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Old 24th July 2011, 06:20 PM   #1
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Default Gainclone 2.1 setup

I built a Gainclone using 2 x LM3875 chips a year ago which I am very happy with. I have just built 2 full range speakers using Tang Band 3 inch drivers in Ikea salad bowls (you know the sort).

Now, obviously the 3 inch speakers have not got much bass, so I was thinking of building a small sub woofer and running it from a LM3886.

Now, my question is, how I go about 'splitting' the input signal into my sub amp...... Obviously the sub amp will take the right and left channel from the input and combine them into one output.

At the moment, each input is connected to a 4.7uF cap, which is then fed through a 220ohm resister into + of the chip. There is also a 22K resister connected to audio ground after the 4.7uF cap.

So, basically I want to know where I get the signal to put into my sub amp, as surely if I just simply got the input signal from the left and right channel, fed them both through 4.7uF caps and then 220 resisters and then connected them together to go into the chip, would I effectively be making the left and the right channel the same, and therefore my original Gainclone would be outputting a mixed mono output?

Would it be better to use 2 x LM3886 in bridged mode and feed the left channel into +ve of one chip and the right channel into -ve of the other chip?
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Old 24th July 2011, 07:21 PM   #2
AndrewT is offline AndrewT  Scotland
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Buy or build a summing amplifier with Low Pass filtering built in.

An MFB active filter is an inverting amp with 2pole low pass filter, all off one opamp.
The input resistor is simply doubled up to create two inputs. These two inputs sum the Left and Right channels.
The output can be fed to an inverting chipamp then to the speaker, or can be fed to a non-inverting chipamp then to the inverted leads of the speaker.
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Old 24th July 2011, 07:59 PM   #3
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Thanks for the reply AndrewT.

Can you explain a little more about the MFB active filter please.

With an MFB active filter, can I leave my original amp as it is and simply build a monoblock with an MFB at the front end and 'feed it' with the same input signal as my original amp?

Thanks again AndrewT, you are always there when I need an answer


Is this the kind of circuit you are talking about..Multiple Feedback Bandpass Filter.

If so, I assume I build this type of circuit and use values of components to give me 0 gain, and then feed the 2 outputs into the front of an LM3886 amp?
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Last edited by portreathbeach; 24th July 2011 at 08:04 PM.
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Old 25th July 2011, 04:14 AM   #4
eccdbb is offline eccdbb  United States
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You could also do a Sallen Key lowpass. Use a 2 gang pot and make the cutoff variable.

http://focus.ti.com.cn/cn/lit/an/sloa024b/sloa024b.pdf
Operational amplifier applications - Wikipedia, the free encyclopedia
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Old 25th July 2011, 08:51 AM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by portreathbeach View Post
Is this the kind of circuit you are talking about..Multiple Feedback Bandpass Filter
yes MFB active filter.
Note it is inverting.
Note that you double up on R1 to give two summing inputs.
gain = 0. NO!!!! Gain = 1times, or 1.5times, or 0.5times, or any other value you require
Note that the sum function adds the two inputs together.
If the stereo signal (two channels) are fed to the sumer and both consist of the same (mono) bass signal then the output is gain * [channel1 + channel2] you might need gain = 0.5times, if you use drivers and amplifiers all with the same sensitivity.

If you want wideband signals to pass to your speakers you can remove the high pass part of the bandpass. However many builders want to remove the sub-bass signals to prevent damage to their gear. You decide how low you want the bandpass to go to sound right for your situation.
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Old 25th July 2011, 03:54 PM   #6
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Sorry AndrewT, when I said 0 gain, I actually meant no amplification of the signal, so I should have said '1'. Anyway, I see your point with the summing of the two channels, so I would have to work out the actual gain I require from the MFB filter.

I also originally asked how I would connect this to my original amp, would the following work?

Click the image to open in full size.
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Old 25th July 2011, 05:58 PM   #7
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Just looking at a few schematics for parallel and bridged LM3886 amps and I notice that the input is fed into 2 or more LM3886 chips.

Could I not simply build a bridged LM3886 amp, feed left channel to + of one chip, right channel to -ve of the other chip? The output from the 2 LM3886s would then be fed through a low pass filter and to the speaker.

So in actual fact I would have a stereo amp, where one channel is normal, the other is inverted and there is only one output.

Doing this, I don't have to alter anything in my original Gainclone to compromise the quality I get from it already. I could simply split the input signal to both amps. Would this work?

Any thoughts.
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Last edited by portreathbeach; 25th July 2011 at 06:06 PM.
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Old 26th July 2011, 04:38 PM   #8
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An MFB is, as the name suggests, a band-pass filter. I don't see how that would help you.

You want a stereo high-pass filter for your 3-inchers and then you can choose from a stereo set of low-pass filters followed by a mixer or a mixer followed by a single low-pass filter for the subwoofer. Something like this. You will have to adapt the frequencies and Q-factors to your needs of course.
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Old 26th July 2011, 04:51 PM   #9
AndrewT is offline AndrewT  Scotland
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MFB is not only a bandpass.
ESP shows the application of his bandpass filter.

The MFB can be used as a high pass or as a low pass.

Have you downloaded Ti filter software? The newer versions are not as easy to use as the old versions, I found about a decade ago.
Butterworth Q = sqrt(2)
L-R Q = 0.5
Two cascaded single pole Butterworth gives a 2pole L-R
Two cascaded 2pole Butterworth gives a 4pole L-R.

The MFB low pass is a very convenient way to sum two (or more) channels and low pass filter with presettable gain, presettable Q and presettable Fr each varied independently. Ideal for a 0.1 low bass speaker and very easy to then adjust for variable Q and variable Fr.
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Last edited by AndrewT; 26th July 2011 at 04:54 PM.
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Old 26th July 2011, 06:50 PM   #10
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Thanks for the advice guys. I've just downloaded TI Filter suggested by AndrewT and installed it on my virtual machine on my iMac. I have to say I am even more confused now than I was when I started.

Here is what I understand...

I have a source which comprises of a left channel and a right channel (2 channels). I want to amplify the signals from these 2 channels to 2 speakers and also want to combine these channels and amplify the mono channel to feed a subwoofer. So, I need to take the signals L and R and then feed them into some kind of active filter arrangement. After the filters, I can end up with a Left channel, R channel and a mixed low frequency channel that I can feed into 3 separate amplifiers.

I have taken a look at the TI Filter software, and to be honest, I don't really understand much of it. I'll keep pursuing...


(edit) I just stumbled across this site... http://www.beis.de/Elektronik/Filter/Filter.html

Seems quite interesting and fairly easy to use. Any thoughts.
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Last edited by portreathbeach; 26th July 2011 at 07:09 PM.
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