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Old 10th November 2003, 08:54 PM   #1
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Default Passive line level low and high pass filter help

I want to filter the input to my sub amp with a simple pair of filters, ~20Hz HPF and ~80Hz LPF.
The output is from a personal CD player, and the input will go to my Gainclone (10k input impedance).
I donít know what slope to use, and would like an idea of what to use: 6db/oct? 12db/oct? 24db/oct?
I have found this site: http://www.t-linespeakers.org/tech/f...ssiveHLxo.html
Can I just put two of these filters together (high pass and low pass) without them affecting each other?
How do I work out the component values? Using a 10k resistor, and 80Hz, I keep getting 0.0000002 is this correct? What units are used? Ohms and farads? 0.0000002 farads is 0.2uf?

Any help appreciated
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Old 11th November 2003, 10:36 AM   #2
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Greetings fellow South West England person

If you must use passive filters then I would just go for 1st order, as higher-order passive filters can be lossy and require careful design.

The standard formula for finding the -3dB point of a 1st-order filter is

F = 1 / 2 * PI * R * C

Where

F is in hertz
R is in ohms
C is in farads (1,000,000 uF = 1F)

So, the calculations you did should be evident now

Are you prepared to mod the Gainclone? You could just turn it into a power active filter and it would give better results. All that would be required would be to change the feedback component values. It's a cinch if you have done an inverting type.

If you don't want to do this, you may have problems driving the filter due to the relatively low input impedance of the Gainclone. Ideally a source should be no less than 1/2 impedance of the load, ideally 1/10. So your filter could end up with an impedance of about 5k, which your CD player may struggle with.

Also, depending on the filter topology, you will either lose some signal level or get worse noise performance. To keep things simple, I just do a high-pass then tag a low-pass after it. This method results in retaining the same nominal level.

HOWEVER, you could be clever and use the 10k input impedance of the Gainclone as one element of the high-pass filter. A 470nF in parallel with 330nF, both in line to the amp input will give you a 20Hz -3dB point. Precede this with a 470nF cap to ground and before that a 4k3 series resistor. Now you have an 80Hz low-pass filter as well.

Also, I think you should really consider a higher-order filter for the low-pass.

A Gainclone for a sub??? I can only assume it's for computer speakers or a portable thingy.
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Old 11th November 2003, 04:10 PM   #3
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Quote:
Ideally a source should be no less than 1/2 impedance of the load, ideally 1/10
Oops! Extraneous 'no' in there, should read

Ideally a source should be less than 1/2 impedance of the load, ideally 1/10.
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Old 11th November 2003, 05:15 PM   #4
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Quote:
Originally posted by richie00boy
Greetings fellow South West England person

If you must use passive filters then I would just go for 1st order, as higher-order passive filters can be lossy and require careful design.

The standard formula for finding the -3dB point of a 1st-order filter is

F = 1 / 2 * PI * R * C

Where

F is in hertz
R is in ohms
C is in farads (1,000,000 uF = 1F)

So, the calculations you did should be evident now

Are you prepared to mod the Gainclone? You could just turn it into a power active filter and it would give better results. All that would be required would be to change the feedback component values. It's a cinch if you have done an inverting type.

If you don't want to do this, you may have problems driving the filter due to the relatively low input impedance of the Gainclone. Ideally a source should be no less than 1/2 impedance of the load, ideally 1/10. So your filter could end up with an impedance of about 5k, which your CD player may struggle with.

Also, depending on the filter topology, you will either lose some signal level or get worse noise performance. To keep things simple, I just do a high-pass then tag a low-pass after it. This method results in retaining the same nominal level.

HOWEVER, you could be clever and use the 10k input impedance of the Gainclone as one element of the high-pass filter. A 470nF in parallel with 330nF, both in line to the amp input will give you a 20Hz -3dB point. Precede this with a 470nF cap to ground and before that a 4k3 series resistor. Now you have an 80Hz low-pass filter as well.

Also, I think you should really consider a higher-order filter for the low-pass.

A Gainclone for a sub??? I can only assume it's for computer speakers or a portable thingy.
returned greetings fellow south west england person

i have made a sub amp gainclone parralleling 4 chips, in inverting config. with 10k input pot, 10k R1 and 220k RF or whatever they are called. will the 10uf DC blocking cap i have in series after the pot effect the filters?

i think i will use your "clever" method, using the 10k pot as part of the filter.

i am not that bothered about sound quality, so the filter's quality it not that important.

what do you mean by a higher-order filter? you mean more db/oct falloff? if so, do you have a diagram?
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Old 12th November 2003, 07:16 AM   #5
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Quote:
Originally posted by richie00boy


Oops! Extraneous 'no' in there, should read

Ideally a source should be less than 1/2 impedance of the load, ideally 1/10.
so i need to measure my source? in ohms?

thanks
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Old 12th November 2003, 08:57 AM   #6
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Quote:
Greetings fellow South West England person
Hey - that's three of us! Where are you Richie? I'm in Burnham.
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Old 12th November 2003, 09:09 AM   #7
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I have to ask why you went to all the trouble and cost of paralleling 4 chips when you could have just built something like a P3A?

By higher-order filter, yes, that means more dB/oct:

6dB/oct = 1st-order
12dB/oct = 2nd-order
18dB/oct = 3rd-order
24dB/oct = 4th-order
etc.

The 10u DC blocking cap with 10k in series will have a -3dB point of 1.6Hz so it's effect will be insignificant. However, because this 10k resistor is in parallel with the 10k pot wiper, the effective input impedance of your amp will depend on the position of the volume control. At full volume the i/p z will be 5k. Now you see why passive filters are less attractive...

If you were to mod the Gainclone to be a power active filter, you could change the 10u cap to the 470n+330n parallel combination I suggested earlier to form the high-pass, and parallel the 220k Rf with 10n to form the low-pass at 72Hz.

On the subject of source impedance, you do not need to measure your source. It will be in the order of <1k unless it is some stupid esoteric contraption, so will be fine. What I meant was, if you place a passive filter before your amp, the filter becomes the source for the amp, and so *it* must meet the loading/impedance criteria. The problem is, you can't lower it by *that* much as the source (CD) still has to drive it.

If you don't want to touch the Gainclone, you can make a passive filter as I described in the earlier post, but you must use 1.5u instead of 470n+330n. The -3dB point will vary from 21Hz at full volume to 11Hz at min vol, but this will actually provide a level of protection as you increase the volume.

Using a passive filter will attenuate the input signal a bit, degrade the noise performance and the -3dB points will not be quite as hoped, but if you're not that fussed it will work...
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Old 12th November 2003, 12:27 PM   #8
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Nuuk, I am half way between Bath and Salisbury.
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Old 12th November 2003, 01:20 PM   #9
Nuuk is offline Nuuk  United Kingdom
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Nuuk, I am half way between Bath and Salisbury.
Not really close enough to 'drop in' then. But we now have the WOE Triangle!
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Old 12th November 2003, 03:48 PM   #10
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Maybe I will 'drop in' on my next visit to the cider farm
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