screening a phono cable

[woodturner-fran]

Hi all,

I have recently improved my analog playback by buying a LOMC (benz-micro L2) cart and adding a phonoclone. Sound is very good - but I have a problem: I get a large amount of hum.

I really don't think its grounding or a miswire in the phonoclone. However, moving the cables around a bit changes the level of hum.

I've spent a large amount of time going through the phonoclone etc and it all checks out. When I use my VSPS I don't get the hum (or its not really audible even at higher listening levels).

The only thing thats left is the tonearm wiring. Commonsense tells me that if it is picking up even a small amount of hum, then the high gain of the phonoclone is gonna make it even bigger. This hum is volume dependant.

So what I want to do is add some kind of screen to my tonearm cable (which is cardas 33awg unscreened - just the 4 wires in 2 teflon tubes grounding wire running with those and with a "decorative" nylon braid outside that again.

I was planning to get some coax cable, cut back the outer sheath and take the foil and copper braid and add it around the tonearm cable and then connect this to the grounding post on the phonoclone.

Would that be the correct procedure? Would I get away with adding the foil (easiest) or copper braid on its own?

Any help would be greatly appreciated!

TIA,
Fran

[SY]

I have one word for you: balanced.

[woodturner-fran]

OK, I'm gonna ask it:


Exactly how would you wire this up?

Fran

[woodturner-fran]

Believe it or not, I've searched and googled quite a bit for how to wire a tonearm cable balanced and not found anything!

But on thinking about this further, the phonostage has rca inputs, I have some nice eichmann bullets on the end of the tonearm cable, not to mention the fact that I don't have any more cardas 33awg for the 3rd wire that balanced would require(?) so it seems to me that the easiest way to go would be to at least try the shielding first.

Thought I had some coax with copper shield and foil, but it turns out I don't so will try and get some tomorrow.

So where should I ground the shield? At the tonearm post or at the grounding point on the phonostage (which is where the tonearm itself is grounded). It seems the shield should only be grounded at one end.

So any opinions anyone?

Fran

[arnoldc]

Thought I had some coax with copper shield and foil, but it turns out I don't so will try and get some tomorrow.

So where should I ground the shield? At the tonearm post or at the grounding point on the phonostage (which is where the tonearm itself is grounded). It seems the shield should only be grounded at one end.

So any opinions anyone?

I have done what you have in mind. Ground ONLY on the phonostage side.

I used to have a problem with radio pickup (FM station) which was solved by re-arranging the equipment.

As for the phonocable, here's what I made for my Clearaudio, using Bullet Plugs and Audience hookup wire-



It's not shielded, it's not grounded, the 'table not grounded to the phonostage. Silent!

ps.

Did you use the PCB of Richard or made your own layout? It matters a lot! My VSPS is silent event at full volume. If you have a hum at listening position (thus swamped at higher volume) you have to re-layout the circuit.

I posted a tiny PCB layout which Richard re-laid using Eagle. That one's silent.

Here's my VSPS-

[woodturner-fran]

thanks arnoldc.

I've tried rearanging the equipment but to no avail, the hum lessens but never drops enough.

Its a phonoclone rather than a VSPS. I'm using my VSPS dual mono supply to power the 2 boards (Richards). I had no hum with the VSPS (or a NAD PP2 which I had to hand) but I do with the phonoclone. I've checked everything 50 times against the construction guide and while I could be wrong, I can't see it. I'm looking at the possibility that the high gain of the phonoclone is amplifying the hum more than the VSPS would have - plus the VSPS was using a HOMC whereas the cart I have for the phonoclone is LOMC.

If I unplug just one of the inputs, the hum is gone. Seems like the tonearm cable is acting like an antenna or something. I tried the phonoclone in a friends system with another cart from the same range (mine is a benz L2, his a benz ruby) and had no hum in his system. So I don't think its the PC itself.

I did a search here too and saw that a few had problems with hum in a phonoclone but it always was down to EMI/RFI rather than the phonoclone itself. So I thought that shielding the cable can't be a bad thing in itself.

Right now I have the cart off the arm and the arm off the TT so its just now a matter of getting some braid and putting it on.

Any other ideas anyone?

Fran

[SY]

Fran, details on balanced tonearm are given in "Building Valve Amplifiers," which IIRC you have. Or was it in "Valve Amplifiers"...? One of them, anyway.

For my arm, I left the arm wiring alone, and replaced the cable with a screened twisted pair fabricated from CAT5. Morgan used teflon-insulated silver wire which makes sense (and I'll do that one of these days). The preamp input connector should be changed to a balanced one, and you need either a balanced preamp or an input transformer (I use the latter).

With my low output MC, I've laid the cable across the turntable's power cord with no audible hum pickup. This is a really effective method.

[woodturner-fran]

Many thanks SY,

yes indeed I do have MJs books - I'll check it out.

It'll be a few days before I get this done, but I'll report back soon.

These - and any other contributions much appreciated!


Fran

[woodturner-fran]

OK, got this done quicker than I thought.

I took some nice copper braid from some coax, slid it up over the teflon tubes that the twisted pairs of each channel are routed through. Brought it all the way up to the tonearm post, but not touching it. The other end goes all the way to the RCA jacks. At a point a little back from the RCA jacks where the two leads exit the nylon braid, I soldered in an extra ground wire That terminates with the tonearm ground wire.

So structure is: twisted pair/teflon/copper coax per channel. Coax grounded one end only at phono preamp.

Damn thing hums the same as ever!

SY - I looked up MJ and found where he talks about balanced. The problem is that I don't know how that would work with the phonoclone. Also, I thought that for balanced you needed 3 wires per channel. Yet in the book he mentions just the 2 per channel. What am I missing here? And how exactly would you wire each channel into a XLR jack?

Fran

[arnoldc]

In this case I would investigate component grounding before you go the balanced route. With much respect to SY, I think your issue can still be fixed.