Digital pulse transformer : Lundahl, Scientific Conversion, Murata or Newava ??? - diyAudio
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Old 3rd October 2011, 11:04 AM   #1
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Default Digital pulse transformer : Lundahl, Scientific Conversion, Murata or Newava ???

Hi there,

I'm using a Lundahl pulse transformer LL1572 (1:1) on the SPDIF unbalanced (coax) output stage of my Goldmund Mimesis 36+ CD transport, and I rather like it : smooth and natural sounding, much better than the stock Coilcraft WB2010.

However, some people seem to prefer Murata DA100 series, Scientific Conversion SC939 or Newava S37211...

Could somebody help me ? Some experience on the topic ? What criteria or rating is important to choose a 1:1 pulse transformer for SPDIF ?
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Old 3rd October 2011, 12:40 PM   #2
marce is offline marce  United Kingdom
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Can I ask how a pulse transformer on a digital signal can effect the sound! If the analogue was going through the transformer I could understand there being some effect, but a digital signal!
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Old 3rd October 2011, 01:01 PM   #3
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Impact on rise times, ringing etc I'd imagine......
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Old 3rd October 2011, 01:03 PM   #4
gmarsh is offline gmarsh  Canada
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Quote:
Originally Posted by marce View Post
Can I ask how a pulse transformer on a digital signal can effect the sound! If the analogue was going through the transformer I could understand there being some effect, but a digital signal!
The end device has to pull a clock out of SPDIF to use for playback, and the less damaged the SPDIF signal is before it gets there, the better the clock recovery is going to be.

Think of SPDIF as an RF signal with sharp edges, and your goal is to preserve those edges. Making sure the interface you come up with doesn't ring and get extra edges, suffer from reflections that can bash subsequent edges, or take too much slope off the edges.

As for transformers... my usual metric for a transformer is a return loss versus frequency plot, you want it to be as low as possible at SPDIF frequencies. My usual transformer for SPDIF is a Mini-Circuits ADT1-6T, chosen for this reason. It's a 50 ohm transformer, and I match a 75 or 110 ohm input to 50 ohms when I use it. Never really used anything else.
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Old 3rd October 2011, 02:53 PM   #5
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Thank you gmarsh for your comment ! Anybody else who tried one of these transformers (Murata, SC, Lundahl, Newava) ??
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Old 3rd October 2011, 04:07 PM   #6
marce is offline marce  United Kingdom
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I am fully conversant with high speed digital, high speed digital layout and signal integrity.
Is there some empirical measurements and data that we can relate to, to say why a pulse transformer can sound different. If the waveform is bad before the pulse transformer then its gonna be bad coming out of the pulse transformer, and if you wave is so bad then there are signal integrity issues that require addressing.
The best way would be collating the digital waveforms before and after the pulse transformer, and also the analogue output of the DAC.
As stated signal integrity issues have to be considered and the only way of knowing what is going on is to capture the waveforms, so that the any problems such as excessive ringing etc can be observered. Though ringing that apears on the top of the wave is nowhere as critical as non monotonicity of the rising (or falling) edge, where the switching threshold is.
So what I am trying to determine is empirical data to support and determine what is happening to the square wave, that will affect the sound, as digital is very tolerant of noise etc. Or is the data just from listening...
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Old 3rd October 2011, 04:49 PM   #7
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Old 3rd October 2011, 05:22 PM   #8
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Quote:
Originally Posted by marce View Post
I am fully conversant with high speed digital, high speed digital layout and signal integrity.
Is there some empirical measurements and data that we can relate to, to say why a pulse transformer can sound different. If the waveform is bad before the pulse transformer then its gonna be bad coming out of the pulse transformer, and if you wave is so bad then there are signal integrity issues that require addressing.
The best way would be collating the digital waveforms before and after the pulse transformer, and also the analogue output of the DAC.
As stated signal integrity issues have to be considered and the only way of knowing what is going on is to capture the waveforms, so that the any problems such as excessive ringing etc can be observered. Though ringing that apears on the top of the wave is nowhere as critical as non monotonicity of the rising (or falling) edge, where the switching threshold is.
So what I am trying to determine is empirical data to support and determine what is happening to the square wave, that will affect the sound, as digital is very tolerant of noise etc. Or is the data just from listening...
It's the way the clock is encoded with the data that makes spdif so susceptible to the interface. There are several papers written by Julian dunn on how bandwidth limitation of the transmitter medium results in data modulated jitter.

Another factor is also that a poor transformer will have quite high leakage inductance making it harder to terminate properly and a poorly terminated interface gives rise to reflections which will also degrade the recovered clock by causing level uncertainties.

However with some care it is possible to approach the limits of what it is possible to measure with 16bit data and the "Jtest" method for measuring data modulated jitter as introduced by dunn.
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Old 3rd October 2011, 08:01 PM   #9
marce is offline marce  United Kingdom
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Its a form of manchester encoding that is used quite widely for data transmission, though the differential method is prefferable.
With digital it is either there or it isn't, SPDIF is quite a slow interface these days, and should with modern devices and what is known about transmitting signals, shouldn't present much of a problem, there are numerous digital interfaces that work at higher speeds, that work quite succesfully. We use custom made pulse transformers for a variet of interfaces as well as planar transformers built into PCB's.
What I am after is the explanation and data on how analogue terminology for the effect on sound can be ascribed to a digital interface, how you ascribe one sounding smooth etc. This would imply that a regular change to the analogue output is being achieved by the transformer! somthing usualy only achieved by DSP in the digital domain.
The artifacts that effect digital signals and the problems of noise are covered quite extensively in a few good tomes, Howard Johnson, High speed digital design for the Signal Integrity and Henry Ott for EMC problems...
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Old 3rd October 2011, 08:29 PM   #10
Angelm is offline Angelm  Bulgaria
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I tried Lundahl LL1572 on S\PDIF output of Musiland 01 USD,
after a day I replaced with Newava. Much better IMHO.
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