Low noise guitar pre-amp / ADC

"Colin Howarth"

** 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.

** 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. ** 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

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", , which looks like something a 6 yr old would draw :-)

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

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?)?

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 :-)

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

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

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'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."

Thanks :-)

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 ?

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.

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.

"Colin Howarth" "Phil Allison" "Colin Howarth"

** 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.

** But high current noise which dominates if the source impedance is more than a few thousand ohms.

Read my reply again.

** I mean use a non-inverting stage.

.... Phil

"Paul Keinanen"

** 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

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.

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

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

The humbuckers are hum cancelling by design.

not really

Thanks,

--colin

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

Colin Howarth is a visitor from another planet

** Nuff said ???

.... Phil

"Colin Howarth"

** Really ??

Most are nothing of the sort.

What kind do you have ???

.... Phil

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.

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.

While those chips produce "24 bit" data words, the SNR figures are at best about 120 dB (20 bits).

or 1 Vpp and 120 dB below that is 1 uV.

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.

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.

:-)

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/µs

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/µs

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

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

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

Some sort of DiMarzio/Ibanez thingies.

--colin

Damn! How did you guess?!!!

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

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.

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

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

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

Thanks for the detailed post.

--colin

Colin Howarth is a visitor from another planet

** Having two heads is a dead give away.

** Plain stupid.

** Yawnnnnnnnn....

.... Phil

"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. "

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

Not one single word of is fact.

.... Phil

"Colin Howarth is a very sick man "

** Why don't you just f*ck off and die ??

.... Phil