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Project
114

BP4078 Class-D Audio Power
Amplifier
Rod Elliott (ESP)

These modules are available directly from ColdAmp - click the PCB image
for details.

Introduction

The ColdAmp Class-D power amp modules are discussed in the PWM
Amplifiers article, but this project description shows specific details of
the modules and how to use them in real life. Because of the design, they are
much easier to use than many competing modules, and for the simplest application
need nothing more than a power supply, and input and output connections.

There are many other options, including ...

Balanced input capability

Volume control connections

External low voltage supplies for lower dissipation

Clipping indicator

Thermal monitor (thermistor)

Synchronisation of two (or more) amplifiers

Comprehensive protection circuits *

The data sheet and application notes are very comprehensive, so armed with
these, you will have no difficulty building a complete system using the modules.
The project here is essentially just an introduction to the modules, showing a
stereo amplifier that I built using prototype units, and giving the basic idea
for a high powered subwoofer amplifier.

Although they are easier to use than many of the other Class-D amps, the
ColdAmp modules sacrifice nothing in terms of sound quality. Distortion,
noise and frequency response are all excellent, and listening tests have
revealed nothing that you would not expect to hear from any high fidelity
amplifier.

Description

The first application has to be a stereo power amp, since that's what I built
using two prototype modules. The reasons for this choice are twofold ... a good
high powered stereo amp is always useful, and it was important to be able to
assess the sound quality in this role. I am at something of a disadvantage in
this, because the only full range speakers I have are bookshelf units. They are
quite good, but obviously don't have deep bass capability and suffer the normal
limitations of most bookshelf designs.

I have long been of the opinion that bookshelf speakers are so called
because one merely has to add a plank of wood to make a convenient bookshelf.
The speakers are best left disconnected.

Having said that, the units I have sound very good with the ColdAmp
modules - there is no discernible difference between the Class-D amps and a
'GainClone' amp (at levels within the range of the GainClone, of course). It
must be admitted that the resolution of the speakers used is not as good as I
would prefer, but at present I don't have anything else available (my main
system is tri-amped, and therefore very limiting for amplifier evaluation).

The simplest possible connection requires only a power supply, plus input and
output connections. While there are many other functions, most will never be
needed for a home audio system - especially at lower supply voltages. Heat is
not a problem - the amps get slightly warm with no load because of the on-board
regulators, and only very slightly warmer in use at normal listening levels.
This is with ±60V supplies (±56V nominal, since I used a pair of 40V
transformers). Remember that the supply voltage will always be higher
than expected at no load, and lower than expected at full load.

Figure 1 - Front View of Completed
Amplifier

The front view is shown above. The amp modules are visible at the back,
mounted on fabricated heatsinks made from some scrap aluminium I had handy.
Normally, the amps would simply be bolted to an aluminium base-plate, and that
would provide all the heatsinking needed for anything short of continuous high
power usage.

Figure 2 - Rear View of Completed
Amplifier

At the rear, you can see the balanced XLR and unbalanced RCA connectors. The
switch selects between balanced and unbalanced (although the balanced input is
disabled - see below for the reasons for this). Output connectors are standard
combination binding post types. In case you were wondering about the apparent
lack of a fuse, not so. The IEC connector is a fused type. Individual amplifier
fuses are located at the power supplies.

The power transformers are mounted at the front of the enclosure to relieve
some of the strain if (when?) I mount the amp in my rack in the workshop. The
power supply is between the transformers and power amps. I used 4 x 8,000uF caps
and a pair of 40+40V 300VA transformers, with the supply arranged as 'dual mono'
(i.e. each amp has its own power supply, sharing only the mains lead, IEC
connector and power switch or soft-start circuit.

This is overkill, and is not warranted for normal use - I did it that way
simply because it was convenient at the time. The modules are small enough that
they will fit easily into a 1RU case (1¾" high). This makes for a potentially
very slim-line high power amp, but the transformers will pose a problem. While a
switchmode supply could be used, this is a more expensive option than a
conventional power transformer based supply.

Construction

Apart from the power supply (described in detail below), the construction of
the amp is primarily a mechanical exercise. Having worked out where everything
will go, and bearing in mind the requirement for good isolation of input and
output leads, it is a matter of drilling the case and connecting everything
together.

The modules use 'fast-on' (also known as quick connect) crimp lugs, although
if you get uninsulated ones they can easily be soldered then shrouded with
heatshrink tubing.

Figure 3 - Module Connections

Figure 3 shows the general layout of the module's PCB. The terminals marked
in red are the only ones you need to get the amp working. Those shown in blue
are optional, and rather than give a complete description here, it's better to
refer to the data sheet [1].
The terminals shown in green are only needed if you must have the lowest
possible amp dissipation. Again, these are described fully in the data
sheet.

While the volume control terminals are a good idea for some applications, in
general I suggest that you do not use them. This is from my own
experience - while everything is fine for low level inputs, if your source can
output more than about 1V RMS, you will run into trouble because the balanced
input stage can clip. When used in balanced or non-inverting single-ended mode,
the stage has a gain of 2, but operates from a ±5V supply. In theory, that gives
you a maximum input level of 1.5V RMS, but with zero headroom. Unless kept very
short, the leads to the volume control also pick up considerable
interference.

Speaking of interference ... all PWM amps have the capacity to create
interference, and the ColdAmp BP4078 is no exception. While I have not
found it to be a problem with the amp in its case, during initial tests my
favourite FM station was almost completely blocked when everything was just
sitting on the workbench. The amplifier modules should always be inside a metal
case of some description, as this provides shielding against RF
interference.

Figure 4 - Recommended Wiring Scheme

The wiring scheme above is the same as I used in the final configuration
shown above. Note that the balanced input capability is disabled (the XLR
connectors are wired as unbalanced). I did use the fully balanced operation
initially, but there was too much noise from the volume control wiring, and I
ran into problems with input stage overload. None of the 'bells and whistles'
were used in this instance, because it was put together to assess the sound
quality of the amp and take measurements. While clipping into a load is easy, my
speakers would explode if I ran the amp to full power into them, and even during
extensive testing, I was never able to get the amps above slightly warm.

Note that the above diagram is intended to be literally interpreted. The
supply is shown in simplified form (see below for all the details), but the
wiring should be done exactly as shown. Do not be tempted to run the speaker
return back to the amp board - take it directly to the supply star grounding
point (the centre tap of the capacitors). Input connectors are standard 3 pin
0.1" headers, and the crimp connectors are readily available. Personally, I
don't recommend that you crimp the connectors at all - they are more reliable if
carefully soldered after performing a rudimentary crimp with a small pair of
pliers.

Modules supplied by ESP will include the quick-connect (Fast-On) terminals
and an input connector. The input cable must be twin shielded microphone
cable, even for unbalanced inputs. The negative input and shield are joined at
the input socket (or the pot if used), and the input connectors should be
isolated from the chassis. In some cases, it may be advantageous to join the
input ground to chassis with a 100nF ceramic capacitor - right at the input RCA
connector. This is something that you must experiment with, because different
layouts will give different results.

Sub-Woofer Amplifier

It is to be expected that BP4078 amps will find themselves in many a
sub-woofer, and they are absolutely ideal for this. Because heatsink
requirements are minimal, a simple flat plate can be used for a sub amp, making
it very simple to build. Incorporating extras like the P48 subwoofer controller
(or P71 Linkwitz transform circuit) and/or the P84 subwoofer equaliser is no
different from adding them to a Class-AB amp, and a suitable scheme is shown
below.

Figure 5 - Subwoofer General Layout

The above is only a suggestion, of course. The idea is to give you a general
scheme that you can work with to suit your own purposes. Because the BP4078 is
so small, you get lots of room to play with on even a relatively modest sized
panel, and with nothing more than a few holes to drill, it is much simpler to
build than an equivalent sized Class-AB amplifier.

I will soon be able to add a photo of my own ColdAmp based subwoofer
amplifier, which will replace the one I have at present. The arrangement will be
almost exactly what you see above, except a P48 board will be used instead of
the P71 shown (since this is what I have now). The other side of the panel needs
a minimum of controls and adjustments - all I have at present is a level
control, and that will not change.

Power Supply

WARNING :Mains
wiring must be done using mains rated cable, which should be separated
from all DC and signal wiring. All mains connections must be protected
using heatshrink tubing to prevent accidental contact. Regulations where
you live may demand that mains wiring be performed only by a qualified
electrician - Do not attempt the power supply unless suitably qualified.
Faulty or incorrect mains wiring may result in death or serious
injury.

The power supply for a PWM amp does not need to be quite as large as that for
an equivalent analogue amplifier, because of the high efficiency. Class-D amps
typically run to about 90% efficiency, versus a maximum of 70% for Class-AB. As
a result, less power is expected from the supply. In general, a transformer
rated at the same power as you expect from the amplifier (on peaks - not
continuous) is a good start, so for the schematic below, a 500VA transformer
will be sufficient for a peak output power of up to 200W per channel (4 or 8
ohms). You will be able to get more, but if sustained for any length of time the
transformer will get hot and may be damaged. If you expect to run the amps to
close to full power continuously into 4 ohms, then you will need a 1kVA power
transformer.

Figure 6 - Suggested Power Supply
Schematic

The supply is fairly conventional, but a notable addition is C5 - this has
not been shown on any other ESP supply to date. The purpose of this cap is to
help reduce conducted emissions - high frequency energy conducted down the mains
lead. You may also find that an EMI filter is helpful - you can get them
integral with the IEC socket.

The fuse rating for F1 needs to be selected according to your local mains
voltage, and it is usually best to check with the transformer manufacturer.
Assuming the use of a 500VA toroidal transformer, you can use the following as a
guide (this also assumes that the P39 soft-start circuit is being used) ...

Supply Voltage

Fuse Rating

110 VAC

10 Amps

120 VAC

8 Amps

220 VAC

5 Amps

240 VAC

4 Amps

In all cases, the fuse must be a slow blow type. Even with the soft-start
circuit, the inrush current may be much higher than the maximum average current.
For safety, use of an IEC socket with integral fuse is recommended.

Testing

Because the modules are fully built and tested, all you need to do is check
your power supply before connecting the amplifier module itself. The modules
have protection circuits that will prevent operation below about ±30V or above
±68V, as well as overcurrent protection. All you can do is connect the amp to a
known working power supply.

For your initial tests, I recommend that you use a lamp (rated for your full
mains voltage) in series with the incoming mains. The lamp should be rated at
about 100W, and will limit the current if there is a problem. You will be able
to operate the amp into a load at low power with the lamp still in series, but
if you try to increase the volume too far the supply voltage will be reduced and
the under-voltage circuit will turn the amp modules off.

It is imperative that you connect the power supply to the BP4078 modules with
the correct polarity, and also make certain that the earth (ground) is solidly
connected to the supply common. Reverse polarity will damage the
module(s) - the supply fuses will blow, but not before damage has been done.

Remember that with the values given for the power supply, you will be able to
get 400W into 4 ohms, so be very careful with your signal input - there is a
real risk of blowing speakers if you apply too much signal level.

Data Sheets & Additional Information

The BP4078
Data Sheet is a must read document before you start. It describes all the
options and also has the full specifications for the modules.

The BP4078 Application Notes provide more information again,
primarily on how to use the modules, additional construction hints, etc. Please
take the time to read this information.

Intending purchasers should contact coldamp directly, or you may send me an e-mail with
your query and I will pass it on. If contacting ColdAmp, please mention that you
obtained the information from ESP.

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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 Copyright © 2005. 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.

Page Created and
Copyright © Rod Elliott 18 Dec 2005