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Old 21st September 2011, 09:03 PM   #1
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Default High freq filtering of a (guitar) preamp?

Hello!

I'm building something like a guitar preamp.. I mean.. It is actually the input buffer for a guitar effects unit and it does not provide any gain.. its purpose is to ensure the minimum loading of the pickups possible. The 0.0047uF decoupling cap provides low frequency roll-off, but i thought that high frequency attenuation is needed as well for 2 reasons: first, the narrower the bandwidth, the lower the thermal noise, and since it is a distortion pedal with about 50 dB of gain, I believe this should not be neglected; second - a guitar is a sensitive device and it tends to pick up any kinds of EMI, so a low-pass filter should bring an improvement. However, I'm not sure how exactly to realize that low-pass filter without sacrifising the large input impedance. I am using an op-amp in a non-inverting configuration, so a cap in the feedback loop is not an option (it would reduce the gain at HF to 1, but the gain is already 1, since we have a follower). Another option I thought of is putting a small shunt cap (in the pF range) across the bias resistor, but i'm not sure if that is reasonable, since it may introduce distortion, and, more importantly, instability. I am really facing a challenge with this low-pass filter. Would you please give your ideas and opinions on what i can do or what is best to do and why? I'd be grateful. Thanks in advance.
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Old 22nd September 2011, 11:12 PM   #2
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Why not simulate it in LTSpice untill you get the response you would like (and not tell us about!) E
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Old 23rd September 2011, 10:58 AM   #3
DF96 is offline DF96  England
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A small cap across the resistor should be fine. Make sure it has very short leads to keep self-inductance low. It forms a CR low pass filter with the 4.3k resistor, and will keep RF out. It won't reduce noise generated within the op-amp but, depending on the souce impedance, it may reduce HF thermal noise from the 2.2M resistor.
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