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In Reply to: RE: Why so few Tube Dacs? posted by Dynobot on July 14, 2012 at 18:11:38
...it's impossible to have a tube DAC where the analog portion of the signal does not spend time in a solid state device. The digital to analog conversion takes place inside of an integrated circuit and comes out as an analog signal.
If one needs tubes to "tame" the signal, why does it matter if that takes place in the DAC unit, a tube buffer or preamp?
That's not to say that some don't prefer the sound of a particular DAC with tubes in the analog line stage, but there are certainly plenty of all solid-state DACs out there that their owners prefer, even if the rest of their system is all-tube.
Audiophiles often tend to ascribe everything they hear to a single characteristic or aspect of a piece of equipment. Things are usually a bit more complex than that with respect to the way things sound.
Follow Ups:
"...it's impossible to have a tube DAC where the analog portion of the signal does not spend time in a solid state device. The digital to analog conversion takes place inside of an integrated circuit and comes out as an analog signal."
Perhaps it amounts to counting angels on the head of a pin, but the digital pulse train created by the switches in a DAC chip are just that, digital. Depending on the way the DAC is implemented, there may be no transistor in the chip that sees anything that looks like an analog audio signal. (As one example, consider a 1 bit DAC where the digital pulses come out of the chip unchanged in form from the way they were input. The analog signal appears only after this pulse train has been filtered and this could be done externally, even by passive components.)
If the I/V circuitry is inside a DAC chip (an optional mode for some) then I would agree completely with your comment.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
> > Perhaps it amounts to counting angels on the head of a pin
That is why, when I proofread my reply, that I put the phrase "practical standpoint" in the subject line. I'm not going to spend time doing research to find if an exception exists, but I suspect strongly 99% plus of the DACs on the market use a standard IC chip from one of the regular companies in the business.
I suspect that the DCS ring DACs use discrete circuitry for their switches. It might not be terribly difficult to reverse engineer how this works, but considering the price not something that I would want to tackle. At least one inmate, Ted Smith, has built a DAC that used a 1 bit design with digital drivers and a passive filter based on a transformer.
From reading the SABRE white paper, especially section V, it seems highly likely that there is not much (if any) active analog circuitry between the DAC switches and the chip output. The clue is the comment about the effect on distortion when used in a current mode vs. voltage mode. This is just a guess on my part. Someone who has designed a DAC based on this chip might know more. There is more detail in the patent. (US7116257)
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
> > > If the I/V circuitry is inside a DAC chip
Wouldn’t that effectively act like an opamp?
Dynobots Audio
Music is the Bridge between Heaven and Earth - 音楽は天国と地球のかけ橋
Function and mechanism are different concepts.
Converting a digital pulse train (current) to an analog signal (voltage) can be done by active or passive components. If the components are active they can be configured as op-amps or in other topologies. Incidentally, there are such things as op-amps made out of vacuum tubes. Back in 1961 I worked with an analog computer that had a few dozen op-amps, all made out of tubes.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
History of Philbrick,
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
I think the Havana Dac although advertised as not having opamps used a Dac chip PCM56P that used internal opamps to do the I/V conversion.
Great Dac but not as resolving as some others.
Dynobots Audio
Music is the Bridge between Heaven and Earth - 音楽は天国と地球のかけ橋
The PCM56P has an opamp on the chip but you don't have to use it, you can connect to the current out if you wish.
I have no idea which way the Havanna did it, it can go either way.
John S.
... that an opamp may be an integrated circuit, but not all integrated circuits are opamps. (Rather like "breeds" v. "dogs". All poodles are dogs, but not all dogs are poodles.)
This ia picture of a vacuum tube op-amp from the analog computer that I used in 1961. The link has a complete description of the system and more pictures. As a 17 year old kid, I really enjoyed twirling the 10 turn precision potentiometers. :-)
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
Ohhhh, sure beats the Heathkit one my H.S. Physics room had. You must have had better connections...
Rick
I got paid $1.00 per hour playing with these toys. I thought this was a much better deal than cutting the neighbors' lawns.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
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Dynobots Audio
Music is the Bridge between Heaven and Earth - 音楽は天国と地球のかけ橋
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