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  Metrum Octave Mod and cable suggestion
Further down the Head Case Forum was this little ditty.

Craig Sawyers
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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!


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.
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Vicdamone  (Threads | Answers | This Thread)

  Responses (1-27 of 27)
Click title to read one, or click date to read all below it.

02-22-12   Mine works fine with rca. of course the digital cable i use ...   Stanwal

02-25-12   Vicdamone: so what are you saying?   Devilboy

02-26-12   Devilboy, this is a post by a diy'er describing his experien ...   Vicdamone

02-27-12   Just got myself a proper 75 ohm bnc connector. my plan is to ...   Elberoth2

04-15-12   Elberoth, have you added the bnc and did you hear any appre ...   Rocker9999

04-16-12   Yes, i did. i think it made a small but noticable difference ...   Elberoth2

04-16-12   burr brown dac8580. datasheet here. best wishes, alex p ...   Aplhifi

04-16-12   Alex, that was easy for you!   Rx8man

10-28-12   I have had my metrum octave for some weeks now and am truly ...   Complin

11-13-12   Has anyone else used the lundahl transformer in their metrum ...   Gopher

12-07-12   Welp, i took the plunge and purchased a lundhl and sent mine ...   Gopher

12-08-12   Gopher, looking forward to your feedback about the lundhal t ...   Lewinskih01

12-08-12   Not really. i am using herbies baby booties under the dac a ...   Gopher

12-09-12   Thanks gopher. does your dac section kind of rings internall ...   Lewinskih01

12-15-12   Do you have the details of the lundhl and where to buy it? h ...   Naggots

12-16-12   Naggots, which bnc connector did you install? cheers   Lewinskih01

12-16-12   Rs components 75ohm, they sent me a few for some reason. i c ...   Naggots

12-16-12   Does anyone know if there's a way to remove the toslink inpu ...   Lespier

12-18-12   So far resolution was significantly improved as has stage wi ...   Gopher

12-21-12   I just received my octave dac back from being modded... repl ...   Srosenberg

12-22-12   Srosenberg, who did the mods? what was the cost?   Vicdamone

12-23-12   @ vicdamone: noble electronics performed the mods. i purcha ...   Srosenberg

12-23-12   Had the house to myself this morning so was able to goose th ...   Srosenberg

12-24-12   Srosenberg, can you put the photos with metrum mod (lundahl ...   Aboaboabo

12-26-12   I'm out of town for the holidays, but wanted to quickly chim ...   Gopher

12-26-12   Gopher, excelent news. gopher, when you compare the two dac, ...   Aboaboabo

12-30-12   If scott can't handle that i will see what i can do. i think ...   Gopher

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