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RC filter component selection
RC filter component selection
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Old 26th October 2017, 06:25 PM   #1
robinlawrie is offline robinlawrie  United Kingdom
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Default RC filter component selection

ok so.. yes.. im a noob.

i need to create a pair of simple lowpass RC filters for line-level signals. lowpass around 2-3khz.

there are many calculators that let you choose values for R, C and frequency obviously, no problems there.

however, it seems to me you would want the lowest value for R as possible, so you dont attenuate the signal more than necessary.

could somebody explain why im not using some 1 ohm resistors?
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Old 26th October 2017, 07:12 PM   #2
richie00boy is offline richie00boy  United Kingdom
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RC filter component selection
Because as you start approaching the roll off point the signal will start seeing a huge load and distortion will go through the roof.
I wouldn't generally go lower than 4.7k in your application. But be mindful of the input impedance of whatever you are driving with the filter.
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Last edited by richie00boy; 26th October 2017 at 07:15 PM.
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Old 26th October 2017, 07:35 PM   #3
flyingfishtw is offline flyingfishtw  Taiwan
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You can, the formula is F-3db = f=1/2piRC, you can shuffle things around to make the equation more suitable for your calculation, the resistor doesn't make the signal smaller in anyway, it's the result of R and capacitor Reactance Xc interaction.

with R 1k ohm F-3db 2khz, C will be 80nF.
with R 1 ohm F-3db 2khz, C will be 80uF.

1. Capacitor will be different types, with smaller C you can choose high quality Film caps with low leakage, better linearity, 80uF, wish you good luck Sir.
2. with 1ohm, the cable you use and pcb board isn't IDEAL conductor, it has resistance as well, also capacitor is far from IDEAL, it has ESR ESL will play bigger role in this situation
3. with small resistor, it need more current to charge capacitor. so you have to worry about your source drive ability

That's just on top of my head, not sure if that's right. Once thing you have to pay attention is, the F-3db is not the starting point of attenuation. Starting point will be for low pass filter, one decade(right word for it?) before F-3db point, so 200hz the signal will start drooping. Also keep in mind, it's easier to find suitable value resistor than caps, and cheaper.
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Old 26th October 2017, 07:38 PM   #4
robinlawrie is offline robinlawrie  United Kingdom
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EDIT: further reply has helped a lot.! arrived while i was typing. so i should not consider the resistor in isolation..

----------------------------------------------------------------------------------------------------

thanks for the reply.

would you mind educating me a touch further?


the amp is the tpa3255EVM here:

TPA3255EVM TPA3255 315W Stereo / 600W Mono Ultra-HD Analog-In Audio Class-D Module | TI.com

but i cannot find the input impedance listed anywhere, and methods to measure it seem complex.

then, assuming i have this information, what to do with it? how does it affect the resistor value i choose.

i cant help thinking a 4.7k resistor will cut down my signal rather a lot? this means one amp channel will see a noticeably quieter signal overall?

Last edited by robinlawrie; 26th October 2017 at 07:41 PM.
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Old 26th October 2017, 08:12 PM   #5
radtech is offline radtech  United States
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Quote:
Originally Posted by robinlawrie View Post
i cant help thinking a 4.7k resistor will cut down my signal rather a lot? this means one amp channel will see a noticeably quieter signal overall?
The amount of loss will depend on the input impedance of the amp. The 4.7K resistor and the amplifier input will form a voltage divider. If your amp had a 4.7K input imp. you'd end up with half the voltage that you'd have had without the resistor (below cutoff frequency, that is).

The formula for a divider is Vout = Vin * R2/(R1+R2), so Vin * 4.7K/(4.7K+4.7K) factors down to Vin * 0.5, you lose 50%.

With a higher input imp. you'd lose less voltage, for instance with 100K you'd get Vin * 100K/(4.7K+100K) which gives you Vin * 0.955, so you've only lost 5%.

I'm not sure why you say one channel would be quieter, wouldn't you put an identical filter in each channel?
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Old 26th October 2017, 08:14 PM   #6
rayma is offline rayma  United States
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Quote:
Originally Posted by robinlawrie View Post
so i should not consider the resistor in isolation..
It's all relative. The series resistor should be much larger than the source impedance,
but also much smaller than the load impedance. In practice, often you can use
Rfilter = sqrt(Rsource x Rload) as the best compromise.

The input impedance appears to be 10k. If your source is 1k,
then use Rseries = sqrt (10k x 1k) = 3.16k.
Then choose the capacitor that sets your desired frequency corner.

There's a more precise way to calculated C if you know both Rsource and R load.

Last edited by rayma; 26th October 2017 at 08:19 PM.
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Old 26th October 2017, 08:15 PM   #7
robinlawrie is offline robinlawrie  United Kingdom
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Quote:
Originally Posted by radtech View Post

I'm not sure why you say one channel would be quieter, wouldn't you put an identical filter in each channel?
its for a 1.5 way dual fullrange cabinet, so there will be two amp channels per side, with one channel/driver on each side with a lowpass.
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Old 26th October 2017, 08:22 PM   #8
rayma is offline rayma  United States
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Quote:
Originally Posted by robinlawrie View Post
its for a 1.5 way dual fullrange cabinet, so there will be two amp channels per side,
with one channel/driver on each side with a lowpass.
If necessary, you can add an input attenuator to match the low pass level.
It would just be a series resistor of the same value as in the low pass, if you use
the same type of amplifier.
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Old 26th October 2017, 08:24 PM   #9
robinlawrie is offline robinlawrie  United Kingdom
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Quote:
Originally Posted by rayma View Post
If necessary, you can add an input attenuator to match the low pass level.
It would just be a series resistor of the same value as in the low pass, if you use
the same type of amplifier.
ha, painfully obvious.. doh.

thanks
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Old 26th October 2017, 08:32 PM   #10
rayma is offline rayma  United States
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Quote:
Originally Posted by robinlawrie View Post
ha, painfully obvious.. doh. thanks
BTW, you'll be losing about 3dB of level from the 3.16k series resistor and the 10k input impedance.
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