few phono preamps with balanced inputs .. why?

by **Steerpike_jhb** » 07 Nov 2007 18:54

Hello Lflieb

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?

Ah, there's the rub, isn't it!? I've been trying to explain here all along that the reason for using balanced lines is to eliminate noise picked up on the cable. If that isn't the source of the noise, then there is no point in invoking a technology designed to fix it.
I did write before that balanced systems are more complex and so have the potential to introduce more noise in the amplifying stages.
This is offset by the fact that balanced systems are usually intended for markets where budgets are very high (broadcast), so they are better designed and use higher quality parts (A Studer doesn't sound better than a Sanyo BECAUSE it uses balanced inputs).

Ben556...
We are talking about each channel individually here, not L and R. EACH channel is routed along two symmetrical conductors with a common screen.

by **tresaino** » 07 Nov 2007 20:03

thanks a lot steerpike, these exchanges are now really taking us a step further in the debate. =D>

by **ben556473** » 07 Nov 2007 20:28

I understand the principle of balanced audio but cannot see how the two equal and opposite signals per channel are taken from the cartridge. If the proposal is that an out of phase signal gives us the second waveform then it would not be equal to the first as it is created from a different part of the groove due to the passage of time.
If you are saying the single channel of the cartridge produces two indentical signals I can't see how. It is the acceleration of the coils through the magnetic field that determines the nature of the signal and this is changing all the time to the point that there is no sinesoidal waveform at all.
Also to say that the signal created by the motion of the 'first' windings of a coil are equal to the signal generated by the 'second' winding of that coil is not true either. They occupy different space in relation to the pivot point of the cantilever and thus pass through greater distance in the magnetic field for the same piece of music.
Help! This is confusing me, in fact I'm going down the gym and forgetting about it for a while.

by **lintwind** » 07 Nov 2007 22:08

Hi Ben,
i think guys are saying that hot(+) and cold(-) (on exit, regardless of field geometry done in stacked windings of the coil, thus already carrying coil stack processed signal) outs of a single channel become the "in-phase" and the "out-of-phase" (ie inverted phase) respectively, both being hot in this scenario (but of opposite phase) and floating against some other shared "ground", 3-wire per channel, thus making the traditional "ground"(-) of the winding behave like a second "hot" carrying the inverted phase.

ps. sorry, i know very little on the subject of magnetics and electricity, excuse the terminology (mis)used :oops:

by **ben556473** » 07 Nov 2007 23:45

Understood, this is where my confusion comes from. I can't see how one channel can make two outputs that are out of phase and identical at the same time. I must be being stupid. Please ignore me and carry on. :oops:

Thanks for your help anyway, I'm sure it will click in a few days time.

by **Steerpike_jhb** » 08 Nov 2007 17:17

I can't see how one channel can make two outputs that are out of phase and identical at the same time. I must be being stupid. Please ignore me and carry on

No need to ignore you! I'm busy writing up a detailed explanation with diagrams that I should have ready for posting next week.

But the scenario you oulined (two outputs that are out of phase and identical at the same time)... that is EXACTLY what happens, and i'll explain how and why.

by **ben556473** » 08 Nov 2007 18:14

Cheers.

by **Gliding Dutchman** » 08 Nov 2007 19:01

There is a good explination on the Van Den Hul site regarding how to connect a cartridge with two stereo phono pre-amps in balanced mode. very straigh forward indeed and worth while testing with say two Pro-Ject Phonoboxes...

GD

by **tresaino** » 08 Nov 2007 20:47

I remember having read strange stuff on the Van Den Hul website, to be taken in with the usual grain of salt. Steerpike, hats off, look forward to your explanations, thanks! But hey, take your time, and take a break when you are getting fed up, after all this is supposed to be about fun. :wink:

by **bo-bo** » 08 Nov 2007 22:03

Ciao Tresaino,

I stumbled upon your post after pondering already a couple of weeks about a configuration for a phono stage. Indeed there are not many advantages for a balanced phono input. AND compared to a true single ended input, with a differential input (necessary to go balanced) one pays a penalty of twice as much equivalent input noise. And more parts of course (= €). But hey, don't we want to hear what's on the discs?
Anyway I have been thinking, and I knew five balanced stages (by heart) when I saw your remark that no five had been mentioned yet. They are:Vacuum State Electronics RTP3, Einstein "the turntables choice (Bal option)", Borbely EB 804, EAR 834P and Manley Steelhead. The latter two are pure tube, and there many truely balanced inputs for phono inputs can be found: a step-up transformer is very nice for balancing! However the impednce of the primary is effectively in series wit the cartridge, so you pay a penalty noise-wise. Noise pick-up by the lines can be a lot less when using balanced interconnects.
The guys struggling with balanced from a single generator (the cartridge) may try imagining this: The generator at the far end of the tonearm really does not care what wires are connected to its exit pins... It just translates lateral needle speed into voltage. Now if the + and - both have the same impedance to ground, we have a balanced line. this means that (stray?) capacitance, inductance and resistance are alle equal toward ground for the two conductors carying the signal. Whoopps... What the heck?? In practice: if you use a twisted pair inside a grounded (but nowhere electrically connected!) shield you have a balanced line. Have a look at good microphone cable. and dissect a piece. Now follows automatically that the 'normal' coaxial wire often used can never transport truly balanced signals.
Well, is it worth it? I conclude yes: getting rid of the coax and installing a balanced interconnect on a turntable gives advantages most of the time, even with a single ended phono input.
Is a balanced input on the amp worth it? Well, ehh, it depends. sometimes it gives a very elegant solution to biasing problems (e.g. as in mentioned VSE RTP3). something like this is what I will build.

just some thoughts... cheers and enjoy the music!!
Bo-bo

by **bo-bo** » 08 Nov 2007 22:25

lfleib wrote:Hi,
This is very confusing for those of us who are not electrical engineers.
........
I have an unbalanced phono stage that I'm quite happy with. It has a SN ratio greater than 90db with MC or MM, and can drive a power amp with a passive line stage.

Hi lfleib,

I am no electrical engineer either. Nor a rocket engineer for that matter.
You have a phono stage that is REALLY impressive. How do I know?
Well just do the math (yuck, no more...). A 'middle of the road' MC cartridge these days gives 400 microVolt and is twelve Ohm (nominally). Your phono stage has a SN ratio greater than 90 dB. That translates to (ref to 400 microV) a noise voltage of 12.7 nV. Or the thermal noise present (@RT) on a fine resistor of just 0.43 Ohm. Hey, I suspect your stage eats noise for breakfast OR (I begin to fear) you use one of those virtual (digital?) signal sources with negative resistance.

regards,
Bobo :lol:

by **Steerpike_jhb** » 09 Nov 2007 01:42

Thanks Tresaino for the words:

hats off, look forward to your explanations, thanks! But hey, take your time, and take a break when you are getting fed up, after all this is supposed to be about fun

Well, it is proving to be quite an in-depth document already. I needed to bring in the concepts of capacitance and inductance, etc; and for those who have not studied physics, I need to explain what those concepts are and do.

As Carl Sagan said: "If you REALLY want to make an apple pie from scratch, you must first create the universe".

He said a good way of saying profound things. Another one, that I think applies very much to turntable playback theory: "There are no penalties for breaking the laws of physics; the universe is just arranged in such a way that you cannot do it"

by **lfleib** » 09 Nov 2007 05:43

Hi Bo-bo,

The tone arm cables that I've taken apart or repaired, do not look like your "normal" coaxial interconnect. They look more like a microphone cable in stereo, with a fifth wire - the tonearm ground. Because of this configuration, it would be easy to connect them in a balanced mode, using the tonearm ground for both channels.

I am interested in the advantages and disadvantages of doing this. Unless your turntable sits in front or on top of a TV set, or some kind of electrically noisy environment, I think it is probably counterproductive. If you set out to design the worlds greatest phono stage, I think you would want the fewest parts of the highest quality.

Yes, I have a KILLER phono stage. It is an AHT - American Hybrid Technology prototype. The SN was higher than the test instruments available. The resistors are Vishay. The signature version had encapsulated Vishays - the same as in the Space Shuttle. Sonically, it disappears, it's like it's not there. I'm not worried about something being better, or looking to replace it. I just wanted to know how balanced lines apply to phono, and why people with home stereos are jumping on the bandwagon. I suspect that for the same money you could get a better sounding unbalanced unit.

by **bo-bo** » 09 Nov 2007 21:14

Hello Lfleib,

I guess you are right. If you have the phono cord as a dual mike + a ground wire, you can go balanced. BUT, what's there to be gained? You have a hybrid stage, that is unbalanced. So, if you go balanced, you would need an input transformer OR an additional, non-inverting input on your stage. Either option would be costly. And it will make things worse: 3 dB extra input noise for a differential input (because we double the input noise resistance. Not 1 "resistor" to put the signal through, but 2). With a transformer it depends, but at least you add the ohmic (DC) resistance to the noise. Suppose primary were 3 ohms (high end!), that would add 1 dB to the ever present noise of the cartridges noise (at Rcart=12 Ohm, some 93nV over 40kHz of bandwidth). And we have to remember that we would be adding components to lose background hum and radio-noise that in most real world circumstances pose no problem for a single ended input. Lfleib, I am nearly certain that going balanced would NOT be worth it in your set-up. As in MOST turntable-phono set-ups. The transformer coupled input is the radiant exception, though!

In most SE phono set-ups, one better takes up something along the line of a direct uninterrupted wire all the way from cart to phono stage input. Get rid of as many as possible connectors (if there are superfluous connectors left of course). Substituting connectors with solder joints (e.g. at the tonearm base) could be a stepping stone. That's reversible by the way: If you don't like the change, you solder the connector back in. Or have it done by a technician with heat-proof fingers :-)

cheers & enjoy your records,
Bo-Bo