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The Critical Q Sub-Woofer

Earlier Sub Plate Amplifiers are no longer available. But PartsExpress makes what looks like a suitable amplifier, part number 300-804. According to the PDF file, the stock unit without any changes will give you good all round performance being -1.5dB @ 20 Hertz. But if you limit the subwoofer for only audiophile use, change R35 from 24K to 15K and R36 from 150K to 470K.

The box cut-out for the amplifier now need to be 216mm by 216mm.


Step-by-Step Changes to AA-0501 Amp Described Below











Input PC Board Module


15K-56K ?

56K 1/4 Watt

Sets Boost to O dB


120K-180K ?

180K 1/4 Watt

Sets LF Extension


On the Output of Input Module


This adds a fixed Low-Pass Filter and is Fitted to the Input Module, see photo

470 Ohm 1/4 Watt

Metal Film 1/8 or 1/4 Watt Rating

0.33uF 63V

Must be Film type - Polypropylene Recommended


Power Amp Module


4.7uF Electro

3.3uF Film 63V min

3.3uF to10uF Film

The C1 is cut away from PCB and the replacement 3.3uF Film type is soldered onto the ends of R1 and R2

Study photo very carefully to see how new C1 is soldered


Step-by-Step and Comments

Input Module: First it needs to be dismounted. Remove all screws from the back plate and also three knobs. Remove knobs carefully using something plastic to lever them of, they are friction fitted only. Behind the knobs are retaining nuts, use a fine pointed needle nose pliers to remove. The PCB may not yet come free, it seems that in some cases the thick gasket is glued and you may need a very sharp blade to cut free.

Desolder and remove R26 and R27 from PCB. The PCB is now remounted, the new R26 and R27 will be fitted on the rear of the PCB. See photo blow. The new R26 (56K) and R27 (180K) can clearly be seen on the left side of PCB.

On the Output of Input Module: Now unsolder the Green Wire that is part of the wiring harness to Power Amp Module, it is right on the end next to the Yellow wire. Solder 470R to the point where you removed wire, now wire Green Wire to other side of 470R. Also from this junction, add 0.33uF to a ground point. Study the photo below carefully. Please note carefully where the 0.33uF Cap is grounded on the PC Board.

Power Amp Module: Finally, and this is a little tricky, we need to replace the yucky 4.7uF electrolytic used input coupling cap. Cut away old C1 with sharp pliers, chances are you will destroy it in the process of removing. Next stage is tricky, wee now need to fit 3.3uF film cap. But this cannot be done unless you would like to remove the whole module, not recommended. Instead it can be fitted to other components with ends exposed enough to be soldered to. These are R1 and R2. Please look carefully at the photo and illustration shown below. You will need a very fine pointed solder tip, pre-solder the correct ends of R1 and R2. Also pre-solder the ends of 3.3uF cap, as this will assist in a tricky job. Do not overheat. Finally use a little bit of silicone glue to mechanically stabilise cap (not shown).


 This requires good soldering skills

The work is now concluded, please check and double-check everything, And when you have done that, then triple-check. Beware that you have no warranty to go back on.

If you have any doubts whether you have the skills, do not proceed. It may be worthwhile to find a qualified technician, show him this page and let him proceed. Please understand YOU are responsible for the end result and YOU have been warned!

See Box Construction Page for detailed Drawings, Cutting Sheet and other Box Diagrams

See User Setup Page for Operating Instructions.


Send mail to joeras@vacuumstate.com with questions or comments about this web site.
Copyright 2003-10 Joe Rasmussen & JLTi
Last modified: Sunday June 07, 2015

Just had a terrible thought. If "intelligent design" is unscientific, then who will design our audio equipment?