Craig Sawyers

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.

And that is a fundamental statement that I agree with 100%

That is really sweet! And hats off to KG for keeping plugging away in the teeth of a cracked knob

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

^ Astronomy picture of the day, http://apod.nasa.gov/apod/ , yesterday.

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

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.

WTF I though - exploding manholes! But then I found it is common. This is the same cover exploding on two separate occasions. http://www.telegraph.co.uk/news/newsvideo/weirdnewsvideo/8909819/Strip-club-CCTV-captures-lucky-couples-escape-from-exploding-manhole.html

What sort of optical, Dread?

I know the '50s and '60s were no joke staring nuclear holocaust in the face - but where is the can?

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.

Yeah - cutting coolant sure smells "characteristic" when it gets an infection.

Just looked at this again - there are 8 DAC's (hence Octave), ie 4 per channel. I think it uses the concept I first saw on Cambrige Audio CD players shortly after CD's were introduced - you time delay the digital signal to each DAC, so they do linear interpolation between the data points. Which gives you an extra two bits. Colloms measured a resolution of 18 bits, which would be the 16 bits of the individual DAC's, plus 2. It may be that they use something more sophisiticated than linear interpolation, but I have absolutely no evidence either way. Notice the lack of anything resembling a PLL or crystal. I have no idea how they recover the clock - it is something in the row of four IC's near to the pulse transformer, but other than that I just don't know.

Yeah - that's the way to do it!

Video skype with text messaging - from 1928

Coming; it is not a pretty sight though! I forgot the headphone systems. Mostly KG stuff. Triode E/S feeding original Lambdas. Dynalo with outboard PSU feeding either AKG K701 or Beyer DT990. SRM-T2 clone feeding 007's from a Tent CD in the bedroom. And a Blue Hawaii in the other bedroom waiting to be wired up and looking for a pair of phones to be fed by it.

A hot-rodded and rather ancient CD player, converted to a transport. Multiple mains tansformers, a regulator glued to the top of every chip, clock board (my design) with OPA623 S/PDIF driver (1ns rise time), and focus and tracking servos run from a dedicated supply. Metrum Octave, Audio Research LS3 pre, Audio Research D125 power, Quad ESL57's

Yes - the board is hand soldered. Nicely hand soldered. I'll bite for the SC transformer (I've just been downloading and speed reading his AES papers; he knows what he's talking about for sure!) - I'll PM you.

Comparing the Newava and Lundahl pulse transformers, the interwinding capacitance is 0.5uH and 1.3uH respectivly, and the interwinding capacitance is the same at 15pF. A figure of merit is root(leakage inductance x interwinding capacitance), and this is proportional to the rise time. So the Newava should have a rise time 62% of the Lundahl - ie it is faster. But the primary inductance of the Newava is only 0.225mH, whereas the Lundahl is 40mH (180 times greater). The winding inductance is related to the droop in the top of the pulse, and smaller inductance gives more droop. Calculation for the Newava shows that the top of the pulse will droop to around 80% for a typical pulse length. The Lundahl has an unmeasurably small droop. Droop gives rise to jitter (because of the variable pulse length encoding of serial digital data) - so it is good to keep it as low as possible. The low winding inductance of the Newava is all to do with use of a ferrite core, which has a relatively low permeability. The more expensive constructions use either spin melt amorphous ribbon, or mumetal, both of which have 200 to 500 times the permeability of most ferrites. And yes - the 50 ohm BNC connectors is just plain stoopid on a 75 ohm system. If that was a RF power amplifier feed to a transmitting antenna, there would be so much backward wave that the amp would blow. And RCA! There are supposed 75 ohm RCA's (WBT do a cable RCA plug and maching chassis socket), but I remain skeptical until I measure the darned things. Might be able to actually do that during the next two weeks. Now 75 ohm N-type! that would be the way to go with a clean sheet of paper! Craig

I bought one of these around 10 days ago, based on Martin Colloms' recommendation in HiFi Critic, in which he billed it as a real giant-killer. Initial thoughts were exceptionally good, and as it warmed up progressively better. But I can't resist getting things right that are wrong. The first problem is common to most DAC's and CD transports - RCA connectors. I did a whole host of measurements using Time Domain Reflectometry, and basically putting a fast pulse into a 75 ohm coax terminated with an RCA, into an RCA socket, with a surface mount 75 ohm resistor tacked onto it - and it is a disaster. Massive reflections - and reflections add jitter. The only way to do things *properly* is to maintain a clinical 75 ohm environment for the whole digital signal chain, and that means 75 ohm BNC connectors throughout. And most digital cables, if not terminated in RCA's are terminated in 50-ohm BNC's! In fact the only audio high-end 75 ohm BNC's are Oyaid. I just use regular clamp-on greenpar and RG302 teflon cored coax. After doing that (and measuring the TDR response into the Octave) the DAC really started to sing. Then I spotted the hokey little pulse transformer (visible in the picture above). This is a $2 part, a standard Murata ferrite cored pulse transformer. I replaced it with a far better (electrically) Lundahl LL1572, which uses an amorphous ribbon core. In the UK this is £35 - so not cheap. After swapping transformers (invalidating the warranty of course) I can honestly say I have never heard a DAC sound that good regardless of cost. I've just ordered an Audio Note toroidal pulse transformer wound on a mumetal ribbon core, so it will be interesting if that improves it further, or otherwise. I can't stop listening to it! Craig PS The DAC's are intersting. They are 16-pin devices (with the type number taken off, of course), so they are definitely not Burr Brown or any of the other audio DAC's, which have far more pins. So I think they must be DAC's intended for instrumentation or data acqisition applications. And they are supposedly R2R ladder DAC's too. I've done an initial trawl of manufacturer's data, but have not been able to find anything that looks like those.

Check.

Could have sworn that I posted to this thread - or am I losing my marbles and imagined it?