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Finally we have put all the components together to create a purpose built, no compromise, highest quality digital to analog system.The basis of the digital conversion is the EXA/Ackodac board which features the EXA U2I USB-I2S convertor and the Ackodac Sabre dac board , 32 bit/384 KHz capable , DSD capable.
The EXA is designed in Canada, the Ackodac in Australia.
This is mounted in it's own "art form" chassis handcrafted from West Australian Jarrah wood (by Jon Thompson Designs in Busselton)
Because of the size of the dac board, the tube output stage has been included in the Supratek 71A DHT line stage preamp, with it's own independent power supply and able to be used independently of the line stage if necessary, eg with digital volume control.
There is a lot of discussion and discord about what is the best dac architecture, 16 bit, 32 bit, R2R output, current or voltage I/V , 44.1KHz or 384KHz sampling rates.
Personally I like the 32 bit Sabre dacs, but the actual implementation of the output stages is in my opinion probably the defining source of a dac sound. Any dac, regardless of it's capability can be easily ruined by a mediocre output stage, whether tube, opamps or direct output.
The majority of tube output stages for dacs are very poor, with cheap power supplies and poorly designed circuits using garden variety tubes not really suited to the application.
I've used a special circuit that uses high tech frame grid pentodes (triode connected), these high performance tubes were designed at the end of the tube era, the E280F, E810F, D3a, 6E6P can all be used without any modification and allow for slight changes in flavor, although the overall circuit design characteristic is accuracy into any load. Magnequest nickel output transformers, lowR biasing, Mundorf Supreme gold/silver/oil caps.
In this configuration the Sabre dac chips sound musical and natural, and most enjoyable for long periods of listening pleasure.
The line stage uses 6H8C input tubes , direct coupled into 71A DHT output tubes, again with output transformer, this time Lundahl amphourous cores. Power supply is in separate chassis as is too big and complex to fit in one chassis. Tube shunt regulation, tube rectification, etc.
I could have achieved the same performance using IDHT tubes, but the 30's era 71A tube has a "cool" factor I enjoy working with , and the problems of getting DHT's quiet enough for line stage use is an interesting challenge.
One unusual feature these days is a balance control, tubes,rooms,speakers rarely provide a perfect balance and being able to fine tune the balance can really snap the imaging into place.
A really well designed and constructed tube line stage will have extraordinary imaging and a 3D effect that gives deep and wide soundstaging - the speakers disappear and realism is paramount.
To get the best out of todays hi-rez recordings the dac output stage and following line stage is very accurate into any load, accuracy is the key to realism ; a warm,romantic tube sound (colouration) that is the result of poor design can sound "nice" but never realistic.
A single 6922 tacked on to a dac with a weak power supply simply can not perform at this level.
This will hopefully be my digital system for a long time, I can't see anything on the immediate horizon to replace it, but digital advances so quickly now, we shall see.
