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Old 11th March 2003, 01:01 PM   #21
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Weird hats aside, some of us have done a lot of research into this. We strongly feel that using isolation transformers function better that direct connections. We have also done a lot of research into what transformers work, and why.

And the bottom line is to use a transformer, and one with as little leakage inductance as possible.

And yes, all the loop filter mods and other tricks are still a bandaid for a poorly conceived interface.

Jocko

Found this quote from Jocko when I did a search for pulse transformer because I remembered this was discussed before. As often Jocko hits the nail on the head. And the simplicity of a good transformer has more sex-appeal to me than a bunch of digital IC's that all have their drawbacks. This does not mean I hate IC's or whatever. Sometimes simple solutions are the most effective. This is such a case. Please consider this is only true in case one uses a real good pulse transformer. Otherwise, don't bother.

I rolled my own before but it just takes too much time and has to be done real, real good. This was the Elektor type as used in their Mini DAC with the PCM1710U.

Then I tried Lundahl LL1566 which is an improvement but it is not perfect. The Lundahl has the advantage that it is easily obtainable. Despite it's technical drawbacks it is a good solution compared to electronics. So I can recommend it even though I know there are better types around. For some strange reason I did like the Lundahl more when used at both sides. In the cdplayer and in the DAC.

I could have lived with the Lundahl but then I saw another type/brand at a suppliers. That one is still in test phase so I won't go in detail for now.

I concur with Elso about the dismissal of SPDIF altogether. If one wants the best; build a DAC and output stage IN the cd-player in a shielded box. Make sure you build separate power supplies ( separated from the cd-players supply that is ) with their own power transformers. It will be better than with SPDIF even when one uses the non-plus-ultra of available transformers.

Regards,

Jean-Paul
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Old 11th March 2003, 01:17 PM   #22
Wombat is offline Wombat  Germany
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Thanks Jean-Paul!

I found no reference so far but as you have some experience i will use this!

regards
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Old 11th March 2003, 01:20 PM   #23
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Hi Wombat,

Please read this thread for information on this subject:

Digital transformer
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Old 11th March 2003, 01:48 PM   #24
tiroth is offline tiroth  United States
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I think Harry fell over laughing the last time someone mentioned LL1566. And Jocko seems to hate those AudioNote ones that some commerical designers seem to think are the best thing since sliced bread.

Honestly, the real hi-end guys are going to be critical of almost everything, since they took the time to find the best components.

If you recall, Jocko hinted that the Schott trannies that Digikey sells aren't a bad place to start.

A transformer isn't all you need in many cases; quite a number of DIR require CMOS level. If you use a 1:2 Scientific Conversions tranny you can get away with no additional circuitry, but I'm not certain you should. A buffer goes a long way towards cleaning up the crud.
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Old 11th March 2003, 04:42 PM   #25
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Fine with me but even LL1566 is better than direct coupling. I did try, I even have a DAC with the part still in it. I don't know the ( in) famous Audio Note part but if I look at the pictures of it I have my doubts about it's electrical capabilities. The winding techniques used seem not the best for a pulse transformer. But as always : it is not fair to judge a book by it's cover. I don't know it and haven't heard it as far as I know. Or it should be DAC 3 that has it ?

What "real" high end guys want I don't know. Some of them criticise everything and quite a few of them only listen to the faults of devices. Being mystical about foggy technical solutions, brand-awareness and the criticism to unknown devices or parts make that the qualification "high end guy" may be good for others but not for me. In the end the high end market is like anything else: it's about money and status. BTW I am just servicing a expensive brand DAC that has a pulse transformer in it with a bandwidth of only 1 MHz !!

The Schott parts seems to be good but I was never able to find one myself. All I know is that the company has a reputation for producing quality pulse transformers.



In this case it is about what is possible to use instead of the electronic solution.

A transformer is useful for galvanic isolation if you don't need it for differences in level. The Lundahl is a 1:1 transformer. No garbage creeps in to your DAC via ground if you use one. Please see the datasheet of CS8412 what it accepts at the input. ( Not the best advice since the readablity of this datasheet ! ).

The Crystal part couples some garbage back to the input. That effect is less if a transformer is used.
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Old 11th March 2003, 06:22 PM   #26
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Default Pulse Transformer

What about using the pulse transformer of a network interface for computers? I suppose the ones mounted on 100 Mbit/s fast ethernet cards should work well.
Marco
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Old 11th March 2003, 07:03 PM   #27
tiroth is offline tiroth  United States
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jean-paul,

I was less concerned about garbage from the ground than general degradation of the waveform. A really good source will deliver a clean signal, but if you have reflections and other problems due to insufficient cable or pulse transformer bandwidth it really helps to have a buffer to clean things up.
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Old 11th March 2003, 08:13 PM   #28
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Ok, Tiroth. But reflections are a reasonably controlable problem. It takes real 75 Ohm cable for a start, not microphone-cable as I saw a bit too often.

So let's assume a clean signal from the source and a well designed output stage of the cd-player. We have real 75 Ohm cable with the right bandwidth ( BTW which 75 Ohm cable hasn't the required bandwidth for SPDIF ? ). We have to start somewhere don't we ? Ok ?

That leaves us the problem of a decent input transformer, right ?

Please explain the need for a buffer instead of a transformer as I really don't see the need for it when all other variables are designed according the rules. Hereby I refer to the schematic as given in this thread. And if the other variables are not according certain rules solving that problems first is a pre in my book.
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Old 11th March 2003, 08:19 PM   #29
tiroth is offline tiroth  United States
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I agree. I just think from an engineering perspective if we can make the interface more robust without degrading quality, then we can only gain fidelity.
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Old 11th March 2003, 08:23 PM   #30
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I agree with you too. But then we should design a better interface than SPDIF in the first place !

And a better connector than cinch for digital transmission would be preferred.
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