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At 07:27 PM 4/19/2006, Tom Fine wrote:
>Hi Richard:
>
>I thought about a turntable-to-preamp patchbay and decided it's just 
>too likely to have problems. If the grounds are all tied together, 
>doesn't the whole of the system become a potential hash/rf pickup? 
>Phono cartridges are crazy about rf fields -- I make sure to keep my 
>cellphone far from the studio and there's only an old-school WECO 
>telephone anywhere near the equipment. Those Blackberry things are 
>horrible about putting out crap -- you hear it all the time on 
>interview shows. I can't understand how they got FCC 
>licensed!  Anyway, though, my end result was, since I don't switch 
>turntables often, just to physically change the RCA and ground wires 
>to the preamp. If I had a setup where multiple turntables were used 
>often, I'd have multiple preamps like a radio station or DJ rig.

Hi, Tom,

The reason that I mentioned that is Kurt is one of the people I 
suspect might have esoteric preamps that are useful in specific 
situations - I don't do discs, remember <smile>.

So, the idea would be specialized turntables and specialized preamps 
having to be routed.

Obviously, the best solution is as you state, a dedicated preamp for 
each cartridge (I don't want to say turntables as I did see the 
Archeaphone (sp?) mentioned which is a cylinder reproducer).

In that model, the processing would be done at line level (even if 
unbalanced) and then fed to the A->D converter.

The solution I was alluding to was very short coax cables from the 
cartridges to an array of BNC connectors and then into the preamps. 
As you recall, I was mixed about whether I'd mount that on a metal or 
insulating plate. Arguments can be made for both, but you do only 
want single point grounds, so you have to decide how you're doing it.

It really takes sitting down and analyzing it which isn't really 
possible without knowing a lot more about what is being done.

The big picture is that you want no gradients on your grounds. 
Everything needs to be at the same potential. Conducted interference 
(the pin-one problem in balanced equipment) is by far the greatest 
challenge to good system design. Capacitive or inductive coupling 
into signal cables is usually much less of a problem.

Oh, and the ground wires that come with most turntables (I've seen 
#22 typically) are inadequate. I generally use anything from #16 to 
$12 or multiple parallelled #22. And here, with a local, single, 
preamp, you're trying to get the turntable chassis, the preamp 
chassis, and the cartridge ground terminal to be all at the same 
potential with the noise voltage on these connections down in the 
sub-microvolt region.

In one project with official "isolated ground" power supplies, we 
measured about 15 millivolts of power between adjacent racks, and 
these racks were bolted to the same base and to each other. Imagine 
how much ground current was flowing! Removing the isolated ground 
conductors and using the conduits as ground (with the formerly 
isolated ground in parallel with the conduit) made all the difference 
in the world. The noise dropped from unacceptable to acceptable on 
video (probably in the 1 millivolt or less region). It is typical in 
video to use unbalanced connections for short runs. Semi-balanced 
"differential" inputs are often used on long runs on the receive side.

Cheers,

Richard


Tape Restoration Seminar:    MAY 9-12, 2006; details at Web site.
Richard L. Hess                   email: [log in to unmask]
Aurora, Ontario, Canada       (905) 713 6733     1-877-TAPE-FIX
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