Metrum Acoustics Octave: A NOS digital filter-less DAC

[Dreadhead]

This is why I use optical

[recstar24]

This is why I use optical

More standardized I presume?

[pigmode]

Thanks.

[Craig Sawyers]

This is why I use optical

What sort of optical, Dread?

[Dreadhead]

What sort of optical, Dread?

Crappy spidif optical. At least I don't need to worry about impedance matching and or ground loop issues (I've had many with conductor based digital). Of course my Bel Canto dac reclocks (the Behringer is slaves to the input signal though).

Edited February 6, 2012 by Dreadhead

[swt61]

Is AES/EBU any different, or is it as susceptible as RCA and BNC?

I ask because it's my method of choice, because I think I hear a subtle difference.

[Dreadhead]

Is AES/EBU any different, or is it as susceptible as RCA and BNC?

I ask because it's my method of choice, because I think I hear a subtle difference.

Appears it's supposed to be a 110ohm twisted pair cable. I don't think people bother with that in non-pro audio though and yes if impedance isn't matched you're going to have issues.

Edited February 6, 2012 by Dreadhead

[Pars]

XLRs aren't 110 ohms either.

[Craig Sawyers]

The whole connector issue in digital audio cabling is a mess. The thing with AES/EBU is that is far less susceptible to common mode noise in long cable runs, typical of recording studios and professional installations. It is actually only partly compatible with SPDIF, because the preamble data is different - so using a 1:1.2 transformer to match impedance to a 75 ohm single ended won't necessarily work.

I find it astonishing that there is so little focus on connector impedance though. With 192kHz sampling, typical in mixing desks, the frequency of the AES/EBU serial data is about 25MHz, and to maintain good square and jitter free edges needs a bandwidth to at least 100MHz, and really 200MHz, and rise times of a nanosecond or so. So matching the connector impedance is rather a big deal! And using XLR is just about as daft as using RCA - both standard practices are trying to spoon broadcast TV bandwidth through connectors designed for audio bandwidth.

Even with vanilla CD data at 44.1kHz sampling, the serial data rate is 5.6MHz and so need at least 50MHz bandwidth to get good edges. So about twice the frequency of the maximum shortwave band frequency at 30MHz - via an audio connector.

[pigmode]

Replaced the RCA jack with a 75 ohm BNC. Just to get it working I initially used a verified 75 ohm cable (RG302 terminated in 75 ohm BNC's) via a BNC to RCA adaptor. But during my swapping the Octave to insulated 75 ohm BNC I tested the cable-adaptor-RCA with 75 ohm termination on my Tektronix 1502 TDR - and it was truly horrendous. It was so bad that it was impossible to see anything approaching the 75 ohms termination resistor. The only way to go is to rigidly stick to everthing matching the characteristic impedance of the cable.

Sorry for the simplistic questions, but I'm trying to understand the steps you are applying. It would be nice to see the same thing done for the Neko DAC.

- The RCA jack on the Octave was replaced with BNC (was this a simple re-solder replacement, or did you have to verify and adjust for 75 ohms)

- The CD transport still had an RCA jack, which you tested with 75ohm BNC cable with RCA adaptor? What is your next step?

[Craig Sawyers]

Sorry for the simplistic questions, but I'm trying to understand the steps you are applying. It would be nice to see the same thing done for the Neko DAC.

- The RCA jack on the Octave was replaced with BNC (was this a simple re-solder replacement, or did you have to verify and adjust for 75 ohms)

- The CD transport still had an RCA jack, which you tested with 75ohm BNC cable with RCA adaptor? What is your next step?

OK - the CD transport is enormously modified (by me), and includes a high stability clock to de-jitter the data stream. That drives the output via an OPA623 wide bandwidth op-amp, with 1ns rise time and precise 75 ohms output impedance. That runs via RG179 75 ohm coax internally to a 75 ohm BNC connector. So it was in that state for years, so no further mods needed. The digital interconnect is RG302 (the big brother of RG179) again wth 75 ohm BNC's.

The mods to the Octave was replacement of the RCA with a 75 ohm BNC. The chassis BNC's that I am using are the insulated ones, and are the same diameter as the hole in the Octave - so it is simply a remove and replace job. And then I replaced the pulse transformer with a much better one, which is a bir more of a fiddly mod, because the size and pin-out is different.

Craig

[Dreadhead]

The whole connector issue in digital audio cabling is a mess. The thing with AES/EBU is that is far less susceptible to common mode noise in long cable runs, typical of recording studios and professional installations. It is actually only partly compatible with SPDIF, because the preamble data is different - so using a 1:1.2 transformer to match impedance to a 75 ohm single ended won't necessarily work.

I find it astonishing that there is so little focus on connector impedance though. With 192kHz sampling, typical in mixing desks, the frequency of the AES/EBU serial data is about 25MHz, and to maintain good square and jitter free edges needs a bandwidth to at least 100MHz, and really 200MHz, and rise times of a nanosecond or so. So matching the connector impedance is rather a big deal! And using XLR is just about as daft as using RCA - both standard practices are trying to spoon broadcast TV bandwidth through connectors designed for audio bandwidth.

Even with vanilla CD data at 44.1kHz sampling, the serial data rate is 5.6MHz and so need at least 50MHz bandwidth to get good edges. So about twice the frequency of the maximum shortwave band frequency at 30MHz - via an audio connector.

If you look at a bunch of the manuals for the good converters they are very clear that all clock and digital signals need to be carried with correctly terminated and impedance matched cables. They do make an effort. Maybe not enough but they do.

[swt61]

Appears it's supposed to be a 110ohm twisted pair cable. I don't think people bother with that in non-pro audio though and yes if impedance isn't matched you're going to have issues.

I did purchase 110 ohm cable, specifically for AES/EBU, and the new AES/EBU 110 ohm XLR connections. Hopefully I'm good to go.

[Craig Sawyers]

I didn't manage to find any 110 ohm XLR's (and I have looked!) - have you got a part number or link?

[luvdunhill]

I didn't manage to find any 110 ohm XLR's (and I have looked!) - have you got a part number or link?

The part is Neutrik NC3MXCC. They are not listed as 110 ohm, but they are made specifically for balanced digital.

[NekoAudio]

Sorry for the simplistic questions, but I'm trying to understand the steps you are applying. It would be nice to see the same thing done for the Neko DAC.

For what it's worth, I have BNC jacks that fit (although not perfectly) in the existing RCA jack cutout so you could use a BNC connector instead of an RCA one. I can send you one, pigmode, to install yourself if you know what you're doing or you could send your unit back to me for resoldering but that might be more than you want to pay for shipping both ways.

Or you can do the same as Dreadhead and use optical since the D100 Mk2 also buffers and reclocks.

[Craig Sawyers]

The part is Neutrik NC3MXCC. They are not listed as 110 ohm, but they are made specifically for balanced digital.

Interesting. The characteristicimpdance of an XLR are fixed by the pin spacing and diameter, and the dielectric constant of the material between them - it essentially looks like a short length of parallel conductor transmission line.

Putting in the pin dimension and spacing (between pins 1 and 2) for an XLR, and the dielectric constant for polyimide (the insulator) into the full equation for characteristic impedance of parallel conductors - I get 109 ohms! I'm not sure of the history behind choosing 110 ohms as the AES/EBU standard, but it seems a odd cooincidence that the (calculated) impedance of an XLR is the same value. Unless I have made a mistake in the calculation, AES/EBU via XLR's seems to provide an impedance-matched system.

[swt61]

It's possible that the 110 ohms was chosen due to the jack. Either way I'm glad there seems to be some science behind my perceived improvement over RCA.

[Craig Sawyers]

That would certainly figure - but I'd sure like to know if that is the way it went for certain. It is surprising that Neutrix don't advertise the fact that their connectors are 110 ohm either.

[NekoAudio]

That would certainly figure - but I'd sure like to know if that is the way it went for certain. It is surprising that Neutrix don't advertise the fact that their connectors are 110 ohm either.

I'm assuming that Neutrik assumes this is a given. There's no particular need to advertise it since it's not a differentiating feature?

Canare also makes RCA connectors that are supposed to be as close to 75ohms as they can, by having the connector's shield and its attachment to the cable shield maintain the correct distance from the center signal line until it reaches your RCA jack. It's their RCAP series.

[Currawong]

Craig: Nice work on the fixes. I'll at least be changing the jack when I receive mine. I considered cancelling the order, but knowing that it's possible to make significant improvements, I might give it a go.

Supposedly the Eichmann Bullet Plugs are 75 Ohm. Then there's that $40 RCA jack which is too. Regarding that Oyaide plug though, I picked up a pair and they fit the Van Den Hul The Professional 75 Ohm cable perfectly, one of which I picked up cheap, so that is what I've been using when I am not using the Audiophilleo or using a cable for one or another reason.

[Pars]

If you can find them (and for a reasonable price), the Trompeter wrench crimp connectors are supposed to be one of the best BNCs available.

http://www.newark.com/trompeter-electronics/upl20-5/rf-coaxial-bnc-plug-str-75ohm-crimp/dp/90B4196?in_merch=Popular%20RF/Coaxials

They've apparently raised their pricing...

[swt61]

http://www.talonix.com/shop/item.aspx?itemid=8920

[Craig Sawyers]

Craig: Nice work on the fixes. I'll at least be changing the jack when I receive mine. I considered cancelling the order, but knowing that it's possible to make significant improvements, I might give it a go.

Supposedly the Eichmann Bullet Plugs are 75 Ohm. Then there's that $40 RCA jack which is too. Regarding that Oyaide plug though, I picked up a pair and they fit the Van Den Hul The Professional 75 Ohm cable perfectly, one of which I picked up cheap, so that is what I've been using when I am not using the Audiophilleo or using a cable for one or another reason.

The only thing to watch is that the BNC needs to be insulated from the chassis.

[Craig Sawyers]

So I did some formal tests of impedance mismatch at BNC 75 ohm and RCA connectors. Measurement system was a Tektronix 7S12/S6/S52 time domain reflectometer - it measures reflected signals from impedance discontinuities. It has a resolution of about 10mm. Set up was a 50-75 ohm matching pad, 75 ohm BNC terminated cable. The other end of this cable was either terminated in a 75 ohm BNC plug/socket with a 75 ohm termination, or Neutrix RCA plug/socket with 75 ohm termination

The pics show the results.

BNC gave a small reflection blip 200 picoseconds long, corresponding to a -9 ohm drop. So for 200ps there is an impedance of 66 ohms. The shortness of this event means that it is entirely insignificant, even at 192kHz sampling.

RCA however gave a much more significant 600ps reflection, corresponding to a -23 ohm drop. For 0.6ns there is an impedance of 52 ohms. In fact if you look at the curve in detail you can see a double dip, where the Tek gear picks up slightly different impedances for the plug and the socket. Since it is likely that both ends of the cable are misterminated in this way, several bounces back and forth will be necessary before the reflection dies away, and since the data stream is pulse length encoded, line resonances from these reflection transients will add a noise and jitter inducing effect.

Craig