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Old 18th February 2008, 07:56 PM   #1
tubee is offline tubee  Netherlands
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Default Low pass filter for simple sub on single rail?

I am busy to build an amp for two subwoofers for in my car. Build a simple 1 chip TDA4552Q amp for them. It sounds allready reasonably good, even on a simple 1A 12V supply. It costs only few bucks, and want to keep it simple.
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I want to add a active low pass filter on 80/100 Hz with a TL072 opamp. Is this simple to do with the single 14.4V car supply? All designs i see on www are with + and - rail supply's. Can i simply pick a low pass design and just hook it on 14V, and neg rail on gnd?

Thanks.
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Old 18th February 2008, 11:42 PM   #2
TheMG is offline TheMG  Canada
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It is fully possible to use one or more op-amps with a single-ended supply to create a low pass filter.

To use an op-amp with a single-ended supply for audio, you will need to bias the input of the op-amp and use AC coupling capacitors on the input and output. DC coupling is not possible when the op-amps are used with a single-ended supply
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Old 19th February 2008, 12:06 AM   #3
tubee is offline tubee  Netherlands
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Thanks for reply TheMG, will use a Sallen-Key 2nd order on 100Hz, and put a big passive 6H cored coil on the output to create a 18dB filter total. And use 0.22uF coupling caps. Does the input impedance of the chip amp influence the behaviour of the lpf? Btw i used a TDA1552Q. How do i bias the signal, put the signal line between two eg 47k resistors, them gnd and +12V rail?

Thanks again.
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Old 19th February 2008, 12:31 AM   #4
Minion is offline Minion  Canada
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Most Active filters use an Inverting opamp and to bias an Inverting opamp to half supply you can use 2 say 33k resistors to create a Voltage divider between + supply and Grnd and then feed the half supply voltage into the Non-inverting input ,this will Bias the input of the Opamp to half supply....

And a output cap will also have to be used to Stop the half supply Dc offset at the output.....


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Old 19th February 2008, 12:35 AM   #5
tubee is offline tubee  Netherlands
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Thanks!

Did some reseach too, found from TI a nice single supply opamp collection: http://instruct1.cit.cornell.edu/cou...gleSupply.pdf.

Also sallen-key is discussed, and the non inverting input of opamp is biased with 2 high value resistors after a coupling cap. Output of opamp does'nt need a cap, the TDA has one allready. Will smoke the iron tomorrow!
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Old 19th February 2008, 05:59 AM   #6
TheMG is offline TheMG  Canada
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The input impedance of the chip amp will only affect the filter if you have any passive filtering elements between the output of the active filter (op amp) and the input of the chip amp.

This is one of the big advantages of an active filter is that load impedance does not affect the performance, so long as you don't load it down too much.
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Old 19th February 2008, 04:44 PM   #7
tubee is offline tubee  Netherlands
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I am calculating the value's for a sallen-key 12 dB with formula's from the TI site. I want lpf 100Hz.


LOW PASS Unity Gain Butterworth
R3 = R4 (HIGH)
R1 = R2
C1 = 2C2
Fo = √2 / (4pR1C2)

Anyone know what is ment by "p" in the calculation ?

btw calculated it on a balanced ps opamp site also (90Hz)
I found for
R1 8.734 kohm
R2 16.275 kohm
C1 220N
C2 100N
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Old 19th February 2008, 04:52 PM   #8
AndrewT is offline AndrewT  Scotland
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p=Pi=3.14159

Most common filters in audio use non-inverting topology.
MFB is the most common exception.
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Old 19th February 2008, 04:54 PM   #9
tubee is offline tubee  Netherlands
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Yes ofcourse!! Thanks!
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Old 21st February 2008, 02:28 AM   #10
tubee is offline tubee  Netherlands
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I have been looking for a very simple clipping indicator, but could not find any. Yes the one from ESP pages, but it needs more stages then my whole amp....

Anyone a suggestion for a simple clip indicator (12V zener to a led ?)
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