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Old 25th February 2010, 04:01 PM   #11
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Originally Posted by abraxalito View Post
I'm wondering what full galvanic isolation might achieve

Me too. I also suspect that it's not quite the same whether you isolate USB, I2S or somewhere further downstream. In my own anecdotal experience using a 2707 into an ASRC and a 1794 with a transformer output, there is no audible difference whether the grounds before and after the transformer are connected. Of course there must be lots of capacitive coupling anyway.
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Old 25th February 2010, 04:21 PM   #12
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Originally Posted by abraxalito View Post
I'm not really sure from this what you're trying to achieve. Do you mean you want to send normal data from I2S into one DAC, and inverted data into another? If so, that will maybe improve SNR by 3dB and cancel out any 2nd order distortion components in the DACs. Such a differential configuration is easily implemented together with the LVDS balancing - to generate + and - data just choose the receivers without the termination resistors (-A rather than -AQ suffix) and correct them with opposite polarities, remembering to add an external termination. The step from SE to BAL (or back again) in digital is quite a lot simpler than in analog - just one of those LVDS chips does the trick. Since the DACs don't have BAL inputs, the conversion from BAL to SE is inevitable.
I was hoping that getting a differential I2S signal as in the attached pic might also have the added benefit of cancelling common mode noise but I may be wrong?
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File Type: jpg I2S Balanced.JPG (42.2 KB, 231 views)
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Old 25th February 2010, 04:24 PM   #13
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I'm wondering what full galvanic isolation might achieve, given that the DAC is going straight into headphones in this case- i.e. not into any other equipment with a ground referenced connection. If the DAC was being fed into a preamp, then on to a power amp it may well be justified. Another thing to try which is way, way simpler is to feed the cables between the computer and DAC through a series of ferrites to absorb RF hash wanting to travel out from the USB port. A few of the clip-on kinds of ferrites will certainly clean things up if applied at right next to the USB port.
Ah well, the real difference in sound between I2S & SPDIF is only noticeable when using a full preamp/amp rig & not noticeable when using the headphones.

Hmm, I'll experiment with ferrites!
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Old 25th February 2010, 08:30 PM   #14
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Originally Posted by abraxalito View Post
.............. Since the DACs don't have BAL inputs, the conversion from BAL to SE is inevitable.
I'm not with you here - I see one I2S stream from the pic above fed to a Vout DAC creating R+ & R- at it's analog outs. And the other DAC would similarly create L+ & L-. These analog signals could be fed to a balanced headphone or balanced preamp/amp.

I imagine that R+/- being generated in one DAC would have better common mode noise cancellation than if R+/L+ was generated in one DAC & R-/L- in another DAC but it may not be worth the extra effort necessary to achieve this?

Last edited by jkeny; 25th February 2010 at 08:36 PM.
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Old 25th February 2010, 10:21 PM   #15
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Default Advantage of balanced mains supply

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Ah well, the real difference in sound between I2S & SPDIF is only noticeable when using a full preamp/amp rig & not noticeable when using the headphones.
Ah, well now it makes more sense to me. I've just upgraded my rig by fitting mains isolation in the power feed to my active speakers and CD player and DAC. This works a treat - means that there's no route for mains hash to get into the system now. I suggest you try the same - in the power feed to your pre/power combination.

I bought two 230V -> 115V 500VA toroids and wired them up to balance the mains feed. So instead of a live wire with 230VAC and a neutral which is close to earth, I now have two 115V wires 180degrees out of phase. The centre point of the two secondaries is connected to earth. This means firstly that incoming RF on the mains gets shunted to ground. But more importantly, the capacitive coupling across the toroids in my active speakers and DAC is considerably reduced, because now they're receiving a balanced power signal rather than a single-ended one. Whereas before I was getting upto 200uA current out of the signal ground into other equipment, now its around 5uA.
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Old 25th February 2010, 10:39 PM   #16
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Default Ground isolation

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Originally Posted by jkeny View Post
I was hoping that getting a differential I2S signal as in the attached pic might also have the added benefit of cancelling common mode noise but I may be wrong?
That's exactly what the LVDS transmitters give you - a differential signal - not just for the data lines, but all of it. It means that ground noise has a much reduced effect. So for example, if you're having trouble in a particular set-up with conducted RF along the ground, you can deal with this with a choke in the ground wire. Or with ferrites over the cable. Using the LVDS means you can now tolerate upto 1V of common mode voltage i.e. upto 1V difference in the ground potentials between transmit and receive sides. Any clearer now?

Full galvanic isolation means you can have potentially a couple of kV difference between two grounds, which looks a bit like overkill in this application where you just want to be rid of some RF!

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I imagine that R+/- being generated in one DAC would have better common mode noise cancellation than if R+/L+ was generated in one DAC & R-/L- in another DAC but it may not be worth the extra effort necessary to achieve this?
Yes, you're right. If you want to use more DACs, that's the way to go. But using more DACs is really only useful if the audible noise of a single one is a problem for you, or there's significant 2nd order distortion in their outputs which is common between them. In this case, you'll get common mode cancellation of the audio band noise, but not common mode cancellation up to RF frequencies which is what I thought you wanted. Perhaps that's been the misunderstanding - when we're talking about common-mode cancellation, there's a difference between audio band cancellation and RF cancellation.

Last edited by abraxalito; 25th February 2010 at 10:46 PM. Reason: forgot 2nd question
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Old 26th February 2010, 11:34 AM   #17
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Originally Posted by abraxalito View Post
That's exactly what the LVDS transmitters give you - a differential signal - not just for the data lines, but all of it. It means that ground noise has a much reduced effect. So for example, if you're having trouble in a particular set-up with conducted RF along the ground, you can deal with this with a choke in the ground wire. Or with ferrites over the cable. Using the LVDS means you can now tolerate upto 1V of common mode voltage i.e. upto 1V difference in the ground potentials between transmit and receive sides. Any clearer now?
But the differential signal coming out of the LVDS transmitter can't be used as a direct feed into a DAC to give balanced feed to the DAC, I presume. It has to go through the LVDS receiver which converts it back to SE & I then have to use some SE to BAL converter to feed the DACs a balanced signal - maybe that is the best way to do it but it seems so roundabout?

Quote:
Full galvanic isolation means you can have potentially a couple of kV difference between two grounds, which looks a bit like overkill in this application where you just want to be rid of some RF!



Yes, you're right. If you want to use more DACs, that's the way to go. But using more DACs is really only useful if the audible noise of a single one is a problem for you, or there's significant 2nd order distortion in their outputs which is common between them. In this case, you'll get common mode cancellation of the audio band noise, but not common mode cancellation up to RF frequencies which is what I thought you wanted. Perhaps that's been the misunderstanding - when we're talking about common-mode cancellation, there's a difference between audio band cancellation and RF cancellation.
Well, yes I wanted CM noise cancellation at all frequencies. Would using a DAC operating from a differential signal not cancel all the common noise, not just in the audio band?


I wanted to go differential into 2 DACs anyway & thought it might also be a way of cancelling any ground noise riding on the signal line - that's all really.

So, I'm looking for the easiest, smartest way of achieving this & I posed this as a question

Last edited by jkeny; 26th February 2010 at 11:36 AM.
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Old 26th February 2010, 11:51 AM   #18
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Thanks for sharing that schematic, its helping me with a design I'm currently thinking about

The HEF4517 is still in full production according to NXP, however from the datasheet that circuit isn't guaranteed to work, though typically it will. It could never be made to work at 192kHz since that requires a bit clock frequency over 10MHz. Increasing the supply to the 4517 to 15V might do the trick though, then level shifters will be needed...

In any case I don't at the moment see how this circuit might take the place of galvanic isolation. If you've got ground currents, once they've been translated into noise at the bit-clock receiver, there's no way to undo the damage. That's because the generated error from the jitter is data-related - a jitter error when you've got digital silence is a different matter from a jitter error at full scale.

I'm currently investigating isolation schemes for my own project so will post something when I've got it to offer... As a first attempt, I think jitter only matters on the bit clock, not the other signals. So we only need to pay attention to that signal - others can have major jitter without any audible effects.
Sorry, Abraxalito, I missed this post & I see that in it, you've answered all my subsequent questions, so sorry for repeating them & appearing dumb
I see what you're saying about ground currents being converted to noise on the bit clock - I guess I was treating the DAC like an op-amp, doh!

I'm interested in hearing about your results with BCK isoaltion
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Old 27th February 2010, 12:20 AM   #19
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Default Dealing with CM noise

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Originally Posted by jkeny View Post
But the differential signal coming out of the LVDS transmitter can't be used as a direct feed into a DAC to give balanced feed to the DAC, I presume. It has to go through the LVDS receiver which converts it back to SE & I then have to use some SE to BAL converter to feed the DACs a balanced signal - maybe that is the best way to do it but it seems so roundabout?
I'm wondering if my terminology isn't precise enough here, so I'm gonna make an attempt to clarify a bit more... By 'DAC' I'm talking about the chip that does the job (like TDA1545 for example), not the whole unit. So yes, given that no DAC chips I've seen have differential inputs, you'd need an LVDS receiver between the differential I2S and the chip. So no, I can't see the need here for 'some SE to BAL converter'. The two (+ and -) DAC chips can be fed from the same differential I2S data line via LVDS receivers connected in opposite polarity.

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Well, yes I wanted CM noise cancellation at all frequencies. Would using a DAC operating from a differential signal not cancel all the common noise, not just in the audio band?
I think I might have been less than clear on this point too. The rejection of CM noise is a function of the receiver, not the transmitter. So when you're feeding headphones, the fact they have no ground connection means they're very good at rejecting CM noise, at all frequencies. But I guess your preamp has a SE input, and that's the problem here. Even running + and - DACs will need a Bal-SE conversion stage before going out to the preamp. Then the CM noise is picked up by the preamp, so all advantage (in CM noise) of running balanced is lost. So to deal with CM noise effectively, we need a systems-wide solution, it can't be fixed just in the DAC unit. Make better sense now?

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I wanted to go differential into 2 DACs anyway & thought it might also be a way of cancelling any ground noise riding on the signal line - that's all really.
Yep, it is, provided the next element in the chain has a balanced input with good CM rejection.
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Old 27th February 2010, 12:51 AM   #20
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I'm wondering if my terminology isn't precise enough here, so I'm gonna make an attempt to clarify a bit more... By 'DAC' I'm talking about the chip that does the job (like TDA1545 for example), not the whole unit. So yes, given that no DAC chips I've seen have differential inputs, you'd need an LVDS receiver between the differential I2S and the chip. So no, I can't see the need here for 'some SE to BAL converter'. The two (+ and -) DAC chips can be fed from the same differential I2S data line via LVDS receivers connected in opposite polarity.
Sorry, yes, I'm being really dumb here - I'm having a brain haze mixing up how balanced analog works using two lines carrying R+/- & two lines carrying L+/- with how balanced digital would work using one line for R+/L+ & one line for R-/L-.


Quote:
I think I might have been less than clear on this point too. The rejection of CM noise is a function of the receiver, not the transmitter. So when you're feeding headphones, the fact they have no ground connection means they're very good at rejecting CM noise, at all frequencies. But I guess your preamp has a SE input, and that's the problem here. Even running + and - DACs will need a Bal-SE conversion stage before going out to the preamp. Then the CM noise is picked up by the preamp, so all advantage (in CM noise) of running balanced is lost. So to deal with CM noise effectively, we need a systems-wide solution, it can't be fixed just in the DAC unit. Make better sense now?


Yep, it is, provided the next element in the chain has a balanced input with good CM rejection.
Yes, I intend to use balanced all the way to the speakers!
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