[Bill Epstein]

I have a phono stage in for review that has balanced inputs and outputs as well as single-ended. The manufacturer states that balanced yields better sound and includes RCA to XLR adapters as somehow even just the XLR connection would be superior.

None of my pre-amps, CAT SL-1, Rogue 66 Magnum and Audible Illusions 3A have XLR inputs.

Before I cut the RCA's off my Cardas phono cables and wire up XLR's, any technical reason to think, besides possibly lower noise, that it would be worth it?

[Steve Eddy]

The whole raison d'etre of balanced interfaces is common-mode noise rejection. So whatever benefit you would end up with would pretty much depend on how much noise you're getting now versus how much less you'd have if you went balanced.

se

[hagtech]

I'm a little confused, as you can't have a balanced signal coming out of an RCA.  So any RCA2XLR converter will result in no changes.  The XLR will have one "hot" and one "return", not two "hot"s of opposite polarity and a return (common).  Same if you go the other way, from XLR2RCA, one of the phases gets ignored.

If you want true balanced operation, you cannot have any RCAs in the path.  The XLR is a 3-wire connection.  I like to use the analogy of a seesaw: two ends and a pivot point.  Relative to the pivot point, the signal at either end is the same, just inverted.  For a cartridge, the pivot point has to be introduced by the receiver (phono input), as the cartridge only has two pins.  This is only possible with a fully balanced input stage using a differential pair with ground reference or similar "instrumentation" type amplifier.  The alternative is to use a center-tapped input transformer (a step-up) with center-tap grounded (or whatever reference voltage you want).

My preference is for the tranny.  The reason is because of the resulting common mode impedance.  For example, with the differential pair, you run 23.5k ohms to ground on each input (both pos and neg).  That gives a differential loading on the cartridge of 47k ohms and a common mode impedance to ground of 11.75k ohms.  That's still a pretty high resistance for referring to ground.  It is still possible to induce some 60Hz hum noise in the common mode, which then has to be rejected by the ac balance of the input stage.  If the stage uses tubes, this balance is not perfect (except for maybe the moment it leaves the factory) and drifts over time and temperature.  You get good rejection, but not perfect.  Anyway, some of that 60Hz hum might get through.  With the tranny, the common mode impedance to ground is only a few ohms.  That's a 1000x improvement.  Ok? 

So first thing you have to do is find out if the phonostage is really balanced or not.  Just because it has an XLR connector means nothing.  Some designers do this for pure marketing, and run the input single-ended.  Don't cut your cables until you know.

jh

[Steve Eddy]

I'm a little confused, as you can't have a balanced signal coming out of an RCA.

Sure you can. Especially if you're talking about a phono cable.

So any RCA2XLR converter will result in no changes.  The XLR will have one "hot" and one "return", not two "hot"s of opposite polarity and a return (common).  Same if you go the other way, from XLR2RCA, one of the phases gets ignored.

That would depend on the converter plug. You'd want it wired with the RCA's center pin going to pin 2 and the RCA's ground collar going to pin 3. The XLR on the phono stage should have pin 1 tied internaly to ground so the ground connection is made via the ground lead on the phono cables (tied to the phono stage's ground lug) instead of the XLR's pin 1.

And in this case, the RCAs provide a balanced signal.

se

[Bill Epstein]

Steve, you're a genius. Of course my measure of a man's intelligence is how much he agrees with me 

So where's the Ingot already. You and Mike still growing the right tree?

I'd given some thought to connecting cart ground to the 3rd pin on the adapter but the supplied Amphenol is sealed up too tight even after removing the collar.
I think I probably will fit XLR's to the tonearm cable at some point. It's just effort after all, reversible and I probably owe it to the manufacturer. I don't know how many purchasers, 99% of whom will have RCA terminations would agree, tho'. 

Jim, the designer says the unit is fully balanced. BTW, do I have to get in line for a Piccolo with everyone else or might there be a review sample somewhere in the near future?
bepstein@dagogo.com

[Steve Eddy]

Steve, you're a genius. Of course my measure of a man's intelligence is how much he agrees with me 

Well then, I agree wholeheartedly.

So where's the Ingot already. You and Mike still growing the right tree?

Yes. It's still just a sapling in the nursery. Sheeesh! Be a little patient!

I'll PM you about that.

I'd given some thought to connecting cart ground to the 3rd pin on the adapter but the supplied Amphenol is sealed up too tight even after removing the collar.

Don't see any reason why you can't just tie the ground leads to the ground post on the phono pre.

By the way, are any of the cartridge pins tied to ground anywhere along the way, either at the TT or in the cables? If so, then that would have to be addressed before you can get a balanced output in the first place.

se

[hagtech]

Ok, but keep in mind RCAs are not well suited for balanced operation.  It's a coaxial connector, outer conductor meant to be a shield.  Sure, you can wire it up to the cart that way and try and run it balanced, but you're exposing a lot of the sensitive conductors to stray fields.  This is fine for single-ended, but not desireable for balanced mode.  Yeah, I know you get the same thing at the cart end, but the wiring is symmetrical so you get a 1st order cancellation.  Best if you can run a twisted pair (2,3) inside an XLR shield (1).

As far a the "cart ground", there is no such thing.  There is a turntable ground wire.  If this is connected to the cart in any way (like a stock Rega), then you cannot run balanced mode.  You first have to separate the TT ground from the cart like Steve mentioned.  Ok, in SE mode, then you can have a cart ground, which is probably best called a "return".

PICCOLO samples have to wait in line.  Half-kits will ship first, in probably about 3 weeks.  Assembled and review units won't be available for another 6-8 weeks, when the painted chassis are due in.  Then they can all ship same time.

jh

[Bill Epstein]

I can't wait that long
Put me down for an assembled unit and let me know when I can click "buy". 

[Steve Eddy]

Ok, but keep in mind RCAs are not well suited for balanced operation.  It's a coaxial connector, outer conductor meant to be a shield.  Sure, you can wire it up to the cart that way and try and run it balanced, but you're exposing a lot of the sensitive conductors to stray fields.  This is fine for single-ended, but not desireable for balanced mode.  Yeah, I know you get the same thing at the cart end, but the wiring is symmetrical so you get a 1st order cancellation.  Best if you can run a twisted pair (2,3) inside an XLR shield (1).

Yeah, and RCA wouldn't necessarily be the ideal connector to use. I wasn't arguing for RCAs but rather simply making the point that balanced interfaces don't inherently require three connections.

XLRs would be a somewhat better connector by virtue of their being a multi-pin connector rather than a coaxial one. But they're just so laughably large I can't bring myself to use them.

And I think shielding is rather overrated in a lot of cases. You gotta ground the turntable to keep its metal parts from acting like big capacitor plates and coupling noise into the lead wires, but I think in many cases a good self-shielding cable geometry and a separate, low impedance connection of the turntable to the phono stage ground is plenty sufficient.

As far a the "cart ground", there is no such thing.  There is a turntable ground wire.  If this is connected to the cart in any way (like a stock Rega), then you cannot run balanced mode.  You first have to separate the TT ground from the cart like Steve mentioned.  Ok, in SE mode, then you can have a cart ground, which is probably best called a "return".

Right. Which is why I was asking if anything tied the cartridge pins to ground.

se

[hagtech]

balanced interfaces don't inherently require three connections

Ok, on this point we disagree.  IMHO balanced operation requires 3 terminals.  In this case, the third terminal (pivot point) is provided inside the phonostage.  I prefer to define "balanced" and "differential" as separate.  Differential only uses 2 terminals.

A cartridge is inherently differential.  Only under very specific conditions can it operate as balanced - when the phono input stage provides a centered pivot point.  Note, that if you shift the pivot point off-center, it becomes unbalanced.  And to the extreme, moving the pivot all the way to one side gives you exactly single-ended.  Even when single-ended, we amplify the difference signal.

jh

[Occam]

Jim,

Dunno if I'm following. If I want a balanced interconnect, with the inherent common mode noise rejection, all I need is a single ended signal, the 'return' doesn't need to be antiphase, either referenced or floating, I just need the source and line impedances to be matched (balanced) for both lines, and then I need the load to have matched impedances and the load input be differential. Then again, I could be wrong, and often am....

FWIW,
Paul

PS - Years ago, when PF was a print magazine, they published a balanced input phono stage using an INA instrumentation amp input with mucho gain, a passive riaa, and an opamp output.

[Steve Eddy]

No, you can still be balanced with just two terminals. The distinction you're making here is simply between fixed balanced and and floating balanced.

If the coil is symmetrical, it will be balanced. The "pivot point" is still there, just that it's internal to the coil instead of being brought out externally in the form of a center tap. In other words, it's floating rather than fixed, but it is indeed balanced.

se

[Steve Eddy]

Damn it, Paul. I thought I could get my post in right after Jim's so I wouldn't have to bother quoting! Grrrrrrrr! 

se

[blakep]

If it's the phono stage I think it is, there are valid reasons to be using the balanced input on the phono stage above and beyond just offering balanced connections even if you cannot continue balanced to your preamp. Even using the adapters supplied by the manufacturer you should be able to ascertain for yourself whether the balanced input is offering superior sound to the RCA's (but keep in mind-once again if it's the stage I'm thinking it is-that you will have to adjust for impedance using the RCA's where you do not with the balanced in). In the end, eliminating the adapters and terminating your phono leads with quality XLR's should improve the sound even further.

[hagtech]

Paul,

You got it right.  If the 2-terminal source is floating, then you can force operation to balanced mode by the way you receive it.  Another example would be a temperature sensor.  There are resistors with value dependent upon temperature (non necessarily linear).    This is a two-terminal device and can be "sensed" just like a cart.

But notice in your post the trick is to balance the impedances on each terminal with respect to some common mode reference (usually circuit 0V).  I'll draw a simplified input circuit:



As we have pointed out, if R1 and R2 are equal (and this extends to generalized impedances), then the floating cart will operate in balanced mode with all of the associated benefits.  The third terminal is shown as ground.  It is provided by the receiver.  If we make R1 unequal to R2, then we unbalance the circuit, which results in both common mode and differential signals.  That is, the pivot to the seesaw is off-center.  Make R2 zero, and we have the typical single-ended connection.

But as you can see, the third terminal is a necessary condition for balanced operation.  One, it creates the seesaw pivot.  Two, it defines the common mode operating voltage which every practical differential input stage requires (just try exceeding their common mode range and see what happens).  Hence, by definition, balance requires three terminals.  Just because the connector might have two does not mean the third does not exist.  It will be found inside the phonostage.  You have to look at the entire picture, not just a snippet.

I also show the other possible solution of receiving via center-tapped tranny.

between fixed balanced and and floating balanced

Steve, you make a good point.  There is indeed a theoretical balance point within the cart itself.  So when disconnected from a circuit, it is balanced.  However, as connection with this point is not possible, in practice it means nothing. 

jh

[Steve Eddy]

You got it right.  If the 2-terminal source is floating, then you can force operation to balanced mode by the way you receive it.

But if the two terminal source is already balanced, say in the form of a symmetrical coil or a symmetrical winding on a transformer, then there's no need to fix (or force) balance. The balance is already there.

Consider:

The secondary of an output transformer (source) and the primary of an input transformer (load). Both are symmetrical and both have center taps.

Connect the one to the other, including the center taps and apply a signal. Because both windings are symmetrical and therefore inherently balanced, there will be no current flowing through the line connecting the two center taps. That being the case, if the line connecting the center taps is removed, the circuit will function just as it did with it there.

So a third terminal isn't required for balanced mode and the benefits it offers. What's needed is symmetry.

Steve, you make a good point.  There is indeed a theoretical balance point within the cart itself.  So when disconnected from a circuit, it is balanced.  However, as connection with this point is not possible, in practice it means nothing.

It means plenty in practice if common-mode rejection is of any meaning. If the coil is symmetrical, there needn't be a physical connection to that center point in order for it to be balanced and to offer the common-mode rejection of a balanced source.

se

[Occam]

.......
But as you can see, the third terminal is a necessary condition for balanced operation.  One, it creates the seesaw pivot.  Two, it defines the common mode operating voltage which every practical differential input stage requires (just try exceeding their common mode range and see what happens).  Hence, by definition, balance requires three terminals.  Just because the connector might have two does not mean the third does not exist.  It will be found inside the phonostage.  You have to look at the entire picture, not just a snippet.
.......
jh

Jim,

You're absolutely right. Even in the context of a single ended output preamp, if I want to accrue those benefits of common mode rejection when connecting to a balanced input load, I'd terminate, at the source end with 2 equal resistors to ground, R1s=R2s and at the differential receiving end similarly R1l=R2l. Even though the 2 lines do not carry anti-phase signals, they are both referenced to the same voltage, ground......

-Paul

[Steve Eddy]

You're absolutely right. Even in the context of a single ended output preamp, if I want to accrue those benefits of common mode rejection when connecting to a balanced input load, I'd terminate, at the source end with 2 equal resistors to ground, R1s=R2s and at the differential receiving end similarly R1l=R2l. Even though the 2 lines do not carry anti-phase signals, they are both referenced to the same voltage, ground......

You'd need more than just the two resistors. You need to mirror the output impedance of the single-ended output, including any output resistors and caps. Here's an example:



se

[Occam]

Welcome back Steve. Yes, you need to balance the impedances, which would include both the resistive and reactive elements. Did you pull that from ePanorama, or directly from Jensen/Whitlock? 

[Steve Eddy]

Welcome back Steve. Yes, you need to balance the impedances, which would include both the resistive and reactive elements. Did you pull that from ePanorama, or directly from Jensen/Whitlock? 

The latter. 

By the way, if your balanced input is in the form of an input transformer, you can just feed it straight from your unbalanced output and still get outstanding common-mode rejection.

se