Adding a Subwoofer Output

So far the amplifiers I’ve built have not included subwoofer outputs. When customers have asked about integrating a sub, I’ve directed them to the Paul McGowan video in which he expresses the view that subwoofers are best fed with speaker-level input signals. If you want to know why, hit up the link and listen to Paul explain it in his usual unique style 🙂

Recently however one customer requested a sub output to run a sub that doesn’t have speaker level input. How to deal with this dilemma?

Thump thump rumble rumble

I briefly contemplated creating an external resistor divider box that would take the speaker input and drop it down to line level with a voltage divider and appropriate plugs and connectors, but quickly dismissed that idea as too makeshift.

Thoughts of a buffered output stage taking signal off the volume control came to mind. This would require two extra bottles on the chassis, with the attendant re-work, not to mention recalculating my power supply and all the other added hassle.

A lot of work to add a simple feature!

Then the idea occurred to me that since the subwoofer has a nasty Class-D amp in it anyway, what did it matter if the sub output stage was tubed at all? Especially since in my amplifier design doesn’t route the input signal straight to the volume control, it’s already tube-buffered by that point (the active tone controls are before the volume control)

So, I got busy with the breadboard and made up a circuit with JFETs – being the closest solid-state device to the behaviour of a tube. The intention is to sum both channels and reduce this to a mono output which is then buffered with a cathode Source Follower.

One important consideration was that I needed the circuit to be powered by 9VDC since I have a 9vac transformer feeding the remote control standby logic board in my design. The idea is to power this with a bridge rectifier feeding a LM7809 linear regulator with appropriate filtering to make the power clean.

After a little bit of experimentation on the breadboard, this is the first circuit I arrived at:

Click to see fullsize

That circuit contains a low-pass filter, not strictly needed since the subwoofer it’ll play into has a turnover frequency control. This circuit provides unity gain if both channels are driven with the same input (which would normally be the case; bass in recordings is usually monophonic)

The only problem with this circuit is it wasn’t exactly all that clean, THD was up around 1.2% which I wasn’t happy with. So I decided on another implementation

This one also includes output filtering, but in this case it’s nested in the NFB loop. On LTSpice, the output looks like this:

Testing on this one yielded a result approximately in line with this prodiction, and a much better distortion result, 0.08% at 1v p-p, with absolutely unity gain (as a result of R16 – amazing what a bit of negative feedback will do!) Also I didn’t bother putting the filter in this one, since a) I was running out of room on the breadboard and b) it’s not really needed anyway.

Also, this circuit inverts phase. As with any subwoofer implementation, using critical listening in lieu of measuring instruments will provide the optimal volume, turnover frequency and phase settings.

How it looked on the breadboard

After the successful breadboard trial, a PCB was designed and ordered, the finished version looks like this and works perfectly:

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