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Old 9th August 2014, 10:48 PM   #1
clm811 is offline clm811  United States
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Default help with 2 pole low pass filter/attenuator

Hello.

I was hoping for some help with a 2 pole low pass filter/attenuator.

I need to provide a low pass input to a woofer amp, but also to attenuate the
signal level a bit to help match with the midrange(less sensitive).

Here is the configuration I was hoping to use, with the amplifier input impedance at 20k ohms.

I calculate a second order low pass at around 400Hz, but about how much attenuation will layout this give,(-6db)??

TIA

-Chas
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Old 9th August 2014, 11:16 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

Thats not a two pole filter. Its a cascaded passive 1st order
buffered with a gain stage you can set to any gain you like.

The input could easily be attached to a 20K volume pot.

rgds, sreten.

Checkout TinaTi :
SPICE-Based Analog Simulation Program - TINA-TI - TI Software Folder
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Last edited by sreten; 9th August 2014 at 11:19 PM.
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Old 10th August 2014, 12:18 AM   #3
clm811 is offline clm811  United States
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OK. Thanks.
So, what characteristics does a cascaded passive first order filter have,
compared to a passive second order filter?
BTW the filter is being attached to the input of a TK2050 based power amplifier(the gain diagram was lifted from somewhere else for illustration purposes).

I read this on a Google search for 2nd order filter:
Quote:
...a first-order low pass filter can be converted into a second-order type by simply adding an additional RC network to it and the more RC stages we add the higher becomes the order of the filter...
-Chas

>Thats not a two pole filter. Its a cascaded passive 1st order ...<

Last edited by clm811; 10th August 2014 at 12:34 AM.
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Old 10th August 2014, 02:10 AM   #4
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Question What's the difference?

Thats not a two pole filter. Its a cascaded passive 1st order

What's the difference?
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Old 10th August 2014, 02:15 AM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by clm811 View Post
OK. Thanks.
So, what characteristics does a cascaded passive first order filter have,
compared to a passive second order filter?>Thats not a two pole filter. Its a cascaded passive 1st order ...<
This is indeed a second order filter, as the ultimate rolloff is 12dB/octave, but the response at the corner is 6dB down. A second order active filter can be designed with more useful behavior.
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Old 10th August 2014, 04:41 AM   #6
sreten is offline sreten  United Kingdom
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Hi,

Your all talking nonsense. Its a poorly cascaded 1st order
that hasn't a hope in hell of replicating a second order
or being barely different to a simple RC first order.

rgds, sreten.

Properly cascaded 1st order filters can approximate
very low Q higher order fiters, but a bucket load
of gain needs to be lost between each stage,
and the eventual roll off is 1st order, always.
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Last edited by sreten; 10th August 2014 at 04:59 AM.
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Old 10th August 2014, 07:14 AM   #7
clm811 is offline clm811  United States
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I researched a bit further, and found this filter calculator:

(Sample) 2nd order CR Low-pass Filter Design Tool - Result -

I will revise the component values to be R1=180ohms, C1=2.2uF, R2=1.8k,
and C2=0.22uF. For these I calculate an Fc of 400Hz and a Q of around 0.5.

True it's a cascaded first order with low Q, but I think I will experiment with it anyway (the loss of gain should approximate 6dB and is acceptable, since my woofer is a few dB more sensitive than my midrange (which has an attenuator in its active crossover, as does the tweeter).

Thanks all for the insights and opinions.

-Chas
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Old 10th August 2014, 11:00 AM   #8
AndrewT is offline AndrewT  Scotland
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Sreten,
clearly OKAWA Electric Design and the other Members all disagree with you.

The two first order filters, giving a Q=0.5, should make this compatible with an LR rolloff of the mid/treble section.
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Old 10th August 2014, 07:24 PM   #9
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
Sreten,
clearly OKAWA Electric Design and the other Members all disagree with you.

The two first order filters, giving a Q=0.5, should make this compatible with an LR rolloff of the mid/treble section.
Hi,

Doesn't mean I'm wrong though. Its cascaded 1st order CR, and
with two identical sections it will not differ much from a simple CR.
(Sim it in TinaTi and see that that is the case.)
Its nowhere near LR second order if that is what your after.

It is not "indeed" a second oder filter, the final roll-off
is not 12dB/octave as stated, it is 6dB/octave.

Any type of active second order can be built around the op-amp.
With a bit of tweaking if you also want / need significant gain.
It can also be easily built into power amplifiers too.
(By changing the unity gain point to the + input.)

rgds, sreten.
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Last edited by sreten; 10th August 2014 at 07:31 PM.
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Old 10th August 2014, 08:01 PM   #10
rayma is offline rayma  United States
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Quote:
Originally Posted by sreten View Post
Hi,

Your all talking nonsense. Its a poorly cascaded 1st order
that hasn't a hope in hell of replicating a second order
or being barely different to a simple RC first order.
Cascaded second order filters like this are common in tube amp and preamp power supplies. The filtering would be very inadequate if they were only first order.
The s domain transfer function is: (1/(1+sRC)) * (1/(1+sRC)), or 1/(1+2sRC+(sRC)^2). This is clearly second order (notice the s^2 term), with a 12dB/octave, or 40dB/decade, asymptote.

Last edited by rayma; 10th August 2014 at 08:20 PM.
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