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 Elliott Sound Products Project 116 

Project 116 Subwoofer Amp

© January 2006, Rod Elliott (ESP)

PCB   Please Note:  PCBs are available for this project, except for the BP4078 (order direct from ColdAmp).  Click the image for PCB details.


As promised in Project 114, here is a complete description of a ColdAmp powered subwoofer amplifier.  There are several differences between the one in the photo and the recommended design - mine is a prototype, but yours will be intended as a finished product to use in your system.  It is a lot easier for me to make major changes to chassis work than for most people, because I have the tools and material on-hand.

Figure 1
Figure 1 - Panel View of Sub Amp

There's not a lot to see, because most of the interesting things are hidden.  You can see a row of holes down the upper middle of the panel, and these are access holes for the pots on a P84 1/3 octave equaliser.  They are sealed by using a piece of plastic suitable drilled out (details of this are shown below).  Although I used a Project 48 Subwoofer Equaliser, you can also use a Project 71 Linkwitz Transform Circuit along with a suitable crossover network.

The only other items are the volume control and phase switch, along with the obvious input RCA connector.  A fused IEC mains connector and power switch complete the accessible operator controls.  There is no provision for speaker level connections, for the simple reason that they are (IMHO) a very bad idea, and rarely (if ever) work properly.

The fins were added to a simple flat aluminium panel, and are made from 20 x 20 x 3mm aluminium angle.  Their purpose is threefold - they improve the performance of the plate as a heatsink (although this is not necessary with the BP4078 PWM amp), they make the panel a lot more rigid, and provide essential protection for the volume control and phase switch.  It also looks better IMO - it looks like it means business, rather than looking like one of those wimpy 50W plate amps you see advertised.

In case you wondered, yes, the edges of the fins have been polished back to metal.  This makes minor scratches virtually invisible, rather than taking off the paint and looking like major scratches.  This is a prototype, after all, and it will get changed about over time, requiring it to lie on the fins while it is fiddled with.


Now, on to the business side.  The ColdAmp module is easily recognised, and the P84 equaliser is trapped inside the aluminium panels that you can see.  One of these is also the support for the P48 board, which is also clearly visible.

The filter caps are under the piece of PCB that holds the fuses.  This was 'etched' using a rotary engraving tool.  Quick and dirty, but it works very well.  The auxiliary transformer is a travesty - it's 5 times bigger than it needs to be, but was the only one I had available at the time with the right voltage.  Because of the space taken up by the transformer, there wasn't enough room for a P39 soft start circuit.

Figure 2
Figure 2 - Electronics View

The P84 board I used is different from the ones I sell (it has a power supply on-board), and that necessitated the relay you can see hiding behind the volume pot plus some other skulduggery to activate a mute function when power is removed.  The recommended P05 supply has this ability built in (using the auxiliary output), so you will (probably) still need the relay, but not the skulduggery Grin.


The overall construction can be gleaned pretty well from the photo.  Chassis details are shown below, but the exact layout of the boards needs to be determined based on the major components you use.  The power transformer and filter caps will use a lot of the available space, and everything else needs to be fitted in around these.  You may find that the suggested chassis details are too limiting, so feel free to make it larger if you think you'll need more space.

The electrical connections are fairly standard for a subwoofer, but Figure 3 shows the various boards, power supply and the interconnections between them all.

Figure 3
Figure 3 - Electrical Interconnections

Note that if you use the P71 (Linkwitz transform circuit) instead of P48, the phase switch is at the input rather than the output.  You may also choose to have the volume control between the P48 (or P71) and the P84 boards, rather than at the end of the signal chain.  This gives better immunity to clipping in the EQ circuit, but at the expense of slightly higher noise.  In general, the input level to the sub should be set so that the volume control is close to maximum.  This is easily achieved with the P48, since there are two level controls that form part of the equalisation circuit.

Each subassembly is fully described in its own project article, and PCBs are available for everything shown.  The relay needs to be hard-wired, but this is very easy to do - there are really only 3 connections - power (DC), signal and ground.  For the mains fuse rating, see the appendix at the bottom of this page.  The P39 soft start circuit is optional.  Although not strictly necessary with a 300VA transformer, you will need it if you use a transformer of a higher VA rating.

The main filter caps should be 8,000uF as a minimum, and should be rated at 75V or more.  Higher values will not hurt, but it is unlikely that anything useful will be gained from more than the suggested values.  The resistor feeding the relay from the Aux output on P05 is simply selected to limit the relay voltage to the nominal coil value.  This is worked out using Ohm's law in the usual way.

It is immediately obvious that this subwoofer amp will be more expensive than typical 'plate' amps that you can buy.  However, it will outperform any of the ready-made units by a wide margin - more power, excellent EQ facilities and totally predictable performance being just a few of the advantages.

Chassis Details

The chassis is quite easy to build, and needs only a flat plate (3mm minimum thickness is recommended) and some 20mm angle.  As noted above, the main purpose of the angle is to reinforce the panel, but it also looks a lot better than a flat plate.  Your controls are also protected against mechanical damage.

Figure 4
Figure 4 - Basic Chassis Details

Although I have shown the location of the P84, you may need to make changes depending on your transformer and filter caps.  Same goes for the IEC connector and mains switch, and as an example, it can be seen in the photos above that I had to move these.

The fins are attached using 3 x 3mm countersunk screws, with the screw heads recessed just below the back (component) surface.  This is important for the BP4078 and transformers, since protruding screw heads may cause damage.  Figure 5 shows a detail view.  Feel free to use more than 3 screws per fin - I used four, but I don't think it really needs that many.

Figure 5
Figure 5 - Fin Mounting Detail

It is very easy to mount the P84 equaliser so that the 8 pot shafts go straight through the panel.  Trouble is, this makes them accessible, so people will fiddle with the settings, possibly wrecking your carefully measured EQ.  To counteract this, I mounted the P84 board onto a piece of thick plastic, drilled so that it seals the pots from behind the panel, and provides access through the holes.  A detail drawing is shown below.  This has the additional advantage that you can screw a panel onto the side (as seen in the photo), where the P48 (or P71) and P05 can be mounted.

Figure 6
Figure 6 - P84 Pot Mounting Block Details

The recessed holes need to be big enough to accept a socket so the pot nuts can be tightened.  The complete assembly needs to be airtight when completed, unless you plan a separate sub-enclosure for the amplifier.  While this is often needed with analogue amps to prevent the amp from heating the inside of the subwoofer box, it is not necessary with the BP4078 because there is so little heat.


Each board should be tested according to the procedure described in the individual project article.  All mains wiring must be checked with a multimeter and visually to ensure that nothing is connected incorrectly.  Mains safety cannot be over-emphasised, and great care is needed to ensure that your finished amplifier is safe.

The final test is to connect the ColdAmp module to the power supply (after verifying that the polarity is correct), and switch it on.  Check for noise (hum or buzz), and rewire as necessary to make the amp totally silent with no signal.

Appendix - Projects Boards Used

The recommended project list depends on whether you plan to use the P48 EAS (Electronically Assisted Subwoofer) or Linkwitz Transform Circuit.  Other boards are common to both implementations.

Also common to all is the ColdAmp BP4078 Class-D amplifier module.  This is described fully in Project 114, which also has links to the data sheet and application notes for the module.  You need to read the article to get a full understanding of the features the BP4078 has to offer.


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Copyright Notice.This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is © 2006.  Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro-mechanical, is strictly prohibited under International Copyright laws.  The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project.  Commercial use is prohibited without express written authorisation from Rod Elliott.
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