New York Speakers Part 4: Drive Electronics

UPDATE 12/31/07: I revised the crossover board to version 2.0. This includes a gain of 6dB in the subwoofer path, to compensate for a mis-calculation I made when I changed the subwoofer amplifier from a bridged amp to a normal amp (thus losing 6dB). I also changed the opamps to TL074 which is a mostly equivalent but lower noise version of the TL084.

If you're the type of person who doesn't recognize the validity of any circuit constructed using devices having more than three terminals (do you make your own water from hydrogen and oxygen too?) read no further. Otherwise, let's talk about subwoofer plate amplifiers. This plate amp's going to need a few things.

1. 3 amplifiers, to drive stereo left right, and the subwoofer.
2. An electronic crossover.
3. A preamplifier, with separate bass control and phase reversal.

While I have built several amplifiers from discretes, both bipolar and MOSFET, class A, B and D, in this case space is a premium and the application isn't honest hi-fi. It's hi-fi given the size, which is why I chose to go with a monolithic amplifier chip, the LM4781. It's a 27-pin TO220 package with three channels of 35W each into 8 Ohms, from National Semiconductor's Overture series. Even with this chip though, I don't have enough room given the enclosure dimensions to fit everything on one board. So I'm going to have to split the circuit into two boards. One board will contain the rectifier/filter and power amplifier, and the other board will hold the preamplifier and crossover. The transformer is mounted inside the cabinet away from the electronics, to reduce EMI and gain space on the plate. The plate amp should look something like this:
The first thing to work on is the crossover board. I'm going to show the schematic, then explain it.
The input comes in through DC blocking capacitors, then goes directly to a header which connects to a dual-gang pot for volume control. It's then buffered and gained by 8dB, the output of which feeds the high-pass and low-pass filters. These are standard 40dB/decade second order filters, the high-pass filter is tuned to about 100Hz, and the low-pass filter is tuned to 250Hz. The reason for the asymmetry is the bandpass box is tuned for a high cutoff of 100Hz, so the low-pass filter is not strictly necessary; it's only there to prevent high-order vent resonances which sometimes happen in these alignments, when you rely entirely on the box geometry to do the filtering.

The subwoofer enclosure crossover is fourth order bandpass, the sattelite enclosures are second order sealed and the satellite electronic crossover is a second order which gives the combined response a fourth order shape as well.

In the subwoofer path there is a gain of 2 in the filter bank, this is to account for the lower sensitivity of the woofer. The gain is accomplished by making the filter values unequal; their values are chosen to maintain a filter Q of 0.5.

Following the low-pass filter is a standard inverter, which in conjunction with the non-inverting signal goes to a DPDT switch, allowing you to choose between 0 and 180 degrees of phase. I toyed briefly with the idea of implementing a continuous phase adjuster, but I've always been wary of the standard analog implementation of that concept (it basically takes the phase and puts it in a blender) so I ultimately decided against it. The output of the DPDT switch goes into a second dual-gang pot, which controls bass level. Resistors R2 and R3 are there to make the linear pot behave logarithmically, or at least somewhat logarithmically. (It's strictly a rational function that looks logarithmic at the volume where our ears are most sensitive.)

Next we have the main amplifier board, but this is uninteresting as it's just three copies of the datasheet suggested application.

You might wonder why I bothered to bring the bass information in as stereo, if I sum it right off the bat. Well, I have in mind to one day make a more powerful amplifier with two stereo amplifier chips, using one of them in bridge mode driven with the stereo bass signal, one in-phase one out-of-phase. By have in mind, I mean I made the PCB already but thanks to a stupid ground loop due to a misunderstanding of how the chip is wired internally, it's useless and I shelved that idea. Note that the 7.5 Ohm output resistors actually have parallel inductors that aren't shown on the schematic, this is just to quench high frequency oscillations although I feel they aren't strictly necessary as these chip amps are very well behaved.

This is pretty much it; the resistor values might look strange but I just chose the numbers I had on hand. You can make liberal substitutions save for perhaps the crossover values without degradation in performance. Just remember to use components rated for the load in the power section, and to use bipolar caps in the feedback resistor path...

Here is the PCB artwork for the amplifier board, and the crossover board. Just send this to AP circuits to get boards made.