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Will this circuit work?
Will this circuit work?
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Old 25th February 2008, 10:06 PM   #1
Kolloi is offline Kolloi  United Kingdom
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Question Will this circuit work?


Just a quick question about this circuit I designed.

The big idea is to take an audio signal, put it through second order low-pass and second order high-pass passive filters so that it's rolled off at 40hz and 15000hz, and then split the signal so that it goes into:

1) a combo of 2nd order low- and high-pass filters rolling off at around 60hz and 125hz; and
2) a combo of 2nd order low- and high-pass filters rolling off at around 2000hz and 4000hz.

After this, the mixed signal from (1) and (2) goes through an op-amp IC in differential configuration to increase the loudness. The feedback and potential divider resistors going to the op-amp are potentiometers i.e. variable resistors.

Please check that the connections are correct, and that the resistances I worked out are correct for the frequencies I want to obtain. The only silver mica capacitors I can obtain are 150pF, so all the caps in this circuit have this capacitance.

Any other comments about the circuit vis a vis its chances of success are welcome. Thanks
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Old 25th February 2008, 11:08 PM   #2
gootee is offline gootee  United States
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Hi Kolloi,

I can't say offhand if it will work as you intend for it to work. You might need to either use active filter (i.e. containing opamps) circuits, or, use opamps as unity-gain buffers in between some of your stages.

Note that there is a good 'automatic active filter design' program available, free, from Texas Instruments, at http://www.ti.com . You can go there and do a search for FilterPro, and download it. I have used it and it works very well. It gives a labeled schematic and plots the frequency response, given your specs, for any of the many types of standard filter responses, from which you can choose.

I strongly recommend that you download the free LTspice software, to simulate your circuit's operation.

You should be able to right-click on the following link and then select 'Save target As', to download the LTspice installation software:


Have fun!
The electrolytic capacitors ARE the signal path: http://www.fullnet.com/~tomg/zoom3a_33kuF.jpg
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Old 26th February 2008, 12:34 AM   #3
Leolabs is offline Leolabs  Malaysia
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Just add an input buffer will do.Also the output buffer should be a JFET OPAMP.
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Old 26th February 2008, 05:20 PM   #4
Kolloi is offline Kolloi  United Kingdom
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Join Date: Dec 2007
Thanks gootee! Those programs look very useful and I will make some use of them in the future for sure.

Leolabs, thanks for your note about the op-amp type. I have added an input buffer stage as per your advice, and it looks like this now:


Hopefully this will ensure that the signal is strong enough for the passive filters to have a meaningful effect, and for the output to be at line level. At the moment the resistors around the 2 op-amps use pots, so that I have a realistic idea of the feasibility when I build it and play around with it.
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