Metrum Octave Mod and cable suggestion


Further down the Head Case Forum was this little ditty.

Craig Sawyers
High Roller

High Rollers
1,072 posts
Posted 04 February 2012 - 11:49 AM
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.
When all is said and done, lots more gets said than done.

If a job is not worth doing, it is not worth doing well.

It's a Norwegian Blue.
vicdamone
RS Components 75ohm, they sent me a few for some reason. I can post 1 to you if you PM me your address.
Does anyone know if there's a way to remove the toslink input and add a second spdif input?
I have 2 digital sources both with coax.
So far resolution was significantly improved as has stage width and seemingly bass impact/tightness. That said it does feel like it needs to do a little more relaxing and I'm hoping over the next hundred hours or so this transformer will mellow.

I think it was purchased through K&K, my buddy ordered a couple and got me one.
i just received my Octave DAC back from being modded... replaced the stock pulse transformer with the Lundahl LL1572, replaced the stock coax input with a 75 ohm BNC, replaced the stock fuse with an AMR gold fuse.

i felt this DAC was a tremendous performer in it's stock form - it is the best of the several i've owned - and now it's gotten considerably better. i really wasn't expecting much from such simple and inexpensive mods, but am extremely impressed. the basic character of the dac remains, the extraordinary clarity and musicality haven't been altered, but now the stage has opened up considerably and the mids/upper bass are fuller. apparant detail, particualrily low level detail, are improved, too.

i'm looking forward to clocking a few more hours and seeing how things develop. right now, i'm very impressed.