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AC analysis Can anybody tell me what I need to do to this circuit to keep the gain at 0dB until 23kHz to 25kHz?

The gain goes to -6dB. I want the gain to remain at 0dB until it's 23kHz to 25kHz, that's when I want the cut-off to start. passive filtering

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    \$\begingroup\$ Can you tell us what your filters are designed for and where you think the problem might be? \$\endgroup\$ – Samuel Feb 11 '16 at 21:27
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    \$\begingroup\$ What are the source and load impedances? \$\endgroup\$ – Brian Drummond Feb 11 '16 at 21:45
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    \$\begingroup\$ The filter response looks like a donkey's hind leg. I notice it has neither source nor load resistance, a filter needs at least one of those to be designable, but it's easier with both. What is the filter prototype response, what was it designed to, or where did the circuit values come from? \$\endgroup\$ – Neil_UK Feb 11 '16 at 22:14
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Your circuit has differential inputs and outputs which are balanced with respect to ground. However you are only measuring the output on one side, so you only see half the voltage (-6dB). To get the total output you need to measure both sides and subtract one from the other, like this:-

enter image description here

The sharp peak is caused by resonance in the LC filter. It will be reduced if you add a realistic load impedance.

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