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Balanced connection/amplification for phono?

Here's a poser I don't know the answer to: Phono cartridges are inherently electrically-balanced devices. They output an extremely low-level signal, which must then be carried on low-loss cabling (both within the tonearm, where cabling must obviously be lightly constructed, and the interconnects leading away from the 'table, where demands of optimal signal-transmission can be at odds with the use of heavy shielding) and pre-preamplified to a high gain factor, both areas susceptible to signal degradation via environmentally-borne RFI/EMI contamination. Yet despite the importance of maintaining high signal-to-noise ratios in these wires and circuits under difficult conditions, few turntables or phonostages offer fully-balanced connection and topology, and most audiophiles must connect and amplify their naturally-balanced phono cartridges in single-ended mode while employing relatively noisy, lossy RCA jacks. Why is it that phono connections and low-level preamplification aren't routinely carried out in electrically-balanced mode, so as to take full advantage of the penalty-free (aside from cost) noise-cancellation properties (regarding both signal transmission, and self-noise/distortion of the high-gain amplification) enjoyed by complementary circuits? This would seem to me to be an easy conclusion, and yet it's something fairly rare to find. Can you guys enlighten me as to why this should be the case?
zaikesman
sayas
190 posts
 
i know an individual who runs 2 krell kpa's, balanced, one per channel. the connection is as you are wondering balanced from cartridge to phono stage to pre to amp...this is what i would call a true balanced system.

the kpa is otherwise a stereo unit

one of the better questions out there and one that requires a manufacturer to question why they go to single ended operation when the balanced is naturally available.
hagtech
173 posts
 
Great question. But phono signals are not balanced. They are floating. A balanced connection requires three signals, positive phase, negative phase, and reference (commonly ground). Cartridge coils have only two wires per channel. For balanced, the coil would have to be center-tapped.

That's the main reason it isn't done. However, one trick will do the job; you can receive with a center-tapped transformer. Tie the center tap to ground and the cart now has a solid low impedance common mode connection and you get positive and negative relative signals.

Many phono stages that have XLR inputs don't run true balanced or differential mode. They just tie one of the phases to ground.

My Trumpet Phono is fully balanced throughout, front to back. I suppose the BAT is too. Not sure anyone else is doing it.

jh
ntscdan
52 posts
 
Audio Research's PH2 was balanced ins and outs only. It is a fine sounding unit.
zaikesman OP
3,017 posts
 
Mr. Hagerman exposes my lack of understanding of some of the finer points. I've read statements to the effect that "all phono cart's are inherently balanced devices" in audio magazines on a few different occasions, though I'm sorry I can't make specific citations. It's self-evident that a cart would generate a +/- voltage swing instead of +/0; that a cart should 'float' does also seem obvious, but the distinction regarding this vs. a 'balanced' condition is apparently where I (and some audio writers) went astray. (Though why the one will give you common-mode noise rejection and the other not is another can of worms I think I'd best leave unopened for the present...)

Having said that, JH's introduction of the topic of transformers seems tantilizing, since of course step-up trannies are commonly employed to add voltage gain for low-output MC's. Since I've always stuck to medium-output models going straight in, I lack the experience with step-ups, but is it common practice for these to be center-tapped and offer true balanced output? Again, sounds like a no-brainer if you're already using the tranny anyway.

P.S. - We could add Boulder to the list of true balanced phono pre's (but we couldn't afford one ;^)
eldartford
4,262 posts
 
Is there a problem with single-ended phono pickup connection? Not that I know of. Why solve a problem that doesn't exist? (Well I guess it's just for fun).

The floating vs balanced thing did occur to me, but if you tie together two of the four pickup terminls it looks like a center tap of the two-winding pickup.
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