Working with Current O/P DAC Chips

[Brian Beck]

Originally posted by Eli Duttman
Brian,

The Vishay S102s seem to stand up well to your criterion. I see a < 0.025 muV. of current noise per applied V. specification.

Oh yeah, these foil resistors are not only good enough, but I think many people would think that they're overkill. So as not to scare off others, can we agree that a good old 1% metal film will work here also? Either way the noise created by the first active device will far exceed any excess noise created by either kind of 4.99 ohm resistor.

[dsavitsk]

A couple of little things to add:

Here's a nice circuit from Gordon Rankin that might be worth considering:

http://www.diyhifi.org/forums/files/simivtube_143.jpg

Also, Sowter makes transformers specifically for this application. http://www.sowter.co.uk/dacs.htm AudioNote does too, but I don't know if they are available to the hobbyist w/o buying a kit. Sowter does suggest putting the I/V resistor on the secondary which would mean the only DCR would be that of the primary.

How about a choke, or an autoformer instead?

Whether they should or not, the I/V resistors make a huge difference in sound Cheap metal films tend to sound bad. Tants and Rikens are nice, but my favorites thus far are ABs. Maybe it is just the structure of the distortion ...

[Brian Beck]

Originally posted by dsavitsk
Here's a nice circuit from Gordon Rankin that might be worth considering:

http://www.diyhifi.org/forums/files/simivtube_143.jpg

Huh? With all due respect to Gordon, either we have circuit that is out of the context of what we’re discussing here, or this circuit misses the boat. Just because the input goes into the cathode doesn't mean that it will see a particularly low input impedance such as we need for the DAC. The plate load impedance is “seen” through the cathode, divided by mu+1. In this circuit, if we load the output transformer with 100K, a typical value for a line stage, then that value is transformed through the output transformer to 2.5M at the plate. In parallel with that is the 225H choke, which at 1 KHz presents 1.4M of inductive reactance to the plate. The combined load will be a complex impedance of very roughly 1M. This value is divided by mu+1 and gives us roughly 30K at the cathode. That’s the load the DAC sees, and it’s also completely dependent on the load on the secondary of the output transformer, as well as being frequency dependent. Compare that to the 5 ohms needed. Sanity check: if the DAC can output +/-1mA (typical), then the voltage will swing by +/- 30 volts at the DAC output. “Pop” goes the DAC. If the DAC doesn’t go “pop” right away, the voltage swing on the plate would be in severe clipping. Gordon has to have been assuming something else altogether. Or, maybe this was just a paper exercise.

[Eli Duttman]

Originally posted by JoshK
I just noticed this thread. Was working on projects last night. I like your ideas, they make sense. But I still have a question with regards to the grid resistance versus the cathode resistance. Didn't we want the load resistance low as well, or does it not matter?

Josh,

The I/P impedance of the cascode is in parallel with the 4.99 Ohm I/V resistor. Since that impedance is several orders of magnitude greater than the I/V resistor, it is effectively "invisible" to the DAC chip.

The O/P of the cascode presents its own issues. A DC coupled voltage follower is needed to buffer the high O/P impedance. Also, PSRR is non-existent in cascodes. So, the B+ must be WELL filtered and regulated.

[Sheldon]

Something like this?

[Eli Duttman]

Sheldon,

Your schematic is reasonably consistent with Allen Wright's phono section FET cascode.

Stopper resistors are needed on the FET's gate and the CF's grid. DC couple the CF's grid to the cascode and adjust its cathode resistor accordingly. Also, you left the low pass cap. across the I/V resistor out.

As Mr. Beck indicated, the gain of a cascode is tied to the transconductance of the lower element. The 10 KOhm load resistor value I gave was based on 6922 sections serving in both the top and bottom positions. A different value is needed with a FET in the bottom position.

The voltage divider that sets the cascode's idle current could be made a pot. A 100 Ohm Riken at the FET's drain would do double duty as a stopper and a test point.

I think this circuit has potential. AW's FET cascode is quiet enough for use with LOMC phono cartridges. The proverbial church mouse is noisier.

BTW, AW sells closely matched pairs of low noise JFETs. "Just what the doctor ordered."

[Sheldon]

Just thought experiments. How bout all tube:

[Eli Duttman]

Originally posted by Sheldon
Just thought experiments. How bout all tube:

All of this is "brainstorming", at least for now.

Loose the grid stopper on the upper triode of the cascode. Remember, the upper triode in a cascode operates GG. The voltage divider at the buffer's I/P grid has to go too. Wire the ends of a pot. between B+ and ground and connect the wiper to the upper grid of the cascode. Put a 100 Ohm Riken between the cascode's lower plate and upper cathode, as a stopper/test point. LED bias for the cascode will work, but I prefer an unbypassed metal film resistor. Local current NFB from degeneration is (IMO) a good thing here. The cascode's load resistor gets tweaked to yield the amount of gain needed.

BTW, a 6922 for the cascode and an ECC99 for the buffer looks good to me.

[Sheldon]

Fixed the pot. The resistors on the buffer grid, Broskie adds as safety resistors, but I may have missapplied them. I left them out for clarity here. So more like this?

Edit: On further checking, realized that John has beat me to it (no surprise there). http://www.tubecad.com/2004/blog0013.htm

[Eli Duttman]

Originally posted by Sheldon
Fixed the pot. The resistors on the buffer grid, Broskie adds as safety resistors, but I may have missapplied them. I left them out for clarity here. So more like this?

Edit: On further checking, realized that John has beat me to it (no surprise there). http://www.tubecad.com/2004/blog0013.htm

Getting close! There is no need for a fixed resistor connecting the "top" of the cascode's current set pot to B+. The value of the current set pot. should be no less than 1 MOhm. The cap. to ground gets connected directly to the upper triode's grid. Think GG!

PSRR in the White CF buffer is good enough to allow connection to well filtered, but unregulated B+. Reserving the regulator to feeding only the cascodes decouples the cascodes from the buffers.