Let me know when you have finished travelling and I can post files where they leave them there for a week.
The Fluke 287 might work for you, set it to 'true TMS' and when the time comes, I will help you through it.
It can vary somewhat. The highest I have seen is 0.82uF and it may be as low as 0.1uF. Get a value and measure it using the steps I will give you, then you will know whether to go up or down in value.
You need a burnt disk with the files (or play the files) and the Fluke 287 - and we can make it work.
Cheers, Joe
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Hello again. I'm sorry but something has come up so at present I've neither got the time or the place for doing this very interesting experiment that your are proposing. I will however try to gather some more information on frequency measurements with DMMs and perhaps invest in a scope if thats necessary (I really should have bought one already). I don't know when everything settles but please check this thread now and then in the coming months.
Kind regards,
Vidar
Hi Vidar, whenever you have the time and feel confident to tackle it. Of course, the offer is also open to anybody else on this thread that wants to try it. Cheers, Joe
Generally works across the phases of the output of the DAC and hence the Primary too (it won't into Virtual Earth I/V as a filter does not work at zero Z).
But with a transformer - yes it should work. Just try different values of caps till you get -1.3dB down at 20KHz.
Then take a listen.
Cheers, Joe
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Joe, I might have a go at this as I have a couple of Sowter transformers sitting on the side.
What's the current recommendation for resistor values to use on the primary (I'll be following your method of bringing each phase to ground)? I think you're using 3R3 from what I can remember. I did fancy trying a few values but given the cost of the Texas Components resistors I might just pick one value.
Cheers.
What's the current recommendation for resistor values to use on the primary (I'll be following your method of bringing each phase to ground)? I think you're using 3R3 from what I can remember. I did fancy trying a few values but given the cost of the Texas Components resistors I might just pick one value.
Cheers.
Are you referring to TX2575 resistors? I have not seen a value that low for sale, could you share the source please?
Thank you Joe! Yes, I would have put them on the board itself, but wanted to make sure of the concept.
TX2352
Actually, they now seem to have a web direct service for us lot: http://webdirect.texascomponents.com/SearchResults.asp?Cat=1822
My experience with the Sabre DACs have mainly been with Oppo Players, the older BDP-83SE and now the newer BDP-95, ES9016 in the former and the ES9018 as in in Buffalo.
I have tried transformers but preferred the I/V approach. As Russ has stated, the 195R output Z means that the ideal load should be a near short or at least a very small fraction of 195R. No doubt that ESS sees Virtual Earth as the solution. I beg to differ, being somebody who wants and explores non-NFB solutions.
Based on what I hear and others have expressed, the transformer/voltage solution gives a lighter tonal balance, but nice and open.
To me, the I/V approach has a better tonal balance, but it requires some serious active circutry and associated power supplies. No wonder transformers look attractice.
But that lighter tonal balance... can something be done?
Something that retains the ease of transformer, yet puts some more body into the presentation.
Please look at the two examples below. It basically boils down to one or the other. You have to choose either Current or Voltage.
A: This represents my preferred way, using two very low value resistors straight to ground. They are a fraction of the value of 195R and so means we have avoided Virtual Earth but this doing it the current way.
But some may note this: There is a 1.65V offset voltage on the output of the DAC, but pulling that straight to ground we have exchanged offset voltage for offset current, the maths is simple:
1.65V/195R = 8.5mA
The beauty of this approach is that I can now make a differential direct-coupled non-NFB amplfier with suitable gain and get the expected 2V RMS output target.
Mind you, I am not one of those who gets nervous if falling short of 2V.
B: Don't you just look at the simplicity and from a DIY point of view, it is so attractive. Try different transformers, you don't need a engineering degree to experiment...
NOW TRY THIS:
C:
What you might call this, I don't know. It is not current I/V in the regular sense. But do note that we now have traded some of the offset voltage for current offset. Again the maths is simple, same as above:
1.65V/(195+330R) = 3.1mA
The offset voltage will be just above 1V DC. That DC will not go through the transformer, only AC current will be seen by the Primary. Looking AC wise, it also looks like a voltage divider:
(330R(195R+330R))*2.1VRMS = 1.3VRMS
But there is also another way of looking at it: The 3.1mA DC current offset is at least less that a tenth of the peak AC current required on the Secondary of the 1:1 transformer in pretty much all circumstances. The Sabre DAC is essentially modulating that current. Think of it as an iddle current being modulated.
Some kind of current processing is going on, but not in the sense of a proper I/V converter - which I still believe to be ideal, but maybe we are not looking for the ideal, just a better compromise for transformer use.
Bottom line, IF only 1.3V RMS output does not ruffle your feathers (it does some, but shouldn't), then like any good soup, how does it taste?
The general impression is that some of the missing sense of body comes back and very little that the transformer does so nicely is lost.
So guys, all it takes is a few resistors!!!
Some transformers will require a Zobel network on the secondary. I have tried my own QuadFilar transformers, they do not require a Zobel. But most others will. I have also tried another and less expensive transformer (my secret for now) that did require 10n/2K7 Zobel. I would recommend 600R:600R transformers, probably about 100R total DCR on Primary and Secondary. Also: That 330R resistor value can of course also be lowered (not much point increasing). But your output will go further down, but try it, maybe find a compromise that suits you.
How about this as another possibility: Use just 3R, as used in my I/V converter above, would give only 31mV RMS. Then you would need 50x gain amp on the Secondary, but it can be a single-ended circuit, ground one side of the Secondary, because the summing has already been done on the Primary. This could be done with tubes. Who said you couldn't do a proper I/V converter with tubes? With a 1:1 transformer you could.
For now, get some hands on four 330R resistors and tell the rest of us what you think, did you get a better tonal balance or anything else?
Cheers, Joe R.
In the scheme, the 195ohm resistors is the internal DAC resistance or real resistors to ad between dac and transformers ?
thanks...
Yes, it is. But keep in mind, this is when the DAC is in stereo mode and all four pairs of phases are paralleled up. Each phase is 781.25 Ohm
Cheers, Joe
Thanks,
I want to keep signal from oppo95 direct from chip out. Can I connect the only 600:600 ohm transformer with the 330/330 ohm resistors like your suggest ?
... i have a tweked oppo95 and i have just 4 connections direct from DAC (phase + / - L channal and phase + / - R channal).
I want use this connections for a transformer output....
P.s. the original oppo output stage was excluded.
Attachments
Should work just as the diagram I posted. On the Secondary you will get about 1.3V if using 330R.
Cheers, Joe
Interesting question, but simple answer. Since the Secondary load is relatively high, the DAC hardly sees the DC resistance of the transformer, if it is reasonably low. I used a 1:1 transformer with a lot higher DCR than that and yet was under 100 Ohm.
The DAC will see the 330R at all times and very little in parallel.
Lundahl 1517 will be OK.
BTW, hate to be pedantic, there is no such thing as Ohms, it is always Ohm (no "s") whether singular or plural.
Cheers, Joe
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.
Hello,
thanks for your answers.
I tried with 330ohm and it sounds good.
Now let me understand this: I connected directly to the output of 30mV rms (with the 3 ohm resistors as shown in the diagram) my amp, but even at low volume there is distortion. Why?
In summary:
DAC 1.65v - 195r - 3ohm resistors to ground - 600:600 ohm transformer - 50K log pots - PCL82 ...
It 's impossible to have a solution to this?
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