The main problem is the convention of using unbalanced systems with
monophonic 6.35 mm plugs. The pick-ups and microphones are by nature
balanced, but for some strange reason, the electric system in a guitar
is unbalanced.
Typically, all metallic parts, including the strings are connected to
the cold side of the jack. This creates a huge "antenna" i.e. a large
capacitance between the guitar and surrounding electric systems,
including triac controlled stage lights etc. containing 50/60 Hz and
quite high harmonics.
The capacitive reactance will allow some current to flow from stage
lights etc. to the guitar body, through the signal cable shield to the
amplifier power cord ground to the utility company.
This is not a problem in the ideal world with ideal connectors and
zero impedance cable shields. Unfortunately the plug/jack interface
and cable shield (especially in old coiled cords) may have a
significant resistance.
Any capacitively coupled interference current flowing through these
resistances will create a noise voltage drop, which is
_directly_added_ to the audio signal, considerably reducing the SNR.
I guess that everyone using an electric guitar would have experienced
the typical sound of hum with a lot of harmonic, if the plug was not
properly inserted or there was a dry joint in the cable/plug.
Most of these problems can be avoided by using stereophonic jacks on
both the guitar and amplifier and using stereophonic patch cords. With
the pickup coils connected to the L and R poles and the guitar
metallic framework connected to the ring and R and ground connected
together inside the amplifier would avoid any capacitively connected
currents from being added to the signal, even with a bad connection on
the shield.
Perhaps Paul (and certainly I) are a victim of that
A little learning is a dangerous thing; drink deep, or taste not the
Pierian spring: there shallow draughts intoxicate the brain, and
drinking largely sobers us again.
thing.
I'm surprised no-one has yet said "what's this obsession with low noise?
An E-guitar isn't a Stradivarius!"
The whole issue is (or I'm making it) a bit more complicated than it may
seem.
Pro audio (not that I'm a pro) is mostly digital these days. My digital
audio workstation (aka computer) likes 192 kHz 24bit input. That may
seem like overkill considering that I don't even hear up to 20 kHz
anymore BUT...
Guitar pickups (resistive/inductive) in combination with cable
capacitance have their own resonance, distortion and filtering
characteristics (ie. sound) and, in the old days, these even change
depending on what effects boxes you plug into, due to varying load
impedance.
I have not been working with electric guitars for a few decades, but I
was a bit surprised that current "high output" pick-ups produce up to
1 Vrms of output. I was used to have something like 100 mV to the
first tube in the amplifier.
If the self resonance frequency with a reasonable cable (200 pF) is in
the order of 10 kHz, that the coil inductance is more than 1 H, is
this really the current situation ?
The traditional design principle has been to try to keep any
mechanical or electrical resonances out of the frequency range of
interest.
What is the point of using "high output" pickups, if this will cause
some nasty resonance peaks within your passband ?
These days an integrated preamplifier is not a problem (e.g. phantom
powered) , thus there is no need to maximize the output voltage and
hence much lower inductances could be used and hence, the self
resonance peak could be moved well above the audio pass band,
especially when the cord capacitance is isolated from the PU.
If the guitar contains multiple pick-ups, each should have an own
preamplifier with possible mixing potentiometer after the
preamplifier.
However, now, ALL the signal modification (including filtering and
distortion) is supposed to be going on in the computer using amp
modelling, equalisation, artificial distortion etc.
If I'm sampling 24 bits, I'd like the input signal to be as clean as
possible. The ADC wants 5.6 Vpp (full scale). That's differential input,
so each signal is supposed to be 2.8 Vpp, ie. around 1 V rms.
While those chips produce "24 bit" data words, the SNR figures are at
best about 120 dB (20 bits).
The pickups output around 300 mV rms.
or 1 Vpp and 120 dB below that is 1 uV.
1 bit of that is, ummmm, about 18 nV.
-> low noise amps, pre-amp as soon as possible.
If you design a guitar amplifier input stage that different kind of
pick-ups can be connected without any gain adjustments, the real 120
dB dynamic range of currently available "24 bit" might barely be
sufficient.