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Old 8th October 2007, 06:34 AM   #1
lykkedk is offline lykkedk  Denmark
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Question Torx digital input selector

Hi.

I am building a new preamplifier, at the moment, and i suddently got the idea, of making digital input selection, along with my analog inputs.

My idea is to have 2 TORX177 reciever's, which are routing through a relay directly into TOTX179 Transmitter module, so that i have the possibility to switch between digital sources at my pre.

There is a lot of reading around the net, regarding digital auto switches, but i rather like this simple approach. Will it work ??? Any suggestion's ???

Jesper.
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Old 8th October 2007, 07:44 PM   #2
lykkedk is offline lykkedk  Denmark
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Anyone ? -- Well maybee it's a just go ?... i don't know, or will i need some kind of buffer to avoid jitter ? pls.. lead me on the right track here.

Jesper.
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Old 8th October 2007, 10:57 PM   #3
rossl is offline rossl  United States
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How about using an IC with quad and gates to switch and buffer.
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Old 8th October 2007, 11:17 PM   #4
rossl is offline rossl  United States
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Oops, that was wrong. The second gate needs to be an OR gate !

or

You can use NAND gates.
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Old 8th October 2007, 11:20 PM   #5
amc184 is offline amc184  New Zealand
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I think it would work, but I don't think it is a good idea. The optical to electrical and back conversion looks like a great way to introduce noise and errors. I would place the input selection into the DAC itself, and use a logic IC, rather than a mechanical switch, but I guess you may not be able to do this with the DAC you have.
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Old 9th October 2007, 07:25 AM   #6
lykkedk is offline lykkedk  Denmark
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Quote:
You can use NAND gates
Ok. the input is high then from torx right ? (+5v digital)
With NAND gate's i only need two gates then.

like this.

input1 (high from torx1) goes into NAND1
input2 (high from torx2) goes into NAND2
With a switch i select feeding either NAND1 or NAND2, if both input on a NAND gate is high the gateoutput goes low. - So if i feed +5v into NAND2, only NAND1 is high on output.

Both outputs can go directly into the TOTX transmitter right ???
If it work's what kind of noise/problem's would it make (teoretically none) ???

Jesper.
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Old 9th October 2007, 06:48 PM   #7
lykkedk is offline lykkedk  Denmark
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OliverD wrote this in an old thread i had found :

Quote:
A simple and yet very good solution is to use relais to switch the inputs. Make sure all inputs are terminated properly, even if not selected. To avoid grounding problems, it might be best to use transformers on all inputs (if you don't use optical connectors, that is)
I will only use toslink, so no need for transformer's. But how do i excatly terminate input ? What doe's it mean

I think i will go for this solution, if i figure this out.

Jesper.
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Old 9th October 2007, 07:55 PM   #8
Nisbeth is offline Nisbeth  Denmark
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Quote:
Originally posted by lykkedk
I will only use toslink, so no need for transformer's. But how do i excatly terminate input ? What doe's it mean

I think i will go for this solution, if i figure this out.

Jesper.
No need to use termination on the optical connections. The electrical SPDIF connection (=via RCA) requires a termination in the form of a 75ohm resistor to work properly. If you use two optical receivers and connect them as shown in the datasheet, and then use a relay (or two) to switch their electrical connections you should be fine.


/U.

PS: If I'm not mistaken, the signals output from the TOSLink receivers are TTL-level, which means you have to remove the 75 ohm termination resistor from the input of your DAC as well - I need someone to verify this though.
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Old 9th October 2007, 08:07 PM   #9
lykkedk is offline lykkedk  Denmark
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Quote:
PS: If I'm not mistaken, the signals output from the TOSLink receivers are TTL-level, which means you have to remove the 75 ohm termination resistor from the input of your DAC as well - I need someone to verify this though.
Thank's... Just what i wanted to hear, well i didn't know that 75ohm to gnd was called termination !
The output from toslink recievers is TTL; i read that somewhere today, somehow also makes sence, since this type of signal are 'easy' to route to gates etc...
Regarding my input on my DAC, i both have toslink and coax, so i just connect the switch to toslink, as i planned from scratch.

Thank's for nice answers.

Jesper.
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Old 13th October 2007, 01:20 PM   #10
lykkedk is offline lykkedk  Denmark
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It's alive!

I have only been testing it with 1 input at the time (missing 1 cable ), but when switching to the unused input, i hear absolutly no noise, or anything like that!, so i guess it's ok... I really don't think, that there will be any crosstalk, but i will ofcause try, when i get my hands on another toslink cable.

The relay's are 'omron g5v-1 - 5vdc', i switch between input 1 / 2 by feeding +5vdc into the solderspear located at the bottom-right the relay (just under the diodes).

Coil's are 47uH, and capacitor's 0,1uF. - PSU = gnd and +5vdc.

Thank's for 'leading' me the right direction.

Jesper.
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