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Old 15th March 2014, 08:21 PM   #1
alibear is offline alibear  United Kingdom
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Default Pi filters

Hi all, I am building a supply for a class A amplifier, the bias current is 1.7A per channel at 42Volts. I am hoping to use a CRC Pi filter consisting of 47,000uF R and 47,000uF. per rail. I am thinking of using a 1ohm 25watt resistor, can anyone see a problem with this value, should I try lower or higher values. What are the drawbacks with using this type of filter. Will there be any degradation to sound quality if R is too high.
Thanks for your help.
Alan
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Old 15th March 2014, 08:33 PM   #2
DF96 is offline DF96  England
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Why 25W?

Large R means too much voltage drop. Small R means less smoothing.
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Old 15th March 2014, 10:53 PM   #3
alibear is offline alibear  United Kingdom
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Hi, 25watt because I have some metal case resistors that I can use .
I think I can aford to drop a volt or two.
But main query is : is a 1ohm resistor too large a value, will it be derogatory to power supply performance?
Thanks
Alan
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Old 15th March 2014, 11:08 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

Depends on the nature / topology of your class A amplifier,
regarding maximum current demands on the power supply.

BTW a proper Pi filter is CLC, not CRC.

Higher R will reduce the voltage available, but TBH @ 1.7A
around 1R seems about right to be useful but not invasive.

rgds, sreten.

A 5W resistor will do the job, no point complicating it.
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Last edited by sreten; 15th March 2014 at 11:11 PM.
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Old 15th March 2014, 11:39 PM   #5
DF96 is offline DF96  England
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Quote:
Originally Posted by alibear
But main query is : is a 1ohm resistor too large a value, will it be derogatory to power supply performance?
Too large a value for what? Performance in what sense?

You need to decide which compromise of voltage drop vs. smoothing best fits your amplifier design. For example, if your amp has very poor PSRR then you need good smoothing and have to tolerate more voltage drop. If your amp has serious second-order distortion then you will have a varying supply current (despite being Class A) and so may need a smaller resistor and have to tolerate more hum.
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Old 16th March 2014, 09:01 PM   #6
alibear is offline alibear  United Kingdom
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Hi everyone, thanks for the informative answers. Another question regarding CRC or CLC filters: what is the result if the first C after the rectifier is much smaller than the C after the R or L. For example, first C 10,000uF , final C 47,000uF. What capacitance does the amplifier actually "see"
Thanks
Alan
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Old 16th March 2014, 10:10 PM   #7
DF96 is offline DF96  England
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That depends on the value of the series element. In most cases it will be large enough that the amplifier will 'see' the final capacitor for AC purposes, but will also see that it is fed from a resistance which is a combination of the actual series resistance and the effect of voltage droop on the first (reservoir) capacitor.
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Old 16th March 2014, 10:34 PM   #8
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For ~1.7amps load, 47Ku x 1ohm x 47Ku, 60Hz

42.48 - 42.84V at diode bridge
41.042 - 41.048 at load ripple ~ 6mv, power loss ~ 1.44V


A CLC always performs better than a CRC.

A CRC has "better than calculated" ripple reduction with Class A bias output stages.

I think a 1 ohm resistor is too high and will drop unnecessary voltage, as shown by the 6mv calculated ripple.
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Old 16th March 2014, 10:39 PM   #9
Sonce is offline Sonce  Macedonia
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Quote:
Originally Posted by alibear View Post
what is the result if the first C after the rectifier is much smaller than the C after the R or L.
Use C1=C2 for the lowest ripple.
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Old 16th March 2014, 10:44 PM   #10
Sonce is offline Sonce  Macedonia
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Quote:
Originally Posted by LineSource View Post
A CLC always performs better than a CRC.
Agreed, but L must pass high current without saturation, so usually it is expensive.
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