P3A Amp: Input Capacitor C1

[dine1967]

I just fired my P3A amp. today and was pleased to find that all was fine . I have the rails running at +/-36.5 and the amp is running very quite. The bias is around 94mv. The DC on the output is around 15mv. This seems a bit low I think. For the input cap(C1) Iam using a 4.7uF bipolar electrolytic capacitor.I have read that its better that one uses polyester/polystyrene cap for the input. Could any of the experienced builders guide me on which would be the best value (1-10uf as per Rod Eillot) and also the type. thanks in advance

 

[PMA]

4.7uF is a good value for input cap, as there is an input resistance of 22kOhm and this will give you 1.5Hz of low frequency limit (-3dB). But you should use foil cap. You can decrease value of C1 a little bit if you want, but I would suggest not to go lower than 2uF.

 

[trwh]

Hi dine1967,

I used a 1uF metallised polyester cap without any problems. 1uF gives a 3dB point of 7.2Hz which will be fine.

Hope this helps,
Tim.

 

[djk]

http://sound.westhost.com/project3a.htm

C1 and R2 comprise the input pole, C3 and R4 comprise the feedback pole, the main filter cap and the load form the output pole.

For best transient response the input pole should be the highest frequency, followed by the feedback pole, and then the power supply pole.

An example:
10,000µF and a 8 ohm load = 2hz
set the feedback pole = 4hz (C3 = 39µF)
set the input pole = 8hz (C1 = 0.9µF)

 

[sam9]

Sam9

I don't think the value is real critical. In general at 1uF the low frequencies will roll off faster and at 10uF quite a bit less. In either case DC is blocked which is the most important thing. If you were going to use this to rub a sub woofer, 10uF or a little less would be better If you will use the amp to drive a satellite speaker with a faily high XO (say 100 Hz), a value in the lower range might make more sense. For general use 4.7uF is probably a good choice.

PS: I'm one of those who get right/left and up/down bassakwards, and I'm writting this on the fly -- so don't be surprise if someone posts that I got it wrong way 'round!