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Old 10th December 2012, 11:57 AM   #381
rolls is offline rolls  Switzerland
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Originally Posted by RollE2k View Post
But, since my line-stage is balanced with 5,4dB gain on each phase - then wouldn't it be better to connect output from each phase with a 2.7R resistor to GND, then maybe the cap between both phases (what does this do in this specific case?) and then into the step-up transformer.
In my case the output from the transformer would go straight into the volume attenuator (50k R2-R log attenuator between the phases not down to GND) with approx 15cm wire, twisted and then maybe 15cm wire into my tube-line-stage.
Thare shouldn't be any trouble running from the step-up into the volume right? since impedance matching seems quite fine ~1.85k into 50k.
Also Joe, wouldn't it be better skipping the GND connection on the output, at least when i run transformer - to keep DAC-GND from my Line-stage GND?

Thanks alot for taking your time helping in this thread so far :-)
Got a bit confused now:
Output of buffalo is loaded with either 2pc of 2.7Ohm or one 2.7Ohm wire with ground in the middle into the transformer, yes?
This is the best way, linestage not involved in the 2.7Ohms?!
Because the secondary is always floating, you can choose what to do.
How have you come to 1.85k with the Lundahl?
Did you already check if the Lundahl can do it?
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Old 10th December 2012, 06:08 PM   #382
RollE2k is offline RollE2k  Sweden
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Output would be loaded with 2 x 2.7R resistors (one for each phase), and GND in between them.
How i come up to approx 1.85K impedance is because i have calculated it.
LL1933 should work according to Lundahl (up to 50mA) and does have primary impedance of 0.8R per winding, and 85R on secondary.
So on the primary side i have 5,4R + transformer 1,6R. Transformer ratio of 1:16 equals 256x impedance ratio. so 7x256 = 1792ohms plus the impedance from the secondary 85/2=42,5R (since they are paralelled) so in total 1834,5R simple as that.

What i may have done wrong with the above calculations is that i'm not shure if the the loading resistors would "seem" in paralell for looking from the secondary of the transformer. If it would be that way, then impedance would only be 938,5R.
Anybody that can calarify this?

Also one last thing, would it be better having the primary floating (center tap not connected to GND) or would it be better to connect it straight to ground?
Also, when running outputs directly from dac to transformer, wouldn't there be any issues with DC-offset? i'm just thinking if coupling cap would be needed for transformers that cannot take DC?
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Old 10th December 2012, 07:04 PM   #383
rolls is offline rolls  Switzerland
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Originally Posted by RollE2k View Post
Output would be loaded with 2 x 2.7R resistors (one for each phase), and GND in between them.
How i come up to approx 1.85K impedance is because i have calculated it.
LL1933 should work according to Lundahl (up to 50mA) and does have primary impedance of 0.8R per winding, and 85R on secondary.
So on the primary side i have 5,4R + transformer 1,6R. Transformer ratio of 1:16 equals 256x impedance ratio. so 7x256 = 1792ohms plus the impedance from the secondary 85/2=42,5R (since they are paralelled) so in total 1834,5R simple as that.
What i may have done wrong with the above calculations is that i'm not shure if the the loading resistors would "seem" in paralell for looking from the secondary of the transformer. If it would be that way, then impedance would only be 938,5R.
Anybody that can calarify this?
1.6 Ohm are primary dc resistance, it does not play a role here, because important are the inductance and reflected secondary load.
If you have a normal loading resistor of 47k, it would reflect (47k :256) about 200Ohms, parallel to your 5.4 Ohms, no problem here.

Also one last thing, would it be better having the primary floating (center tap not connected to GND) or would it be better to connect it straight to ground?
It would be helpful if you carefully read my post 372!
That was the main reason I wrote the post.


Also, when running outputs directly from dac to transformer, wouldn't there be any issues with DC-offset? i'm just thinking if coupling cap would be needed for transformers that cannot take DC?
Definitely not, had been discussed as well.
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Old 10th December 2012, 08:53 PM   #384
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Quote:
Originally Posted by rolls View Post
I would like to thank Joe Rasmussen.
Thanks.

Quote:
Originally Posted by rolls View Post
It is a pity that The Sabre chip sounds o.k. even in voltage mod, so often people use 1.1 trannies, which is still much better than most of the active I/V stages, but sounds just broken compared to (nearly) passive current mode, but there are people paying 1000 Dollars for a transformer to run a Sabre in voltage mode....
Yep.

Whether using step-up or 1:1 Txs, that Z on the Primary needs to be as low as possible - and it benefits BOTH the DAC and Txs. But in your case, step-up makes it even more important as I see it.

I suspect that using a step-up transformer with 1K on the Primary is not so good because the source Z of the DAC is way too high into the Primary. In that case a step-up transformer would indeed sound "broken" - so the Lo Z is also good for step-up transformers as the need that Lo Z on the Primary may be even greater with step-ups. And that Z can be lower than 1:1 - so do it.

It really doesn't matter what the Tx post-DAC solution is (if using Txs), the basic rule is the same. Get the Z on the Primary as low as possible, and don't worry about grounding the Sabre DAC - and that passive I/V is the way to go, even partially loading down 1:1 Txs. It just comes down to whether you have enough output.

Even with non-Tx solution, Lo Z passive I/V is the way to go. But here the benefit is only to the DAC and very important too. But using Txs, they should always sound better with Lo Z on the Primary.

Hope I haven't sounded to repititious.

Cheers, Joe
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Old 10th December 2012, 09:05 PM   #385
RollE2k is offline RollE2k  Sweden
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According to Lundahl at least does the transformer resistance count into total impedance, and logically i do not see why it wouldn't?

Anyway, have i understand right that from the beginning you just had a 1,2R "resistor/wire" between the positive and negative phases, and then you split it connecting the "middle" to the transformer Center Tap? but you did never connect to DAC-GND?

Which of these solutions did you use?
Click the image to open in full size.

Joe: did you see my questions at post #380

Last edited by RollE2k; 10th December 2012 at 09:18 PM.
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Old 10th December 2012, 09:21 PM   #386
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Quote:
Originally Posted by RollE2k View Post
According to Lundahl at least does the transformer resistance count into total impedance, and logically i do not see why it wouldn't?

Anyway, have i understand right that from the beginning you just had a 1,2R "resistor/wire" between the positive and negative phases, and then you split it connecting the "middle" to the transformer Center Tap? but you did never connect to DAC-GND?

Which of these solutions did you use? Click the image to open in full size.

Joe: did you see my questions at post #380
No, both are not good. The second is worst as you will get DC through transformer.

Only connect the Ground to the centre of where the resistors meet.

Connect nothing to the transformer's centre tap.

You do not want any potential DC to go through the transformer - the DC resistance of the Primary (or any part of it) must not be in parallel with the resistors, or else some DC current will flow through that part of the Primary. You was all the DC current to go through the resistors, to ground.

If the two resistors are well matched, then both extremities of the Primary will sit at the same voltage. That means no DC current through the Primary of the transformer.

Is that clear enough?

Re Secondary. In theory, best not to be grounded. Especially not grounded when balanced used in balanced mode. If unbalanced, then still best to let it float at the transformer end (if possible or practical) and ground at the receiving end, whether that be a pot/volume control or not. That way you get "galvanic isolation" - that is that the ground of the DAC is isolated from the ground of the preamp or whatever is at the receiving end. Isolating grounds are good in principle.

Cheers, Joe
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Last edited by Joe Rasmussen; 10th December 2012 at 09:33 PM.
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Old 10th December 2012, 09:40 PM   #387
RollE2k is offline RollE2k  Sweden
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Originally Posted by Joe Rasmussen View Post
Only connect the Ground to the centre of where the resistors meet.

Connect nothing to the transformer's centre tap.

You do not want any potential DC to go through the transformer - the DC resistance of the Primary (or any part of it) must not be in parallel with the resistors, or else some DC current will flow through that part of the Primary. You was all the DC current to go through the resistors.

If the two resistors are well matched, then both extremities of the Primary will sit at the same voltage. That means no DC current through the Primary of the transformer.

Is that clear enough?

Re Secondary. In theory, best not to be grounded. Especially not grounded when balanced used in balanced mode. If unbalanced, then still best to let it float at the transformer end (if possible or practical) and ground at the receiving end, whether that be a pot/volume control or not. That way you get "galvanic isolation" - that is that the ground of the DAC is isolated from the ground of the preamp or whatever is at the receiving end. Isolating grounds are good in principle.

Cheers, Joe
Yes, i think so. Like my second example, but remove the center tap from the middle/gnd between the resistors.
But what is the cap for, the 0.33uF one you had between positive and negative phases?
Also when running primary side this way and you look from the secondary side (1:16). Does the reflected impedance look like (2,7+2,7)x256=1138,4R or (2,7/2)x256=345,6R - that is, do they look like series OR parallel?
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Old 10th December 2012, 09:52 PM   #388
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Quote:
Originally Posted by RollE2k View Post
But what is the cap for, the 0.33uF
Please note that was a NON-transformer solution. That cap was specific to that circuit and not transformers. The transformer is a bandpass filter and does not need the cap.

Cheers, Joe
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Old 11th December 2012, 12:13 AM   #389
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Originally Posted by Joe Rasmussen View Post
No, both are not good. The second is worst as you will get DC through transformer.

Only connect the Ground to the centre of where the resistors meet.

Connect nothing to the transformer's centre tap.

You do not want any potential DC to go through the transformer - the DC resistance of the Primary (or any part of it) must not be in parallel with the resistors, or else some DC current will flow through that part of the Primary. You was all the DC current to go through the resistors, to ground.

If the two resistors are well matched, then both extremities of the Primary will sit at the same voltage. That means no DC current through the Primary of the transformer.

Is that clear enough?

Re Secondary. In theory, best not to be grounded. Especially not grounded when balanced used in balanced mode. If unbalanced, then still best to let it float at the transformer end (if possible or practical) and ground at the receiving end, whether that be a pot/volume control or not. That way you get "galvanic isolation" - that is that the ground of the DAC is isolated from the ground of the preamp or whatever is at the receiving end. Isolating grounds are good in principle.

Cheers, Joe
Hi Joe, do the resistors need to be terminated to ground? I have bought a Buffalo DAC, haven't assembled it yet but for an IV convertor I was planning on an approach shown in the diagram ie I will feed the DAC output into my Lundahl transformer, and as I prefer not to use a preamp, I will vary level by use of a switched resistor (24 position switch) also across the output - its easier if I can just use a single resistoer across the 2 phases rather than having 2 resistors (which then requires a double pole switch). What do you think?
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Old 11th December 2012, 02:48 AM   #390
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Originally Posted by hazard500 View Post
Hi Joe, do the resistors need to be terminated to ground? I have bought a Buffalo DAC
Yes, in my opinion that is what will get you the max results. It does two things, converts DC voltage offset to current offset, and creates the lowest impedance possible (our topic is Txs) into the Primary.

Transformers works at their best when driven from a low impedance. The output impedance of the Sabre DAC is quite high, many times higher than voltage DACs. So the low impedance cannot come from the DAC itself, it in a sense has to come from the Ground. Simple as that. (Current type DACs like B-B does this naturally as the have a very high output Z and relies on Ground to make it low). And Sabre DACs are also at their most linear when seeing a Lo Z current mode. The usual "common garden" solution is to use an opamp with a Virtual Ground approaching Zero Ohm. But this is a feedback trick. Normally with "current" DACs with DC voltage offset, this means that the Zero Ohm has to actually float at the same DC voltage offset of the DAC (usually 1.65V).

But the Sabre DAC is very different beast, it has a fixed output Z of 780 Ohm. Paralleling four phases will give you 195 Ohm combined, eight will give you 97 Ohm. These are all relative high - not good for Txs. But ahh... that output impedance also acts as a current limiter when shortened to ground, and in the Sabre DAC means we can do something we cannot do with any other DAC: Pull it to Ground! The DC offset current is 1.65V/780R = 2.1mA per phase. It cannot go higher than that - so we use that to advantage. We can now construct a near REAL EARTH rather than a Virtual one.

The downside? Only slightly more heat generated by the DAC's outputs and that about 3.5mW phase per side. Negligible. In total about 50mW.

The DC offset current we have created must go through the resistors and not the transformers. The resistors should also be 1% or better matched.

Cheers, Joe
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