Please help.
I've been putting off fitting bncs for ages even though I've read many times that they're bound to sound much better than phon-bnc adaptors.
Anyway, I've finally found the courage to get round to fitting the bncs to my OL modded RB300, but I'm a bit confused as to what to solder to where.
I have looked at the online preamp manual and at least this has confirmed I passed the IQ test that is the assembly of a bnc plug. The fitting description is less ambiguous.
Inside the cable ther are two wires, signal and ground, and the cable is screened with braid.
The signal wire will obviously be connected to that sharp pin thingy, but the ground wire is what's confusing me. From the manual:
'Comb out the copper braid and insert the ferrule'
And there is no mention of a ground wire. In fact, the description seems to fit more with a TV coax cable than normal audio interconnect cables, so please help.
Does this mean that the braid then becomes the ground, and what was the ground wire before is left unattached?
Or is it the ground wire that I should solder to the ferrule?
I underatsand little of this electrical stuff, so could somebody please tell this poor TEFL teacher what should be soldered to where.
Many, many TIA.
Jon
Posted on: 20 August 2001 by Jon Moxon
"
[/I][I]Exactly what do you have? "
What I have is two discrete cables; each contains two normal-looking wires, these surrounded by cotton wadding type stuff, and this in turn surrounded by the braid.
On the phono plugs currently fitted, one of the wires is connected to the signal and the other to the ground sleeve. The earth braid hangs free.
Does any of this sound familiar?
What goes where? I may have a go a bit later if I can clarify this.:-(
Thanks again.
Jon
Posted on: 20 August 2001 by Andrew L. Weekes
quote:
What I have is two discrete cables; each contains two normal-looking wires, these surrounded by cotton wadding type stuff, and this in turn surrounded by the braid.On the phono plugs currently fitted, one of the wires is connected to the signal and the other to the ground sleeve. The earth braid hangs free.
Hmmm... I guess what you have is similar to a twisted pair cable, with overall screen.
Is the braid connected to anything at the other end to the Phono's (turntable end)?
You'll have trouble connecting the two discrete wires, since the ferrule referred to is designed to be a tight fit between the inner insulation and the braid of a coaxial type cable (centre conductor / braid).
If the screen is not terminated at the turntable end, I would connect the the wire that goes to the outer of the phono plug, to the braid, then insert the ferrule over the two inner conductors and push it between them and the braid, if it fits snugly. To do this you will need to terminate the screen and inner conductor away from the connector, by removing a length of outer insulation, and convering in heatshrink, in the same manner Naim does with the SNAIC's.
The other inner conductor is then connected to the pin of the BNC.
To be honest the cable type you're talking about is unlikely to be of the correct impedance (50 or 75 Ohm) to match the BNC's, thereby negating any benefit you may have got from using them.
If you cannot make a satisfactory termination, I'd stick to phono's.
Andy.
Posted on: 20 August 2001 by Jon Moxon
Thanks everyone for taking the time to reply.
What started off as a little afternoon odd-job while the kid was asleep seems to have turned into major structural remodelling.
Martin, I take on board your point and will contact OL directly for advice, especially in the light of Andrew's comments about cable/plug impedance matching. Thanks Andrew and Richard for the detailed advice on how to proceed anyway.
The screen , BTW, is not connected at the turntable end, and seems to serve no purpose other than to confuse unsuspecting plug-changers.
I am now left with one phono plug removed. Good thing I was doing some work on the bloody thing abyway, or I would otherwise be fairly p***ed.
Jon
Posted on: 20 August 2001 by Chris L
Jon,
Just wondered how you find the OL RB300? I'm soon going to be getting my RB250 done, and would welcome feedback from a current user (other than, try and OL to fit the BNC plugs 
)
Chris L
Posted on: 21 August 2001 by Chris L
Geoff,
Thanks for the feedback. OL seem to be on holiday just now, so I'll probably leave it for a few weeks before sending the arm in.
They seem to prefer the RB250 as a basis for mods because the bearing is supported at both sides on the 250 but only at one side on the other RB arms.
Will post a proper review once the things fitted.
Chris L
Posted on: 21 August 2001 by david skinner
Jon,
I fitted an OL cable to my OL modded RB250 and because OL do not supply BNC terminated cable, I had the same problem as yourself.
The two inner conductors of each lead are hot and cold connections to the cartridge and the cold connection is designed to be connected to the BNC outer body (and hence system earth at the preamp. It should not be connected to the outer braid unless, as Andrew remarks, the braid is not terminated to the turntable earth. However the arm cable should have a seperate earth lead (blue insulation on my cable) which is both connected to the outer braid at the deck end of the cable and also internally to the arm itself. This cable is designed to be connected to the preamp chassis earth to star earth the cable shield (braid) and the arm itself. The only problem I experienced with the cable as supplied was that the cable shield did not extend back to the arm base and there was some residual hum from the deck motor. I have since corrected this with some additional braiding.
Hope this helps explain the situation.
David
Posted on: 21 August 2001 by Milan
I am also thinking of the same mod. I have the same problem. Also the BNC plugs I have do not seem to have anywhere to connect the cable that links to the phono outercase. These seem to rely on the outer braid carrying that connection!
John,
How did you find the OL mod. Is their % expression on the web site accurate?
Regards
Milan
Posted on: 22 August 2001 by bam
I personally don't think the impedance match between the cable and the connector matters one iota.
The output source impedance of the cartridge and the input impedance of the pre-amp are, as far as I know, neither 50 nor 75 ohms nor are they purely resistive. So the connector is the least of the problems. Nor are reflections in this frequency range relevant.
In my opinion the most critical factors are the quality of the electrical connection and correct grounding. Check with OL about whether the ground conductor should be connected to the shield at the plug end. Ultimatley to achieve best performance avoid the BNCs altogether and solder the TT leads directly to the preamp (some issues of flexibility and warranty perhaps).
Just my 2p worth.
Posted on: 22 August 2001 by Andrew L. Weekes
I'm tempted to agree with your observations w.r.t. impedance at these frequencies being irrelevant.
It is though the reason Naim quote for all of their connector choices, and when the .5 upgrades were introduced Naim stated that unless phono-equipped pre's were modded to add BNC's, the full benefits would not be realised.
A moving coil cartridge will exhibit low impedance (100R v.s 47k for MM), the cable will be of a similar impedance, and the BNC will represent less of an impedance step than a phono (50/75R vs 300R).
I'm doubtful, like you, but Naim imply there's something in it!
Andy.
Posted on: 22 August 2001 by david skinner
Andrew D,
" David, how do you mean "designed to be connected to the BNC"? "
I stand corrected, I was suggesting that the cold conductor should be connected to the outer body of the BNC, that is if you are using BNCs. The fact that OL do not supply with BNCs implies that the cable is not designed with them in mind.
However I still believe that fitting BNCs is a better alternative to using an additional phono to BNC adapter.
The most important issue with this cable is that both hot and cold conductors are shielded and that the cold conductor is seperated from the arm/system earth (shield)until the point of connection to the preamp chassis. With this in mind I agree,there should be no connection between cold conductor and shield at the preamp end of the cable.
I have previously enquired with my dealer re purchase of Aro cable for this job, but he is unable to supply. The other problem is that the plug in the base of the arm which supports the cable for termination to the arm internal wiring is moulded to the cable and would require a fair degree of butchery to accommodate a different cable.
David
[This message was edited by david skinner on WEDNESDAY 22 August 2001 at 14:17.]
Posted on: 22 August 2001 by bam
Hi Andy,
That's interesting. I'm a little surprised that Naim claim this since it seems to me to be an erroneous justification for using BNCs rather than phonos. Marketing motives aside
, I would understand if they made this claim on the basis of superior connection quality and reliability - most professional test equipment I have used uses BNCs. I encourage their use but I cannot support the characteristic impedance argument.
Besides, if Naim truely believes this is of significance then wouldn't they make a similar effort to match CD output to pre-amp input and pre-amp output to amp input. The implied importance of reflections is independent of signal amplitude.
BAM
Posted on: 22 August 2001 by Martin Payne
quote:
Originally posted by bam:
Besides, if Naim truely believes this is of significance then wouldn't they make a similar effort to match CD output to pre-amp input and pre-amp output to amp input. The implied importance of reflections is independent of signal amplitude.BAM
BAM,
the output from the CD player is presumably buffered.
The connection to the cartridge is attempting to carry almost NanoVolt signals, and the impedance of arm lead, etc will affect the signal which is driven from the magnetic coils of the cart. I'm sure it's a much more fragile signal.
cheers, Martin
Posted on: 22 August 2001 by bam
Martin,
I don't think the size of the signal matters. I agree that the signal is "fragile" in the sense that it is very small ans susceptible to noise sources, but in terms of the affects of characteristic impedance matching the signal size is not important.
The low frequency impedance (which is not what Andrew and I were talking about) is very important and this is what I think you are refering to - eg: lead inductance. But the capacitance and inductance differences between a phono socket and a BNC are insignificant in this context. Perhaps the quality of the electrical and mechanical connection between the two types is significant.
The characteristic impedance is usually discussed in the context of transmission lines - at much, much higher frequencies than audio. In general, whenever there is a change in characteristic impedance between two parts of a transmission line some % of a transmitted signals energy will reflect back to the source. This can cause really big problems in radio transmission systems for example.
From the data provided in this thread I would surmise that the transmission line impedance mismatch is much, much higher between CD and pre-amp. A CD o/p impedance is usually less than 10 ohms and the input of the pre-amp is usually more than 20kohms. With the cartridge/pre-amp line the impedances seem much closer, although they are complex - containing inductance and capacitance - so it's hard to be sure without measuring them.
I suggest that if the main reason for using BNCs for phono inputs is to achieve better characteristic impedance matching then the same applies, even more so, to the other transmission lines in the system. The fact that Naim don't deal with these other lines suggests to me that they use and promote BNCs for phono stages for OTHER reasons.
BAM
[This message was edited by bam on WEDNESDAY 22 August 2001 at 23:35.]
Posted on: 23 August 2001 by Andrew L. Weekes
quote:
The fact that Naim don't deal with these other lines suggests to me that they use and promote BNCs for phono stages for OTHER reasons.
Naim also use the same argument for Din's as well, along with the single earth.
One of the reasons given by Naim for the 'Black SNAIC' improvement is the cable is of more suitable impedance to match connectors.
I, like you, find this a difficult argument when faced with the obviously much larger impedance mismatch between driving source, and amplifier input. We're not talking transmission lines in audio.
I do not believe the BNC offers dramatically better electrical connections than a well-engineered phono, so assuming the BNC's really do work, there has to be something in it.
Let's face it, we cannot measure cable directionality, but it's a very real phenomenon to my ears.
It's just these discussions that JV used to contribute to so skillfully - it's a shame Naim's R&D are too busy to contribute here.
Andy
