An Unconventional Loudspeaker system Latest change 2019-01-17
Filter Board FLT
The filterboard is more or less the
heart of the system. It contains the crossover filter, the
corrections for the loadspeaker's low-frequency drop and
the level control circuits. Also it contains the Arduino
microprocessor which monitors and controls several
You may download
the board's schematic
in PDF and / or the zipped Eagle files.
Note that the size of this board exceeds the free licence
limits of Eagle. You can view it but you cannot modify the
board without a licence.
(actually this is FLT-3
with some minor modifications, which are corrected in
Before mounting I removed the screen of the RJ45
Description of the functionality: (pls. download the schema in
The audio signal enters the board on the RJ45 connector J2
as a differential signal on pins 1 and 2. R16,17 and C7,11
reduce possible interference above 100 kHz. U6 is an
amplifier with high common mode suppression, it helps in
reducing audio frequent disturbances and effects of a
possible ground loop.
U7A is the second-order lowpass filter for the Bass
channel (145 Hz -3dB), U7D the second-order highpass
filter for the Tweeter (1307 Hz -3dB). U7C is the
subtraction stage and produces the Midrange signal.
The reason that U7D has gain is as follows. It turned out
that the power amplifier for the tweeter was not happy
with the high capacitive load of the electrostatic
loudspeaker, approximately 6uF, so I put a 2 Ohm resistor
in series with the step-up transformer. This gives a
lowpass filter with 12 kHz -3dB so it had to be
compensated for. This compensation is done with C25 and
R36,39 on the FLT board and required compensation of the
The signals for the Bass and Midrange channels go through
U8, which is a 2-channel volume control with a range of
-96 .. +31 dB in steps of 0.5 dB. Here the output levels
of the Bass and Midrange speakers can be matched with the
U7B is the first-order low-up filter for the Bass channel,
U9B is the anti-resonator together with the Gyrator U9A.
For details refer to the Elektor article about the Time
Correct Bass Reproducer.
The low-up filter for the midrange channel is combined
with the subtractor U7C. U9C and the gyrator U9D are the
U10,11,12 are the differential drivers to the power
At several locations DC blocking capacitors are present.
The low-up and anti-resonator circuits have very high gain
at DC and so will amplify offset voltages to unacceptable
values when everything is DC-coupled.
All opamps and drivers used have extremely good distorsion
specifications and are driven from relatively low
impedance sources so the effect of the non-linear input
capacitance is negligible. Important
On pin 6 of the RJ45 connector J2 a 5Volt signal CR+
enters the FLT-board. This signal turns on the solid-state
relay on the Power Board and awakes the Arduino through
D1. After a few seconds, when all power is ok the Arduino
takes the Power Amplifiers out of reset, and the music can
Near the power board sits a neon indicator showing that
mains power is available. In series with that neon bulb is
an optocoupler which generates the MAINSOK signal, a 50 Hz
square wave. When this signal does not change polarity in
15 ms or when the CR+ signal goes down, the Arduino brings
the Power Amplifiers into reset mode.
The board is supplied with +12 Volt. On board DC/DC
converters derive +5D for the digital circuits, +5A and
-5A for the analogue cicuits.
With U3 a 2.5 volts precision reference voltage for the
measurement of the supply voltages is derived.
For details of Communication, Error handling and Monitoring
see the Software page and / or the Messages page.
U4 converts the RS422 signals from/to the Control
Amplifier to 5Volt levels acceptable for the Arduino.
Under normal listening conditions this communication is
silent. The only digital activity outside the
microprocessor are the slow blinking of the blue LED3
and the 50 Hz square wave MAINSOK from the Powerboard.
To adjust the output levels for the Mid and Bass
channels a PC/Laptop program has to be started which
talks to the Control Amplifier over the local area
network. The Control Amplifier conveys the information
to the FLT boards.
The TPA3255 in the power amplifiers can produce several types of
errors (see the datasheet of
TPA3255 for a complete overview).
The most important are:
If the output signal clips to GND or PVDD a series of low-pulses
are generated on the line CLIP_OTW. In the software on the FLT
board these events are counted in 2 ClipCounters, one for each
TPA board. A change of one of these counters will trigger a
message to the Control Amplifier where an error led will flash
shortly. The ClipCounters are visible in the Configuration Tool
when communication with the loudspeakers is established.
A chip temperature over 125 °C
brings the line CLIP_OTW continously low. This condition will be
sent to the Control Amplifier where the error led(s) will be
flashing as long as this condition lasts. If the temperature
lowers below 125 °C the
error reporting stops automatically.
This pin reports a multitude of error conditions. Some are
latched, which means that the TPA must be reset, others
disappear when the error disappears.
The software on the FLT board will send an error message to the
Control Amplifier, bring the TPA in reset mode for a while and
after that checks whether the error has gone. If not error
messages are continued. The orange error led will be on as long
as the error exists.
The Arduino on the FLT monitors the supply voltages and the zero
crossings of the mains voltage. If out of range or absent the
blue led on the FLT will flash at a fast rate, the TPA will be
reset, and error messages will be sent to the Control Amplifier
where the error led(s) will flash at a fast rate. The voltages
and the mains condition are visible in the Configuration Tool.
Note that if the mains voltage fails the Arduino on the FLT
stays alive as long as the Control Amplifier provides the CR+
simulation of the cross over. Green = Bass, Red =
Tweeter, Cyan = Midrange, Yellow = sum(Bass, Midrange, Tweeter)
The -3dB frequencies are 145 Hz for the bass channel and 1307 Hz
for the tweeter.
Below the pSpice model. Component numbering is according to the
FLT-4 schema where relevant.
* 3 way subtractive
* SallenKey topology
* Signal source
V 1 0 AC 1
.AC dec 150 10
* Low pass
R24 1 12 3.6k
R26 12 13 43k
C20 12 14 220n
C21 13 0 33n
E7A 14 0 13 14 1E3
* output i node 14
* High pass
C18 1 22 33n
C19 22 23 6.8n
R25 22 24 8.2k
R27 23 0 8.2k
R28 25 0 3k
R29 24 25 8.2k
E7D 24 0 23 25 1E3
* output is node 24
R1 1 31 10k
R2 31 32 10k
E1 32 0 0 31 1E3
* output is node 32
R30 24 33 37.3k
R31 32 33 10k
R32 14 33 10k
R34 33 34 10k
E7C 34 0 0 33 1E3
* output is node 34
* Summed output
R101 14 100 10k
R102 24 100 37.3k
R103 34 100 10k
R110 100 101 10k
E100 101 0 0 100 1E3
* output is node 101