high gain 2nd order high pass active filter topology

[consort_ee_um]

Here is a circuit which conforms to the above It gives 6db of boost on a gain of 27dB. The ratio of R1 and R2 defines the amount of boost. Can use any power op-amp with the proviso that the input resistance is high. ST quote 5Mohms for the TDA2030A. The naughty people at National do not quote an input resistance for the LM3866 but they do say in the notes that a 100k resistor across the input defines the input resistance as 100K thus implying the resistance is high enough not to affect the filter.
This circuit in combination with an underdamped vented box design would make a better sub-woofer IMHO than the popular closed box and Linkwitz transform design. It will have lower cone excursion and voice coil temperature for the same acoustic power.

[djk]

Nice to see a drawing of what I said in post #8

Post #8
Just pretend the feedback node of the power amp is the output node of a unity gain opamp. If the lower arm feedback resistor is low enough then you won't have any problems.

If the lower arm feedback resistor is a high value, then just add a voltage divider at the output of the amp that has the same ratio as the feedback resistor pair. Sony did this around a 30dB preamp line stage, works fine.

The NAD 3020 integrated amp uses this on the main power amp input too, the lower arm feedback resistor is 27R.