Buffalo II & transformers

[markusA]

Let's talk transformers and the B-II.

Running the B-II in current output mode and using a trafo and resistor for I/V.
The Lundahl LL1674 seams like a popular choice.

How do you hook it up and what are your results?

What impedances are you seeing, gains, voltages, ...?
Pros and cons for using a trafo?



Other transformers are welcome as well

[markusA]

The Rakk Dac do it this way... (LL1674)
I guess that'll work if you want SE output?

[alboran]

Hello
I can tell my personal experience with buffalo dac (es9018) + transformer.
I have tried many Transformer, Current and Voltage ..
Stevens & Billington TX-103 - Audio Tekne LT-8310 - UTC HA-108 (UTC A20) - UTC HA-101 - Jensen JT11-P-1 and others ...
the best sound I got with the two Jensen, I recommend trying these very Jensen.
wishes for the tests, good plays

[fierce_freak]

I've read in a few places that some feel the sabre chip sounds better through transformers when it's forced into voltage-out mode. I think something like a 10K:10K transformer with 2-5k resistance across the primaries would work well, but I'm sure there are others way more knowledgeable about this that can correct me.

[ianp]

Just like to report that my Buffalo II DAC using Sowter 9454E transformers is now alive and well.

I have the DAC output going directly into the Sowters. All seems well.

The 9545 transformers were previously used in a modified SRC2496. Actually, I have another SRC2496 with upgraded 4396 DAC chip and (more) Sowter 9545 transformers. I will be doing a direct comparison between this and the Buffalo II.

ianp

[Marek]

Originally Posted by fierce_freak

I've read in a few places that some feel the sabre chip sounds better through transformers when it's forced into voltage-out mode. (...)

Thats interesting. the intuition tell me that it shouldn't be true although I do not have Sabre DAC...yet. Transformer is a current element because magnetic flux inducted in primary winding is proportional to current flowing through it.
But as I said this is theory...

I recomend to read this lecture http://www.analog.com/static/importe...tes/AN_912.pdf

[markusA]

Isn't the wole idea of using a trafo to have the dac in current mode?
If used in voltage mode you don't need it?

And the reason for that is improved THD which need a really low impedance load, as close to zero ohms as possible?

[markusA]

Some comments made by Russ White about current output and load.
Buffalo II (About loading)
Buffalo II (Output impedance)
Buffalo II (Current vs V mode and THD)

And please read the link Marek supplied.
http://www.analog.com/static/importe...tes/AN_912.pdf (A paper on transformers)

After reading all this I'm confused.
Acording to Russ any loadimpedance will be bad for the distortion figures, even a 10ohm load.
Somewhere in the 700ohms region the dac shifts from current out to voltage out.
Using a transformer the load impedance will be reflected through the trafo onto the dac. Reaching zero ohms or even fairly close doesn't really seam like an option with a trafo?
So if a load quickly pushes the THD up in current mode, why even bother?
It sounds a little like "go op-amp or use voltage out"?
And using voltage out you don't really need the trafo if you have a buffer.

Also, reading the paper they talk about R0, would that be the dac output impedance (195ohms)?

[Marek]

markusG,

R0 is termination resistors of DAC. It seems that those resistors change impedance seen from secondary (output impedance) because Zs=2*N^2*R0.
For now I'm not using those resistors but I'm thinking about that - maybe it will decrease THD, but for sure will decrease output level . For me output level is a problem because I have a AD1853 based DAC which has only 3mA output current and I use LL1517 (1:1) transformer. Unfortunetally I haven't LL1674 or LL7903/5.....

[markusA]

Yeah, ok? Termination resistors... I'm a little electronically challenged, trying to learn though.
In the model, output impedance of the dac isn't mentioned.
Couldn't one say the output impedance of the dac is referenced to ground?
Ergo it becomes R0 in the model?

I'm a little worried using a LL1674 will give me to high output level? 1:4 would make the 1.65VAC into +6VAC and you only need 2V to drive most amps to clipping. The buffer will take care of the current.
So, I was thinking a 1:1 or 1:2 ratio would be better? However the lower the ratio the higher the load impedance to dac will see.
It feels like going in a circle and no one's the winner?

How high should the target output voltage be? (Passive vs into a buffer?)
Is it even possible to use the dac sucessfully in current mode with a trafo?
Since I'm adding a buffer, do I even need the trafo or is it redundant? Is the trafo only needed for truly passive output?
Is voltage output really any worse than curent out? -120dB to -108dB THD is really only a factor 4. The distortion is 4 times larger with voltage out, still in the 0.000x% THD range.
My tube buffer will surly have more than that?