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Balanced phono carts

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Posted on March 15, 2017 at 09:09:07
Lee of Omaha
Dealer

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Location: Omaha NE
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Why on earth are phono cables NOT twisted pair? Common mode rejection of differential amps is the reason for existence of twisted pairs used everywhere for data.

Yes, there would be some extra cable capacitance which could be addressed in a couple of ways, the best being putting the phono preamp in the turntable (short leads, low capacitance).

I suggested a six-wire cartridge to Grado (center ground and two opposite-phase signal leads) which was promptly ignored. That would be backward compatible (with no mods you could use a four-wire connection).

Hum sucks, and it seems easy to prevent by adopting existing twisted pair methods.

 

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The reason Grado rejected that is because, posted on March 15, 2017 at 09:39:07
Ralph
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Location: Minnesota
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All magnetic and MC cartridges are already balanced sources. They don't need 6 connections!

If a center tap were added, it would reduce the CMRR (Common Mode Rejection Ratio) of the differential amplifier that the cartridge drives.

We sorted out all this about 30 years ago so when we introduced our MP-1 preamp; it was the first balanced line preamp that could properly accept the phono signal in the balanced (and differential) domain.

That ground wire that all single-ended phono systems use? That's the shield connection that ties to pin 1 of the XLRs of both channels. That wire (which no other single-ended system seems to need) is one way you can tell that you are working with a balanced source while using it single-ended. You have to do something with the ground; like any true balanced source, the ground is ignored and is only used for shielding.

 

RE: The reason Grado rejected that is because, posted on March 15, 2017 at 10:20:12
Lee of Omaha
Dealer

Posts: 1301
Location: Omaha NE
Joined: September 8, 2006
The problem is the shield. Only shielding ONE lead makes the noise (hum) introduced into the two leads inherently unbalanced and NOT common mode, so it can't be cancelled in the differential amp. By using XLR, you're right, problem prevented, but hardly any turntables use XLRs (which I think should be universal in audio.)

 

Its not the turntables, posted on March 15, 2017 at 11:25:30
Ralph
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Location: Minnesota
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Its the connection that occurs at the output of the tone arm, which is usually balanced.

There are 5 connections- the 5th being ground. Because of this you can set up an old BSR or Dual with a balanced connection simply by changing out the interconnect cable.

The ground ties to the shield of both channels. At the output of the arm.

The arm is equipped with a 5-pin connection (like an SME arm) then this is really easy to deal with as you just plug in the new cable.

 

RE: Its not the turntables, posted on March 15, 2017 at 11:36:58
Tre'
Industry Professional

Posts: 12026
Location: So. Cal.
Joined: February 9, 2002

Only one channel shown.

The two signal conductors would be twisted (twist not shown).

Ground is just for shielding.



Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Right! nt, posted on March 15, 2017 at 12:43:38
Ralph
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Location: Minnesota
Joined: April 24, 2002
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April 1, 2002
-

 

Lo-Z microphones have always been wired that way NT, posted on March 15, 2017 at 17:41:12
Alpha Al
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nt

 

RE: Lo-Z microphones have always been wired that way NT, posted on March 15, 2017 at 19:11:12
Tre'
Industry Professional

Posts: 12026
Location: So. Cal.
Joined: February 9, 2002
Yes, balanced.

Microphones connect to a transformer primary or a differential circuit and they are connected using a balanced connection.

A differential circuit accepts a balanced connection.

A transformer primary accepts a balanced connection.

Some people confuse the terms "balanced" and "differential".

The term "differential" describes a circuit.

The term "balanced" describes a way of connecting a source to a differential circuit or a transformer.

With a transformer you can have a fully balanced connection without having a differential circuit.

Some equipment has a input stage circuit and a output stage circuit that are differential while the rest of the circuit in the unit is single ended.

Other equipment uses input and output transformers while the rest of the circuit in the unit is single ended.

Both of these type units accept fully balanced input and output connections.

Some special equipment is fully differential from input to output.

Some using input and output transformers with fully differential circuits in between and some using active differential circuits from input to output and no transformers.

All of these will accept fully balanced connections.

In the studio and at home I have used, maintained and repaired units using all of the type circuits described above.


Tre'






Have Fun and Enjoy the Music
"Still Working the Problem"

 

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