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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