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Old 23rd April 2008, 06:16 PM   #591
johnm is offline johnm  United Kingdom
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Kevin I've just ordered a set of 4 75-ohm BNC connectors from Maplin. Already have some 75-ohm cable with connectors installed somewhere in my box of wires (THAT should take a day to find lol!).

I'll also replace the phono socket on my DPA DAC with the 75-ohm BNC connector (easy to do - just taken a look at it).

So as I'm technically installing a 'perfect' 75-ohm cable and connectors, which values do you think are the best to use at the SPDIF output? I'm currently using 291 / 100 ohms. Do you think I'd be better off with the 392 / 93.1 ohms combo?

Thanks,

- John
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Old 23rd April 2008, 06:23 PM   #592
Puffin is offline Puffin  United Kingdom
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I am using a BNC nicked from an old CD player. I am not using any resistors at the moment and it souds o.k. Should I use some ?
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Old 23rd April 2008, 06:51 PM   #593
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by Puffin
I am using a BNC nicked from an old CD player. I am not using any resistors at the moment and it souds o.k. Should I use some ?

Yes, see my previous replies for why.
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Old 23rd April 2008, 06:58 PM   #594
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Quote:
Originally posted by johnm
Could someone please tell me - in layman's terms if possible - why it's important to set up the digital output to support a 75 ohm impedence, when it's accepted that a phono socket is not 75 ohms (even if the cable might well be)? Shouldn't it take into account the two phono connectors?

Perhaps you should try have a look here

The pictures tell a lot about the problem. Every part has an effect on mismatching of the impedance. Connectors, cable, pcb tracks etc...
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Old 23rd April 2008, 06:59 PM   #595
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by johnm
Kevin I've just ordered a set of 4 75-ohm BNC connectors from Maplin. Already have some 75-ohm cable with connectors installed somewhere in my box of wires (THAT should take a day to find lol!).

I'll also replace the phono socket on my DPA DAC with the 75-ohm BNC connector (easy to do - just taken a look at it).

So as I'm technically installing a 'perfect' 75-ohm cable and connectors, which values do you think are the best to use at the SPDIF output? I'm currently using 291 / 100 ohms. Do you think I'd be better off with the 392 / 93.1 ohms combo?

Thanks,

- John
I've tried several combinations, but my primary concern is to match the coax and present a load to the chip that is within its safe output current capability. It may take you a while to find the combination that works best for you - I'm still working on that myself.

The 392/93.1 ohm combination is pretty standard for 5V cmos logic levels. 301/100 also gives you almost exactly 75 ohms - I've tried this combo and it seems to work without issues, output level is a bit hot, but I think within spec, and the current required of Dout is acceptable. (I'm currently using cheap 330 and 92 ohm 5% 1/2W CF resistors bought at Radio Shack - and I think I like these better than the 301/100 Holco combo, but I am not sure about that. )
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Old 23rd April 2008, 07:30 PM   #596
johnm is offline johnm  United Kingdom
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Thanks for the link Trilithium (Trek fan perchance?) - the pictures explain it well, though much of the txt went way over my head

I guess the best we can do is ensure a steady 75 ohm output, transmission line, and receiver. I'll definately go with those 75 ohm BNC's. Maplin make a superb cable (for digital and analogue transmission) which I've been using for my digital connections recently.

"Manufacturer: Bandridge

Double-shielded satellite cable
Improved shielding for reduced interference
99.96% OFC conductor for crystal clear pictures and sound
Air foam PE insulation layer for purest sound transmission
Heavy-duty PVC jacket"

http://www.maplin.co.uk/module.aspx?...98026&doy=23m4

I use a long run of this cable to reduce the effect of any reflections when used as a digital interconnect. Definately the best cable I've used, and the cheapest!

Thanks for those figures Kevin. If it isn't too much trouble to write up, would you mind explaining how one comes up with those figures in the first place? I know the output of the 31 mech is -12mA. Min rating is listed as 0.9VDD. Erm..... how do I work this out?

- John

P.S. Couldn't resist - seeing as the Black Gates and Silmics seem to have burnt in sufficiently - I decided to have another go with that 0.1uF Auricap in the C906 position. MUCH more successful this time I'm pleased to say. Seems to have made the presentation more 'analogue' sounding.... more fluid and natural. This time the overall sound hasn't suffered. So please ignore what I said a few pages back folks - experiment with the 0.1uF cap, but ONLY after the caps have had a chance to burn in first.
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Old 23rd April 2008, 08:37 PM   #597
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by johnm
Thanks for the link Trilithium (Trek fan perchance?) - the pictures explain it well, though much of the txt went way over my head

I guess the best we can do is ensure a steady 75 ohm output, transmission line, and receiver. I'll definately go with those 75 ohm BNC's. Maplin make a superb cable (for digital and analogue transmission) which I've been using for my digital connections recently.

"Manufacturer: Bandridge

Double-shielded satellite cable
Improved shielding for reduced interference
99.96% OFC conductor for crystal clear pictures and sound
Air foam PE insulation layer for purest sound transmission
Heavy-duty PVC jacket"

http://www.maplin.co.uk/module.aspx?...98026&doy=23m4

I use a long run of this cable to reduce the effect of any reflections when used as a digital interconnect. Definately the best cable I've used, and the cheapest!

Thanks for those figures Kevin. If it isn't too much trouble to write up, would you mind explaining how one comes up with those figures in the first place? I know the output of the 31 mech is -12mA. Min rating is listed as 0.9VDD. Erm..... how do I work this out?

- John

P.S. Couldn't resist - seeing as the Black Gates and Silmics seem to have burnt in sufficiently - I decided to have another go with that 0.1uF Auricap in the C906 position. MUCH more successful this time I'm pleased to say. Seems to have made the presentation more 'analogue' sounding.... more fluid and natural. This time the overall sound hasn't suffered. So please ignore what I said a few pages back folks - experiment with the 0.1uF cap, but ONLY after the caps have had a chance to burn in first.
Hi John,
I'll check and see if the Bandridge or equivalent cable is available here - is it flexible?

On selecting the resistors 3 criteria apply, first look at the minimum overall resistance values that do not call for more than 12mA, then look at the ratios that give you 75 ohm source impedance and result in output voltages that are within the allowable window (0.5Vpp when terminated 1Vpp when unterminated - note that I believe levels of 0.5Vpp - 1Vpp are considered acceptable when terminated. I've never seen spdif at levels significantly above 0.5Vpp in any commercial gear I've owned when terminated. Just math really...

I have more BG on the way and plan to wait to tweak that tracking servo cap until the rest of them are broken in..
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Old 23rd April 2008, 08:59 PM   #598
Zen Mod is offline Zen Mod  Serbia
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if one can use sat cable as digital one , then one can use F connectors, too .

what's more logical .......

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Old 23rd April 2008, 09:28 PM   #599
OneyedK is offline OneyedK  Belgium
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Quote:
Originally posted by Zen Mod
if one can use sat cable as digital one , then one can use F connectors, too .

what's more logical .......

From an RF point of view, F-connectors are superior...
But if you want to be able to connect/disconnect the cable often,
you would destroy it in no time.
So BNC is the more logical choice.
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Old 23rd April 2008, 09:30 PM   #600
johnm is offline johnm  United Kingdom
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Hi Kevin!

Thanks for the explanation.

The cable is fairly flexible - about the same flexibility as any old standard solid-core coax really.

Just been looking at my board, and the bridging cable on Okapi's PDF between the + pin of C916, to JW906. I'm having trouble tracing this on the circuit diagram - does this bypass C952 by any chance? (therfore making my 'upgrade' of an Elna Silmic 470uF in this position a waste of time?)

Thanks,

- John
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