triode with a relatively low plate resistance of 1.2K. I am loading it with a IXTP08N100D2 / DN2540 cascode CCS. The tubes are battery biased with a 1.2VDC battery between the SUT secondary and ground. I am using a high-performing HV regulator for the B+ supply. Also using a DC heater regulator for the 6E6P-DRY to keep coupled AC noise as low as possible.
Here is a look at the I/V and output stage. Have some anti-parallel zeners on the output to limit DC transients on startup / shutdown as the output caps charge / discharge. As you can see, the output tubes / CCS are setup as a hybrid mu follower with the output taken from the source of the DN2540. This gets the midband output impedance of the stage down to around 135ohm.
Here is a look at the 6E6P-DRY triode curves from my curve tracer. Doesn't get much more linear than this for a triode.
So that's the design at a high level. Here are some pictures of the final build. Need to shorten and tidy the digital input wiring.
Here is a FFT of the left channel, 1kHz -6dB undithered sine wave. Did well to minimize LF mains noise, even a bit better in the right channel. Worth noting that the PCM56 is rated for THD at -94dB, the -87dB H2 is due to the output tube.
Here is the frequency response. The blip at 3.5kHz is an artifact of my soundcard measurement setup using the MOTU M4, along with the "waviness" of the FR from 10kHz-20kHz, seen in all of my measurements.
Would you believe me if I told you this DAC sounds incredible? Maybe not, but to me it does! Really happy with the sound