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fez79 24th September 2012 07:01 PM

Preamp circuit THEORY
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Hello. I have a question pertaining to analog circuit THEORY/sense, using a guitar preamp circuits just as an excuse. I will use the MXR MicroAmp schematic found here: MXR MicoAmp. I'm doing things on these lines and plan to continue and evolve on them, however I'd like to know the why's along with that what's as well.

1. In engineering, compromise is always present. I can see R7 and R8 being used to bias the input to half the DC voltage, but why do they have the values that they do? Why not higher, say 1M? Wouldn't that result in even LESS power consumption? Potential dividers are used a lot so this a very important question. In its crux, the question becomes: in circuits dependent on RATIOS of impedances, what is the drawback to having higher impedances? What is the advantage and disadvantage to having them lower (aside from higher power consumption)?

2. Most the caps here are coupling or decoupling. But my question is similar to the previous one - why do they have the values that they do? Why not higher, or lower? Also, what purpose does C3 serve?

3. What purpose does R1 serve? Wouldn't it be better to leave it out? It only ends up loading the guitar (tonal degradation). Also, what is the point to R9 and R10? They, too, only end up loading the opamp (current/power consumption). What is the purpose to R3?

richie00boy 24th September 2012 07:30 PM

1. higher impedance means more sensitivity to noise pickup and also higher self noise.

2. a trade off between size, cost and cut-off frequency.

3. R1 and R10 allows the cap to charge and reduce the severity of clicks when the switch is moved. R9 is to isolate the op-amp from cable capacitance.

jcx 24th September 2012 08:19 PM

guitar preamps are rather specialized - the pickups have Henries of inductance, resonate in the audio band with any cable, preamp input C, preamp input resistance can change the damping - hugely changing the peaking

all of which depends on the pickup details as well - you can't build a guitar preamp, choose cable independent of the exact pickup model, desired response

in most other preamp applications in audio the preamps are not usually considered integral to the sound of the musical instrument itself

HJWeedon 24th September 2012 11:06 PM

Hi fez79.

Besides the comments of richie00boy I would make the following additional comment: R11 is your biggest power consumer dissipating as much as 8mA of current by itself. Leave out the LED feature and your battery will last about 10 times longer. The amplifier itself draws no more than 0.3mA and with signal, probably no more than 0.8mA from the 9V battery.

I do not believe that an input impedance of about 8Meg Ohms R2 parallel with R1 makes any difference to the sound whatsoever. I disagree that higher impedance in this circuit make any difference at all because the input impedance is electrically shorted out by the source impedance of the guitar pickup. Without that, the amplifier might be a little noisy without an input connected.

Good luck

Hans J Weedon

fez79 1st October 2012 07:10 PM

Thanks for the replies! I have a problem with the preamp now though - when I attach headphones to the output, I hear a somewhat distorted sound. However, when I remove the output coupling capacitor, replacing it with a short, the sound is better! (Somehow my headphones didn't burst or break at absorbing a DC voltage...). What sorcery is THIS, now?

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