Fullscreen HTML5Med Res (??MB)HTML4

Part 1: by Nicholas Vinen & Tim Blythman Touchscreen & 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 analog design. It achieves that by using classic Baxandall style volume and tone control circuitry with op amps, incorporating high-quality digital potentiometers to provide the adjustments. M ost of our remote-controlled preamplifiers to date 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 also can 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 hifi, and most of those have been discontinued. They also can be pretty expensive and difficult to obtain. And 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 came up with would have 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) 38 Silicon Chip 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) Ideally, an intuitive and attractive colour touchscreen interface for direct control We achieved 1) through 4) by using two quad Analog Devices AD8403ARZ10 digital potentiometer ICs. While these are not especially cheap at around $10 each, they are still quite reasonably priced compared to hifi-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. Australia’s electronics magazine 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, loading/ saving presets – the whole nine yards. It’s just the go 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 siliconchip.com.au 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 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 & Bluray 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 siliconchip.com.au Features • • • • • • Four input stereo preamp with a colour touchscreen and remote control Bass, mid & 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 Frequency response: 20Hz-20kHz +0,-0.5dB Channel separation: >75dB, 20Hz-20kHz Signal handling: 0.1-2.5V RMS Volume control range: approximately 78dB Gain range: -50dB to +27.6dB (0.003 times to 24 times) Input impedance: 100kW || 470pF Bass tone control: ±12.5dB centred around 20Hz (±11.5dB <at> 50Hz, ±8.5dB <at> 100Hz) Midrange tone control: ±11dB centred around 440Hz (±7.5dB <at> 200Hz & 1kHz) Treble tone control: ±11.5dB centred around about 20kHz (±10.5dB <at> 10kHz, ±9dB <at> 5kHz) Power supply: 12-15V AC, 24-30V AC CT or ±15V DC Current draw: typically around 200mA with touchscreen on and <50mA 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, and the unit can handle over 2.5V RMS at its inputs before clipping. 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). Australia’s electronics magazine September 2021  39 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. 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. controls at their extremes, as shown in that plot. Fig.5 shows some more realistic tone control settings (mauve, orange & blue) along with some examples of what happens if you set multiple controls to their maximum extents (red, green & cyan). Note how there is some interaction between the controls. For example, the treble boost is reduced when a lot of 40 Silicon Chip 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. Circuit details The Digital Preamp circuit is shown in Fig.6. Signals are fed into one of four pairs of RCA sockets, CON1A-D Australia’s electronics magazine and CON2A-D. These have individual 100kW 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 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 100W series resistor, a ferrite bead and a 470pF capacitor to ground for each channel. This RC low-pass filter has a -3dB point of 3.4MHz, while the ferrite bead helps to eliminate much higher frequency signals which could otherwise be rectified by the following buffer stage, inducing unwanted signals into the audio. 1kW 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 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 & 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 Bluray 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 2.2kW fixed series resistors connected to pin 3 of the two digital pot ICs. These combine with the digital pots’ 10kW 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 2.2kW resistors also limit siliconchip.com.au 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. 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 & IC4a are buffers, while IC2b & 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 2.2kW 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 50-100W. 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 current to around 50mA, so neither the diodes nor the op amp will be damaged during clipping. As the signal siliconchip.com.au Parts List – Touchscreen Digital Preamp 1 Micromite LCD BackPack programmed with 0110319A.HEX (2.8in display) or 0110319B.HEX (3.5in display) [SC3321, SC4237 or SC5082] 1 double-sided PCB coded 01103191, 206 x 53mm 2 double-sided PCBs coded 01103192, 12.5 x 45.5mm 1 universal IR remote control (optional) [Jaycar XC3718 / Altronics A1012A] 3 EA2-12 DPDT 12V DC coil telecom relays (RLY1-RLY3) 2 500W 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) [SC5912, 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) 1 W04M bridge rectifier (BR1) 12 BAT42 schottky small-signal diodes (D1-D12) 3 1N4148 silicon small-signal diodes (D13-D15) Capacitors 2 1000μF 25V electrolytic 3 220μF 16V electrolytic 3 100μF 16V electrolytic 2 47μF 16V electrolytic 2 22μF 16V electrolytic 3 10μF 16V electrolytic 2 1μF 63V MKT 2 220nF 63V MKT 4 150nF 63V MKT 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 100kW 6 2.2kW 1 110W 6 47kW 13 1kW 5 100W 2 22kW 1 910W 2 10W 1W 5% resistors OR 2 10kW 11 680W 4 4.7W 1W 5% (see text) 2 4.7kW 1 560W Australia’s electronics magazine September 2021  41 Fig.6: the Digital Remote Controlled Preamp circuit, plus its attached infrared receiver. Besides those components, everything is mounted on one board, which mounts on a small 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. 42 Silicon Chip Australia’s electronics magazine siliconchip.com.au siliconchip.com.au Australia’s electronics magazine September 2021  43 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 adjacent). 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 44 Silicon Chip 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 & IC5a are no longer connected to the digital pots. They are instead connected to the centre taps of pairs of 4.7kW 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 the DC bias, then amplified by a factor of two by op amps IC3b & IC5b. This allows the output amplitude to be above 1.9V RMS if desired, up to about Australia’s electronics magazine 3.8V RMS before clipping. The 100W 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 installed). The other pair consists of vertical connectors on the board, more suited for internal connections to amplifier modules. It should be 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 common reset (RS) and shutdown (SHDN) lines. That’s a total of seven digital lines required to control both ICs. 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 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 siliconchip.com.au 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). 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 on etc. 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 siliconchip.com.au 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 & IC7. The +2.75V mid-supply rail is derived from the +5.5V rail using a resistive divider and trimmed using 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 etc. Links LK1-LK3 are provided for testing because IC6 and IC7 are SMDs. Australia’s electronics magazine 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 10W 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 4.7W 1W resistors mounted vertically instead of horizontally would spread out the heat load. 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. 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 September 2021  45 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. 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. 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. 46 Silicon Chip Australia’s electronics magazine 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 button is also highlighted when active. Three more buttons provide MUTE, VOLUME UP and VOLUME DOWN functions. These nine buttons correspond one-to-one to the functions that are available via the IR remote control. The volume level is also displayed numerically. At the top right is a much smaller button marked SAVE. Pressing this 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 low-brightness idle mode. Two more buttons provide access to the settings. The EQ SETTINGS screen (Screen 2) is used 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 potentiometer travel. siliconchip.com.au Screen 5: once you press the Enter (Ent) key, it confirms the new name you have typed for the preset. The graph is characterised by arrays of values which provide a value for the midpoint response and another value for the difference per potentiometer step at ten different frequencies. These are the values you would need to change if you wanted to characterise your device precisely. The default values should be acceptable for most users. The SETTINGS (Screen 3) screen 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 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. We’ll explain the particulars of setup and operation next month after going over the construction and testing details. SC siliconchip.com.au Screen 6: this simpler numeric keypad is used to enter backlight brightness percentage values. There’s one setting for when you are actively using the touchscreen, and another dimmer setting after the timeout. For best audio performance, we suggest using 0% (backlight off) as the timeout value. Screen 7: the confirmation message that appears when you have adjusted one of the brightness settings. Screen 8: if you enter an invalid value, an error message will be displayed. Australia’s electronics magazine September 2021  47