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From: Colin Howarth on 12 Apr 2010 21:14
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.

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.

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


tip ------------------------ +
| |
R |
| |
GND C
| |
R |
| |
sleeve ------------------------ -



or do I have to do the dual op-amp single to differential conversion
thing?


Thanks,

colin
From: Phil Allison on 12 Apr 2010 21:34

"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 noise -
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.


> 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.


> 2. the guitar output is single ended.


** So one connection is ground and PSU common and the other is signal.

Use the basic non-inverting stage.



..... Phil




From: MK on 13 Apr 2010 04:25

"Phil Allison" <phil_a(a)tpg.com.au> wrote in message
news:82i020FkcrU1(a)mid.individual.net...
>
> "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.
>
>
>> 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.
>
>
>> 2. the guitar output is single ended.
>
>
> ** So one connection is ground and PSU common and the other is signal.
>
> Use the basic non-inverting stage.
>
>
>
> .... Phil
>
>
>
>
Look at the Cirrus schematic for the evaluation board for this chip and use
that as a reference design. It is NOT easy to get these converters to work
at anything like the full quoted performance - you will need very careful
attention to board layout, component types etc.

I found them to be very sensitive to noise on the VQ pin and that much
larger decoupling caps on that pin were needed than Cirrus suggest.

Good luck !

Michael Kellett


From: Colin Howarth on 13 Apr 2010 15:20
In article <82i020FkcrU1(a)mid.individual.net>,
"Phil Allison" <phil_a(a)tpg.com.au> wrote:

> "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 noise -
> 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)?"



As it happens I leave the volume pot on maximum. The resistance is then
8.5 k. Say, do people remove the volume (and tone) pots? They seem to be
useless. (I think the tone control is just a variable (500k) resistor in
series with a 22 nF capacitor to ground ? - it's hard to tell from the
"circuit diagram", http://www.dimarzio.com//media/diagrams/C.pdf , which
looks like something a 6 yr old would draw :-)

The cable is coax, with 76 pF/m and 24.4 m�/m.


> > 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 NatSemi's LM4562 seems
to have much lower overall THD+N.

By the way, what do they mean by "Differential Input Impedance (30 k�)
and CM Input impedance (1 G�)?


> > 2. the guitar output is single ended.
>
>
> ** So one connection is ground and PSU common and the other is signal.


OK. One connection is ground and the other is signal. I suppose one
could one day add electronics in the guitar body for true differential
signalling...

Maybe there's no point if one just plays wham, bang, wham, bang, bang,
wham and feeds that into phase distortion and bitcrusher units :-)


> 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 have to study a bit to prepare my next question (which will be "how
many amplifier/buffer stages should I use?" :-)

--colin
From: Colin Howarth on 13 Apr 2010 15:30
In article <ccKdnRniis3zt1nWnZ2dnUVZ8gqdnZ2d(a)bt.com>,
"MK" <mk(a)nospam.please> wrote:

> Look at the Cirrus schematic for the evaluation board for this chip and use
> that as a reference design.

I'm looking at the reference design, the evaluation board and some other
circuits...

> It is NOT easy to get these converters to work
> at anything like the full quoted performance - you will need very careful
> attention to board layout, component types etc.

That's why I'm checking here on board layout, component types etc. :-)

Some text books pretend that analogue stuff isn't really black magic at
all, if one grasps a couple of basic concepts. But we know better, hey?
:-)

> I found them to be very sensitive to noise on the VQ pin and that much
> larger decoupling caps on that pin were needed than Cirrus suggest.

I'll remember that. Their appnote, AN241 has one topology with an opamp
just for buffering VQ. The whole input buffer "provides proper biasing,
isolation from the switched capacitor currents, low output impedance,
and anti-alias filtering."


> Good luck !

Thanks :-)
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