Passive line level low and high pass filter help

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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 don’t 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/filters/passiveHLxo.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? :bawling:

Any help appreciated
 
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.
 
richie00boy said:
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 :D

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?
 
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...
 
Nuuk said:


Hey - that's three of us! ;) Where are you Richie? I'm in Burnham.

there are also 2 other matt's from bristol. (my name is matt)

richie00boy said:
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...

i have spent about £15 total on the amp so far, so it would not have been cheaper to make a P3A. case=free PSU=free chips=free caps/resistors=~£15

so you are suggesting the active filter will effect the sound less? so for that option, i change the input capacitor to a 470nf and a 330nf cap paralleled, and parrellel a 10nf cap with the 220k resistor?

which option would you 1) use, and 2) suggest i use?
 
Hi again,

OK, I see your point on the Gainclone :)

Are you prepared to mod the Gainclone? You could just turn it into a power active filter and it would give better results.

And

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...

Thought that might have given my preference away ;)

Any details on the sub you are working with? And what you are pairing it with/application?
 
Have you the thiele/small parameters for that sub (and can post)?

THAT BOX IS FLIPPIN' MASSIVE!!!

What are your main speakers?

As you are using a large speaker you will probably get away with the 50W or so that you will squeeze out of your chip amp. Which chip are you using BTW? You may not need to parallel as many as four together.

I would recommend that you increase the slope of the low-pass filter though, you will get too much leakage with 1st-order in music application. You could add the passive filter made from 4k3 and 470n as I described earlier IN ADDITION to the Gainclone mods to make a 2nd-order filter (1st+1st = 2nd).

Then when you have built this and got you addicted we can get you onto making a proper sub ;)
 
richie00boy said:
Have you the thiele/small parameters for that sub (and can post)?

THAT BOX IS FLIPPIN' MASSIVE!!!

What are your main speakers?

As you are using a large speaker you will probably get away with the 50W or so that you will squeeze out of your chip amp. Which chip are you using BTW? You may not need to parallel as many as four together.

I would recommend that you increase the slope of the low-pass filter though, you will get too much leakage with 1st-order in music application. You could add the passive filter made from 4k3 and 470n as I described earlier IN ADDITION to the Gainclone mods to make a 2nd-order filter (1st+1st = 2nd).

Then when you have built this and got you addicted we can get you onto making a proper sub ;)

the T/S values are here: http://www.bkelec.com/Professional/Loudspeaker Drivers/Eminence/PDF's/beta15.pdf

thats not big, i was going to build a ~500L one until a load of people talked me out of it (this is my first sub by the way).

mains? it says "Goodmans Mezzo" on the back, and has a "Goodmans" badge on the front, but thats all i know.

OPA549. i will be getting ~60W from the four chips parralleled, and to get ~120W i may parrallel both channels together (8 chips). this is peak by the way.

i was going to try the "active" solution you suggested. what "order" would this be, or dont active filters have orders? if not, what slope would this have?

i do want to build a proper sub, but i have a lack of finaces. i can only make this sub as SimonTY sent me the Eminence Beta, and the MDF will only cost ~£30. im hoping to get some more cash, and get a higher quality 18" driver, for about £300 and make a PROPER sub, or maybe a horn. :devilr:
 
matttcattt,

I thought this might be your first sub :)

I thought your box was way too big, but now that I've done some quick checks you aren't too bad.

Simulation of the beast

If you're using IE5.5 or higher, make sure you open the window to full size or it will re-size the image to be unreadable.

There are more pics if you wanna check 'em out - just change the beast3.jpg in the open window to 1-6 :cool:

As you can see, 60W peak as you are intending to use would be about right for the excursion limitations of this design. In fact, had you gone for the 500 litre monster you would actually have been limited to even less power handling, but it would be more efficient, so the net difference wouldn't be that great.

The order of a filter is exactly the same whether it's active or passive, it's to do with how many 'poles' it has and consequently the roll-off.

You could add the passive filter made from 4k3 and 470n as I described earlier IN ADDITION to the Gainclone mods to make a 2nd-order filter (1st+1st = 2nd).

So the high-pass would be 1st-order (perfectly adequate for this particular application), and the low-pass would be formed from two similar 1st-order filters - one passive and one active - therefore it would be 2nd-order.

Save your pennies and don't buy that crazy 18 incher just yet, you can have some fun playing and learning with this one, and you never know, you might learn a few things and realise that maybe the 18 incher is actually no good...

As for horns, I wouldn't bother unless you are limited to amplification of less than 10 watts and you don't mind having a cabinet take up 90% of your lounge.

The next step is to add EQ to the sub then, after that make the box about 60 litres and add a high-power amp... ;)
 
richie00boy said:
matttcattt,

I thought this might be your first sub :)

I thought your box was way too big, but now that I've done some quick checks you aren't too bad.

Simulation of the beast

If you're using IE5.5 or higher, make sure you open the window to full size or it will re-size the image to be unreadable.

There are more pics if you wanna check 'em out - just change the beast3.jpg in the open window to 1-6 :cool:

As you can see, 60W peak as you are intending to use would be about right for the excursion limitations of this design. In fact, had you gone for the 500 litre monster you would actually have been limited to even less power handling, but it would be more efficient, so the net difference wouldn't be that great.

The order of a filter is exactly the same whether it's active or passive, it's to do with how many 'poles' it has and consequently the roll-off.



So the high-pass would be 1st-order (perfectly adequate for this particular application), and the low-pass would be formed from two similar 1st-order filters - one passive and one active - therefore it would be 2nd-order.

Save your pennies and don't buy that crazy 18 incher just yet, you can have some fun playing and learning with this one, and you never know, you might learn a few things and realise that maybe the 18 incher is actually no good...

As for horns, I wouldn't bother unless you are limited to amplification of less than 10 watts and you don't mind having a cabinet take up 90% of your lounge.

The next step is to add EQ to the sub then, after that make the box about 60 litres and add a high-power amp... ;)

what program did you use for the graphs?

i have IE6, but i have turned off auto imnage re-sizing.

i will make a diagram later, when i have a chance, to make sure i have everthing clear. then i can order parts. :D

EQ? an equalizer? i want to try one, yes. i think ill probably try the Linkwitz Transform First. :)
 
The modelling was done using a home made program.

By EQ, yes, I meant equaliser. I'm a massive fan of the Linkwitz Transform, it's the only EQ worth using IMO. I have designed several PCBs for this kind of thing. A bare LW transform with adjustable subsonic filter and my latest one is as above but with L+R summing inputs, LFE input, adjustable phase control and adjustable low-pass filter as well :D

BTW, how did you get those OPA chips for free?

What supply are you using for your chip amp?
 
richie00boy said:
The modelling was done using a home made program.

By EQ, yes, I meant equaliser. I'm a massive fan of the Linkwitz Transform, it's the only EQ worth using IMO. I have designed several PCBs for this kind of thing. A bare LW transform with adjustable subsonic filter and my latest one is as above but with L+R summing inputs, LFE input, adjustable phase control and adjustable low-pass filter as well :D

BTW, how did you get those OPA chips for free?

What supply are you using for your chip amp?

i am going to make the linkwitz transform circuit in project 71 by rod elliot. (http://sound.westhost.com/project71.htm) do you think i need the buffer circuit? i am just going to sum the inputs using this circuit if i dont.

i ordered the chips on the Texas Instroments sample program. it took me 3 orders to get them all (3 each time). i wont be ordering any more. :dead:

voltage? +/-15v
or do you mean what an i using? 2 15v regulated power sluppies connected together. they are ~5A each max. when i tried them with the amp, i turned the pot up, and the volume went down. the rails sag massivly, so i am building a new power supply, 25A max, based on rob elliot's project 77 (http://sound.westhost.com/project77.htm). the circuit it on my website (or it will be soon, as i have only just thought of putting on there, and have only hosted whilst posting this :p).
 

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