PART 1: BY NICHOLAS VINEN AND TIM BLYTHMAN
TOUCHSCREEN AND REMOTE DIGITAL
PREAMP WITH TONE CONTROLS
This preamp has the
best of both worlds: the benefits
of digital control, such as an intuitive
touchscreen interface, presets and remote control, along
with the low noise and distortion of an analogue design. It does this
by using classic Baxandall-style volume and tone control circuitry with op
amps, using high-quality digital potentiometers to provide the adjustments.
T
o date, most of our remote-
controlled preamplifiers have
used motorised potentiometers.
While these have many benefits, such
as low noise and distortion, and the
ability to simply turn the knob if you
are close to the preamp, they are quite
expensive and can be hard to obtain.
They can also fail and wear out.
Digital volume control ICs are an
attractive alternative, but there have
only been a few of these with performance that we would call ‘Hi-Fi’, and
most of those have been discontinued.
They also can be pretty expensive and
difficult to obtain.
Futhermore, since they only adjust
the audio level, we need separate
arrangements for input switching (as
any self-respecting preamp needs at
least a few pairs of inputs) and tone controls. Those are a frequently requested
feature for preamps, and we agree that
they can be handy. For example, they
can compensate for loudspeaker shortcomings, such as a lack of bass or treble, or too much treble. So, any digital
preamp we want to desisgn would need
to tick the following boxes:
1. Decently low distortion and noise (at
least CD quality, and ideally better)
2. Tone controls (ideally with at least
three bands for flexibility)
3. A wide volume control range operating in a logarithmic manner
4. Adjustable gain to suit a wide range
of signal sources
5. Infrared remote control
6. Input switching
7. Intuitive and attractive colour touchscreen interface for direct control.
16
We achieved 1) through 4) by using two
quad Analog Devices AD8403ARZ10
digital potentiometer ICs. While these
are not especially cheap at around £8
each, they are still quite reasonably
priced compared to Hi-Fi-quality volume control chips. The eight potentiometers they include let us adjust the
volume, bass, mid and treble levels in
both channels using just two chips.
These devices have impressive specifications, borne out by our testing,
with a rated THD+N figure of 0.003%
at 1V RMS/1kHz (they tested considerably better than that), a −3dB bandwidth of 600kHz and an impressively
low noise level of 9nV per √Hz. So
they are well suited to audio signal
processing tasks.
Because each chip has all four potentiometers needed for a channel, the digital pot and its associated op amps are
laid out all in one area, simplifying the
PCB design and minimising crosstalk
between channels.
The input switching is handled by
three telecom-style relays, which has
worked well for us in the past, as these
mechanical devices have minimal
impact on signal quality.
Finally, the control interface is handled by a Micromite LCD BackPack
with either a 2.8-inch, 320x240 or
3.5-inch, 480x320 colour touchscreen.
This provides many benefits, such as
a nice clear volume readout when you
adjust it via the remote, the ability to
show the actual frequency response
for any given tone control setting and
loading/saving presets – the whole
nine yards.
It’s just what you want for a modern
preamplifier or amplifier, without compromising the sound quality.
Besides the BackPack, which would
generally mount on the unit’s front
panel (along with the IR receiver), all
this circuitry is packed onto a modestly-sized PCB at 206 x 53mm. It has four
pairs of onboard RCA inputs, so that it
can be mounted at the back of the unit.
It can be powered from a separate AC
or split DC supply or an internal transformer with suitable windings. That
includes transformers with high-voltage windings to power amplifier modules, and low-voltage secondaries for
preamps like this one.
For standalone use, the power input
can be an onboard socket on the back,
near the inputs, along with the optional
rear panel pre-outs. These are in parallel with a pair of internal RCA sockets,
which can feed the preamp’s output
signals to a couple of internal amplifier
modules, making a complete preamp/
amplifier combination.
Performance
The performance of the preamp is
summarised in Figs.1-4. Fig.1 shows a
plot of total harmonic distortion plus
noise (THD+N) against frequency for
an input signal level of 1.5V RMS and
an output level of 3V RMS. As the
final stage has a gain of two times, this
means that the volume control section
is set for unity gain.
The 20Hz-22kHz bandwidth plot
(in cyan) gives the best indication of
audible performance. This shows a
total harmonic distortion level of less
Practical Electronics | September | 2022
than 0.001% from around 35Hz up to
2.3kHz. The distortion level rises above
1kHz, with the dashed line showing
how the curve would look if the harmonics weren’t rolled off at the upper
end by the bandpass filter.
As a good CD player is generally
expected to have a THD+N figure of
less than 0.0018% at 1kHz, we’d say
that this preamp exceeds CD quality.
That’s also indicated by its signal-tonoise ratio of over 100dB, with CDs
being limited to 96dB by their 16-bit
sampling resolution.
Fig.2 shows how THD+N varies with
signal level for some typical gain settings. The rise in distortion at the low
end is due to noise being a larger component of the signal for small signals,
while the rapid rise at the upper end is
where the preamp has run out of headroom and has started clipping.
The best performance is around 2V
RMS, a typical level for many playback systems such as CD, DVD and
Blu-ray players.
Fig.3 shows how the channel separation varies with frequency. We consider
this an excellent result, with worst-case
crosstalk of −75dB at 20kHz.
Fig.4 shows the preamp’s frequency
response with the controls set flat,
which only varies by about 0.5dB
across the whole audio spectrum, rolling off slightly towards the 20Hz end.
It also shows plots with the bass/
mid/treble controls set to their extremes
individually. This should give you an
idea of the adjustment range that the
preamp permits. Of course, you would
usually not use the controls at their
extremes, as shown in that plot.
Fig.5 shows some more realistic
tone control settings (mauve, orange
and blue) along with some examples
of what happens if you set multiple
controls to their maximum extents
(red, green and cyan).
Note how there is some interaction
between the controls. For example, the
treble boost is reduced when a lot of
bass or mid boost is introduced. These
are somewhat odd situations, though,
since you would typically be better off
with bass cut instead of using a lot of
mid/treble boost, and mid cut instead
of a lot of bass/treble boost.
down to two pairs, and these then go
to a third DPDT relay which makes
the final selection of which stereo signal reaches the RF filter. The RF filter comprises a 100Ω series resistor, a
ferrite bead and a 470pF capacitor to
ground for each channel. This RC lowpass filter has a −3dB point of 3.4MHz,
while the ferrite bead helps to eliminate much higher frequency signals
Features
•
•
•
•
•
•
Four input stereo preamp with a colour touchscreen and remote control
Bass, mid and treble adjustments with presets, plus volume control
Better than CD quality
Four external stereo inputs (one active at any time)
Two stereo outputs, one internal and one external
Optional loudness control automatically adjusts tone with volume
Specifications
•
•
•
•
•
•
•
•
•
•
•
•
•
THD+N: typically less than 0.001%; see Fig.1
Signal-to-noise ratio: typically around 104dB with respect to 2V RMS input
Fre uenc response 0 - 0k
0 0
hannel separation
0 - 0k
Si nal han lin 0 MS
Volume control range: approximately 78dB
ain ran e
0 to
0 00 ti es to
ti es
Input impedance: 100kΩ || 470pF
ass tone control
centre aroun 0
0
8
00
Mi ran e tone control
centre aroun
0
00 an
1kHz)
reble tone control
centre aroun about 0k
0
0k
k
ower suppl
- 0
or
urrent raw t picall aroun
00
with touchscreen on an
0
with it off
Fig.1: harmonic
distortion plus noise
plotted against
frequency for two
different analyser
bandwidths. The blue
plot with the dashed
line is the most realistic
representation of the
performance, which
we think is meritable.
1.5V RMS gives the best
performance, but it’s still
pretty good at around
1V RMS full-scale. The
unit can handle over
2.5V RMS at its inputs
before clipping.
Circuit details
The Digital Preamp circuit is shown
in Fig.6. Signals are fed into one of
four pairs of RCA sockets, CON1A-D
and CON2A-D. These have individual
100kΩ termination resistors to prevent
signals from deselected devices from
floating and causing a thump when
switching inputs.
These go to the contacts of a pair of
DPDT relays which narrow the signals
Fig.2: a plot of distortion
versus signal level for a
1kHz tone, confirming
that distortion rises
at lower signal levels
due to noise. This also
shows the onset of
clipping for high signal
levels, but note that
there are two reasons
for clipping; either the
input signal rises above
2.5V RMS (as is the
case with lower gain
settings), or the output
runs into clipping at
about 4V RMS (higher
gain settings).
Practical Electronics | September | 2022
17
Fig.3: the channel
separation of the
preamp is excellent,
with very little of one
channel leaking into
the other channel,
especially below 5kHz.
The input separation is
even better, exceeding
100dB in most cases.
digital pot ICs. These combine with
the digital pots’ 10kΩ track resistance
to reduce the input signals by 18%.
So a 2.3V RMS signal is diminished
to 1.89V RMS, just within the 1.94V
RMS capabilities of the digital pots
running from 5.5V.
This is easily compensated for by
adding extra gain in the volume control
stage. Those 2.2kΩ resistors also limit
the current that op amp IC1a needs to
deliver if the signal is clipped by diodes
D1-D4. IC1a runs from ±12V regulated
rails for best performance, so its maximum output swing is about ±10.5V,
enough to damage the digital pots without current limiting and clamping.
Fig.4: with all the
tone settings at 0, the
preamp’s frequency
response is very flat,
dropping by only about
0.5dB at 20Hz. The other
curves show the result
of each tone control
being individually set
to maximum boost or
cut. They indicate how
much adjustment you
can make and over what
frequency range each
band operates.
Fig.5: there is a bit of
unavoidable interaction
between the controls
if you make large
adjustments in more
than one band. The
cyan, red and green
curves demonstrate this.
The other three curves
show the results of much
subtler simultaneous
bass and treble boost
settings of various
magnitudes. You can see
from those curves that
there is essentially no
interaction at
those levels.
which could otherwise be rectified by
the following buffer stage, inducing
unwanted signals into the audio. 1kΩ
stopper resistors further help eliminate RF coupling and also protect op
amp IC1 from damage in case a high
amplitude signal (or static discharge)
is fed into one of the input connectors.
Op amp IC1 buffers the selected
stereo signal, and its outputs are ACcoupled to the gain control section via
10µF capacitors. Note that the input
side of IC1 is not AC-coupled; it is
expected that signals applied to the preamp are reasonably close to 0V DC bias.
The signals from the outputs of IC1
are DC-biased to +2.75V and clamped
18
to be within the range −0.3V to +5.8V.
This is done by a pair of schottky
small-signal diodes for each channel,
connected to ground and a +5.5V rail.
This +5.5V rail is also used to power
the quad digital pot ICs, IC6 and IC7.
This is their maximum recommended
supply voltage (the absolute maximum
is +8V). We have done this so it can
handle the maximum expected RMS
signal voltage from a signal source
like a Blu-ray player, which is usually
around 2.2-2.3V RMS.
To achieve this, we’ve had to slightly
attenuate the signals being fed to the
digital pots, using 2.2kΩ fixed series
resistors connected to pin 3 of the two
Volume control
The Baxandall volume control for
the left channel consists of dual op
amp IC2 plus digital potentiometer
#2 within IC6. Similarly, for the right
channel, it is op amp IC4 and digital
pot #2 in IC7.
Op amps IC2a and IC4a are buffers,
while IC2b and IC4b are configured as
inverting amplifiers with fixed gains of
14.7 times. The digital pots are then
connected within the feedback loop
between the output of IC2b/IC4b and
the input of IC2a/IC4a. As a result,
IC2a/IC4a are fed a signal voltage
between that of the input signal and the
inverted and amplified output signal.
The net result of this is, with the
digital pot ‘wiper’ (pin 4) all the way
at the input (A) end of the ‘track’, the
full input signal is applied to the pair of
op amps, so the maximum gain of 14.7
times occurs (actually about 12 times
or +21.6dB when you consider the
attenuation due to the 2.2kΩ resistors).
As the ‘wiper’ moves towards the
output (B) end of the ‘track’, the gain
reduces logarithmically, eventually
to almost zero. The minimum gain
(actually attenuation) is limited only
by the digital pots’ wiper resistances
of around 50-100Ω. Our tests show
that the lowest gain setting gives about
1.5% of the input signal at the outputs
of the volume control section, equivalent to −56dB.
This means that with the volume
control at zero, you will still get a little
sound out of the preamp, but it will be
very quiet. To fully mute the audio, the
digital pots have a shutdown feature
that disconnects each pot’s ‘A’ terminal entirely. This is where our input
signal connects; hence, we can fully
mute the outputs if desired.
The output signals from IC2b and
IC4b are again clamped to the supply rails by pairs of schottky smallsignal diodes, protecting the digital pots from damage if you set the
gain too high. The op amps limit the
Practical Electronics | September | 2022
current to around 50mA, so neither the
diodes nor the op amp will be damaged during clipping. As the signal
is AC-coupled, this will only ever be
intermittent anyway.
Tone control
This output signal is AC-coupled to
the tone control section via a pair of
47µF capacitors.
The tone control section is the classic Baxandall feedback-based tone
control using op amp IC3a for the left
channel and IC5a for the right channel. Digital pots #1, #3 and #4 are connected in the negative feedback loops
of these op amps, with capacitors connected such that each controls the
amount of feedback over a particular
range of frequencies.
With these pots all centred, the tone
control section has virtually no effect
on the signal, basically just acting as
an amplifier with a gain of −1. When
the pot wipers move off-centre in one
direction, signal components in that
frequency range are amplified, producing bass, midrange or treble boost.
When they move in the opposite direction, signals in those frequency ranges
are attenuated (cut) instead.
As the tone control stage is inverting,
and the volume control stage is too, the
phase of signals fed through the preamp is maintained. Since the outputs
of op amps IC3a and IC5a are fed back
to the digital pot ICs, they once again
are clamped to the supply rails using
schottky diodes. The 100pF capacitors
directly connecting the outputs to the
inverting inputs ensure stability.
Relay RLY4 is the bypass relay.
When it is energised, the inverting
inputs of op amps IC3a and IC5a are
no longer connected to the digital pots.
They are instead connected to the centre taps of pairs of 4.7kΩ resistors connecting from the output of the volume
control stage to the output of the tone
control stage. This configures these
two op amps as fixed signal inverters.
The idea behind this is to eliminate
any distortion or noise that might be
introduced by the digital pots or the
associated passive components when a
flat response is desired. In practice, the
performance of the tone control stage is
good enough that this is not necessary.
While we have left provision for
RLY4 and its associated components
on the board (there would be no real
benefit to modifying it to remove them),
we don’t think the extra cost and complexity is justified. So our parts list
and construction details (to come next
month) will omit these components.
The output signals from the tone
control stages are AC-coupled again,
to remove DC bias, then amplified
Practical Electronics | September | 2022
Parts List – Touchscreen Digital Preamp
1 Micromite LCD BackPack programmed with 0110319A.HEX (2.8in display)
or 0110319B.HEX (3.5in display) – see below
1 double-sided PCB coded 01103191, 206 x 53mm (from the PE PCB Service)
2 double-sided PCBs coded 01103192, 12.5 x 45.5mm (from the PE PCB Service)
1 universal IR remote control (optional) [Jaycar XC3718 / Altronics A1012A]
3 EA2-12 DPDT 12V DC coil telecom relays (RLY1-RLY3)
2 500Ω mini horizontal trimpots (VR1,VR2)
2 small slip-on ferrite beads (FB1, FB2)
3 2-pin headers with shorting blocks (LK1-LK3)
2 quad right-angle RCA socket assemblies (CON1, CON2) [Altronics P0214]
1 dual vertical right-angle RCA socket pair (CON3) [Altronics P0212]
1 white vertical PCB-mount RCA socket (CON4) [Altronics P0131]
1 red vertical PCB-mount RCA socket (CON5) [Altronics P0132]
1 3-way mini screw terminal block, 5.08mm pitch (CON6)
1 PCB-mount barrel socket (optional) (CON7)
1 18-pin header (CON8)
2 18-pin socket strips
2 16-pin box headers
2 16-pin IDC sockets
1 length of 16-way ribbon cable to suit installation
1 3-pin infrared receiver (IRR1)
1 12-15V AC plugpack/transformer or 24-30V AC centre-tapped transformer
with associated wiring, fuse, mains plug etc (to power preamp board)
1 M3 x 6mm machine screw, washer and nut (for mounting REG4)
3 tapped spacers plus 6 machine screws (length to suit installation)
Semiconductors
5 LM833 low-noise dual op amps (IC1-IC5)
2 AD8403ARZ10 quad digital potentiometer chips, SOIC-24 (IC6, IC7)
[Digi-Key, Mouser, RS]
1 78L12 +12V 100mA linear regulator, TO-92 (REG1)
1 79L12 -12V 100mA linear regulator, TO-92 (REG2)
1 LM317L 100mA adjustable linear regulator, TO-92 (REG3)
1 7805 +5V 1A linear regulator, TO-220 (REG4)
3 PN200 or equivalent PNP transistors (Q1-Q3)
3 PN100 or equivalent NPN transistors (Q5-Q7)
1 through-hole LED (LED1; 3mm or 5mm, any colour)
1 5.6V 1W zener diode (ZD1)
0 M bri e recti er
12 BAT42 schottky small-signal diodes (D1-D12)
3 1N4148 silicon small-signal diodes (D13-D15)
Capacitors
000μF
electrol tic
0μF
electrol tic
Micromite LCD BackPack
00μF
electrol tic
We covered the Micromite BackPack
μF
electrol tic
in PE issues May 2017 (V1), May 2018
μF
electrol tic
(V2) and August 2020 (V3). BackPack
0μF
electrol tic
V2 rendered V1 obsolete so the PE PCB
μF
M
Service now only stock PCBs for V2 and
2 220nF 63V MKT
V3. For all other Micromite requirements
4 150nF 63V MKT
we recommend visiting micromite.org
5 100nF 63V MKT
2 33nF 63V MKT
2 470pF ceramic disc
4 100pF C0G/NP0 ceramic disc
Resistors (all 1% ¼W axial metal film unless otherwise stated)
11 100kΩ
6 2.2kΩ
1 110Ω
6 47kΩ
13 1kΩ
5 100Ω
2 22kΩ
1 910Ω
2 10Ω 1W 5% resistors OR
2 10kΩ
11 680Ω
4 4.7Ω 1W 5% (see text)
2 4.7kΩ
1 560Ω
by a factor of times-two by op amps
IC3b and IC5b. This allows the output
amplitude to be above 1.9V RMS if
wanted, up to about 3.8V RMS before
clipping. The 100Ω series resistors
prevent cable capacitance from affecting those gain stages.
The two outputs are connected
in parallel; one is available at the
rear panel (if those connectors are
19
Fig.6: the Digital Remote Controlled Preamp circuit, plus its attached infrared receiver. Besides those
components, everything is mounted on one board that plugs into the Micromite LCD BackPack. The
components shown in red could be installed but we recommend you leave them off, as our testing
shows that they don’t provide any benefits.
installed). The other pair consists
of vertical connectors on the board,
more suited for internal connections
to amplifier modules. It should be
20
possible to use both at once, given that
the output impedance is relatively low.
This could be the case in an integrated
amplifier that provides pre-out signals.
Control by Micromite
The digital pots are controlled using
an SPI serial bus, with one CS (chip
select) line each, plus active-low
Practical Electronics | September | 2022
Micromite-controlled Digital Preamp with Tone Controls
common reset (RS) and shutdown
(SHDN) lines. That’s a total of seven
digital lines required to control both
of the ICs.
Practical Electronics | September | 2022
We also have four relays to control.
An NPN transistor drives each relay
coil with a back-EMF clamping diode.
These relays have 12V DC coils, and
somewhat unusually, are powered
from the −12V rail. This is because the
+5.5V rail is derived from the +12V
rail, so we are driving the relays from
21
Reproduced by arrangement with
SILICON CHIP magazine 2022.
www.siliconchip.com.au
This photo shows the completed preamp board without the
LCD BackPack. We have fitted RLY4 and associated components as
it is a prototype; we expect most constructors will leave these off and link
out RLY4, as explained next month in the construction section. A small adaptor board
(shown inset) converts the SIL header to a DIL type more easily connected to a ribbon cable, and
this same board at the other end also provides somewhere to mount the IR receiver and its supply filter
components (shown below).
the negative rail to better balance out
the current draw. This means that
all the relay coil positives are connected to GND, and the negative ends
are switched to −12V. Some clamp
diodes connect to GND and some to
+12V depending on PCB routing convenience; either way, they will still
absorb back-EMF spikes and prevent
damage to the transistors on switch-off.
PNP transistors Q1-Q4 level shift the
0-3.3V digital relay control signals to
allow the NPN transistors with their
emitters connected to the −12V rails
to be switched normally. So the relays
activate when the associated control
line is pulled down to 0V, and are off
if that control line is at +3.3V or floating (high-impedance).
22
These 11 total control lines are
wired back to SIL header CON8, in
positions suitable for being directly
wired to the I/O header on a Micromite LCD BackPack module.
There are two additional connections: one to allow the BackPack to
illuminate or flash the onboard LED
(LED1) in response to remote control commands and to indicate that
power is supplied. This LED could
also be duplicated on the front panel,
if desired, along with a series current-limiting resistor. The other connection is for infrared reception, at
pin 8 of the I/O header.
While the IR receiver and its supply RC filter are shown on the circuit
diagram, they are mounted on a small
board attached to the BackPack, as the
receiver needs to be mounted behind
a hole on the front panel of the unit.
Power supply
The power supply is pretty basic;
AC is applied to either barrel socket
CON7 or terminal block CON6. If a
centre-tapped transformer is used,
this would typically be wired to
CON6, with the tap to the middle terminal. DC split rails can also be fed
to CON6. If AC is applied, this is rectified by bridge rectifier BR1 and filtered by a pair of 1000µF capacitors.
The pulsating DC across these
capacitors is then regulated to smooth
±12V DC rails by REG1 and REG2. We
have chosen 12V rather than the commonly-seen 15V because the performance is much the same, and we don’t
need the extra signal swing given the
5.5V limitation of the digital pots.
This also provides more headroom
for regulation.
The +12V rail is dropped to +5.5V
using adjustable regulator REG3. This
is adjustable so that it can be set to
precisely +5.5V; to be safe, we don’t
want to exceed the maximum recommended supply voltage for IC6 or
IC7 (even though the absolute maximum rating is much higher). A series
fixed resistor is provided to limit the
adjustment range.
Zener diode ZD1 acts as a safety so
that if the output of REG3 is much too
high for some reason, it should conduct and prevent damage to IC6 and
IC7. The +2.75V mid-supply rail is
derived from the +5.5V rail using a
resistive divider and trimmed using
Practical Electronics | September | 2022
Screen 4:
if you decide
to name
one of the
presets,
you will be
presented
with this
basic
QWERTY
keyboard
so you can
enter a new
name or
change the
existing one.
Screen 1: the main screen has buttons to quickly
load one of six presets, change the volume, mute the
output or go to one of two settings screens (presets
and tone/EQ adjustments).
Screen 2: the tone control/equaliser (EQ) adjustment
screen. Here you can set the bass/mid/treble boost/
cut values as well as a volume adjustment (PRE+/−),
and it shows you an approximation of the resulting
frequency response below. You can also switch
between the inputs, adjust the loudness control,
reset the settings or store them to the current preset.
Software
As the control module
is a Micromite, the software is written in BASIC
(MMBasic, to be exact).
The control program for the Digital
Preamp is quite small compared to
other Micromite-based projects. This
is mainly due to the relatively simple
functions it provides, with the hardware doing most of the work.
Screen 3: in the preset screen, you can switch between
the six presets, give them names, view their settings
and adjust the backlight brightnesses and timeout.
VR2 so that signal clipping to the supply rails is symmetrical.
It’s filtered using a 220µF capacitor
so that the source impedance seen by
the rest of the circuit is low, preventing unwanted crosstalk.
Practical Electronics | September | 2022
Links LK1-LK3 are provided for testing because
IC6 and IC7 are SMDs.
They can be left out while
the supply voltages are
checked, and IC6, IC7
and the op amps will not
receive power. Once the
supply voltages have been
verified as correct, they
can be inserted, and the
unit powered back up.
Finally, regulator REG4
provides a 5V DC supply
to run the BackPack control circuitry. Two series
10Ω 1W resistors have
been provided to prevent this regulator from
overheating due to the
relatively high current
required by the BackPack,
and the large difference in
the input (12V) and output (5V) voltages.
This works, although
these resistors run fairly
hot if you have the BackPack LCD backlight turned
up to a high brightness setting. If you find this to be a
problem, there isn’t room
to fit a heatsink to REG4,
but you could add more
dropper resistors. For
example, four 4.7Ω 1W
resistors mounted vertically instead of horizontally would spread out the
heat load.
The Micromite processor controls
the four relays and the two digital
potentiometer ICs, which have four
potentiometers each, for a total of
eight. The Micromite also commands
the LED and receives signals from the
infrared receiver.
While the MMBasic code provides
an interrupt that is triggered when an
IR code is received, we simply use
this to set a flag, as other operations
could be occurring when the interrupt
is triggered. The received command is
processed later, when the Micromite
would otherwise be idle.
We think that many constructors
will want to use the 2.8-inch touchscreen (eg, as used in the original
BackPack or BackPack V2) because it
will be a better fit on the front panel
of many cases suitable for a preamp.
However, you can use the Micromite LCD BackPack V3 with its higherresolution, larger 3.5-inch touchscreen
if you have room. The software has
been designed so that it can use either
screen with just minor changes to the
code, and we will provide both versions (BASIC code and HEX files) in
the download package for this project.
User interface
As with other projects using the
Micromite BackPacks, several different screens are provided for various
features. The MAIN screen offers the
features that will be used most often,
while two other screens allow the settings to be customised.
The MAIN screen (Screen 1) has six
buttons corresponding to six presets.
While there are only four inputs, some
readers might have these connected
to other devices with more inputs,
so multiple presets can use the same
input to provide various custom tone
profiles for each input. You might
also want different sound profiles for
the same device (eg, to suit movies or
music playback).
If one of the presets is selected,
its button is highlighted; the MUTE
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Screen 8:
if you enter
an invalid
value,
an error
message
will be
displayed.
Screen 5: once you press the Enter (Ent) key, it
confirms the new name you have typed for the preset.
will cause the current
settings to be saved to
Flash memory if they have
changed. There is also
an automatic timed save
feature. To help conserve
Flash memory longevity,
this defaults to 10 minutes
(of unsaved changes), but
you can alter that.
The SAVE button is red
if there are any unsaved
changes; otherwise, it is
grey. Below the SAVE button is a timer showing the
number of seconds before
the screen changes to a
Screen 6: this simpler numeric keypad is used
low-brightness idle mode.
to enter backlight brightness percentage values.
Two more buttons proThere’s one setting for when you are actively using
vide access to the setthe touchscreen, and another dimmer setting after
the timeout. For best audio performance, we suggest tings. The EQ SETTINGS
screen (Screen 2) is used
using 0% (backlight off) as the timeout value.
to set the tone controls
and input selection. This
screen also shows an
approximate frequency
response graph of the current settings.
The graph is based on
tests conducted with our
prototype, so it will not
reflect variances due to
component tolerances.
The response calculation assumes that the frequency response of each
stage is linear, which does
not apply at the extreme
ends of the potentiomeScreen 7: the confirmation message that appears when
ter travel.
you have adjusted one of the brightness settings.
The graph is characterbutton is also highlighted when it ised by arrays of values which provide
is active.
a value for the midpoint response and
Three more buttons provide MUTE, another value for the difference per
VOLUME UP and VOLUME DOWN potentiometer step at ten different
functions. These nine buttons corre- frequencies. These are the values you
spond one-to-one to the functions that
would need to change if you wanted
are available via the IR remote control. to characterise your device precisely.
The level of the volume is also dis- The default values should be acceptplayed numerically.
able for most users.
At the top right is a much smaller
The SETTINGS (Screen 3) screen
button marked SAVE. Pressing this allows the currently set tone controls
to be allocated to a preset and for these
presets to be renamed. The parameters
for each preset are displayed next to
their buttons. These are shown in raw
digital potentiometer steps ranging
from −127 to +127, with zero denoting the midpoint.
Each of the six presets can be
renamed by pressing the corresponding RENAME button. This brings up
a keypad allowing capital letters and
numbers to be entered (Screen 4). To
make good use of the available space,
only a limited set of keys is provided.
Backspace, Enter and Cancel buttons are also provided. Upon pressing Enter, the new name is displayed
briefly (Screen 5).
Finally, there are buttons to allow
for numeric entry of three backlight
settings (normal intensity, idle intensity and idle timeout) and the save
timeout setting. Pressing the corresponding button displays a numeric
keypad for entering a new value, with
the prompt containing a range for valid
values (Screen 6).
Entering a value also displays a brief
popup indicating the entered value
(Screen 7) or noting an error if an
entered number is out of range (Screen
8). For simplicity, only positive integer values are supported.
The normal backlight values range
from 1-100%, while the idle backlight
extends the lower limit to 0%, blanking the display completely. This is
handy if you don’t wish the display to
interfere with, for example, viewing a
movie in a dark room.
The idle backlight is only activated
on the MAIN screen, so it does not
interfere with changing the settings.
A touch anywhere on the screen will
awaken it; you can use the title area at
the top of the screen to be sure of not
changing any parameters.
Next month, in Part 2, we will go
through the particulars of setup and
operation of the Digital Preamp after
going over the construction and testing details.
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Practical Electronics | September | 2022
