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From: Colin Howarth on 14 Apr 2010 19:13
In article <hq4edg$5rh$1(a)news-01.bur.connect.com.au>,
"Phil Allison" <phil_a(a)tpg.com.au> wrote:

> Colin Howarth is a visitor from another planet


Damn! How did you guess?!!!


> > If I'm sampling 24 bits,
>
> > The pickups output around 300 mV rms.
> >
> > 1 bit of that is, ummmm, about 18 nV.
>
>
> ** Nuff said ???


My calculations are wrong? But to err is human is it not?


All I meant was that (for fun) I'd like the pre-amp to be as low noise
as possible and that means it should be as close as possible to the
pickup, and that fully differential signalling would be nice.

--colin
From: Colin Howarth on 14 Apr 2010 19:53
In article <suacs5t4gtski9d0foo1nuh3lmj942h5r5(a)4ax.com>,
Paul Keinanen <keinanen(a)sci.fi> wrote:

> On Wed, 14 Apr 2010 12:21:39 +0200, Colin Howarth <colin(a)howarth.de>
> wrote:
>
> >In article <82khg3Fc4vU1(a)mid.individual.net>,
> > "Phil Allison" <phil_a(a)tpg.com.au> wrote:
> >
> >> "Paul Keinanen"
> >>
> >> > In order to avoid noise and hum problems, I would definitively want to
> >> > build the amplifier into the guitar,
> >>
> >> ** Why ?
> >>
> >> Don't you believe that a few metres of well shielded co-axial cable can
> >> deliver the signal from a guitar PU without introducing hum or noise ?
> >>
> >> It's the guitar's PUs ( and any unshielded internal wiring) that are
> >> sensitive to electric and magnetic hum fields, not the connecting cable.
>
> 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 ?


A quick google shows that Fender pickups are between 2.4 and 4 H which
would put the resonance at around 5 kHz. Don't know about my particular
pickups.


> 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.


.... which it would be with a pre-amp. Then high output would give a
better SNR?


> 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).

How's that SNR arrived at? Is that an "in practice" figure?

> >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.

It's all just for fun. And for my pickups.

Thanks for the detailed post.

--colin
From: Phil Allison on 14 Apr 2010 21:59
Colin Howarth is a visitor from another planet
>
>
> Damn! How did you guess?!!!


** Having two heads is a dead give away.



>> > If I'm sampling 24 bits,
>>
>> > The pickups output around 300 mV rms.
>> >
>> > 1 bit of that is, ummmm, about 18 nV.
>>
>>
>> ** Nuff said ???
>
>
> My calculations are wrong?


** Plain stupid.


> All I meant was that (for fun) I'd like the pre-amp to be as low noise
> as possible and that means it should be as close as possible to the
> pickup,


** Yawnnnnnnnn....



..... Phil




From: Phil Allison on 14 Apr 2010 22:07

"Paul Keinanen lives in a crater Mars "

Phil Allison:

" Don't you believe that a few metres of well shielded co-axial cable can
deliver the signal from a guitar PU without introducing hum or noise ?

It's the guitar's PUs ( and any unshielded internal wiring) that are
sensitive to electric and magnetic hum fields, not the connecting cable. "


> 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.


** What a load of absolute and utter TWADDLE !!!!

Not one single word of is fact.



..... Phil



From: Paul Keinanen on 15 Apr 2010 02:33
On Thu, 15 Apr 2010 01:53:57 +0200, Colin Howarth <colin(a)howarth.de>
wrote:

>>
>> While those chips produce "24 bit" data words, the SNR figures are at
>> best about 120 dB (20 bits).
>
>How's that SNR arrived at? Is that an "in practice" figure?

I was just trying to point out that 24 bits would theoretically be
24*6=144 dB. In practice the best claims in manufacturer data sheets
for ADCs and DACs is about 120 dB, typically with 192 kHz sampling
rate but measurement bandwidth to something like 10 Hz - 20 kHz (so
there is some noise shaping :-).

As others have pointed out, the layout is critical, probably requiring
a multilayer board, perhaps one or two contiguous ground plane layers
etc.

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