few phono preamps with balanced inputs .. why?

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tresaino
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few phono preamps with balanced inputs .. why?

Post by tresaino » 07 Sep 2007 12:37

I’ve always asked myself why there are only few phono preamps on the market with balanced inputs. This is somewhat surprising as people generally acknowledge that cartridges’ outputs are actually balanced and it would therefore make perfect sense to go down this route. Some models on the market have XLR outputs, but how many have XLR inputs? I think we would be hard pressed to come up with the name of five companies that produce fully balanced models – but please correct me if I’m wrong. :?

Most of us would state that XLR can have significant advantages over RCA, it starts already with the superior plugs.. Anyway, the professional world still uses XLR connections and circuits, and pro turntables from the 70s and 80s used balanced circuits and connections – EMT 930s and 950s models come to mind (I think their phono circuits were also fully balanced, but have not verified before posting this).

So a few questions come to mind. Why has high-end phono not adopted superior balanced circuits and connections? Perhaps because this is not the case? Or is the benefit simply not worth the extra effort, perhaps because of the usually rather short interconnects used between tonearms and phono preamps? Or is a fully balanced phono circuit too complex to build? Or too expensive to be profitable? :?

Anyway, lots of questions fellas… look forward to hearing from you. :wink:

roberto

delcam1n0
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Post by delcam1n0 » 07 Sep 2007 13:26

Treisano,

Methinks...

Because 99.999%(?) of home audio users over the past 30 or so
years didn't even know that XLR existed, plus, whatever demand
existed for high Q balanced phono preamps in studios was well
taken care of by the companies like EMT, Telefunken and Philips
studio equipment, at prices most of "us" could/would not afford.

Guest

Post by Guest » 07 Sep 2007 14:47

Because balanced circuits solve a problem that doesn't exist. Is something wrong with your own system that would be fixed by balanced phono inputs?

FWIW, the best components I've heard did NOT have balanced connections.

Steerpike_jhb
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Post by Steerpike_jhb » 07 Sep 2007 16:20

The main reason for using a balanced line is when the line is long and/or likely to pick up interference.
Balanced inputs with a GOOD common-mode rejection ratio (i.e., good noise immunity) are expensive to make. Balanced systems ONLY reduce noise picked up by the cable itself, not any other noise sources. On short runs, a good unbalanced cable is a negligeable source of noise, and active balanced-input circuits GENERATES more noise internally than a single ended input, so if the cable isn't a problem, you're better off with single ended.

A balanced cable presents a higher loading capacitance, which some cartridges may not like.

Transformers are inherently balanced loads, so anyone using an MC step-up transformer has a balanced input to experiment with.

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Post by jstraw » 07 Sep 2007 16:39

My DJ rig...for streaming music on the 'net, uses a Presonus BlueTube as a phono preamp. It's has XLR ins and outs. It also has seperate gain and tube drive controls for each channels.

http://www.presonus.com/bluetube.html

Guest

Post by Guest » 07 Sep 2007 19:05

Hello, I am not sure whether that preamplifier is a fully balanced circuit or a suedo balanced design so I have contacted the manufacturer to find out.
Can I ask how a cartridge output is balanced? I thought balanced was where a 'negative' signal was produced and sent with the original signal then a circuit used both of these to identify the interferance picked up by the cable and subtracted this from one of the signals. It uses summation of the signals, I think, to void the added RFI. I had this explained clearly to me once but unfortunately I cannot remmember it fully.
If someone would be good enough to explain it I would be very greatfull, cheers Ben.

blakep
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Balanced phono....

Post by blakep » 07 Sep 2007 21:37

Here is a link to a thread at Audiogon in which Ralph Karsten from Atmasphere explains the advantages of using balanced circuits in phono preamps (he has apparently been building them that way since the late 80's).

http://forum.audiogon.com/cgi-bin/fr.pl ... Atmasphere

I'm much more of a listener than a technical person, but I can say that I've been really pleased with the Aqvox 2Ci which I have and its performance is notably better in balanced mode than single ended, quite a huge difference in fact. I'm running fully balanced through the phono stage directly to my integrated amp and would not return to single ended.

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Post by Steerpike_jhb » 07 Sep 2007 23:30

Can I ask how a cartridge output is balanced? I thought balanced was where a 'negative' signal was produced and sent with the original signal then a circuit used both of these to identify the interferance picked up by the cable and subtracted this from one of the signals. It uses summation of the signals, I think, to void the added RFI. I had this explained clearly to me once but unfortunately I cannot remmember it fully.
A balanced input amplifies, or detects, only the difference between the two signals, usually named 'in-phase' and 'return'.
Any interference - unless of extremely high frequency - will affect both conductors and inputs equally, and so be rejected by the input circuit.
The ability of the balanced input to do this is called its common-mode-rejection-ratio (CMRR), the bettter the system, the higher the number. Any manufacturer that doesn't quote a CMRR is probably a little embarassed by the performance in this area.

Balanced lines are very commonly used on mobile/DJ/PA equipment for another reason - to eliminate ground-loop problems. That is the main reason why such equipment has XLR connectors. A relatively poor noise rejection figure is achieved, but that is not an issue for them, as long as they have eliminated the potential for ground loops. (1)

Balanced cables usually have an outer braid to add extra screening. An almost-as-good solution can be achieved by twisting the two wires together very tightly, as is done on telephone connections and ethernet computer networks (ethernet uses USTP cable = unshielded twisted pair)

The balanced line receiving can be done by means of a transformer (centre tapped or not, as described later). Or it can be done by means of a differntial amplifier circuit. These usually need to be more complex than a single op-amp, because all practical op-amps present a different load impedance on their '+' and '-' inputs, which cannot be 'fixed' purely by feedback resistors. This impedance disparity completely defeats the purpose of a balanced system. Some form of buffer circuit must preceed the differencing op-amp, and the whole configuration of 3 op-amps is then called an 'instrumentation amplifier'.

It is convenient to *imagine* the cartridge winding as centre-tapped, this being a ground reference. Each half winding then generates an equal, but opposite phase signal with respect to this ground reference.
However, if the various currents are measured - or computed theoretically, you find that there is zero current flowing in the ground reference - no current flows into or out of the 'centre tap' of the cartridge. If there is no current flow, you can sever the connection with no effect.
You can infer from this that the halfway point of the cartridge winding - when driving a balanced load - always remains at ground potential.

Anyone familiar with 3-phase power distribution will know you can send three phase power over 3 wires - no neutral conductor is required - if the load is balanced. The reasons and principle are exactly the same for a cartridge.

(1) a ground loop (often called a hum loop since it mostly injects 50- or 60Hz hum into the system) is formed when two connected pieces of equipment share more than one ground connection; either their cabinets are touching, or they share a mains earth through the building wiring, or they just have two screened audio cables. The two ground connections then form a complete loop, which acts as a single-turn transformer, susceptible to any magnetic fields, especially those of the transformers in the equipment.

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Post by lintwind » 08 Sep 2007 00:04

Steerpike, +1
i've lots of pro kit which takes XLR along with unbalanced, and as a rule, i always go for mono-jack (half TRS) (unbalanced) in the studio, and XLR through DI box(es) on site.

Stacked XLRs (2 or more XLRs piped together) are always likely to develop out-of-phase noise, at least to my experience.

I'd never use a balanced XLR in my bedroom nor in studio setup, unless absolutely necessary (phantom-powered mics and such)

The true XLR implementation of any stereo cartridge output would take a passive circuit to negate the phase of each of its channels, as in "mic" type of XLR output, thus probably adding to the load.

ps. ground loops are twice as likely to occur in all-XLR setup, Steerpike's absolutely spot on there, i've learned this phenomenon the hard way, by spending hours to eliminate each and single one before recording sessions :)

Guest

A Very Helpfull Reply

Post by Guest » 08 Sep 2007 08:19

Here is the reply from the question I asked of the manufacturers of the previously mentioned phono stage.
Hi Ben,



Our preamps all have a balanced, class A input buffer and an active balanced output; with the exception of the ADL, which has a transformer balanced output. Some of our preamps have transformer balanced inputs, some are transformerless. Some have fully balanced outputs, some are impedance-balanced only. Nothing we make has a fully balanced circuit path from front to back; but I would hardly call this a ‘pseudo’ design as it is quite the norm in engineering. The only preamp that I personally know of that utilizes a fully balanced circuit path front to back is the True Systems Precision 8. There may be others, but I do not know of them. That approach has its merits but can be very costly and tricky to implement correctly. If the components are not matched perfectly you will have problems and this type of design lends itself to more odd-order harmonics, which are considered less pleasant. This type of design essentially maintains the balanced signal from in to out and often passes along the duty of common-mode rejection (hum/buzz, and noise removal) to the next piece of gear in a chain. Like I said, this approach has its merits and can be implemented wonderfully; but it is not our approach and is not the normal design approach in the industry. Thanks!



Chad Kelly

Dealer Support Coordinator

PreSonus Audio Electronics, Inc.

As you can see this gentleman has been very helpfull and explained why his company and almost all others to his knowledge do not utilize fully balanced circuits in their designs.

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Re: few phono preamps with balanced inputs .. why?

Post by dgotlse » 08 Sep 2007 09:46

tresaino wrote:I’ve always asked myself why there are only few phono preamps on the market with balanced inputs. This is somewhat surprising as people generally acknowledge that cartridges’ outputs are actually balanced and it would therefore make perfect sense to go down this route. Some models on the market have XLR outputs, but how many have XLR inputs? I think we would be hard pressed to come up with the name of five companies that produce fully balanced models – but please correct me if I’m wrong. :?

roberto
The reason is that most of torn arm are not wired with balanced cables.

Just look at http://www.aqvox.de, it explains how to modify the wiring of a RB300 for a balnced version.

blakep
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Balanced

Post by blakep » 08 Sep 2007 15:35

Actually, the Rega is in the minority with respect to needing this modification to work in balanced mode with the Aqvox. Most tonearm cables can be very simply reterminated with XLR's by a qualified tech or someone with a bit of knowledge in this area and soldering capability.

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Post by dcg69 » 08 Sep 2007 19:57

Hi,

I agree with Blakep, as most of the tonearms will keep the negative poles separated from each other. If they have RCA connections, an RCA-XLR connector will provide you the balanced output. Some tonearm makers, like Mr. Shroeder, provide on request the XLR connectors directly wired from the tonearm.
Going back to the original question, I guess a possible answer is that on short distances the theoretical advantage of balanced vs unbalanced becomes in practice neglectable. A good shielding is cheap and it might just do the trick ...

Cheers,
Carmelo

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Post by tresaino » 09 Sep 2007 11:58

It appears that most of us tend to agree that a balanced circuit is more complex (but fascinating) and most probably also more expensive to build. Where opinions start to diverge is when it comes to assessing whether the results are worth the extra effort.

Steerpike, thanks a lot for sharing your technical knowledge, great stuff. Two observations and one question:

(a) cable length can matter also in home audio - if I count all the cables needed from tonearm to phono preamp to normal preamp to amps there are quite some meters. Not as in the pro world of course but still.

(b) thanks to blakep we got Atmasphere’s response, one of the very few manufacturers of truly balanced phono preamps: “The result is lower noise throughout the phono system. In our case this allowed us to eliminate a stage of gain. That made the preamp more transparent, as it now makes less noise and distortion with wider bandwidth. IOW the signal path is actually simpler, not more complex, quite the opposite of the usual drone of balanced circuits being more complex!” Let us take note that at least Atmasphere believes that there can be additional advantages with balanced circuits.

(c) what are in your view good CMRR values? I am asking because I am about to receive a balanced phono preamp on trial, will check with the manufacturer.

Blakep, I am also more of a listener than a technical person, so here’s a question for you: you found that with the Aqvox balanced sounds “notably better” than single ended .. can you try to describe what you mean by “a huge difference”?

And to Dgotise, well .. I cannot imagine that people decided not to go down the balanced route because of the cable wiring. I think it went the other way round: most people are not selling balanced tonearm cables because there is almost no market for them at present. And hey, no, I don’t think it’s a chicken-egg problem. :D

roberto

PS:
lanny wrote:Because balanced circuits solve a problem that doesn't exist. Is something wrong with your own system that would be fixed by balanced phono inputs?
Lanny, you must be joking. The issue is not to fix something that is wrong, rather to explore whether results can be further improved. :wink:

blakep
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Balanced phono....

Post by blakep » 09 Sep 2007 23:19

Roberto: In the TNT review of the Aqvox, the reviewer suggested that the Aqvox sounded approximately 20% better in balanced than in single-ended mode and I'd be inclined to agree with that. That is not a small number, at least from my perspective in "audiophile" terms when describing an improvement.

What Ralph Karsten of Atmasphere describes as less noise and/or lower noise floor is quite apparent in the Aqvox in balanced vs. single ended. It is really uncanny and very different when you first hear it. It's hard to describe, but there is just more of a sense of listening to real music as opposed to an amplified reproduction. It's (IMO) less of a "hi-fi" sound; there's more of a sense of the performance as a whole as opposed to a lot of different instruments and/or voices being separated and thrown at you in the listening experience. I think there is a much greater sense of the acoustic environment in balanced vs. single ended and much greater front to back depth in particular. Frankly, I've never been obsessed with what I perceive to be audiophile qualities like detail, imaging and front to back or side to side soundstaging, but the balanced mode really excels in all these areas and the end result is something that is just more musically satisfying to listen to, but not in what you would consider to be (to many people at least) the normal audiophile or hi-fi way.

I think the lack of noise in balanced mode is critcal. While I've never been a "detail" chaser (I think that many systems that really chase the detail rainbow end up sounding clinical and mechanical and not very appealing over the long term), I think the Aqvox is actually more detailed, but also more relaxed and natural sounding at the same time in the balanced mode. "Natural" is a really great way of describing the sound. I've found that many systems/products that I've listened to in the past that are detailed are unpleasant in the long run; I'm only guessing that there is some kind of high frequency noise that hypes certain qualities/types of sounds with many of these systems and/or phono stages to give a sense of detail but ultimately it just ends up being irritating over the long term. I just don't find that in balanced mode with the Aqvox; everything seems to emerge from a very quiet background/acoustic and it is very even handed.

I'll take smoothness/refinement over what I'd describe as detail/excitement any day and to this end I've changed the power cord on the Aqvox-it is detachable-(not with anything really exotic-a $75 DIY power cord done with DH Labs power cable terminated with Marinco connectors) to tailor the Aqvox to be slightly warmer (it was described as being just a little bit "thin" compared to the much more expensive-and also balanced-Ayre phono stage by one reviewer who did a head to head with them), but don't get me wrong: I don't want "slow and syrupy", just something on the slightly warmer side of neutral.

Ironically, the one criticism that I have of the Aqvox is that it seems to be more susceptible to power line noise than previous phono stages that I've used. I have no dedicated lines, but do have the Aqvox plugged into a decent line conditioner (as I've had other stages plugged into). I'm not sure that the switch mode power supply on the Aqvox contributes to this, but I can hear (slightly and when music is not playing) when things like the refrigerator compressor or the furnace motor kick in. This manifests itself as a faint kind of clicking noise through the phono stage without music playing when such a device kicks on, but I do not hear anything with music playing. Nonetheless, this does seem a bit at odds with my description of the Aqvox as being incredibly quiet. To be fair, my Aqvox is a MK 1 and the MK II version is supposed to be further improved in terms of being even more quiet; perhaps Aqvox has addressed power line noise coming through in the new version. In any event, it is a minor criticism from me and not something that would have me looking at other phono stages.

Hope this helps and I'm curious to hear about your experiences with the balanced unit you have coming for review.

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