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From: Paul Keinanen on 13 Apr 2010 16:35
On Tue, 13 Apr 2010 03:14:15 +0200, Colin Howarth <colin(a)howarth.de>
wrote:

>Hi,
>
>I'm (trying to) design a dedicated single channel pre-amp / ADC for my
>guitar. This is a hobby/learning exercise.
>
>The ADC will probably be the 24bit cirrus logic CS5381. I'd be sampling
>at 192 kHz.

Is this pre-amp situated within the guitar body or is it in a separate
box ?

If the pre-amp is within the guitar, where is the ADC, is it within
the guitar or as a separate box ?

>A couple of questions:
>
>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? For example, the Linear LT1115 datasheet
>refers to source resistance, not source impedance.

The wire resistance within the pick-up coil will generate some white
noise voltage, when operated above absolute zero (0 K) temperatures.
The unloaded noise voltage can be easily calculated from temperature
and resistance. The loaded noise voltage depends on the load
resistance (some of the noise may be converted to current noise), but
at room temperatures, the noise power density about -174 dBm/Hz can be
assumed.

The inductive or capacitively reactance does not generate any noise at
the amplifier input terminals, but the amplifier itself may generate
more noise when mismatched.

>2. the guitar output is single ended. Should/can I treat this as what
>Analog's SSM2019 datasheet calls pseudo-differential, like this:

Quite nice specifications for that chip.

Do you have room for this amplifier and for at least two, preferably
four 9 V batteries within the guitar body ?

Or would it be acceptable to connect the guitar to the ADC using a
CAT5 cable, one pair feeding +/- 15 V to the guitar and the other pair
outputting the amplified single ended signal plus the signal ground ?

In order to avoid noise and hum problems, I would definitively want to
build the amplifier into the guitar, even if the ADC is in a separate
box.

If the volume/tone controls are required at the guitar, first amplify
the weak pick-up signal and then use potentiometers to attenuate the
signals. If the original potentiometers are designed or high impedance
levels, you may have to change these potentiometers to units with
lower resistance.

From: Phil Allison on 13 Apr 2010 20:18

"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.

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.


>> 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.



..... Phil


From: Phil Allison on 13 Apr 2010 20:44

"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.



..... Phil




From: Colin Howarth on 14 Apr 2010 05:47
In article <eri9s5t8oc5bl58b4t725aqu2n84ighqs1(a)4ax.com>,
Paul Keinanen <keinanen(a)sci.fi> wrote:

> On Tue, 13 Apr 2010 03:14:15 +0200, Colin Howarth <colin(a)howarth.de>
> wrote:
>
> >Hi,
> >
> >I'm (trying to) design a dedicated single channel pre-amp / ADC for my
> >guitar. This is a hobby/learning exercise.
> >
> >The ADC will probably be the 24bit cirrus logic CS5381. I'd be sampling
> >at 192 kHz.
>
> Is this pre-amp situated within the guitar body or is it in a separate
> box ?
>
> If the pre-amp is within the guitar, where is the ADC, is it within
> the guitar or as a separate box ?


Both were going to be in a separate box, but the more I look at it, the
more I think having the pre-amp as close as possible to the pick-ups and
having it generate a differential signal makes sense. I'd then simply
use microphone cable, XLR connectors and phantom power.


> >A couple of questions:
> >
> >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? For example, the Linear LT1115 datasheet
> >refers to source resistance, not source impedance.
>
> The wire resistance within the pick-up coil will generate some white
> noise voltage, when operated above absolute zero (0 K) temperatures.
> The unloaded noise voltage can be easily calculated from temperature
> and resistance. The loaded noise voltage depends on the load
> resistance (some of the noise may be converted to current noise), but
> at room temperatures, the noise power density about -174 dBm/Hz can be
> assumed.
>
> The inductive or capacitively reactance does not generate any noise at
> the amplifier input terminals, but the amplifier itself may generate
> more noise when mismatched.
>
> >2. the guitar output is single ended. Should/can I treat this as what
> >Analog's SSM2019 datasheet calls pseudo-differential, like this:
>
> Quite nice specifications for that chip.
>
> Do you have room for this amplifier and for at least two, preferably
> four 9 V batteries within the guitar body ?


For a pre-amp circuit - yes. For two 9 V batteries - perhaps, but a bit
impractical. For 4 - no way.


> Or would it be acceptable to connect the guitar to the ADC using a
> CAT5 cable, one pair feeding +/- 15 V to the guitar and the other pair
> outputting the amplified single ended signal plus the signal ground ?


See above. Cat 5 cable isn't really flexible enough anyway.


> In order to avoid noise and hum problems, I would definitively want to
> build the amplifier into the guitar, even if the ADC is in a separate
> box.


The humbuckers are hum cancelling by design.


> If the volume/tone controls are required at the guitar, ...


not really


> ... first amplify
> the weak pick-up signal and then use potentiometers to attenuate the
> signals. If the original potentiometers are designed or high impedance
> levels, you may have to change these potentiometers to units with
> lower resistance.



Thanks,

--colin
From: Colin Howarth on 14 Apr 2010 06:21
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.


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.

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.

The pickups output around 300 mV rms.

1 bit of that is, ummmm, about 18 nV.

-> low noise amps, pre-amp as soon as possible.


--colin
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