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From: Phil Allison on 14 Apr 2010 09:02

Colin Howarth is a visitor from another planet

>
> If I'm sampling 24 bits,

> The pickups output around 300 mV rms.
>
> 1 bit of that is, ummmm, about 18 nV.


** Nuff said ???




..... Phil


From: Phil Allison on 14 Apr 2010 09:04

"Colin Howarth"
>
> The humbuckers are hum cancelling by design.


** Really ??

Most are nothing of the sort.

What kind do you have ???



..... Phil


From: Paul Keinanen on 14 Apr 2010 17:21
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 ?

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.

From: Colin Howarth on 14 Apr 2010 18:45
In article <82kg6oF5vrU1(a)mid.individual.net>,
"Phil Allison" <phil_a(a)tpg.com.au> wrote:

> "Colin Howarth"
> "Phil Allison"
> "Colin Howarth"
> >>>
> >> > 1. the pickups (Humbucker) have a normal resistance but also a hefty
> >> > inductance. For noise calculations (& op-amp choice etc) is it only the
> >> > resistance that is relevant?
> >>
> >> ** Nope.
> >>
> >> The volume pots on your guitar are the dominant sources of circuit
> >> oise -
> >> only at full setting will residual noise be from the PU. The source
> >> impedance of such PUs is a function of frequency with a large peak in
> >> value
> >> around 5 to 10 kHz - depends a lot on the capacitance of the lead in use.
> >>
> >> Imagine the source resistance to be 50kohms and bandwidth to be 7kHz and
> >> you
> >> are in the ball park.
> >
> > Hmmm. Not sure if you (mis)read the original as
> >
> > "the pickups have normal resistance... is it THE ONLY resistance that is
> > relevant?"
> >
> > whereas what I meant was
> >
> > "the pickups have DC resistance but also a hefty inductance. For noise
> > calculations is it ONLY THE resistance that is relevant (Johnson noise)?"
>
>
> ** Read my reply again - it is a complete answer.


You mean the total noise is sqrt( e.n^2 + (i.n * R.s)^2 + e.t^2 )
where

e.n is the amp voltage noise
i.n is the amp current noise
R.s is the source IMPEDANCE
e.t is the source RESISTANCE thermal noise

So only the DC RESISTANCE is relevant for Johnson noise whilst the
IMPEDANCE (* current noise) gives another contribution and the voltage
noise is another contribution.

:-)


> The winding inductance of magnetic guitar PU's resonates with stray and
> cable C in the audio band so the source impedance rises, plateaus and then
> falls. This high impedance at high audio frequencies means you need an
> op-amp with low current noise - something all FET types have.
>
>
>
>
> >> > For example, the Linear LT1115 datasheet
> >> > refers to source resistance, not source impedance.
> >>
> >> ** That IC should be OK - but the best op-amps for magnetic guitar PUs
> >> are
> >> low noise FET types cos the source impedance varies from 10k to 100
> >> kohms.
> >
> >
> > The LT1115 has extremely low voltage noise,
>
> ** But high current noise which dominates if the source impedance is more
> than a few thousand ohms.
>
> Read my reply again.


Looks like Analog's ADA4627 is really nice (and slightly expensive)

e.n (@ 1 kHz) = 6.1 nV/rtHz
i.n (@ 100 Hz) = 1.6 fA/rtHz
THD+N (@ 1 kHz, A.v = 1) = 0.000045 %
CMRR = 116 dB
PSRR = 112 dB
SR = 56+ V/�s

Those are comparable values to NatSemi's LM4562 I was thinking of using

e.n (@ 1 kHz) = 2.7 nV/rtHz
i.n (@ 10 Hz, 1000 Hz) = 3.1 , 1.6 pA/rtHz
THD+N = (A.v = 1) = 0.00003 %
CMRR = 120 dB
PSRR = 120 dB
SR = 20 V/�s

except that ADI's i.n is 1000 times better :-)



> >> Use the basic non-inverting stage.
> >
> >
> > Do you mean the "Single-Ended Input Buffer with Dedicated Reference
> > Pins" referred to in Cirrus's AppNote, AN241? Or the "Single-Ended to
> > Differential Input Buffer"? Or something else?
>
>
> ** I mean use a non-inverting stage.


Oh. You mean use a non-inverting stage?

How will using a non-inverting stage (say, with a gain of +3) give me
the differential input that the CS5381 requires?

--colin
From: Colin Howarth on 14 Apr 2010 18:57
In article <hq4eg7$61s$1(a)news-01.bur.connect.com.au>,
"Phil Allison" <phil_a(a)tpg.com.au> wrote:

> "Colin Howarth"
> >
> > The humbuckers are hum cancelling by design.
>
>
> ** Really ??
>
> Most are nothing of the sort.

Well, they're supposed to be, aren't they? I thought that was the point?

> What kind do you have ???

Some sort of DiMarzio/Ibanez thingies.


--colin
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