Post by KeithL on Feb 6, 2014 12:38:07 GMT -5
The XPS-1 itself is very quiet... and so is the provided wall wart. (Because it is a high frequency switcher, any noise that the supply actually produced would be far outside the range of audibility. In other words, there's almost no noise there, and you wouldn't hear it if there was - which is why we chose to design it that way ). The XPS-1 also has a secondary power supply and filtering inside the unit itself. The main benefit to using a battery with the XPS-1 would NOT be that the power supply itself is quieter; the benefit would be that the XPS-1 would be totally isolated from the electrical power system (the wall wart is isolated, but no isolation is perfect).
Hum is a problem with phono circuitry for two reasons. First, the signal levels involved are very low, and so huge amounts of gain must be used. Second, the RIAA equalization required to listen to records adds significant additional boost at low frequencies. So, you're starting with a very low level signal, which is very sensitive to interference, and boosting the low frequencies where hum resides even more. All this means that the grounding on all of your components, and especially the turntable, and the shielding on all of them, is very critical for phono signals. To make matters worse, most phono cartridges work by using magnetic fields - which makes them sensitive to the magnetic fields all around us. Because of this, many cartridges themselves are sensitive to hum, as is the wiring in many turntables, so moving the turntable, or re-routing the wires between it and the phono preamp will often help. Sometimes simply rotating the turntable 90 degrees will help. Likewise, the wires from the turntable to the phono preamp should be kept as short as possible, and as far as possible from power transformers, other equipment, fluorescent lights, and even power lines in the walls.
An important thing to remember (unfortunately) is that hum in a phono preamp usually ISN'T specifically related to the "quality" of the preamp itself; it's simply a matter of synergy between how the individual components are grounded and how they are connected together. (While a poorly designed phono preamp might always hum, there is no such thing as one that is well enough designed or expensive enough that it can guarantee NOT to hum). Batteries may help because they totally avoid a connection to the power grid, and often this can help (but not always). You also avoid the cable between the power brick and the preamp (and, under some circumstances, any wire can act like an antenna and "pick up" the hum that saturates the modern world).
Incidentally, our wall wart sends DC power to the XPS-1, so you really can't get any hum "finding its way in" from the power supply itself.
Hiss is a natural "byproduct" of all electronic circuits (even a six inch length of wire produces hiss, and a phono cartridge produces quite a bit of it - relatively speaking). There is no such thing as a "totally quiet" component; the best anybody can do is to ensure that the noise generated by any particular component is far lower than the signal level - and far lower than you can hear. Hiss is usually more noticeable on a phono preamp because the maximum amount of gain in a phono preamp is typically between 100x and 1000x as much as in a normal preamp (so, when you turn your preamp all the way up, then switch to the phono input, you are actually increasing the gain by several hundred times - kind of like putting your "clean" kitchen table under a microscope). And trying to compare phono preamps by "lifting the arm and turning the volume all the way up" is pointless... because you will definitely hear hiss - but the amount you hear will depends on things like the gain of the amp - which will NOT be the same for the various units you're comparing - so it is not a "fair" comparison.
If you want to do a legitimate comparison of hiss.....
1) play a record
2) turn it up as loud as you normally ever would (and note the level setting)
3) now turn down the volume and lift the arm
4) now return the volume setting to the previous level you noted and listen for hiss
By doing this, you can hear how much hiss there is WITH THAT PHONO PREAMP - WITH EVERYTHING SET AT YOUR NORMAL MAXIMUM LISTENING LEVEL
Now, if you want to try a different phono preamp, repeat steps 1-4 but DO NOT USE THE SAME VOLUME SETTING AS YOU DID WITH THE PREVIOUS PHONO PREAMP.
At step #2 you will probably end up at a different volume setting on your preamp (but it will be the one that delivers the same OVERALL GAIN with the new phono preamp).
The point here is that you want to compare this hiss of various phono preamps when the volume levels on the preamp they are connected to are set so that they each produce the same output level from the same record.
(You may well find that the output level of one phono preamp is such that it is playing loudly when the volume of the preamp it is connected to is set to "2", while the output of another is playing equally loudly when your preamp is set to "6". If so, then they have different gains - but this doesn't suggest that either one is "better". The trick is that you have to compare the amount of hiss each makes WHEN THEY ARE SET TO PLAY AT EQUAL LEVELS. In other words, a phono preamp with lower gain will indeed make less hiss for a given setting of that volume control, but, when you use it, you will have to turn the volume control up higher to reach the same listening level. As long as both can be set so that they play at the levels you want, and sound good doing so, neither is "better" or "worse" because you end up with the volume control on your preamp set higher or lower.)
The S/N (signal-to-noise) spec on a phono preamp is the number that tells you how much hiss there will be.
However, in order to compare that number, YOU MUST USE S/N NUMBERS REFERENCED TO THE SAME SIGNAL LEVEL.
If the reference level isn't specified, or if they are different for two specs, then the specs are NOT comparable.
(If the specs use different reference levels, you can calculate how they compare, but the numbers don't compare directly.)
By any such legitimate comparison, the XPS-1 is very quiet - and far quieter than any actual record which you might play. (People seem to lose track of the fact that the single largest source of noise when playing a record - at least with a good phono preamp - is almost always the surface noise on the record itself; and no preamp can reduce or even minimize that.)