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Old 23rd July 2014, 03:58 AM   #1
klackto is offline klackto  Australia
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Default Metrum Octave ... change toslink input to coax

Hi Am after some advice. I have a 1st generation Metrum octave that has coax and toslink inputs that are switchable via front panel toggle but I have 2 sources that I would like to use via coax outputs.Would it be possible to remove the toslink receiver and instead wire in a BNC to the point where the toslink receiver outputs the electrical spdif signal? Is it that simple? I gather a pulse transformer may be necessary. Any advice would be much appreciated.

Last edited by klackto; 23rd July 2014 at 10:44 PM. Reason: Mistake in title
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Old 23rd July 2014, 07:38 PM   #2
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Originally Posted by klackto View Post
Hi Am after some advice. I have a 1st generation Metrum octave that has coax and toslink inputs that are switchable via front panel toggle but I have 2 sources that I would like to use via coax outputs.Would it be possible to remove the toslink receiver and instead wire in a BNC to the point where the toslink receiver outputs the electrical spdif signal? Is it that simple? I gather a pulse transformer may be necessary. Any advice would be much appreciated.
Without seeing the schematic, or having an Metrum Octave, my advice will entail a fair degree of guesswork. So, here goes. I suggest you not physically remove the TOSLINK receiver, as you may want to reconnect it in the future. Drill a hole in the chassis to mount the 75 ohm BNC connector. Terminate the connector with a 75 ohm resistor soldered directly across the connector pins. Solder a tightly twisted pair of wires from the BNC connector to the PCB. The exact location on the PCB for soldering the twisted pair will depend on how the Octave's selector switch is routing the two input signals - which, I don't know. You should avoid cutting PCB traces to disconnect the TOSLINK unit output in favor of desoldering that connection if possible. While a pulse transformer would be useful to include for blocking common-mode currents should there be room for one, it's not a necessity.
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Last edited by Ken Newton; 23rd July 2014 at 07:45 PM.
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Old 23rd July 2014, 10:42 PM   #3
klackto is offline klackto  Australia
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Hi Ken

Thanks for the reply.
At the moment I am just trying to ascertain whether it would be possible.
Space will be limited, especially if I leave the toslink receiver in place so am thinking of omitting the pulse transformer. Both my sources have a transformer on the spdif out in any case.
I need to have a good look at the pcb to see if I can trace how the signal is routed.
If I leave the receiver in place and say I desolder and isolate the output pin then I should be able to find a point slightly further upstream to solder the twisted wire to. Do you think this will work?
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Old 24th July 2014, 12:49 PM   #4
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Yes, it sounds like your plan to desolder the TOSLINK receiver output pin will work. Just be sure to connect the BNC leads at some point before the input selector switch (or relay) contacts, so that input selection still functions. Alternatively, Metrum may be utilizing an DIR (Digital Input Receiver) chip featuring independent input pins selected by that toggle switch, meaning, the input signal would then not be routed through any switch or relay contacts.

Also, there almost certainly will be a small value (around 100nF) high frequency A.C. coupling capacitor located just before the DIR chip's input pin. Be certain that you are not bypassing this coupling capacitor when you solder the BNC leads to the PCB. The capacitor is there to keep D.C. current from flowing in or out of the DIR input pin.
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Last edited by Ken Newton; 24th July 2014 at 01:03 PM.
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Old 24th July 2014, 01:36 PM   #5
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Don't use twisted wires but use thin coax/shielded cable and use a SPDIF transformer as the original SPDIF input also has one (mmm, my Metrum Quad had one, I don't know the Octave that well). That is if you want equal performing SPDIF inputs and galvanic isolation. BTW you could make the input cap of the receiver slightly larger in value. For an unknown reason this sounds better, I was pointed to this by a member concerning our V3 DAC and it indeed sounds a tad better. No explanation...

If you will be using plain wires better keep them aligned and don't twist them. Twisting nearly always is better but not so with SPDIF.

I take you use good 75 Ohm BNC connectors ? The Tyco 1-1478048-0 fitted exactly in the RCA mounting hole of my Metrum Quad and it performs excellent.
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Last edited by jean-paul; 24th July 2014 at 01:40 PM.
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Old 24th July 2014, 02:36 PM   #6
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Originally Posted by jean-paul View Post
Don't use twisted wires but use thin coax/shielded cable...If you will be using plain wires better keep them aligned and don't twist them. Twisting nearly always is better but not so with SPDIF.
Yes, properly attached 75 ohm coax would be best, except that mounting the 75 ohm terminating resistor directly on the BNC connector may very well interfere with proper connection of an coax cable to the connector. Directly mounting the terminating resistor on the PCB may or may not be practical with the PCB layout.

Twisting a plain wire pair will reduce parasitic inductance while increasing parasitic capacitance. It's a guess as to which will provide the least signal edge deformation. However, twisting is usually preferred for wideband signal transmission. One of the reasons I recommended connecting the terminating resistor directly to the BNC connector is that it mitigates the impedance matching issues associated with then attaching leads to the PCB.
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Old 25th July 2014, 05:35 AM   #7
klackto is offline klackto  Australia
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I have some fine teflon 75ohm (as per Allen Wright's cable
cookbook that I use as a digital interconnect)coax and
intend to use that for any internal wiring.
The original coax input has already been changed to bnc
and I also plan to use a bnc for the second coax.
Interesting re: the capacitor so I will look into that also.
Thankyou both for the help.... much appreciated.
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