bassxtreme
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Post by bassxtreme on Dec 14, 2010 3:39:58 GMT -5
I built this for bass and I was wondering what the C2 22pF capacitor is for. It looks like it filters something from the C3, R3, TR1 boost circuit. how would different values like 100pF, 220pF, or 500pF affect the sound?  |
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Post by JohnH on Dec 14, 2010 14:36:04 GMT -5
hi be
C2 is in the feedback loop of the first op-amp stage. The higher the frequency, the more it reduces the signal. It is a very small value, and it is in parallel with a 100k pot. So it will only start to cut the signal at very high frequency, 70 kHz or higher (based on working out what frequency its impedance matches the 100k).
So I'd say it is for cutting out unwanted high frequencies above audio range, such s those derived from rf interference
John
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bassxtreme
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Post by bassxtreme on Dec 14, 2010 15:55:39 GMT -5
hi be C2 is in the feedback loop of the first op-amp stage. The higher the frequency, the more it reduces the signal. It is a very small value, and it is in parallel with a 100k pot. So it will only start to cut the signal at very high frequency, 70 kHz or higher (based on working out what frequency its impedance matches the 100k). So I'd say it is for cutting out unwanted high frequencies above audio range, such s those derived from rf interference John I put a 120pF cap there thinking 22pF is a too small value for a bass preamp. what would be the frequency that would be cut with that cap? |
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Post by JohnH on Dec 14, 2010 16:28:03 GMT -5
It depends on the setting of the pot. At minimum gain, this cap is fully bypassed and does nothing. At max gain, it will be cutting from a frequency dependent on the cap. Yours is 5.5 x larger in value, so therefore from 70/5.5 = 13kHz upwards
J
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bassxtreme
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Post by bassxtreme on Dec 14, 2010 17:03:28 GMT -5
It depends on the setting of the pot. At minimum gain, this cap is fully bypassed and does nothing. At max gain, it will be cutting from a frequency dependent on the cap. Yours is 5.5 x larger in value, so therefore from 70/5.5 = 13kHz upwards J I have the pot at 30% (30K) so that seems about right. Anything past 30% makes the signal way too hot. I never seen a cap value lower than 100pF in a bass preamp. Also the sound was way too bright and harsh so I changed C4 & C5 to .022uF and it took off the harsh high end. thanks for the info ;D |
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