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Old 15th May 2011, 03:07 PM   #1
skibum is offline skibum  France
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Default Isolation transformer for SPDIF input - ?

Hello,


What is the correct technique for using a 1:1 TX for isolating a S/PDIF input signal?


The common S/PDIF connection is via a Phono (RCA) plug. ( I know BNC is better but I don't want to get into that here).


The input impedance is suppose to be 75 ohms while the input resistance of a TX is about 1 ohm.


When you are not using a TX generally a 75 ohm resistor is placed across the input of the RCA connector.



But if the TX is placed across the input of the RCA connector than the input resistance will be basically 1 ohm no matter what resistance is placed in parallel with it. ???


I also want to put a 0.1uF Cap in. Should that go on the input side of the TX?


Thanks,


Skibum
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Old 15th May 2011, 04:13 PM   #2
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Others may disagree, but I'm not sold on the efficacy of using an input transformer with an RCA connected unbalanced S/PDIF input signal. You lose the common-mode rejection benefits of having a transformer - which you could regain by switching to a true balanced XLR connection. While a transformer will break any D.C. ground current loop, so will an inexpensive coupling cap.

That 1 ohm from the input transformer you mention is undoubtedly the D.C. resistance of the coil. What matters with transformers is their A.C. impedance. Ideally, a 74 ohm resistor would be located on the secondary side of the transformer. A 1:1 transformer will then reflect that 74 ohms to the primary side when it is driven with A.C. input signal. The net load impedance presented to the input signal will be 74 ohms + 1 ohm(coil R) = 75.

No offense is intended, but you don't stike me as having the requesite electronics knowledge to design your own input circuit from scratch. I recommend that you copy the input circuitry from some available existing DAC schematic.
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Last edited by Ken Newton; 15th May 2011 at 04:33 PM.
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Old 15th May 2011, 04:52 PM   #3
skibum is offline skibum  France
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Quote:
Originally Posted by Ken Newton View Post
Others may disagree, but I'm not sold on the efficacy of using an input transformer with an RCA connected unbalanced S/PDIF input signal. You lose the common-mode rejection benefits of having a transformer - which you could regain by switching to a true balanced XLR connection. While a transformer will break any D.C. ground current loop, so will an inexpensive coupling cap.

That 1 ohm from the input transformer you mention is undoubtedly the D.C. resistance of the coil. What matters with transformers is their A.C. impedance. Ideally, a 74 ohm resistor would be located on the secondary side of the transformer. A 1:1 transformer will then reflect that 74 ohms to the primary side when it is driven with A.C. input signal. The net load impedance presented to the input signal will be 74 ohms + 1 ohm(coil R) = 75.

No offense is intended, but you don't stike me as having the requesite electronics knowledge to design your own input circuit from scratch. I recommend that you copy the input circuitry from some available existing DAC schematic.

Thanks Ken,

I am using a Murata DA101C that Murata says is "suitable for 75 and 110 ohm circuits".

I will put a 74 ohm resistor on the secondary side and give it a try.

You are right - I am new at all this! I am modding a cheap DAC I bought on Ebay. I really do not mind copying an input circuit from some available existing DAC schematic but I want to understand how it works - that's why I am asking here on this site. I appreciate your input, I now understand that my resistance measurement I made of the TX coil with my multimeter is pretty much meaningless. Thanks!
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Old 15th May 2011, 05:16 PM   #4
qusp is offline qusp  Australia
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yes, you want impedance, which will be different at differing frequencies. all too often resistance and impedance are lumped together
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Old 16th May 2011, 12:46 PM   #5
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FYI - The link below is to a short discussion on transformer basics. I suggest reading some of basic tutorial white papers written by Bill Whitlock on the Jensen Transformers website.

AK4396: best solution for output stage
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Old 16th May 2011, 01:57 PM   #6
infinia is offline infinia  United States
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Quote:
Originally Posted by skibum View Post



The input impedance is suppose to be 75 ohms while the input resistance of a TX is about 1 ohm.


But if the TX is placed across the input of the RCA connector than the input resistance will be basically 1 ohm no matter what resistance is placed in parallel with it. ???

why do you talk about 1 ohm.

at low frequencies the sending (Thevenin) impedance is usually approximated by using Rds (on) of the drivers mosfets. In an IC this can be determined by looking at The parts data sheet. Also circuit ideas to answer your Q's can be optained by looking the data sheets application schematics. usually Rds of 20-50 ohms is typical and varies with temperature. At high frequencies RF parasitics come to play.
Ideally a 2:1 transformer is used in my example Rds ~ 1/2 (75 ohm)
Start collecting application data sheets of pulse XFMRs and typical ICs used.
Thévenin's theorem - Wikipedia, the free encyclopedia
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Old 16th May 2011, 05:13 PM   #7
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Hello

It is good that you want to understand what is going on - you probably have a circuit in mind but you can also look here (on the schematic page) for a transformer coupled co-ax input.

The γ1 Modular Miniature DAC

I have built this and it works pretty well so the basic circuit is good.

GR
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Old 17th May 2011, 04:20 PM   #8
skibum is offline skibum  France
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Thanks for all the replies. I am going to go with a 0.1uF cap between the RCA and the TX input side and a 74R resistor across the output side of the TX.
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Old 17th May 2011, 04:30 PM   #9
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by skibum View Post
Thanks for all the replies. I am going to go with a 0.1uF cap between the RCA and the TX input side and a 74R resistor across the output side of the TX.
Use a 75 ohm resistor, the 1 ohm transformer dcr appears in series with the winding reactance and is basically irrelevant.(the reactance should be quite large at the frequencies of interest and appears in parallel with the 75 ohm load resistor, note that the winding dcr and any leakage inductance actuallly do appear in series with the winding inductance, and these parasitic elements along with any interwinding/stray capacitance also form a frequency dependent voltage divider with the load) See comments about thevenin in previous posts. Generally the transformer and the coax are terminated into 75 ohms assuming a 1:1 transformer.. Note that the source impedance driving the transformer and coax should also be 75 ohms - depending on signal levels involved this may just be a simple network (see my comments in the shigaclone thread on this subject)
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