few phono preamps with balanced inputs .. why?

[tresaino]

the build quality and the choice of internal parts is more important than adhering to a single dogmatic view that only encompasses one aspect of a whole raft of considerations.

Hi LPspinner, no dogmatism intended from my side, we’re only trying to find out what’s behind the story of balanced versus unbalanced phono preamps. The list of false debates is already too long, push-pull against single ended, tube versus solid state, analog versus digital, and so on. Fortunately some of this debate takes place elsewhere, not here.

I also agree with you that comments by manufacturers need to be read with a grain of salt. However, I assume you would agree that this is also true for comments made in a public forum such as this one.

Now, thanks to steerpike and others we are now learning that carts do actually not deliver balanced signals, so the question comes up whether it really makes sense to choose a fully balanced design in a phono preamp. Heck, does this mean that all the manufacturers and magazines have been BS-ting us stupid, technically unexperienced, audiophiles? Is this the whole truth, or is there perhaps something else that we have overlooked in the debate so far?

This is very confusing for those of us who are not electrical engineers. Mr. Karsten is saying the opposite of what most others say. I'm referring to using the cartridge as a differential balanced source, and a balanced system using fewer parts.

I believe ifleib has a point here. I can only take note that the engineering world is not too different from ours, people don’t necessarily agree on all the choices one can make. I would love to see a reply from Karsten-Athmasphere to one of the key points raised in this exchange - that carts are actually not balanced as is often claimed.

Anyway, here’s the graph from the Italian mag that I had promised. The article actually started with a good question: if one starts from the assumption that normal line-out line-in connections have less degradation because of balanced cabling and plugs, does it not make even more sense to use this type of balanced connection also between carts and phono preamps, where the signal is even smaller and therefore more sensitive to interference?

Here’s the graph:



What they did: they put the same signal through a balanced cable into the balanced and unbalanced MC entries of the AM Audio phono preamp (unbalanced by adding XLR-RCA plugs to the same cable). The cable length was not specified, but it was long enough to be put on a TV cathode tube monitor before reaching the preamp. The results are astounding, particularly if one looks at the two graphs on the right, when the TV monitor was on.

Now, one could perfectly argue that no serious audiophile would be foolish enough to use a phono interconnect that is long enough to cross a TV monitor on its way to the phono preamp. Agreed. However, it still give us food for thought.

It appears that balanced connections and phono stages can have advantages despite the fact that carts are actually not balanced as is often wrongly claimed.

[ben556473]

Hello, could you be kind enough to explain the graphs a little as I cannot make out the scale and also I'm a little uncertain as to which is balanced and balanced but with a unbalanced connector at one end. If abilanciato is unbalanced then it shows a lower reading (noise?) than the bilanciato (balanced) at all points bar the first twothirds of the first square (going left along the X axis) without the monitor. With the noise source connected the balanced shows better results.
Am I right in saying this experiment is meant to show that the balanced cable is better at carrying a signal through a noisey environment? If so it does but also shows that an unbalanced cable (balanced but with unbalnaced connector at one end) is better at carrying the signal in a non noisey environment by a small amount.
Have I missunderstood? Is the monitor placed near the cable to induce noise in it as the signal passes the monitor?

[LPspinner]

Hi Guys:

First of all to draw a final decision based on these graphs alone without fully understanding the method of data collection would amount to bad engineering practice - however: here goes.

Looking at the graphs I think the “Y” axis starts at -120db at the origin and reads -120db, -100db -80db, -60db, -40,db -20db and 0db at the uppermost graduation. Along the “X” axis it looks like it reads 0 KHz, 2.0 KHz, 4.0 KHz, and 6.0 KHz. 8.0 KHz and goes up to 20 KHz in 2 KHz steps. According to my very poor school boy Italian the graphs on the Left hand side are with the noise source turned off (in this case a Monitor). The upper Left hand graph is unbalanced (abilanciato) and the lower Left hand is Balanced (Bilanciato) inputs (ingresso).

The Right hand side graphs display the results with the noise source (the monitor) turned on. Again the Upper Right hand shows the unbalanced input and the lower Right hand shows the balanced configuration. In my experience a monitor seems to represent a fairly extreme example of induced noise and certainly not typical of the sort of environment a phono lead would encounter (Again not knowing the exact methodology I would like to preface my conclusion with “based on what I think they are measuring”). It is also interesting to observe that with the monitor turned on the increase in the noise floor on the unbalanced cable is around 4-6db above that of the balanced cable for frequencies above 4 KHz. Below 4 Khz the general trend of the two right hand graphs is pretty much the same. Given that the noise floor above 4 KHz is already at about 95db the variance seems pretty academic in my book and most listeners would not be aware of the difference when you consider that the general noise floor of a typical high quality vinyl play back system could not hope to achieve these sorts of figures.

It is also interesting to note the quietest of all four graphs (although not by much) is the top left hand graph, the unbalanced input with the noise source turned off. At the end of the day it appears that the best solution is to pay careful attention to cable routing and to keep the cable as short as possible there by eliminating the added complexity of the balanced input circuitry.

As I discussed in my previous post, balanced configuration is good for a recording studio or other harsh electrical environments but I think in the domestic audio situation the added complexity required for balanced operation can counteract any potential advantages.

Tresaino wrote:
The list of false debates is already too long, push-pull against single ended, tube versus solid state, analogue versus digital, and so on. Fortunately some of this debate takes place elsewhere…

Nothing personnel Tresaino but in my book these debates were closed along time ago …. Single ended triodes, folded horns and valves were great back in the days before we had better technology. These days we now know how to manufacture stable and robust complementary NPN and PNP bipolar devices that provide a very good low distortion amplifier with low output impedance and the ability to deliver large amounts of current into difficult loads. I see no need to use the old technology when the newer can do it so much better and can do for less money... Sorry I’ll get off my soap box now.

LPSpinner

[tresaino]

Hi to all. And thanks to LPspinner for having responded to Ben - both for the interpretation of the graphs and for the translation. Congratulations on your Doctor Honoris Causa in Italian!

Am using my lunch break to read this, will check the magazine again tonight at home. The only thing I recall for certain is that the folks in the magazine concluded that balanced was somewhat better also with the monitor turned off - although looking at the graphs as a non-technical guy I had your same impression, that the differences non-balanced and balanced were very very small. If I look again carefully, then balanced+monitor off (left bottom) appears to be very slightly better than unbalanced, although the word "better" is probably wrong here. Am having my glasses on right now: LPspinner, are you sure the top left graph gives better results than bottom left?

However, comparing the two bottom graphs tells us that balanced (bottom right) made quite a difference when the monitor was on. That said, I would agree with LPspinner that this scenario is not very realistic.

And I would also agree with you on the soap box comments.

[Steerpike_jhb]

Now, thanks to steerpike and others we are now learning that carts do actually not deliver balanced signals

No! I didn't say that! I said precicely the converse!
A phono cartridge is indeed a true balance source. Although as confirmed, some like Shure's M97 use one of the pins as a screen in the assumption that it will be connected in an unbalanced configuration. This makes it impossible to use in a balanced configuration.

I don't know how else to describe why it's truely balanced, other than my previous post in which I described the creation of a virtual ground half-way along the cartridge winding. You do not need a hard-wired earth/ground wire for balanced operation; in fact in most cases a 'centre-tap' ground connection is undesirable, and fidelity (phase and frequency linearity) is better if you allow the circuit to self-balance.

Would you accept moving-coil microphones as true balanced sources, when wired up as such. These have no centre-tapped coil. The generator is inherently balanced - just like the coil in a cartridge.

Th 300-Ohm feeder to tuners is balanced - that has no grounded centre tap.

Look at the schematics of any number of studio tape decks, CD players etc. which feature transformer balanced outputs: the output transformer is a single winding with no centre tap. A hard-wired centre tap serves no purpose (refer to the electrical theory in my earlier post) and can cause other undesirable effects.

If you still need to see it with your own eyes, you'll have to borrow a dual trace oscilloscope: connect one channel of the cartridge pins (+) and (-) to the 'scope probe inputs 1 and 2, load the cartridge correctly with a 47k 1% resistor. Don't connect the 'scope probe screens at the cartridge end.
What you will see on the screen is two identical, but oppositely phased signals - proof indeed that you have a balanced source.

[tresaino]

No! I didn't say that! I said precisely the converse! A phono cartridge is indeed a true balance source.

Oops, sorry I misunderstood you! Truth is that your 28 October response to Ifleib was not openly taking issue with his statement that:

I really don't understand how anyone can say that the output of a phono cartridge is balanced. I know that this was already stated in this thread, but it was not addressed. With one hot lead and one ground per channel, the output is obviously unbalanced.

And LPspinner agreed and reinforced his view:

Yes quite right lfleib. I fail to see the advantage of using a balanced input for a phono stage when nearly all phono cartridges use a single coil for each channel and therefore unbalanced by nature.

The problem is that many of us don’t have the necessary background to understand all the technical nuances here. I’m more a listener and empirical guy like blakep, trying to translate some of the technical explanations and differences of view into popular and human jargon. Well, I utterly failed this time round.

If you still need to see it with your own eyes, you'll have to borrow a dual trace oscilloscope: connect one channel of the cartridge pins (+) and (-) to the 'scope probe inputs 1 and 2, load the cartridge correctly with a 47k 1% resistor. Don't connect the 'scope probe screens at the cartridge end. What you will see on the screen is two identical, but oppositely phased signals - proof indeed that you have a balanced source.

Will definitely do so in December, when I will pay a visit to the AM Audio factory.

In the meantime, I called the factory today, had a short friendly chat with the engineer who created the MM-MC Reference preamp, and invited him to read this interesting exchange and perhaps comment on some of the issues raised. I hope he will find the time!

[Steerpike_jhb]

I'll try and write up a simplified explanation with diagrams and some elementary maths in the next few days.

[ben556473]

Here is a good place to start, I often go searching the net to read up on stuff that's being mentioned here.
http://en.wikipedia.org/wiki/Balanced_audio

[lfleib]

From a theoretical point of view, wouldn't the potential benefit of balanced audio also have the potential for losses? Because electrical parts, connections and wires (and their lengths) differ, at least to some small degree, couldn't some information, not equally transmitted, be rejected as noise?

Balanced audio obviously works, but considerations are different in home than in studio. I don't have an answer, just more questions.

[ben556473]

I think the answer lies in what it was designed to do.
If your listening room is full of guitar amps that are on and lots and lots of other electrical equipment that is on while you are listening then balanced connections are required, if however you do not have this situation then unbalanced connections will offer a better sound, and cost you alot less money. Which could be spent elsewhere in your system.

[LPspinner]

.....

And LPspinner agreed and reinforced his view:

Yes quite right lfleib. I fail to see the advantage of using a balanced input for a phono stage when nearly all phono cartridges use a single coil for each channel and therefore unbalanced by nature.

....

While I will agree that that a phono cartridge can be configured to work into a balanced input if you wire the coil outputs across the Positive “Hot” and the Negative “Cold” balanced inputs and leave the coils to float above the earth potential but I fail to see how this configuration offers the advantage of a true dual signal “balanced” complementary voltage source that allows cable borne interference to nulled out when the complementary signals are summed at the input end of the pre-amp or phono stage. To my mind this is the only reason you would bother with a balanced configuration in the first place.

Also consider that Most Moving magnet cartridges will have one end of their coil assemblies earthed to the metal cartridge body in order to shield the coil assembly from noise and the effects of the outside electrical environment. This arrangement makes the “float above earth” quasi-balanced configuration no longer achievable.

Low output Moving Coil cartridges are a different case altogether, due to their very low source impedance (usually less than 10 turns of wire around the armature), the Moving Coil cartridge will act as a curent source rather than a voltage source and because of the low impedance most noise generated within the cable will be shorted to earth.

As a side issue it is also worth pointing out that with Moving Coil cartridges most noise issues are generated within earlier amplification stages due to the high levels of gain required and not from in noise induced into the cable form an electrically noisy environment for the same reasons listed above. However these days internally generated noise issues are no longer an issue as modern electronics will easily provide very quiet Moving Coil head amps.

I think here we are confusing two different conditions when it comes to our discussion about balanced configurations. Steerpike_jhb is talking about the impedance matching conditions and considering the cable transmission system as a “transmission line” in order to optimise the curent and voltage relationships within that transmission system and from this perspective his theory is quite sound and valid. I would consider this type of balanced operation as an “impedance-balanced line”

In the situation I am considering as balanced I am referring to the fact that two symmetrical voltages are required with respect to an earth or Zero potential in order to cancel, null, or balance out induced interference within a cable carrying a signal. As I said above most Moving Coils have an output impedance that approaches zero, so in my book considering this as an “impedance-balanced line” doesn’t seem quite right, at least not without introducing a transformer or impedance matching head amp at the cartridge end.

LPSpinner.

[Steerpike_jhb]

but I fail to see how this configuration offers the advantage of a true dual signal “balanced” complementary voltage source that allows cable borne interference to nulled out when the complementary signals are summed at the input

That is EXACTLY what DOES happen. Perhaps when i've written it up with some simple maths it will become apparent.

Steerpike_jhb is talking about the impedance matching conditions and considering the cable transmission system as a “transmission line” in order to optimise the curent and voltage relationships

No, I'm talking about the cartridge generating two antiphase signals relative to the pre-amplifier ground. Hence all noise picked up by the cable is seen as Common-mode, and strongly rejected by the amplifier.
At the sort of frequencies used in audio, transmission line principles are irrelevant - unless you plan on running an audio cable several kilometres.

In the situation I am considering as balanced I am referring to the fact that two symmetrical voltages are required with respect to an earth or Zero potential in order to cancel

And that is exactly what a cartridge gives you, EVEN THOUGH THERE IS NO CENTRE TAP ON THE COIL. YOU DON'T NEED A CENTRE TAP, and as I wrote before, a centre tap on the balanced source can create problems and so is more often than not, omitted.

[ben556473]

I thought the two signals coming from a stereo cartridge were different and so could not be used to cancel interference. Please excuse me if I'm showing a lack of understanding of what is being proposed.
In fact I'm going to do some more reading as I can't understand what is being said here, I have assumed, up until now, that the output from a cartridge is not symetrical in anyway not even across the X-axis.

[lfleib]

Hi Steerpike_jhb,

Thanks for your posts on this subject. I see how a phono cartridge can be used as a balanced source, just like a microphone (except those that are internally grounded).

My question now is about the advantages of using a balanced configuration in a phono stage. If you're trying to make the circuitry sound as good as possible, for any given price point, wouldn't the simpler path with fewer higher quality parts, tend to sound better?