I fully agree: this is exactly what I will do !. I hope that soldering some wires to an outboard connector will not create major problems (such as noise for ex), but my hope is that it will be "good enough" to verify that the board works, or to debug it.
I have still not managed to find an equivalent of this schematics on the web... this would male me more confident.
I have always almost completed the design of a new PCB with all types of inputs, assuming that my board would work... 😉 😉
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if you take the time to draw it out, I (and likely many others on diyAudio) would be glad to help review it for you.
keep the wires to the off-board connector short (a few cm) and twisted together to minimize noise coupling. BNC s/pdif is 1.0Vpp digital signal, so not as fragile as signal from an MM phono cartridge which is a few mV.
With the exception of the 22R in series with the Transmitter, the circuit shown is exactly what AVAGO advises for a 100mA Transmitter current, .
However for the application to transmit SPDIF, they advise to use ca. 50mA.
That's why the designer of this circuit used the additional 22R, which still results in ca. 60mA.
It's hard to see from a schematic how much current is actually flowing, but it can be influenced by varying the value of this 22R and should be measured with a high speed scope.
Hans
Keep in mind that SPDIF has nsec switching flanks, leading to reflections if not properly terminated, unlike MM signals.
Hans
where have you seen a identical circuit using a 74AC14 ?? can you give us URL of the Avago circuit recommendation ? (the Avago circuits that I have see, do not use the 74AC14)/
I would be happy to find an Avago circuit using the 74AC14, but none of the circuits I have seen use the 74AC14....
your explanation is excellent: this explains the presence of the 22R resistor.
But I still try to find a circuit using the other parts before the 22R resistor....
I have a scope but currently no time to test it...
You are right and that"s my fear: I can use wires from an external BNC connector (fed by a signal generator), but I am not sure that the wires will not damage the signals...
Anyway, this approach should be "good enough" for testing the board...(I hope so)
When the board has been tested and its circuit is validated, my goal is to design a full PCB, with many different inputs (BNC, Toslink, AES-EBU, maybe ST) and many different outputs, including a ST optical output.
I am "furious" that the ST standard has been pushed out of the high-end audio market, so I will build my own conversion switch... 😉 🙂
I work on it, but I cannot go further until the actual board has not been validated as "a good circuit" for BNC/ST conversion....
a suitable AES termination resistor should be present on the BNC receiver input of your board.
100 base ethernet, runs at even higher (100Mbps) data rates / transition speeds than s/pdif, using unshielded twisted pair cabling.
74AC00 quad nand gates with one input tied high will function very similarly to the 74AC14 hex schmitt trigger inverters, namely a logic inversion function.
can you draw the remaining circuits of your existing board that has BNC or TTL input(s) and the ST optical transmitter output?
100 base ethernet, runs at even higher (100Mbps) data rates / transition speeds than s/pdif, using unshielded twisted pair cabling.
74AC00 quad nand gates with one input tied high will function very similarly to the 74AC14 hex schmitt trigger inverters, namely a logic inversion function.
can you draw the remaining circuits of your existing board that has BNC or TTL input(s) and the ST optical transmitter output?
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Id be more annoyed with the Wadia 9 only having ST inputs. What's the advantage of ST over other digital inputs?
This project may take a while.
Why dont you just set up your PC as a media server that accepts and controls all inputs?
primary benefit of optical ST connection is electrical isolation (no ground loops) between the digital source (e.g. an electrically noisy music server or pc) and the DAC & analog electronics. generally, toslink optical is less desirable than ST because of lower / insufficient bandwidth.
the choice of optical instead of electrical input interfaces may have also been driven from an FCC regulatory approval standpoint (back then, digital circuits ran at higher voltages and currents on a per gate basis, creating more digital RF energy).
the choice of optical instead of electrical input interfaces may have also been driven from an FCC regulatory approval standpoint (back then, digital circuits ran at higher voltages and currents on a per gate basis, creating more digital RF energy).
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Tincanear has replied to your question: "generally, toslink optical is less desirable than ST because of lower / insufficient bandwidth."
I confirm that from a sound quality viewpoint, ST is far better than Toslink, and all the persons having compared the two standards conclude the same.
It will, but as soon as the BNC/ST board is validated, I can progress quickly: I use to design such devices.
This does not seem a quicker solution to my eyes: I will have to convert to ST in this approach too...
I am not familiar with the PC media servers, but I am quite an expert in IT and PCs...
you are probably right but I don't know the FCC rules processes...
The only thing that I know is the vast superiority of ST from an sound quality viewpoint...
There is (small) chance that I can get in touch with the engineer who originally designed the board: if so, he could quickly validate the board...
- Termination should be 75R, the characteristic cable impedance.
- Termination of characteristic impedance is what counts, in case of twisted pair 120R, not the shielding and not the Mbps.
- Yes I agree, it makes no difference at all to the ST circuit using a 74AC00 or a 74AC14, as long as their input signal has a well defined transition and that's the part that is still not revealed.
Hans
- As mentioned, the 74AC14 plays no magic role. It could just have been a fast 74AC00 or whatever gate that can supply much current in low and in high output state to steer the HFBR1414.
- For your circuit, see https://nl.mouser.com/datasheet/2/678/av02-0525en-1827973.pdf, page 17 for the calculation and page 18 for the circuit diagram.
- In all cases you will need a scope to verify the correctness of the waveforms.
Hans
+1 about the twisted pair having a different characteristic impedance than 75 ohms. alternatively, for testing, the off-board BNC to board connection could be made with a short section of 75 ohm coaxial cable (RG-59, RG-179, etc)
understood: thank you !
Thank you tincanear,
Agreed : I currently only have TV coax in my stock (which I supposed is 50 ohms ?) but I order RG-59/RG179 soon. I have a lot of 75 ohms BNC connectors, so I just need the cable.
Now have a look at the ST receiver side (HFBR-2416). The Avago datasheet gives a not complete connection diagram (at least for me): see attachment.
What are the recommended values for C2, C3, C4, IC1 and IC2 in your opinion ?
Attachments
You initial need was to have BNC to ST fiber, true ?
So, why don’t you finalise this first.
It’s all on the board, you only have to reconstruct the circuit diagram.
Hans
perfect ...
When you say "short cable" I assume that 50cm is OK (I have such a cable in my stock); if too long I will assemble a shorter one.
True, but how to verify that the onboard ST emitter works properly and generates the right signals, if I don't have a receiver to check what it emits ?
All you need is a proper scope to watch the signals.
And you have perfect references with your Wadia and your Bel Canto ST transmitter.
Hans
So I assume that I have to find a way to plug my scope on the Avago Emitter pins (?): will have to solder a piece of wire directly to the PCB...
I hope to begin this test next week