I am trying to design/build a preamp for a piezo pickup for my guitar, and I just wanted to check a couple things first. It'll just be a simple opamp design. The output Z of the pickup is 1.3 Meg at 100 Hz, so I figure I'll need a bifet like a TL071, with an input resistor of about 5-10 Meg. I want to be able to control the volume from the guitar. Would a pot of 500k be suitable (this is the normal value for a guitar, but do the higher impedances affect this value)? Also, I have seen a 100ohm resistor in series with the output on a few designs. Does anyone know its purpose?
Thanks
Oh - something just occurred to me: I suppose I could also use a JFET instead of an opamp, provided I could get sufficient gain from it. Any thoughts on that?
Ideally, a non - inverting amp with bootstrap feedback to the input to keep the input impedance high. The other thing is to keep the first stage op amp gain low to get maximum headroom, consistent with enough gain. Some ceramic pickups can give a volt or more peak - peak, so you need to make sure that the output doesn't clip worst case. Would think a gain of 10 max for the first stage and run the pickup directly into this to maximise s/n ratio. Then, the level pot goes on the output of the first stage...
Hmmm.... I like the idea Chris, but I can't put the pot in between the 2 stages. The pot will be on my guitar, and the preamp will be outboard, mounted inside my pedalboard. I was thinking of something a little simpler - like 1 stage of gain, but if I do 2, what would you set the gain at for the 2nd stage? Also, if you're thinking about 1V for an output, we're not too far off line level already. Are you sure they get that hot? I've emailed the company that made my pickup, and hopefully they should have some output specs for me.
A bootstrap input scheme is where you have a low impedance input amplifer and use positive feedback to bootstrap, or "follow" the input, thereby effectively increasing the input impedance. It was used in the early transistor record players etc, which used ordinary junction transistors. In the original scheme, the transistor base is biased as per normal, but the upper or lower leg of the bias network is split into two resistors, with a capacitor connected between the transistor emitter and the junction of the two resistors. I think that's right, but difficult to describe when it could be drawn on paper in seconds. The same technique can be used with op-amps by running non-inverting and using a cap feedback between the output of the op amp and a split input resistor to ground. If you are running the op amp at > unity gain, you need to ensure that the overall + gain doesn't exceed unity, otherwise the circuit can become an oscillator :-). The advantage is that the input resistors can be normal range values, rather than megohms.
If the volume control is on the guitar, then I would try a single stage op amp, with or withouit bootstrap feedback. You shouldn't need much gain and the main function of the preamp is to translate high to low impedance, where it's easier to drive lines and control noise etc. One thing to note is that a 100 ohm or so stopper resistor should be in series with the output jack socket, as some op amps don't like driving capacitive loads...
The 500K pot should be OK. Acting against the .001 uF capacitance (estimated) of the the pickup you get a gentle first-order high-pass filter with a knee frequency at 2000 Hz.
I think I would use the normal 500k guitar volume control pot, and build the preamp with a base boost that compensates for the rolled off lower part of the spectrum, to as low as you want to go, since this preamp will be dedicated to this guitar. The amplifier would have an impedance4 a bit higher than 500k, at least at the low end of its response.
Op amp amplifiers often have 100 ohm output resisters added downstream of the feedback components, to isolate the output from the capacitive load of cables, which tends to make the opamps go unstable. At audio frequencies and normal cable lengths, this produces no audible frequency response changes.
In fact, bootsrap circuits aren't noisy if properly designed, but ymmv.
I can only say what we found from experience, though it is over twenty years since I worked in pro audio mixer design. The TLOxx series were well known for instability and a stopper resistor in the output is just good practice. You never know how much cable will be on the output and mic cable is quite capacitive. TL0xx series don't like overly capacitive loads, especially at low gains. As I said, it's just good design practice...
** You have **completely** missed the technical point. The very nearly positive feedback applied to bootstrapped input load resistor increases the input circuit's noise level.
PLUS - the source device here is effectively a 1200pF cap which will nearly eliminate ( by shunting to ground) the self noise of a 1.5 Mohm load resistor. It will not similarly eliminate the noise generated by a much lower value resistor despite it being bootstrapped.
There is simply no need for such bootstrapping with FET op-amps.
** That is a separate issue & utterly irrelevant to anything I made comment on.
Ok, don't want to start an argumentative thread here, because that's not what the op asked for :-). TLOxx series op amps are quite noisy at high input impedances though and if you calculate the reactance of a 1200 pf cap, You get the following figures:
So, at low frequencies, where flicker noise dominates, you probably won't hear much audible effect on the noise at all.
While you may not *need* to bootstrap using an fet op amp, there can be benefits in terms of frequency response control and ime, you can get a better overall noise figure. That's not to say that it's a one size fits all though. For most guitar type circuitry, a jfet will get the job done as well, but it's nothing like as good as an op amp in terms of linearity / distortion. That's subjective though and the soft limiting might be just what's wanted in some cases. My advice would be to try both approaches and see which has the better overall sound, as this is what matters to a musician. High impedance pickups are always difficult to do signal conditioning for, so a bit of experimenation may be needed.
As for the second part of the post, apologies if it wasn't relevant, it was a reply to another posters comments...
** They actually have their best noise figure at high input impedances - but the effective input impedance *includes* that of the source.
** You have not even the faintest clue.
The noise from a resistor is " white " noise with energy proportional to bandwidth - so the noise energy in the band below 10 kHz is the same as that between 10 kHz and 20 kHz. The audible portion is between 1 kHz and 5 Khz where the ear is most sensitive.
That 1200 pF piezo pickup will attenuate the * audible noise range * of a
1.3 Mohm resistor by 10 to 50 times or 20 dB to 34 dB soon as it is connected.
A higher value load resistor will result in further noise improvements ( see below).
** You simply *cannot better* the noise of a resistor with no DC bias - it is set by a law of nature and depends only on the resistance value and temperature.
En squared = 4.K.T.B.R
En = noise voltage. K = Boltzman's constant. T = temp in K B = measurement bandwidth. R = resistance value in ohms
In circumstances where the input source is capacitive - the higher the load resistor value the better.
The math is simple:
Double the resistor value and its self noise increases by 3 dB.
Double the resistor value and the noise attenuation by a capacitive source increases by dab.
Ergo - you gain 3 dB improvement for each doubling in load resistor value.
** More irrelevant stuff.
Just as silly to bootstrap a JFET input as a FET op-amp input.
** More irrelevant stuff.
** JFET inputs are close to ideal for piezo and magnetic guitar pickups. Used as a source follower, there is no overload issue since the input level may be up to several volts.
BTW
Ever considered why condenser mics use hard to get resistors in the 1 to 5 Gohm range to DC bias the capsule and the input FET's gate ??
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