few phono preamps with balanced inputs .. why?

[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.

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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

[Werner]

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.

Darn. In my book 20% is a small improvement: nice to have, nothing to lie awake for.

And I was that reviewer ...

[blakep]

Sorry if I misinterpreted you Werner. I felt that 20% was fairly significant and some other reviewers have stated, without numbers, that the performance is obviously enhanced in balanced mode.

Having had a chance to audition the Aqvox before buying it, I can say that I doubt I would have purchased it if I'd been unable to use it in balanced mode. There was something clearly special happening there, although it's difficult to say just how much of that can be attributed to balanced operation versus the current mode when using the XLR's. It's quite possibly a combination of both features and having a cartridge that matches up well with the current mode as well.

[Werner]

Well, I'm definitely not one of those reviewers who deal in hyperbole


Ever noticed how often in one article a certain UK editor/reviewer uses the adjective 'far', and not in the context of measuring a distance?