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Almighty Subwoofers

Part III - Filter details

These are the details of the filter circuit. The different filter stages are in the same order as shown on the block schematic.

Filter schematic

The OP's I used was the quadruple LM837, which makes one piece of silicon enough for a complete filter channel (two needed for stereo). Better devices (possibly better, I have not A/B tested them) are Linear Technology LT1115CN8 and Analog Devices AD846. Both types are much more expensive than a LM837, and they both contain only one single OP.

The OPs may need offset adjustment to avoid DC potensial at the output. This is somewhat dependant on the OP type you choose. The LM837 will need it, but the LT1115 should not. The offset adjustment is best carried out by feeding the negative input of the last OP the required DC voltage for zero output offset through a high value resitor. Alternatively, you can put in a large high quality cap at the output. The value of this must be large enough to avoid low frequency cut-off, 5 Hz is a good target to aim for. If your power amp has an imput impedance of 47kOhm this cap should be at least 0.68 µF.

Resistors must be 1% metal film types, and the caps should be some good polyprolyplene type. It's difficult to find 1% or 2% caps that sounds good. Instead, I buy a large amount (hundreds) of caps, and measure them to pick out the ones that fall within 2% tolerance. My favourite cap is Roederstein ERO MPK1841.

A power supply is of course necessary. This must be a regulated type, and its voltage must not exceed +/- 18 V. If it does, your OP-AMPs will be joining Elvis. The simplest solution is to put in a pair of 78xx / 79xx regulators, and feed them from the main powersupply of your power amplifiler. What I have done is to use two 12V lead-acid batteries, and trickle charge them from a dedicated charger. This is the best sounding power supply solution.

This filter concept is quite flexible, and will work with just about any subwoofer. The component values must of course be altered when different cut-off frequencies and filter Q's are needed.

Stig Erik Tangen

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