Yes, the signal will just be quite small.
in general how large a signal from a DAC should be bofore to enter a (input) buffered well done tube stage with a *regular* gain?
Pierre
I know where you are coming from. But the Sabre DAC is different from any other DAC, not just that it can be used in both modes - but arguably sounds best in Current mode IMO.
If you go back through the posts, see the ones that discusses the lighter tonal balance of the Sabre DAC is Voltage mode.
If you want to use Voltage, then transformer is the way to go. Many have heard me say that many times over, that with a differential voltage output where any DC offset will not be seen by the Primary of the transformer. But the Sabre DAC is not a conventional Voltage DAC.
The Sabre DAC has a much higher output Z than is usual for a Voltage DAC and hence there is reason to point to it as the cause of the light tonal balance when used as such. We need to reduce that Z and also force the DAC into current.
So, Voltage mode, use:
The two resistors force the Sabre DAC into 3.14mA current offset while reducing offset voltage from 1.65V to about 1V. The output will be 1.3V RMS.
But how could this be applied to using Tubes and Current mode.
Pretty much all the post-DAC Tube designs I have seen avoid full-on Current mode. But with the Sabre DAC there is and opportunity if employing above suggestion repeated here:
We can force the Sabre DAC into Current mode by pulling it virtually down to 0V DC and then get an offset current of 8mA. Using the same 1:1 transformer, it now opens up the way to use Current mode with Tubes.
So the above example shows how it can be done.
The point is that the Sabre DAC has flexibilities in the way that no other DAC has and using it in Plain Vanilla Voltage mode can be avoided as IMO it gives worst performance.
Cheers, Joe R.
Yes, the signal will just be quite small.
Hi Russ
Indeed it will. The maths are simple. With the example shown 30mV RMS and if a full output of 2V is required, then the tube gain needs to be 66 approx. But most of us can get away with less than that. The 3R value can be tweaked too.
I am not about to do it myself, at least not for now but might a little further down the road, or likely help a fellow DIY'er to do it, that being more likely.
Cheers, Joe R.
Hi Joe
so if I got you right, suppose I'll go for "C", you are saying that with 1.3V RMS I go straight to my tube preamp (?)
Best
Pierre
...
So, Voltage mode, use:
The two resistors force the Sabre DAC into 3.14mA current offset while reducing offset voltage from 1.65V to about 1V. The output will be 1.3V RMS.
so if I got you right, suppose I'll go for "C", you are saying that with 1.3V RMS I go straight to my tube preamp (?)
Best
Pierre
I use Sabre in voltage mode without transformer directly to amp. Only positive and GND pinouts. My guess no need to convert from differential to SE, as negative signal is the same, only opposite phase. Benefit of using transformer can be with current mode.
Could you suggest suitable transformer parameters for ES9018 dac in current mode?
Three points, you miss out on the better performance using both phases; you need a coupling capacitor; and you only get 1V output when using ES9018 in stereo mode.
You will need a loading resistor across either Primary (I would think preferably) or Secondary and a 1:10 (or more?) step-up. A good step-up transformer needs to be chosen with some care.
Others may choose to comment on this?
Cheers, Joe R.
Better performance when using long cables, when short cable to SE amp input no difference.
I'm not using stereo, 4 single channels not paralleled. I don't know if I require 2V RMS, but with current output 50% of signal level is more than enough for me.
What does it mean 1:10 transformer prim-sec ratio? What kind of core shall it have, it's wattage, how many turns, what wire?
Have you mentioned voltage output in your schematic? Why should we increase input resistance of the primary with those 195R resistors? Does lower input resistance worse in case of current output?
I this is differential conversion, GND return shall be taken directly from ES9018 out? If I have SE amp, can this transformer convert differential current output to SE voltage output?
Regards.
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Sorry, but this is completely incorrect.
the reason is that all of the stuff that is common mode and designed to be rejected by I/V stage will simply pass right through unfiltered.You will never get the most out of the DAC this way. Both THD and DNR will suffer greatly, and the noise floor will be much higher.
In my opinion it is simply is not possible to run directly (even with resistors) into *any* transformer and keep the DAC in "current mode". This is because I would never consider anything more than 1ohm seeing a 195ohm source impedance as approximating an infinite impedance ratio. Your still modulating the outputs of the DACs far more than is desirable even at 5-10 ohms. It is about ratios. 195R Is low output impedance. So to make 195R look like a current source the load has to be an incredibly small impedance. I am sorry to say this but even 2 ohms in not small when looked at in those terms. That ratio is only ~100. Certainly not large enough.
On a personal note - having listened to the ES9008/12/18 for as long or longer than anybody on earth (well other than possibly Dustin and crew) I can say emphatically that I absolutely hear a difference when the DAC is loaded into a virtual ground as apposed to even a smallish resistance (2-10 ohms). Very subtle but it does lose something in my opinion. I have heard hundreds of output stages on the Sabre and designed quite a few (only some of them make it onto these pages).
The bottom line is you absolutely shouldn't choose to use transformers because you want current mode. You simply can't get it. Choose them because you like the way they sound! It is the same argument for tubes. It is perfectly OK to prefer any output stage on the basis of things that go beyond ideal measurements and extend to taste.
Now I do think the smaller the impedance the better, this of course extends to transformers, but you are *always* going to be using the DAC like a voltage source when using transformers. There is nothing wrong with that.
Happy Thanks Giving to all!
Cheers!
Russ
It is about ratios. 195R Is low output impedance. So to make 195R look like a current source the load has to be an incredibly small impedance. I am sorry to say this but even 2 ohms in not small when looked at in those terms. That ratio is only ~100. Certainly not large enough.On a personal note - having listened to the ES9008/12/18 for as long or longer than anybody on earth (well other than possibly Dustin and crew) I can say emphatically that I absolutely hear a difference when the DAC is loaded into a virtual ground as apposed to even a smallish resistance (2-10 ohms). Very subtle but it does lose something in my opinion. I have heard hundreds of output stages on the Sabre and designed quite a few (only some of them make it onto these pages).
The bottom line is you absolutely shouldn't choose to use transformers because you want current mode. You simply can't get it. Choose them because you like the way they sound! It is the same argument for tubes. It is perfectly OK to prefer any output stage on the basis of things that go beyond ideal measurements and extend to taste.
Now I do think the smaller the impedance the better, this of course extends to transformers, but you are *always* going to be using the DAC like a voltage source when using transformers. There is nothing wrong with that.
Happy Thanks Giving to all!
Cheers!
Russ
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Absolutely agree with that, it virtually has to see zero Ohms and indeed IF zero Ohms then, and only then, would you achieve infinite ratio.
So those resistors and their values are about compromises and the degree to which we make those and what the benefits are. If you want to use transformers, then it will never be true current mode. No question about that.
Joe R.
That is up to you. I suggest that you buy a suitable transformer like maybe Sowter Type 9545 DAC INTERFACE TRANSFORMER.
No, there are no added 195R resistors, these represents the internal impedance of the ES9018 when used in Stereo mode.
That is pretty much the idea. Works both balanced and unbalanced. If unbalanced, use a floating RCA and let the Interconnect provide the grounding at the preamp end. That way you get galvanic isolation even if unbalanced - the two grounds will be kept separate.
Cheers, Joe R.
For sure. Those who use so-called passive preamps may find -4dB noticeable, but use any proper preamp and -4dB is nothing to worry about.
Cheers, Joe R.
Who says you can't use a transformer in 'current mode' ?
Do a search on "zero field transfomer".
There are various implementations and I've even cooked up some of my own
sans opamp. With mixed feedback you can make the IP Z whatever you like.
Without it, just use a very low winding resistance transformer.
T
Who says you can't use a transformer in 'current mode' ?
Do a search on "zero field transfomer".
Hi Terry
Shows you can do a lot with feedback. But as a wise man once said "all things may be possible, even lawful, but not all things are advantageous." Seems the guy's name was Saul or Paul or something.
Come around and listen to the Oppo 95 here, puts a smile on my face. Need two clocks though, makes a real difference.
Cheers, Joe R.
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