amplifiers UcD vs. Hysteresis
As stated above the construction
on Phillips' application note. There are though several differences.
Input resistance increased
to 10k Ohm, to make it work wit any normal preamp.
Choice of transistors. The
goal was to make the amp really small and inexpensive. The use of BC846 for
Q12/Q13 will limit the use of supply voltage to some +-30VDC giving a
theoretical output of 56 W/8 Ohm. It is though fairly easy to change these to
transistors with higher voltage capability to make the amp work for higher
DC offset is handled by
trimmer pot R29. The DC offset seems quite low even without the pot, and this
can easily be omitted to lower cost.
Most components are
different from the application note.
The resistors R15/R19
should be omitted. First of all these where wrongly placed on the wrong base of
the input pair. They are supposed to help getting a better PSRR but seems to do
more damage instead (picking up noise), this also when placed at the right base
When running in UcD
configuration R1, R2 and C1 should be mounted.
When running in Hysteresis
mode, R4, R14 and C6 should be mounted.
Click here for a pdf version
The PCB is double sided
and measures just 3.3 cm x 7.8 cm.
Six of these amps will fit
on one euro card (160 x 100 mm).
download the Gerber files needed for the
production of the class-d amp PCBs here.
I have had god experience
with Olimex, who has a very good offer on fast prototype boards. One double
sides euro board is 33 USD, which means that each of the amp PCBs are 5.5 USD.
It's easy convenient and cheap. Now I simply wouldnt think of trying to etch my
own boards any more! Another unique point is that you can mix diferent designs
on one eour card as long as there is space enough. Cutting out the PCBs are free
Gerber viewer can be downloaded from GraphiCode.
Comments when running in
There is overall a bit too
much noise at the output. At first the following components where used:
R18 3k3, R9 = 10k, R4 = 270k, R14 =
1M, C6 = 220 pF.
This results in a
switching frequency a bit over 250 kHz.
There is a bit of 50 Hz
noise from the PSU. This can be lowered by using bigger caps in the PSU, and is
overall a result of a rather low PSRR (which is quite normal for a class-d amp).
The noise from on the
output (besides the 50 Hz) is quite broad band and is probably coming form the
high switching currents/voltages. I think that this can be reduced overall by
applying snubbers over the mosfets, and on the output, further more fast diodes
could be mounted over the mosfets. Also the PSRR, and general rejection of
induced noise could be improved by increasing the open loop gain. But all this
was omitted to make the amp as small as possible and to require as few
components as possible.
Improvements could also be
made to the PCB layout, by placing the feedback components as close to the input
as possible. As it is these components are actually placed close to the output!
Changing the following
components, noise was though decreased considerably and is now at a level where
it is not a concern anymore for normal use.
R18 = 3k3, R9 = 3k3, R4 = 100k,
R14 = 470k, C6 = 1 nF.
R29 was not mounted, but
the DC output was always less than 100mV.
R6 gets a bit hot. Must be
considered if running from a higher PSU voltage.
Note 13/5 2007: Actually
mounting snubbers (10R 1206 + 1nF 100V COG) on the mosfets, did nothing to the
output noise, but the ringing on the fets was naturally almost eliminated.
with the above mentioned components. Fs = 245 kHz.
Power supply board
Power supply board.
On the first assembled unit I used a coil
wound on a
Micrometals T130-2 core. This is overkill in this small an
application. Therefore I will try out the smaller version
As seen from the analysis 40 windings with Ø 1 mm
wire (windings counted on outside diameter not including leads) will give 21,5
uH, which should be sufficient (1,7 m wire was used per. coil).
The two coils, wound using
banking to minimize capacity
What could be improved?
What are the learning points?:
The overall open loop gain
is to little to give a proper PSRR. This is making the amp sensitive to self
generated noise (EMI), ripple on the supply lines etc.
The output coil is
unnecessary large. For powers less than 100W it is quite over dimensioned.
Q1 for the 12 V supply, is
getting quite hot. Think it is ok for the +-24 V DC used, but both a heat sink,
and another device should be considered if aiming for higher power levels.
R6 is there for easing
startup. It gets quite hot and must be reconsidered for higher power levels.
The feedback network R1,
R2, R4, C1, should be placed near the inputs (basis of Q6 and Q7) and not near
the output as it is now. This will help reducing noise induction.
Ground, should be laid out
with more care. The layer is to fragmented, and there is not a good separation
between the input ground and the power ground.
The of set adjustment
seems unnecessary. R29 can therefore be omitted.
For higher powers some
current limiting should be implemented.
To make the construction
smaller and easier to handle, it should be considered to place 2 amps and a PSU
on a single board. This would eliminate a lot of tiresome and clumsy cabling.
For the presented power
level, the mosfets does not get more than a little warm .... therefore no heat
sink is really necessary. SMD mosfets should be considered as well.