I've been working on a fully-balanced input moving magnet cartridge preamp. I'll post details about it soon. One of the issues I ran into during development were the limitations of coaxial cable and RCA plugs.
A fully-balanced input benefits from fully-balanced wiring. Though most tonearms seem to be internally balanced up to the DIN connector, the rubber ultimately hits the road as a coaxial RCA connection. To get the most benefit, shielded twisted pair cable is required.
Have you priced 5 pin DIN/XLR "tonearm" cables? I hadn't until recently and was shocked at how much they cost. The connectors alone seem to be unavailable on ebay from the US with international ones beginning at $35 each.
This post (which I originally posted at the proaudiodesignforum) explains a simple, low-cost way to make your own balanced, shielded twisted pair DIN tonearm cable.A Fully-Balanced DIN Tonearm harness for the Balanced-Input Flat/RIAA Phono Preamplifier.
The Lumberg KV50/6 is a 240 degree, 5 pin DIN connector available from Allied Electronics.
By removing the pin block and filing it to fit inside the tonearm you can "bootleg" an expensive over-priced "tonearm" connector.
Allied has the Lumberg KV50/6 for about $9: http://www.alliedelec.com/lumberg-kv-50-6/70151666/
All Electronics currently have them surplus for around $3: http://www.allelectronics.com/make-a-store/item/din-5f/female-5-pin-din-connector-in-line-locking/1.html
Based on the prices of DIN females specifically made for tonearms the above seem very reasonable.
The connector pin block has to be modified slightly to be a DIN tonearm connector.
It takes a couple of minutes at most.Lumberg KV50/6 5 Pin 240 Degree Female Locking DIN Connector
The pin block has to be removed from the outer metal shell which will be discarded.
Tabs extending from the back of the pin block interlock into an insulating plastic shell that is also not used.
The tabs interfere with insertion of the connector body into the tone arm.
Filing them off to make the end of the pin block round allows clearance.
Apparently this is what makes DIN tone arm connectors "special."Lumberg KV50/6 5 Pin 240 Degree Female Locking DIN Connector Modified Pin Block
Once the pin block has had the plastic tabs filed down with the body completely circular it can be used to make balanced phono cables:Lumberg KV50/6 5 Pin 240 Degree Female Locking DIN Connector Modified Pin Block With Leads
The assembly slips into the Audio Dynamics tonearm.I realize that the Brand-Rex cable I used isn't made of unobtainium: I could have paid $229 if I wanted unobtainium.This is a comparison of unbalanced coaxial RCA cables to fully-balanced shielded twisted pair cables using a Stanton 681 dummy cart mounted in a tonearm:Unbalanced RCA connections with a grounded differential input:Balanced Flat Phono Preamp with RIAA EQ Applied, Noise Floor, Unbalanced coaxial RCA Connections, Grounded Differential Input, Stanton 681Balanced shielded twisted pair connections with a fully differential input:Balanced Flat Phono Preamp with RIAA EQ Applied, Noise Floor, Balanced Shielded Twisted Pair Connections, Fully Differential Input, Stanton 681
The difference in "hash" between 1-10 kHz is significant.
The 60 Hz hum is about 6 dB less.
The performance with the grounded differential instrumentation amp input is good, but given the modest investment to make a balanced phono preamp cable I'd say the improvement from a fully-balanced connection is worth it.