Silicon Chip – May 2019

siliconchip.com.au Australia’s electronics magazine May 2019 39

Bargain Class-D Stereo +

Subwoofer Amplifier

Modules

By

Allan Linton-Smith

It will reject 70dB of ripple, mean-

ing you can have up to 200mV peak-

to-peak ripple before you’re likely to

notice any buzz or hum creeping into

the audio outputs.

For testing, we used a 24V 7A DC

plugpack which cost $33 including

postage. 24V x 7A = 168W so with a

90% claimed peak amplifier efficiency,

we should get a total theoretical out-

put of around 150W RMS, ie, around

2 x 38W into 4Ω for the left and right

channels and about 75W into a 2Ω sub-

woofer.

The efficiency of the device varies

significantly with supply voltage and

output power (see Fig.1). It is typi-

cally 40-70% at low power levels, ie,

below 5W. If you only require power

levels up to 10W into 4Ω speakers you

are better off with a 6-12V DC supply

because this will give you 70-90% ef-

ficiency and it won’t cause any over-

heating problems (see Fig.1).

So your best approach is to decide

what power output you need and then

choose your power supply to deliver

this with the highest efficien-

cy. Otherwise, the device may

overheat and automatically

shut down during use.

Power output figures

The measured power for

this module is good but not

quite up to the claim of 2 x 50W

+ 100W.

During testing, we did manage to

get 2 x 50W into 4Ω and 2 x 30W into

8 Ω as expected. But we were not able

to get the full 100W into 2Ω from the

subwoofer output because the device

protection circuit sent the output into

high impedance and it cut out. We

were only able to get about 50W into

the sub.

This is no doubt due to poor design

of the subwoofer section; we suspect

that the IC has not been correctly con-

figured for mono operation. It may be

possible to fix this by changing some

of the passive components connected

to the subwoofer amplifier IC, but we

haven’t tried that.

So basically, you can expect to get

about the same amount of power from

the subwoofer channel as you can from

the left and right channels, taking into

account the possibility that your sub

may have a different impedance from

the other speakers.

Frequency response

The quoted frequency re-

sponse by the supplier is 20Hz

to 20kHz with no plus/minus decibel

figure, which is quite common to see

but also a pretty-much useless state-

ment. So we decided to measure the

frequency response accurately.

First, we did a listening test which

exposed a lack of treble with cymbals,

triangles and slightly muffled brass.

The measured response, as shown

in Fig.2, confirms our subjective im-

pression.

There is a significant drop-off in the

output above 1kHz. We did this test at

1W and 5W output levels, using a 12V

DC supply for convenience.

So the out-of-the-box response is

poor, and you can clearly hear the lack

of treble. It’s down by 8dB by

20kHz.

Even if

you don’t

need the two

extra outputs, as

long as you can live

with the extra size (and

cost), this module has two

benefits: no need for mods,

and built-in Bluetooth support.

If you’re clever, and you only need

two or three channels, you’ll take the

left output from one chip and the right

output from the other chip to spread out

the heat load between all the devices.

The

Bluetooth

module is

supplied

already attached to

the main board.

PCB size is

167 x 116mm.