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Old 9th July 2003, 03:38 PM   #1
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Default Gainclone Low Pass Filter

I want to have a low pass filter on the input of my gainclone amp. it is used to drive my subwoofer, and i thouht that it would be easier to have a LPF on the input, than to make on for the output. i am aiming for approx 80hz cutoff. im guessing this would be done with a capacitor and resistor?
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Old 9th July 2003, 03:47 PM   #2
Paulr is offline Paulr  United Kingdom
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Default try this

Its been posted before, search for pllxo, but I think this is one of the sites the thread refers to

http://www.t-linespeakers.org/tech/f...ssiveHLxo.html

Thought about this for the car "sub" amp - they're 6" 4R 125watt nominal cheapies & a capacitor from the network point to phone converters in the office (PABX or secondary device selector) is polypropelene 185nF 250v which would give a F of 86Hz on 10k resistor & 172Hz on 5k

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Paul
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Old 9th July 2003, 05:12 PM   #3
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this is for the input to the amp, not the output?
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Old 10th July 2003, 12:36 AM   #4
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Check out this link
http://www.analog.com/Analog_Root/st...er/filter.html

It shows you how to make up to an 8th order filter with op amps. Might be overkill for you, but I thought it was interesting.

But yes, you can make a simple 2nd order filter just using a RC network.

Corner frequency = 1/2*pi*R*C

I think the resistor comes first in a low pass filter, but I won't swear to it, I was never very good at analog stuff.

Randy
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Old 10th July 2003, 12:48 AM   #5
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Default Re: Gainclone Low Pass Filter

Quote:
Originally posted by Matttcattt
I want to have a low pass filter on the input of my gainclone amp.
just treat the chip as an op-amp and you will find plenty of examples.
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Old 11th July 2003, 08:11 AM   #6
Paulr is offline Paulr  United Kingdom
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Hi Matt,

The link I posted is for putting in-front of the amp, it can just about be built into a matchbox size box with inputs for your rca leads & outputs that plug straight into your amp.

Component count is 1 cap & 1 resistor so its cheap as hell to try it & since the frequency point changes with the resistance you can even try a 10k resistor in parallel with a 10k & a 50k pot - that would give you a variable resistance from 5k to about 8.5k for experimentation. On my car setup idea that would result in a frequency cutoff of 172 to 100 Hz on a 185nF capacitor.

As always I haven't got round to trying this yet. I asked Santa for a few more hours a day for Christmas.

Cheers

Paul
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Old 11th July 2003, 10:35 AM   #7
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Quote:
Originally posted by Paulr
Hi Matt,

The link I posted is for putting in-front of the amp, it can just about be built into a matchbox size box with inputs for your rca leads & outputs that plug straight into your amp.
a high-order filter usually is done on the feedback loop. get a National OPamp databook and it should have plenty of them.
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Old 11th July 2003, 06:34 PM   #8
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Quote:
Originally posted by millwood


a high-order filter usually is done on the feedback loop. get a National OPamp databook and it should have plenty of them.
Or you can go to the link I gave. It's an interactive page for opamp based filters, let's you define for 2-8 pole filter, define different filter types, gain, freqency, and maybe a few other things, then spits out a schematic, parts values etc.

randy
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Old 11th July 2003, 06:38 PM   #9
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randy, that's a good suggestion.

Alternatively, you can get the opamp book everyone is talking about from National and it has plenty of examples for active filters.
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Old 11th July 2003, 06:53 PM   #10
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Hi Millwood,
Yeah I saw that too. I downloaded it already, I plan to print it out later when most people have left work, since it's sooo long.

My analog knowledge is pretty weak, so I'm sure I'll learn alot from it.

Randy
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