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Old 21st March 2005, 01:34 AM   #1
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Default Active BSC and RC circuit - help needed

I am fine tuning my active XO / EQ. I have a number of circuits that use something like the following to perform BSC or other shelving low pass filters (opamps are OPA2134).


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Questions:

1. I want to scale up / down the values in order to obtain smaller capacitor values (I can get some cheap, high quality polystyrine caps) and higher resistor values because I suspect that anything less than 5k for R1 may drive the opamp into clipping more easily. In the circuit above (the values are arbitary), Can I replace R1 from 1k to 10k, R2 from 2k to 20k and C from 0.22uF to 0.022uF without affecting the function of the filter?

2. From your experience, what would be the lowest R1 value you would use so that the opamp is not driven too hard, and what would be the highest R1 value you would use so that noise is not a problem?

3. It would be ideal if the BSC circuit has a pot to fine-tune the dB gain on a fixed capacitor value. Do you know if there is such a circuit and where is the link?


Many thanks in advance.

Regards,
Bill
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Old 21st March 2005, 08:52 PM   #2
Svante is offline Svante  Sweden
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1. Yes.

2. I think I would keep R1+R2 to about 5-10 k, but the data sheet of the opamp could help you here.

3. I have seen schematics of such circuits on the web, but I don't remember the link right now.

Another thing; the circuit you have drawn would give a 9.5 dB raise (3 times gain), but the typical BSC should be 6 db (2 times) or less. So, typically R1 should equal R2. All assuming that you plan to use the + input as the input of the circuit, and that the - input is not connected to anything but the feedback circuit.
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Old 21st March 2005, 11:30 PM   #3
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Svante,

Thank you so much. I am a 9 months old DIY newbie and have no EE background but I worked out from maths that it would be fine. It was good to be confirmed by an expert like you.

The values in the circuit are for illustration purpose and of course it is not a BSC. I now believe that there can not be a circuit that allows adjusting the gain of BSC by using only one pot. It would change the F0 point. Two pots make it possible, but require the use of a pre-defined set of values for the pots for different gains.

I read a lot of your posts and learnt a great deal from them. There was only one thing that I might not have understood properly what you meant. That was in a series of discussions of back-EMF in woofers connected in serial or parrallel. You proved that back-EMF is the same when connected in either way. I searched this entire forum a few months ago when I was learning the best method of connecting my woofers (4 per channel in a tower speaker). While back-EMF may be the same, I have heard many times and my experiments showed that connecting woofers in parrallel sounds substantially better than connecting them in series. It was an active system and no passive components were connected to the woofers. The amplifier had a very stiff power supply and could deliver an ample amount of current. I guess that this problem may not be due to back-EMF. Probably no amplifiers like high inductance. Serial connections double up the inductance while parallel connections reduce it. With 4 woofers per channel, I have to use 2 power amplifiers to power them.

Best regards,
Bill
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