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
@ Vicdamone: Noble Electronics performed the mods. i purchased all the necessary parts and sent them along with the dac. K&K Audio stocks the pulse transformer, avatar sells the fuses and the bnc is widely available. noble's work was flawless and timely - highly recommended.

i've now put some time on the dac and it has smoothed out nicely - it really has the most analog sound i've ever heard from a dac. difficult to describe the presentation - finely detailed, yet with an extraordinary musicality. this one one incredible sounding piece now.
had the house to myself this morning so was able to goose the gain quite a bit. i agree with gopher - there is a solid improvement in the low end. robert plant / allison krauss please read the letter from raising sand is playing now - the opening has never sounded this powerful and controlled.
Srosenberg,
Can you put the photos with Metrum Mod (Lundahl LL1572 pulse transformer)?
Whatt's the AMR gold fuse that you use in this mod?
Thanks a lot.
I'm out of town for the holidays, but wanted to quickly chime in that things were coming around in my system before I left.

As with many changes to your rig some coaxing may be necessary to reaxhieve your desired balance. In my case I moved my amp out of an oyide r1 outlet and into a Maestro outlet and my Metrum out of a Maestro and into a Furutech FPX gold.

Sounds tweaky/fringy I realize but they made a notable improvement. Im planning on buying a nice fuse, probably AMR shortly.

While i was still undecided about the mod, i purchased another stock Metrum so ill have that to compare snd ecperiment with in a week or so.
Gopher, Excelent news.
Gopher, when you compare the two DAC, we have the dimension of improvement with these changes (Mod).
Gopher, Can you put the photos with Metrum Mod (Lundahl LL1572 pulse transformer)?
Thanks a lot.