Balanced in phono stages preamp?

Have you ever wondered why the phono is the only hookup in your system that requires an extra wire for grounding (else it hums)? This is the result of trying to operate a balanced source in single-ended mode.

That is simply not true. It is not required. A cartridge connected to a single ended input sometimes will hum with the ground connected and sometimes without. Sometimes the ground connection makes no difference. After playing records for almost 40 years I have proven this to be true.

While it may be true that balanced equipment might have been used in the recording process, the groove in the disc is not balanced. Balanced has 2 signals of opposite polarity per channel so a stereo signal would require 4 separate signals. There is no medium (tape, vinyl, digital, etc.) that provides these balanced signals so all sources are single ended. Why even imply that vinyl is balanced?

Dear Herman: Our design is the Essential 3150 Phonolinepreamp that you can follow at this link:
http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1166047312&openflup&49&4#49

The Essential 3150 is fully diferential input to output and has two gain phono stages with out step-up transformers.

Ralph, I understand your cartridge balanced explanation but it is not clear to me, how a coil/inductor can be balanced?

Regards and enjoy the music.
Raul.

>>Proof? Invert the phase of one channel of your cartridge in your system and see if it hums<<

You're kidding, right?

>>Balanced is hype<<

There is more to balanced circuitry than is obvious to the casual observer. One significant advantage is PSRR. Or the way it interacts with a power supply. Think holistically for a moment, everything is inter-related to everything else. I can walk through an example here.

What is the best type of regulation? Many will say shunt. Why? Because it offers a lot of line rejection and can be made very fast. It is also class A. What is better than shunt regulation? What if (pretend it is possible) instead of reacting to a load variation, as a shunt regulator does, that you could know in advance what the variation will be? What if the regulator acted precisely at the same moment and without phase delay to a load transient? And without feedback. Would that not be better? Imagine correcting the supply rail perfectly in response to the music. Well, that is basically what a balanced differential stage does. One way to look at it is a single-ended stage mated with a shunt regulator. You can actually run it that way. There isn't any other topology out there that can top that.

Now take it even further. You can add passive line regulation (no feedback). In a balanced phonostage I once designed you could take a variac on the ac line and crank it up and down (simulating line disturbances), and the dc output level from the amplifier doesn't budge. Yes, the power supply rails move in response to the variac, but the output doesn't. Well, at least not at a 1Hz variac rate. The point is, there are tricks you can pull with differential topologies that you can't use anywhere else. It ain't hype. Unfortunately, there is a big price to pay. It takes double the number of devices. Cost goes way up.

jh

Atmasphere, agreed. The notion that balanced circuits are more complex that single-ended is a myth. In fact, balanced operation in comparison gives you twice the voltage (+6 dB) than single-ended, which in many stages would allow you to eliminate one gain stage (or several, with multiple stages)! Also, a symetrical circuit presents supply current variations in opposing polarities, so cancellation takes out and supply demands are drastically reduced. Balanced circuits are more elegant from an engineering standpoint, etc. As a final bonus, the signal-to-noise ratio improves by +3 dB (not +6 dB). You can definitely notice that.

Speaking of a phono cartridge, it's just a floating coil with 2 wires. As the coil is floating, you have the choice to operate it as either single-ended or as a true balanced signal source. If you connect the negative terminal to ground, and use the positive terminal as signal, then you have normal single-ended mode.

To better understand the advantage of a cartridge working in balanced mode, I will quote a paragraph from my book "Noise Reduction Techniques in Electronic Systems" by Henry W. Ott: "A balanced circuit is a two-conductor circuit in which both conductors and all circuits connected to them have the same impedance with respect to ground and to all other conductors. The purpose of balancing is to make the noise pickup equal in both conductors, in which case it will be a common-mode signal which can be made to cancel out in the load".

According to this definition, we can confidently say that a pickup cartridge truly acts as a balanced source, because when the cartridge is connected to a normal phono cable (with 2 wires and 1 grounded shield), the impedance of both wires with respect to ground is the same. Further, the noise pickup is equal in both wires, forming a common-mode signal that a differential input stage (or a center-tapped transformer) will easily cancel out.

Please notice that differential mode is not the same as balanced mode. In fact, an input circuit working in differential mode is a pre-requisite for balanced mode. As said, this circuit could be: 1) A center-tapped transformer, or 2) a differential input stage (tubes or solid-state). As you can see, balanced mode has obvious advantages over single-ended mode in phono cartridges.