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In Reply to: Charles I meant if I wired mine like an Incognito with..... posted by mothman on May 3, 2007 at 16:49:26:
I'm not familiar with the Incognito, so that doesn't help me much. I'm not sure I'm following your question clearly so I'll try from the start.On the tonearm side, there are normally 5 wires in total. 4 are to the cartridge pins and the 5th one is attached to the tonearm to act as a shield. This latter connection should be connected to the ground of the preamp in order to fulfill its shielding function.
At the same time, the cable(s) going from the TT to the preamp must also have their shield(s) attached to preamp ground so that they can fulfill their shielding function.
The most straightforward way to do this is with a tonearm that has a 5-pin mini-DIN connector and then use the appropriate Ayre tonearm cable. In this case the cable has four conductors inside a shielded cable. This acts *exactly* like an extension of the tonearm itself, where there are 4 conductors inside a shielded tonearm tube.
Unfortunately for people that want to use a balanced connection, "termination boxes" became fashionable about 20 years ago. Now there is kind a a mess, because it's not immediately clear how to connect things. Here is the story:
Remember that the preamp ground should connect to the cable shields (two in the case of a termination box) and also the tonearm grounding point, BUT only with a single wire in order to avoid ground loops.
If you study the schematic of our RCA - XLR cables, you will see that this is exactly what happens. The shield of the cables is attached at the preamp end in order to shield the cables. But at the RCA end, there is *no* connection for the shield. Instead, a single external ground wire connects from the preamp ground post to the ground post on the termination box, which in turn connects to the tonearm tube to provide shielding there. The RCA connectors are wired *as if* they were balanced, and the center pin and shell are connected to the twin-axial wires of the (balanced) cables. This is what I would recommend doing in your situation. The fact that there is a slight imbalance for 1/2" due to the asymmetric construction of the RCA connector will have no practical consequence.
It sounds like your are bent on modifying your termination box. Unfortunately, I can't quite figure out what you are trying to do, so I am at somewhat of a loss as to what to suggest.
If you really want to have a go at it, probably the best thing to do is to change out the RCA connectors for XLR connectors on the termination box. (It will be something of a mechanical nightmare, unless you have a very well stocked toolbox.) Then you are free to swap in different sets of standard XLR cables in an attempt to fine tune your system.
The best way to do this would be as follows:
a) Connect the two wires from each channel of the phono cartridge to pins 2 and 3 of the XLR. (Red to pin 2 of the right channel XLR, Green to pin 3 of the right channel XLR, White to pin 2 of the left channel XLR, and Blue to pin 3 of the left channel XLR.)
b) There should be no connection to pin 1 of either channel of the XLR connectors at the termination box. Instead the 5th wire (which is connected to the tonearm tube) should go to the grounding post that is presumably already present on the termination box. Then run a separate external wire to the ground post on the preamp.
Now there will be a single connection (the external ground wire) between your TT and preamp. In this way there will be no chance to introduce a ground loop. The XLR cables themselves will be shielded because pin 1 will be grounded at the preamp end of things.
Hopefully I've answered your question, but let me know if I still haven't.
Follow Ups:
regarding how to connect the tonearm ground and cable shields.I have an extra termination box and have already replaced the rca's and hardwired the phono IC's to the box allowing me to connect directly to the tonearm wires.This also gives me the opportunity to 'star'connect the cable shields and the tonearm ground and then a single wire from this star ground in the termination box directly to the preamp.This eliminates connecting the cable shields separately via pin 1 on the preamp side.Also this is how most of the Rega re-wire kits,like the Incognito,handle the shield connections.Again sorry to labor the point and I hope I've clarified for you what my intentions are.Thanks,
OK, I think I am following you. I will go through this step-by-step. If I am missing something let me know.1) There are 5 wires coming into the termination box. 4 from the cartridge and one from the tonearm grounding connection.
2) You have two shielded twinaxial (two symmetric conductors inside a shield) interconnects leaving the box. They are terminated with XLRs at the preamp end.
If this is correct, here is what you want to do:
a) The Red cartridge lead should connect to pin 2 of the right XLR.
b) The Green cartridge lead should connect to pin 3 of the right XLR.
c) The White cartridge lead should connect to pin 2 of the left XLR.
d) The Blue cartridge lead should connect to pin 3 of the left XLR.
e) The tonearm ground wire should connect to the shield of both interconnects *and* the grounding wire that connects to the post on the preamp. (This will be a bit of pain to achieve in actual practice, as there will be four wires attached together at one point.)
f) Pin 1 of both XLRs should be left unconnected.I think this is what you are planning, but just want to make sure. This should work fine. Let me know if it solves your hum problem, and also how it sounds.
for staying with me on this.You have helped me and many others who will search this topic and find your generous post on the subject.I will let you know the results!
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