DIY Servo Subwoofer Amp Build

These are some build pictures of the subwoofer amp and feedback filter.

Enclosure

The enclosure came from AliExpress, and is the most expensive part of the project. It is a very solid box. Worth the money, I think. I picked it because it was large enough for my last project, a 150W per channel MOSFET amp, and I wanted matching boxes for the two. This one with a knob, and the last one without. Obviously you could put it in any box or use any mono power amplifier. The amplifier is not modified in any way.

This is the layout I ended up with. It offers the most direct path between modules. The box marked POT at the bottom is the stepped attenuator I chose for the input level control. I decided on it because the steps are discreet, and when I bump it, it won't change the level. The level is for adjusting the volume to match the main stereo amps and speakers. If I change speakers or amplifiers, I'll have to turn that knob.

Power Supply

The 400VA transformer is from AnTek. It has 2x 34V@4.9A secondaries and 2x 15V@2A secondaries. The big ones feed the amplifier through the rectifier board (right), and the 15V secondaries feed the power supply on the VFB filter board. The main supply is ±48VDC for the amplifier, which will just allow for 140W into 8 ohms. 140W into 8 ohms yields 4.18A, at a voltage of 33.46V_RMS.

Transformer at AnTek

I've had bad luck with some imported capacitors, so I got 80V caps, just to be safe. The rectifier is rated "120 Amp", but in fact it is 2 15A diodes in parallel, at any given time. It is adequate for this project, but don't count on 120A, or you will be disappointed.

Electronics

The amplifier is a 200W amp from AliExpress, which has 2 pairs of 2SA1943/2SC5200 output devices. I used a 5.9 amp 34V 400VA transformer, which will provide ±48V for 140W @ 8Ω. I think if it is called on to produce 140W, the subwoofer will melt - it's only good for 40W per voice coil, and I've taken one for feedback. But then it had 100W on it with the bridged amp, and didn't have any trouble.

I ordered a pair of amplifiers, so I could have one spare. They were only $15 each, made with used power transistors. I picked up a few pairs of transistors and replaced the outputs and 2 capacitors that were in bad shape. One of them was swollen from overheating or over-voltage.

The velocity feedback board has its own power supply, using the 15VAC windings on the transformer. It takes the 2 low-level inputs and one of the two subwoofer speaker cables as inputs, and outputs an error signal to drive the power amp. The amplifier drives the other speaker cable. See Velocity Feedback for specifics, like how to adjust the circuit.

I couldn't find a delay-only relay board that I liked, so I built this one to run off of 18VAC and do most of the things the μPC1237 does, and a little more. The delay is programmable via the RPi Pico. For this project, I wanted a 3-second delay to allow the output to stabilize before connecting the speaker. It controls 2 relays - one for the speakers and one for the power transformer. You can see the speaker protector project to get further details on what it does. The relays are mounted on the rear panel of the box.

The schematic of the delay PCB. The RPi Pico handles the smarts for getting the heatsink temperatures, reading the power switch, and reading the output of the μPC1237.

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