Low-pass for Active woofer Gainclone

Atilla · 23rd November 2010, 05:53 PM

Just a quick question here. What if I wanted to make an active woofer, using a pretty standard LM3886 and a simple low-pass. Can't I just increase the size of the cap between the positive and negative input, so I make a low-pass filter like that? Say, between 0.05 and 0.005uF, depending on what I'm doing? And make it a good polyprop cap, while at it.

richie00boy · 23rd November 2010, 06:01 PM

Not that cap, that one is used to reject RF hash. You are best off putting an active filter before the amp. You can use a passive filter, but their response is too droopy I feel.

The worst way would be to add a cap in parallel with the feedback resistor. In non-inverting mode you would still get treble bleedthrough as the lowest gain you can have is 1 and the roll-off is just 6dB/octave which is pretty poor. Also the chip is probably not stable at unity gain.

Atilla · 06:16 PM

If I put a filter before the amp, I'd make a good, active filter. But really - I couldn't think of any side-effect of increasing the inputs cap to the point where it'd attenuate enough to get down in the audio band. So I'm looking to find one, so I can focus on the active low-pass then

I wouldn't filter an LM3886 trough the feedback no. IIRC, it's far from unity-gain stable. Thanks for reminding me though, considering what I'm asking, I shouldn't be forgetting anything about it.

planet10 · 23rd November 2010, 10:12 PM

Can you post the schema you are using?

I can't recall seeing a shunt cap on a gain clone.

A chipamp is just a high powered opamp, so it can be configured as a 2nd order LP (or HP) filter (or even fudged into a 3rd order).

Body

dave

abraxalito · 23rd November 2010, 11:06 PM

Quote:

Originally Posted by planet10

A chipamp is just a high powered opamp, so it can be configured as a 2nd order LP (or HP) filter (or even fudged into a 3rd order).

True to some degree only - chip amps are not unity gain stable without some fudging networks around them - the K value in your diagram is unity for a typical Sallen-Key configuration.

sangram · 24th November 2010, 04:19 AM

I use a 6dB network in front of the chipamp, and another in front of the preamp (if present/necessary). Works well enough - if you need steeper slopes, active filtering is the way to go.

High pass is significantly easier, I just use a small feedback capcitor (Ci) to provide additional 6dB rolloff, plus the 6dB in front.

The one time I messed with the series feedback path I had severe issues with howling feedback, so I didn't pursue that road (STK chip and speakers both survived the ordeal).

The cap between the inputs is also a feedback path, IINM. I'd be interested in the results of this increase, it does seem like a good place to provide rolloff but never tried it myself.

Atilla · 11:24 AM

Well, I'll give it a shot and I'll see how it goes. It would be nice to measure freq. response with something though, so I can confirm how it works.

I'm talking about Cs in a configuration like this:

That's not the schematic I'm doing, but it'll do as an example. Just bump Cs to a suitable value for my woofer?

richie00boy · 24th November 2010, 12:19 PM

You would be better putting a cap from the non-inverting input to ground, in conjunction with the 330R it will form a traditional low-pass filter.

Atilla · 24th November 2010, 04:11 PM

Well, that is a fair point, I wouldn't need a bigger cap for that.. Oh well

I'll either install whatever is easier to fit on the board I've got, or I'll place an active filter before it.

Thanks!

richie00boy · 24th November 2010, 05:44 PM

Do the active filter, 6dB/octave is rubbish on a sub.

23rd November 2010, 05:53 PM	#1
Atilla Soldering Gun Fanatic diyAudio Member Join Date: Jul 2008	Low-pass for Active woofer Gainclone Just a quick question here. What if I wanted to make an active woofer, using a pretty standard LM3886 and a simple low-pass. Can't I just increase the size of the cap between the positive and negative input, so I make a low-pass filter like that? Say, between 0.05 and 0.005uF, depending on what I'm doing? And make it a good polyprop cap, while at it.

23rd November 2010, 06:01 PM	#2
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	Not that cap, that one is used to reject RF hash. You are best off putting an active filter before the amp. You can use a passive filter, but their response is too droopy I feel. The worst way would be to add a cap in parallel with the feedback resistor. In non-inverting mode you would still get treble bleedthrough as the lowest gain you can have is 1 and the roll-off is just 6dB/octave which is pretty poor. Also the chip is probably not stable at unity gain. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

23rd November 2010, 06:14 PM	#3
Atilla Soldering Gun Fanatic diyAudio Member Join Date: Jul 2008	If I put a filter before the amp, I'd make a good, active filter. But really - I couldn't think of any side-effect of increasing the inputs cap to the point where it'd attenuate enough to get down in the audio band. So I'm looking to find one, so I can focus on the active low-pass then I wouldn't filter an LM3886 trough the feedback no. IIRC, it's far from unity-gain stable. Thanks for reminding me though, considering what I'm asking, I shouldn't be forgetting anything about it. Last edited by Atilla; 23rd November 2010 at 06:16 PM.

23rd November 2010, 10:12 PM	#4
planet10 frugal-phile(tm) diyAudio Moderator Join Date: Oct 2001 Location: Victoria, BC, NA, Sol III Blog Entries: 4	Can you post the schema you are using? I can't recall seeing a shunt cap on a gain clone. A chipamp is just a high powered opamp, so it can be configured as a 2nd order LP (or HP) filter (or even fudged into a 3rd order). Body dave __________________ community sites t-linespeakers.org, frugal-horn.com ........ commercial site planet10-HiFi p10-hifi forum here at diyA

24th November 2010, 11:19 AM	#7
Atilla Soldering Gun Fanatic diyAudio Member Join Date: Jul 2008	Well, I'll give it a shot and I'll see how it goes. It would be nice to measure freq. response with something though, so I can confirm how it works. I'm talking about Cs in a configuration like this: That's not the schematic I'm doing, but it'll do as an example. Just bump Cs to a suitable value for my woofer? Last edited by Atilla; 24th November 2010 at 11:24 AM.


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24th November 2010, 04:19 AM	#6
sangram diyAudio Member Join Date: Sep 2002 Location: India	I use a 6dB network in front of the chipamp, and another in front of the preamp (if present/necessary). Works well enough - if you need steeper slopes, active filtering is the way to go. High pass is significantly easier, I just use a small feedback capcitor (Ci) to provide additional 6dB rolloff, plus the 6dB in front. The one time I messed with the series feedback path I had severe issues with howling feedback, so I didn't pursue that road (STK chip and speakers both survived the ordeal). The cap between the inputs is also a feedback path, IINM. I'd be interested in the results of this increase, it does seem like a good place to provide rolloff but never tried it myself.

24th November 2010, 12:19 PM	#8
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	You would be better putting a cap from the non-inverting input to ground, in conjunction with the 330R it will form a traditional low-pass filter. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

24th November 2010, 04:11 PM	#9
Atilla Soldering Gun Fanatic diyAudio Member Join Date: Jul 2008	Well, that is a fair point, I wouldn't need a bigger cap for that.. Oh well I'll either install whatever is easier to fit on the board I've got, or I'll place an active filter before it. Thanks!

24th November 2010, 05:44 PM	#10
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	Do the active filter, 6dB/octave is rubbish on a sub. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

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