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Old 16th January 2005, 05:22 PM   #1
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Location: Sussex
Default Adding Digital Inputs ?!

In addition to CDP there are many digital sources around these days. These can benefit if fed through a DAC and hi-fi system. But many DACs, have only one electrical digital input.

To solve this problem of limited number of digital inputs/outputs, I am thinking of making a simple 4-1 Selector Box with these bits items from Maplins:

1 off, FF74R Rotary Selector SW6B
4 off, L83AW Gld Chassis Phono Skt White
1 off, L84AW Gld Chassis Phono Skt Yellow
1 off, RX01B Knob NK2
1 off, LF12N Aluminium Box AB11
1 set, BP58N Stick On Feet, Domed

Some foil screened, silver plated cable that I have left over will be used for internal wiring.

Any comments? Suggestions? Alternatives?
Thanx
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Old 17th January 2005, 12:52 AM   #2
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There is several issues you will have to address if you are routing digital signals.

Firstly are you using standard phono connectors? If you are then routing SPDIF you must terminate the line in 75ohms or you will introduce reflections and your signal will turn to custard!

Secondly you will need to pay close attention to the lengths of cables in the box - keep them as short as possible!

Your method will probably work ok, however it is far from optimal.

You are using an all passive solution? Rather than that I would suggest that you use high quality gold plated hermetically sealed signal relays and use your cruddy rotary switch (and trust me - they are terribly noisy) to select which relay to turn on.

Make a double sided PCB, and mount the relays it and solder the pcb to the connectors directly. This ensures that the signal path has low inductance and the switch just selects a relay...

Hope this helps

apollyon25
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Old 25th January 2005, 06:42 PM   #3
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Thank you for your post. Yes, I am using phono connectors. I use 75 Ohm terminators for all unused SPDIF digital outputs of CDPs, DVD players, etc. But I donít understand the need for them here, cos whichever output is in use will be fed into the DAC. The other sources, not in use are likely to be switched off.
Inside the aluminium box the cables will be about 60 mm each. The output cable will about 300mm long and will be connected to the DAC directly; rather than through a RCA socket and plug in the box.

I like your idea of using relays. What sort of power supply will it require? I have a 9VAC PSU for the DAC that I can tap from. Any suggestions as to make and model of relays to use?

Thx
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Old 25th January 2005, 07:12 PM   #4
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You are welcome.

SPDIF must be terminated correctly or it fails the SPDIF electrical specification this is especially required when the signal is actually being used.

The SPDIF signals behave as specified only when correctly terminated. The SPDIF is effectively a transmission line (albeit at the low end of HF) and thus requires to be terminated correctly.
Correctly terminating a transmission line (ie a pair of wires carrying a signal with any fundamental or harmonic content exceeding about 1MHz) is essential to eliminate voltage standing waves (google: VSWR (voltage standing wave ratio)) and the reflections caused by impedance mismatches.
Reflections and standing waves cause really nasty looking waveforms to appear rather than nice 'square' waveforms. This is particularly relevant to digital audio as the SPDIF receiver (in whatever form) uses the signals to extract the embedded clock and this, if the signals are all distorted, will cause jitter on either the extracted bit clock BCK, left/right clock LRCK (not usually so critical) or the DATA itself. This can then propagate into the DAC and reduce SNR and increase THD+N.

UNLESS the input of your DAC comes with internal 75ohm termination you should really terminate the SPDIF signal.

Of course this is an audio forum (and audiophiles abound) so if you think it sounds better unterminated - go for your life.
But using an (industry standard) AudioPrecisionII test set shows increased THD and a worsening SNR if the SPDIF signal is not terminated correctly.

If you are very cunning, you can make a pcb and connect the RCA plugs to the PCB directly.

300mm long for the output cable should be ok, however if you can mount the unit inside the DAC it would be better.
Depends of course on the internals of the DAC...

Ok - Relays. You want to look for low voltage (talking about signal voltage here NOT relay-coil energisation-voltage) and these typically have gold plated contacts as this fixes an oxidation problem that high voltage relays get around by using back-EMF arcs to clean the contacts.
9Vac will probably be enough (once rectified and filtered) to give you enough to drive a 12Vdc relay although I would power the unit with its own supply to prevent any contact noise from getting into the PSU for the DAC (but that is just me).

Minimum specs for the relay should then be something like -
coil voltage: 12Vdc
signal voltage: up to 50V (ish) - low voltage signals...
contacts: gold plated

As for manufacturer - there are heaps... do a search for relays in Farnell, Newark, RS Components, Digikey etc...

apollyon25
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