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TOPIC: Preamp with JFET

Preamp with JFET 5 months 2 weeks ago #1815

  • gonzalososa095
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Hi, I have an Arduino Uno and I want to start doing this pedal. The problem is that in my country I do not get an op-amp rail to rail. My first question is why the op-amp has to have rail to rail output? And what would happen in case of not having it?. This question is because i saw that the pedalShieldDue's project uses a TL072.

The other doubt is if a JFET preamplifier could work instead of the proposed circuit.

Thank you !
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Preamp with JFET 5 months 2 weeks ago #1816

  • Ray
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Hi,

The main idea after the rail to rail op-amp is that with a "tight" power supply (Arduino only gives us +5V) is we use a normal op-amp, the signal output signal will be clipped (not being able to reach 0 and +5V). Using a TL072 with a +5V supply gives an output swing of +1.5 to +2.5V.
www.ti.com/lit/ds/symlink/tl074b.pdf

In the pedalshield DUE we use a "normal" op-amp (TL072) because we also have a MAX1044 that creates a 9V power supply. With 9V power supply, we do not have problems with the signal output swing.

The pedalSHIELD UNO circuit is a bit more minimalist, so instead of using a MAX1044 + extra circuitry we just use a rail to rail op-amp (TL972, MCP6002 or similar) that will ensure that the output signal will reach +5V and 0V so the ADC reads the best signal.

You can do any kind of preamplifier, you can use the DUE schematic for the UNO input stage (although the circuit will be more complex) or even a JFET stage.
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Preamp with JFET 5 months 2 weeks ago #1819

  • gonzalososa095
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Thank you very much!
According to what you say, if I increase the power supply, I could use a TL072 on the pedalSHIELD UNO.
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Preamp with JFET 4 months 4 weeks ago #1824

  • Ray
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yes, if you can power it with external 9V you can use a TL072 but make sure that the 2.5V are there (you would probably need to modify the voltage divider).
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