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I want to build this electric guitar output to phone mic input converter: irigschematic
(source: altervista.org)

From my limited knowledge on the subject, it looks like a simple JFET preamplifier (actually, I've simulated the circuit in LT Spice and it seems to attenuate the signal, not amplify), but I do not understand the role of the 100pF (gate to ground) and 470 pF (drain to ground) capacitors, are they used for some kind of filtering of the signal ? Will it work without them ?

The circuit I used for simulation in LT Spice is:

lt spice simulation

(the parts in the red rectangle should be the circuity inside the phone, I used the reference circuitry for soundblaster mic input thinking the phone has something similar inside)

So, my question is: what is the role of C2 and C3 ?

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  • \$\begingroup\$ Can you provide a link to the source of the circuit \$\endgroup\$
    – Andy aka
    Dec 2, 2013 at 14:28
  • \$\begingroup\$ @Andy: The link is also in the image, link, but the page doesn't contain more information (except for a youtube movie of the working thing) \$\endgroup\$
    – cobru
    Dec 2, 2013 at 17:45
  • \$\begingroup\$ "it seems to attenuate the signal, not amplify". My guesstimate would be that attenuation (voltage) shouldn't differ too much from 1, but yes I believe you when you say it attenuates. Do notice though that the output impedance of you amplifier is much lower than that of the guitar. The current amplification therefore is probably much higher than 1. \$\endgroup\$
    – jippie
    Dec 2, 2013 at 19:23

1 Answer 1

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It looks like those capacitors and the optional ferrite bead are only meant to attenuate high frequencies.

It is fine to squash everything above 20 kHz in a audio signal because you can't hear above that anyway. The audio circuit in the phone probably does that already anyway. However, if really high frequencies get in there, then you can't rely on active electronics still working as intended. Pickup from radio stations and the like can be so high that the circuitry intended for audio frequencies can't actively deal with and properly reject the signal. Put another way, the amplifier no longer acts linearly at these high frequencies, so distortions in the frequency range you can hear can result. All it takes is a little assymetry in dealing with the signal, and you get a little AM demodulation, for example. There are other effects too.

Look at the rolloff frequency of C3 and R3, which is F = 1 / (2 π R C) = 72 kHz. C2 is somewhat misguided in that it forms a capacitive voltage divider with C1. It doesn't attenuate high frequencies by itself, and relies on the input signal having some finite impedance.

This is not a great design, which of course shouldn't be surprsing for something you found on the web.

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