Legato 3, Common Mode DC and You. How do you choose to deal with it?

[Russ White]

Originally Posted by 4est

If you are going to use a transformer, why use a Legato at all?

One reason would be to present the DAC with a far lower input impedance than you could with just a transformer. This makes the DAC behave more like a true current source.

The more the voltage modulates at the DAC outputs (the more it acts like a voltage source) the worse it performs in terms of both IMD and THD. So keeping that impedance as close to zero as possible is very important.

[4est]

OK, I buy that to a point, but wonder if the overall complexities would negate the improvements, and worth the risk of floating the ground or DC imbalances at the balanced outputs.

As a TVC user, I keep worrying about the DC balance drift of the Legato's output saturating the transformer on my volume control. I prefer the body/meat (distortion?) the TVC gives me over amp direct, and have considered trying a transformer I-V in lieu or in conjunction with my present TVC (Bent Audio TAP).

[barrows]

The transformer will just reflect the input impedanc of the following stage (amp) so the output of the ESS would be loaded with 10K-200K, or whatever your amp is.
Plus, I have not looked, but is the voltage and current output enough to drive the input of a transformer with low distortion? In other words, what is the output of the ESS in V when loaded with, say, 100K, and how much current is available vs the Legato's output set for 2V?

[Russ White]

In stereo mode the ES9018 DAC has 195 Ohms output impedance.

The Legato's output impedance is slightly less.

The output of the DAC will be just a bit less than 2VMRS input no load. You can calculate the actual swing using ohms law against your load impedance.

Cheers!
Russ

[Russ White]

The key thing is the way the DAC reacts to voltage at it's outputs. That is the reason for an I/V converter. When used as a voltage output DAC the ES9018 is only a little better than average.

When used as a current output DAC it is simply stellar.

[staccatiss]

Hi Russ,

I have a question about the loading of the ESS9018 in your Legatos. The two inputs "sit" at half of VACC (around 1,65VDC), and this means, I think, that the ESS9018 does not give out any DC current Only AC current (the music). Using a resistive load on the other hand would draw DC current out from the ESS9018 chip. Using a transformer will not draw any DC current either.

I wonder, what does the ESS9018 prefer - that it outputs both DC and AC current or only AC current?



/S

Originally Posted by Russ White

One reason would be to present the DAC with a far lower input impedance than you could with just a transformer. This makes the DAC behave more like a true current source.

The more the voltage modulates at the DAC outputs (the more it acts like a voltage source) the worse it performs in terms of both IMD and THD. So keeping that impedance as close to zero as possible is very important.

[Russ White]

Current will flow from it always - it just depends on where the load is sitting.

You can calculate the current using Ohms law.

For example with a AVCC at 3.3V and a virtual ground (0V) at outputs you will get ~ 8ma static current.

My experience about the static/bias current is that the DAC is not particularly picky - it is just that the voltage at the outputs should remain as constant as possible.

I have also found it does not really matter much if the static current is sunk or sourced.

[4est]

Originally Posted by barrows

The transformer will just reflect the input impedanc of the following stage (amp) so the output of the ESS would be loaded with 10K-200K, or whatever your amp is.
Plus, I have not looked, but is the voltage and current output enough to drive the input of a transformer with low distortion? In other words, what is the output of the ESS in V when loaded with, say, 100K, and how much current is available vs the Legato's output set for 2V?

Well, that depends upon the turns ratio. The reflected load is the secondary load times the squared primary/secondary ratio. Furthermore, if there is a resistor in parallel to the primary, the reflected load will be a combination of the impedances in parallel. In either event, not the 100k described.
The 9018 has plenty of drive to get 2v out via transformer btw.

[barrows]

Originally Posted by 4est

Well, that depends upon the turns ratio. The reflected load is the secondary load times the squared primary/secondary ratio. Furthermore, if there is a resistor in parallel to the primary, the reflected load will be a combination of the impedances in parallel. In either event, not the 100k described.
The 9018 has plenty of drive to get 2v out via transformer btw.

Those are the details. My point was, that in any case, the load is going to be much closer to that of the amplifier than the <1 ohm the ESS 9018 wants to see for best performance. And, thanks to Russ' answer which does confirm that the direct output of the DAC chip will produce enough voltage and current, barely (just as the Legato does, barely) to drive the transformer well. But it does appear that if one wants 2V output, and adequate current to drive an amplifier directly, a 1:1 transformer would be the only suitable choice for use with most amps. Of course, we digress! There is already a huge thread on direct coupling the Buffalo's output to transformers:
4est, are you using caps with your L3? If so what has your experience been?
I am leaning more an more to just running with the G unconnected at the output of the L3 into the nCore amp, hoping that the nCore is going to beat out my Pass this time... But I do not think this appraoch will work with my Classic 250, as it has TVS diodes at the inputs...

[4est]

Originally Posted by barrows

Those are the details. My point was, that in any case, the load is going to be much closer to that of the amplifier than the <1 ohm the ESS 9018 wants to see for best performance. And, thanks to Russ' answer which does confirm that the direct output of the DAC chip will produce enough voltage and current, barely (just as the Legato does, barely) to drive the transformer well. But it does appear that if one wants 2V output, and adequate current to drive an amplifier directly, a 1:1 transformer would be the only suitable choice for use with most amps. Of course, we digress! There is already a huge thread on direct coupling the Buffalo's output to transformers:
4est, are you using caps with your L3? If so what has your experience been?
I am leaning more an more to just running with the G unconnected at the output of the L3 into the nCore amp, hoping that the nCore is going to beat out my Pass this time... But I do not think this appraoch will work with my Classic 250, as it has TVS diodes at the inputs...

I do not think that it is so cut and dried, otherwise there would a clear consensus on what transformer to use, and 10 ohm is closer to 1 than it is to 100k while eliminating a bunch of other parts and supplies, but yes enough of that!

I briefly ran mine with 10uf polyprope/tin before I dared hook it up direct. I needed to know that it wouldn't drift during break in. It still sounded good, but a cap is a cap and I took it out first chance I got, and then in again to briefly test it amp direct with the volumite. I struggle with the idea of lifting the ground and floating 8-9v on the input pins, seems as if begging for trouble.