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Old 11th April 2012, 09:26 AM   #1
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Lightbulb Is this filtering really needed ?

Hi
In my power amplifier RCA inputs i noticed that there is a filter consisted of a 330ohm resistor in series & a 47pf capacitor connected to ground as shown on the attached schematic. I removed the 47pf (to experiment actually) and connected a 33nf. It made the extension of high frequencies worse, so i put the 47pf back to their place again.
I was wondering, what will happen if i remove the capacitor completely & sort the resistor ?
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Old 11th April 2012, 09:36 AM   #2
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The R-C filter in the schematic is a low pass filter with the center freq of 10MHz. I do not believe there should have been a perceptible difference with 33pF but you should leave the filter in place. The cutoff is high enough for it to affect the audio band. Most filters i've seen are far lower than that. If you remove the cap you remove the filter, and if you feed the amp with a high freq signal you might destroy the amp or the speakers as the amp will begin to oscillate.
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Old 11th April 2012, 09:36 AM   #3
AndrewT is online now AndrewT  Scotland
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keep the 47pF.
It is there to attenuate RF interference.
Try moving it to directly connect to the RCA barrel and RCA hot pin. You can delete the 300r. Try comparing with and shorted.

I don't know if you are referring to 33nF or now I see 33pF in a reply.
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Old 11th April 2012, 09:49 AM   #4
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Hi & thank you for your reply.
I was referring to a 33nf (replaced the 47pf). It is definitely an RF filter.
What really made me curious is that it did affect the audio band so that is why i put the 47pf in its place again. I will sort the 330ohm resistor to see what happens.
After this filter there is another filter that goes to a 2SK389 differential amplifier.



Quote:
Originally Posted by AndrewT View Post
keep the 47pF.
It is there to attenuate RF interference.
Try moving it to directly connect to the RCA barrel and RCA hot pin. You can delete the 300r. Try comparing with and shorted.

I don't know if you are referring to 33nF or now I see 33pF in a reply.
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Old 11th April 2012, 11:10 AM   #5
DF96 is offline DF96  England
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The designer is, wisely, trying to keep RF out of his circuit. Changing 47pF to 33nF will change the frequency rolloff by a factor of about 700. Unsurprisingly, this is audible.

The first stage filter keeps out high RF, such as VHF and microwaves. This will probably need short connections. The second stage filter stops low RF such as AM broadcast. If you remove or significantly modify these filters you will either introduce interference (e.g. from nearby mobile phones) or damage the audio frequency response of your unit. Is this your aim?
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Old 11th April 2012, 11:35 AM   #6
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So better not touch anything here.
Thank you so much for your help. I really appreciate it.

Quote:
Originally Posted by DF96 View Post
The designer is, wisely, trying to keep RF out of his circuit. Changing 47pF to 33nF will change the frequency rolloff by a factor of about 700. Unsurprisingly, this is audible.

The first stage filter keeps out high RF, such as VHF and microwaves. This will probably need short connections. The second stage filter stops low RF such as AM broadcast. If you remove or significantly modify these filters you will either introduce interference (e.g. from nearby mobile phones) or damage the audio frequency response of your unit. Is this your aim?
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Old 11th April 2012, 04:32 PM   #7
AndrewT is online now AndrewT  Scotland
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The RC time constant of the 3k9 and 470pF is quite high.
It may be worth experimenting with this value. Changing either the resistor or the capacitor to a lower value will move the roll off further away from the audio band.
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