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
Has anyone else used the Lundahl transformer in their Metrum? It seems like an affordable modification which yielded a very nice upgrade for Craig and I want to do this modification to mine...
Welp, I took the plunge and purchased a Lundhl and sent mine out for modification. The surgery was a success and it's due back on Tuesday... I'm pretty excited to hear the improvements though my transport's PSU is in the shp for some tweaking...
Gopher, looking forward to your feedback about the Lundhal tweak. Have you tweaked your Octave in any other way?
Not really. I am using herbies baby booties under the dac and psu which actually made a nice little improvement. I'm pretty much down for now except for an AMR fuse which i'm planning on ordering.
Thanks Gopher. Does your DAC section kind of rings internally? When I knock gently on the top of my unit I hear things vibrating inside, and I'm wondering if there is room for improvement by treating those somehow.

Replacing the RCA for a BNC connector is one tweak I have noted a couple folks claim worked well for them.

Using an after market power cable of course is another one. I'm still using stock. What are you using?