few phono preamps with balanced inputs .. why?

[tresaino]

Hi analogous, you know I actually like tubes, but there is not one single tube in my circuit !

And steerpike, thanks for the offer to eventually post your paper, you must have felt to be off the hook.

I spent the evening yesterday reading this whole thread again (I printed the thread a few months ago and then filed it, so that was easy), and actually think I know the way forward: have the + and - leads go through the step-ups without any connection to the ground of the arm nor of the turntable, nor to any metallic screening parts of the cartridge (the last should be already allright). All the cable from the cartridge to the preamplifier must have the active + and - leads independent from the ground and from the screening of the cable.

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.

Steerpike, I found your earlier comment interesting because it raises the question whether it is actually worth changing the Bent step-up transformers into XLR. Cable length and therefore noise would not be a problem, as my second TT (a splendid Thorens TD 124 with FR-64S and B-60 currently in assembly) will be rather close to the phono preamp. But I think it is still worth doing, because I am currently obliged to use the phono preamp in RCA output mode as the power amps are not balanced. Not yet, because eventually they will be, so the whole chain from the cartridge to the amps will be fully balanced. So better start preparing for the future now.

[analogous]

tresaino, my bad. Evidently mixed you up with someone else. And here you think you know somebody

[Steerpike_jhb]

I've put up the *start* of the theory on balanced interconnects here:

http://web.eject.co.za/s8nspawn/hifi/bal/balanced.htm

.. if anyone wants to comment on the format or degree of simplicity. I'm not sure what level of complexity people are looking for in it.

(You may not like the colours! I'm editing it still, so the colours are those that are best for my eyes. I can re-colour it when it's all finished.)

[tresaino]

Oh my, this will be weekend reading for me, am too busy during the week. I would suggest inverting the colors, it makes it easier to print out and read anywhere. Actually, will read in the train to Paris on saturday, there's a hifi show there and haven't visited one for a couple of years.

And hey, thanks!

[tresaino]

Steerpike, thanks for this useful background, the first part is Carl Sagan, the second one a bit less so. If I may I would have a couple of – probably stupid – questions:

“Some authors claim that balanced line amplifiers inherently cancel even-order harmonic distortion. This may or may not be true: it is a consequence of the specific circuit design, and not an automatic consequence of it being balanced.”

Does this mean the claim is wrong? Or can balanced circuits actually really help in canceling even-order harmonic distortion?

Also, could you explain briefly what the 600 Ohm standard is all about? For example, does this have any implications for the selection of output and input impedances on pre and power amplifiers? There is no standard for those in XLR electronics, or is there?

A final comment: I would have perhaps added that XLR connectors appear superior to coaxial ones, but people may have different views.

[analogous]

A balanced circuit, consisting of +, - (i.e. inverted +) and ground, will reduce even order harmonics. To what degree comes down to how symmetrical the design is. And it will accumulate odd order harmonics.

The balanced circuit is far from the best topology to reduce distortion. An active load reduces all harmonics evenly. You can use both. And then you have negative feedback. 20db NFB, which is nothing for solid-state, reduces distortion by a factor of 10.

Distortion is not important. What matters is noise-floor (i.e. thermal noise). Not so much for MM, since you would have to be a klutz to design a MM phono preamp with too high self-noise. But it is a challenge for a low-output MC cart. The noise-floor of a low-output cart with an internal resistance of 2ohms is the thermal noise generated by the current through 2 ohm resistance!

600ohms has its origin in telephone lines in the beginning of the previous century. It's only used in audio today and just barely. Everything else is 50 or 75 ohms. 600ohms only means that the output impedance is less than 600ohms. (Transformers are measured in henries and not impedance.) 600ohms into 600ohms means you lose half the signal. The output of solid-state gear is usually in the tens of ohms, often twice that for balanced outputs. The input impedance can be expected to be between 10k and 22k.

No, there's no standard for XLRs. But there is common practice.

Yes, the XLR connector appears to be superior. But then, everything seems to be superior to the lowly RCA connector. The RCA connector was designed as a low-price mass-market connector in the 1930s, when virtually all tube gear suffered from hum and had a frequency response of around 50Hz to 5kHz.

[Steerpike_jhb]

A balanced circuit, consisting of +, - (i.e. inverted +) and ground, will reduce even order harmonics.

Ah... I will stir up a lot of contention now I'm sure.
Certain amplifier topologies cancel even-order harmonics that they themselves generate. No amplifier will cancel harmonic distortions presented to its input, since this implies the amplifier can magically detect distortion from wanted signal & somehow remove it.

Put another way, certain amplifier topologies generate less even-order harmonic distortion than others. This has much to do with the internal symmetry (as Analogous said) of the amplifier and not all balanced-input amplifiers are fully symmetrical, and so do not self-cancel these distortions. But balanced input amplifiers that are symmetrical will generate less even-order harmonic distortion than equivalent quality non-symmetrical amplifiers.

The even-order-harmonic rejection is a property of other factors in the amplifier, and not solely on whether it's balanced.

----

50 and 75 Ohm connections are used for radio-frequency interconnects (and video too). The importance of correct matching here is that the wavelength is short enough that it starts to match the length of the cable. This can lead to standing waves being set up on the cable, which can do damage to transmitters, or rob systems of power - which is important in antenna systems where power levels of a few femto-Watts are being detected.

Impedance matching is generaly not critical in audio since the wavelengths are very long, and signal levels are almost always high enough to make the cable loss negligeable.
What is usually done is to have the load impedance higher than the source impedance, to prevent overloading the source. Overlooking this can give you hassles if you were to say connect a modern cassette deck (input impedance 33k or so) to a tube tuner - which would have been designed to plug into a tube amp, and would expect a load impedance of 1Meg or greater.

On impedance matching, a notable exception is pick-up cartridges and moving-coil microphones.
Both of these have a significant inherent inductance; if incorrectly matched to the load, the configuration results in a filter - and you lose part of the frequency response.

Tube/Valve power amplifiers are also very sensitive to correct speaker impedance matching.

Recording studios and radio stations still stick with 600 ohm references since absolute measuring units such as the dBv and dBm are defined in terms of a 600 ohm load. When absolute level monitoring is critical, you need to know the load impedance precicely. Not relevant to home hi-fi .

XLR connectors are physically very robust, and withstand musicians & roadies stomping on them regularly. But I don't believe that ELECTRICALLY an XLR is any better than a good-quality RCA.

[tresaino]

XLR connectors are physically very robust, and withstand musicians & roadies stomping on them regularly. But I don't believe that ELECTRICALLY an XLR is any better than a good-quality RCA.

electrically perhaps not as in both RCA and XLR single wires within a cable have to be soldered on to a connector. However, I would have thought that the contact between a male and female XLR connector can be more solid/stable with XLR.

[Steerpike_jhb]

However, I would have thought that the contact between a male and female XLR connector can be more solid/stable with XLR.

Obviously you get crappy RCAs but you also get crappy XLRs. But good quality RCAs - gold plated, machine-turned nickel body, split sleeves etc. - perform exceptionally well. Especially if they aren't being plugged/unplugged twice a day every day of the year, which is a situation XLRs are often subjected to in industry.

RCAs have larger pins too, so in principal, the contact area is greater, supposedly a good thing - particularly when mild corrosion does start to set in. One NICE thing about RCAs is that, if they don't make good contact, you can rotate them a few times in the socket - kind of like 'screwing them in', which scrapes the contacts clean. Cleaning XLR pins is a tedious disassebly.

[ben556473]

I have a pair of Furutech RCAs and they are superb with a filament pin and tiny gold plated allen headed screws for keeping the wires in (no solder joint at all). They do actually sound very good.

[analogous]

No amplifier will cancel harmonic distortions presented to its input, since this implies the amplifier can magically detect distortion from wanted signal & somehow remove it.

Magic to me is cables and connectors that have sound to them and defy all physics known to man.

[analogaudio]

For the balanced thing to work you have to twist the wires together, then you get the benefit of cancellation of interfering signals at the receiving end. If you twist together the thin wires of a tonearm you get a single thicker stiffer wire in a place where that is not wanted, it would interfere with the pivoting motions.

The capacitance between thin wires in a metal tonearm tube is quite low, so is the capacitance of thin shielded cable, phono cartridges are sensitive to the amount of load capacitance. If you use the shielded twin wire format common in balanced interconnects you raise the capacitance and that's not a good idea.

The distance between cartridge and phono preamp input is short, deliberately so, longer cables pick up more interference. In the normally benign domestic environment there is not enough radiated energy in the air to disturb the signals in the normal unbalanced shielded cable, the energy is too weak and the cable too short for there to be a problem, usually, unless you live next door to a taxi transmitter maybe, bad luck.

Interference of the common sort in homes from domestic appliances and lighting is usually conducted to the preamp along the AC supply wiring rather than radiated into the phono cable, this means that replacing the unbalanced shielded phono cable with a balanced shielded one would not fix the problem because the problem is not radiated energy it is conducted energy in the AC supply.

Balanced circuits require either transformers (large, heavy, expensive) or active electronics (small, cheap, noisy) to terminate the circuit from the cartridge. Since there is no benefit from going balanced the penalty becomes significant.

[Steerpike_jhb]

I cannot recall the brand, but I do have somewhere a review of an amplifier made in the mid 80s, with a *transistor* RIAA stage driven from a power supply of +150V/-150V to ensure it would never run into clipping. I'd call that slightly excessive engineering though.

[analogaudio]

"Recording studios and radio stations still stick with 600 ohm references since absolute measuring units such as the dBv and dBm are defined in terms of a 600 ohm load. When absolute level monitoring is critical, you need to know the load impedance precicely. Not relevant to home hi-fi ."


600 Ohms was once used in some broadcast and studio systems but went out in the 1950s I think. In thirty years experience with studios and radio I came across one mixer that had 600 Ohm output impedance everything else used and still use the system of low output impedance and high input impedance so that level is independant of impedance.

That one mixer drove everyone nuts because whenever the system configuration was changed the level rose or fell, a small amount but enough to bother engineer types. The problem was solved by removing the 600 Ohm build-out resistors put there to provide the 600 Ohm output impedance. Low out-high in rules.

I agree the dBm power specification requires knowledge of the impedance and 600 Ohms is the standard there.

Ted