please explain tube dac's

[superjojo]

Ive been messing around with a Jolida tube dac III. Uses 2x 12ax7.

For a budget piece of gear, it seems to be well built and I think it sounds ok in my system. However, I don't have any reference to compare with high end ss or tube dacs.

Can someone explain to me how tubes work in a DAC circuit?

and

At this price point is a tube dac just a gimmick?

 

[kward]

The tube is employed after the digital signal is converted to analog. In the Jolida, the 12AX7 is probably used as a buffer. A buffer isolates the frontend portion of the circuit from the output stage portion of the circuit. A tube buffer can also add a little warmth and I expect this is the true purpose of it in the Jolida. (could have made a buffer stage equally well out of a transistor, for example). It's not a gimmick in the sense that the tube is in the audio circuit and the DAC won't play without it. Some people don't like tubes in their digital frontend. Just depends on your point of view whether it's a good thing or not.

Jolida could have used any number of tubes for this purpose--12AX7, 12AU7, 6DJ8, 6FQ7, 6H30, etc, etc. Now this is pure speculation, but it wouldn't surprise me that they used the 12AX7 becaue of the recognition factor of that particular tube.

 

[BinaryMike]

The output of the DAC chip is an audio current, rather than an audio voltage. It's usually converted to voltage by an op-amp configured for this purpose, but "tube DACs" often do this with a resistor instead. The resistor has to be very low in value for the sake of conversion linearity --- so low that tube noise comes into play. Also, tube DACs often ignore the DAC chip's datasheet recommendations for sample-rate filtering, which can lead to some 'interesting' sonic artifacts.

 

[superjojo]

Both great replies!

The resistor has to be very low in value for the sake of conversion linearity --- so low that tube noise comes into play.

Does this mean they are much more sensitive to tube rolling?

Also, tube DACs often ignore the DAC chip's datasheet recommendations for sample-rate filtering, which can lead to some 'interesting' sonic artifacts.

What type of artifacts?

 

[BinaryMike]

I haven't had any desire to fool around with tube DACs myself, but I've seen at least one thread where the artifacts boiled down to excessive white/pink noise. If there's any opportunity for intermodulation downstream, due to nonlinearity at ultrasonic frequencies, then serious distortion could result. It's unwise to allow any ultrasonic garbage into a preamp, and pretty much cuckoo to allow it into a power amplifier. Amplifier designers rarely consider the effects of such outrageous misuse scenarios.

Tube rolling could have some effects, inasmuch as the stage following the conversion resistor is generally open-loop and tubes vary with respect to gain and noise. I wouldn't expect distortion changes unless tube biasing goes off the rails, which you should be checking with every tube change.

 

[superjojo]

Some people don't like tubes in their digital frontend

Im getting the notion that you feel this way BinaryMike? (ps thanks again for the great replies).

Since this is one of the few tube dacs I even see being marketed, and certainly at this price point (500 bucks), im getting the feeling that indeed its a gimmicky thing.

 

[BinaryMike]

I don't have a pooch in this race. I prefer tube amplification, but I've worked with other technologies professionally and learned that it's risky to flush good engineering practice in the search for an edge in performance. I would be interested in evaluating a tube DAC that's filtered well enough for full compatibility with commodity amplifiers.

 

[mroboto]

Tube output of my TubeDAC-11 sounds better to me, more lively than SS output, it was bright and quasi harsh sounding in the beginning but I upgraded the capacitors, sounds better than any other DAC I owned.

Still have not yet finished the mods, I wait for additional Elna Cerafine capacitors 47uf 100V to add to the new, existing 100uf 100V Nichicon Fine Gold for the tube, but already sounds very good.

And I like to be able to experiment and compare different 6DJ8 tubes.

 

[PakProtector]

Since there is 4 triodes, I would suspect an amplifier, and buffer stage per channel. A 12AX7 is just adequate for that, though sensitive to cable capacitance due to its high output Z. The usual output is current, which is turned to voltage across a resistor. This current output can also drive a transformer, but its load needs to be chosen carefully as its voltage output is limited, and running to its rails will not sound good...LOL Doing an I-V conversion and then using that voltage to feed an amp can be done quite well.
cheers,
Douglas