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How to measure
harmonic distortion
I recently found out about your
magazine, and I bought several of your
older issues that were very helpful.
Do you have a magazine where you
described how to measure THD (total
harmonic distortion) on audio amplifiers? (B. G., Neu-Isenburg, Germany)
• We have not published a dedicated article on the topic, although we
have touched on it several times over
the years.
To measure THD of an amplifier,
you need to use a distortion analyser
and a low-distortion signal (sinewave)
generator. We have published several
articles on these in the past.
The most recent article is the USB
SuperCodec (August-October 2020;
siliconchip.com.au/Series/349) and
the matching Balanced Input Attenuator (November-December 2020;
siliconchip.com.au/Series/349). Those
articles also included details on analysis software that can be used in combination with the SuperCodec for measuring THD+N, signal-to-noise ratio,
channel separation etc.
In terms of a build-it-yourself instrument for distortion measurements,
the USB SuperCodec will be hard to
beat. We use an Audio Precision System Two, which cost quite a bit new
(and they still go for a fair bit of money used).
What is a PC stake?
I am building the High Power Ultrasonic Cleaner (September & October
2020; siliconchip.com.au/Series/350),
and I have a question about the parts
list. What is a PC stake? What does it
look like, what is it made of and what
is its purpose? Can I make one myself
(out of thick copper wire)? I can find
no mention of them on Jaycar’s website, for instance. (D. P., Noumea)
• A PC stake is a small hardened pin
(typically 0.9mm to 1mm in diameter)
that fits into a hole in a printed circuit
board (PCB). It is used to allow wires
to be easily soldered, or as a point
siliconchip.com.au
for connecting multimeter or oscilloscope probes.
They are generally optional, and
in this case, can be left off if you are
happy to directly solder wire or hold
a probe to the PCB pad. Jaycar sell PC
stakes, Cat HP1250 (they call them
“PCB pins”, which is the same thing).
Identifying charred
component for repair
I have an Ozito SNG-956 Staple/Nail
Gun that has a burnt-out resistor R1.
The circuit appears to use the typical
capacitor/resistor in series from the
mains to a bridge rectifier to provide
DC to the rest of the circuit.
I wonder if anyone knows where I
can find a circuit diagram for it, or the
value of R1.
This is quite an old tool, and unfortunately, Ozito can’t get the circuit
diagram from the manufacturer. so at
the moment, I’m stuck with not being
able to repair it. (B. P., Dundathu, Qld)
• If the resistor is in series with the capacitor feeding the bridge rectifier, it’s
likely to be a relatively low value like
1-10W. Its primary purpose would be
to limit the inrush current when power
is first applied, and perhaps act as a
‘fuse’ of sorts (it sounds like it did...).
It might also exist to drop some of
the voltage (although we don’t think
that is a very wise design decision), in
which case using a low value could
cause other components to overheat.
Increasing feedback for
Motor Speed Controller
I purchased an old gem facet machine with a ¼hp induction drive. It
shook, rattled and totally stuffed up
soft gem faceting. I have modified it
to work with a small universal motor
and your Full Wave 230V Universal
Motor Speed Controller (March 2018;
siliconchip.com.au/Article/10998).
Actually, I’m using two universal
motors, both running via the same
type of speed controller. One starts
and runs perfectly; I just needed to
Australia’s electronics magazine
adjust the feedback pot (VR2). The
feedback works well, maintaining a
fixed speed. This is important for a
faceting machine when cutting soft
gemstones; unlike diamonds that can
handle speed variations, the speed is
more critical for soft gems.
But the other universal motor will
not start, no matter the setting on VR2.
I connected a desk lamp in parallel
with the motor, and off it went. The
speed control was great; remove the
lamp load, and the motor continues
to run. However, there is a slow-speed
point where this motor stops rotating.
The only way to get it going again is
to have the lamp connected in parallel with the motor. I note that the
slow speed point on the other motor
is much slower.
It appears to be a back-EMF sensitivity constraint; this motor requires
a lower feedback threshold to start
and rotate as slowly as the other motor. Can I change the CT feedback loop
components to improve the sensitivity
for motors such as this?
Also, can some of the feedback
bridge’s values and its associated RC
network be adjusted to increase the
voltage feedback to the PIC? (J. T., Teneriffe, Qld)
• You would need to add more turns
of the mains wire through the transformer. That might be difficult as the
hole is a small diameter. Select 10A
mains wire that will allow more turns
through.
You could also increase the 510W
loading resistor that is across the
AX1000 transformer coil. A larger
value will increase the output voltage. We used 510W but, for example,
a 2.2kW resistor would give a higher
feedback voltage. There is a limit to
the output versus current response,
and it becomes non-linear with greater
resistance values.
Note: J. T. got back to us and said:
“I increased the 510W loading resistor
that is across the AX1000 transformer
coil to 1kW, and doubled the number
of winding through the CT. It works
perfectly now.”
March 2021 107
Larger display for RPi
Tide Clock
I want to build the Raspberry Pi Tide
Chart from July 2018 (siliconchip.com.
au/Article/11142), but I want a larger
display than the 2.8in TJCTM24028
screen you used. Do you know if there
is a larger screen that is compatible,
which could be plugged or wired in
to replace the existing small LCD? I
would like to put it in a frame that can
be placed on the wall for everyone to
see. (R.W., Mt Eliza, Vic)
• 3.2in and 3.5in variants of the
ILI9341-based 2.8in display we used
in that project are available, with the
same pinout as the TJCTM24028. We
suspect that the mounting holes and
SD card reader would not match, but
the 14-way header for the LCD and
touch appears to be the same in each
case (which is all that is needed for
the Tide Clock).
For example, see siliconchip.com.
au/link/ab6y and siliconchip.com.
au/link/ab6z
We haven’t tested any of these, so
we can’t comment with any certainty that they would work. Using any
other display controller (instead of
an ILI9341) would require a major rewrite of the code.
Note, though, that you would have
many more options for larger (and
cheaper) screens if you used one with
an HDMI input (ie, a small computer
monitor), which is natively supported
by the Raspberry Pi.
Effect of changing
crossover inductor
The recommended inductor for the
Majestic loudspeaker crossover (June
& September 2014; siliconchip.com.
au/Series/275), in series with the woofer, is a 2.7mH inductor. The Jaycar Cat
LF1330 inductor that was recommended is no longer available. With COVID
restrictions, most European suppliers
aren’t exporting down under.
I can get a 2.5mH air-cored inductor locally. Would this be suitable, or
would I need to make other changes?
(P. S., Hamilton, NZ)
• We doubt you would notice the difference. That is only a 7.4% difference
in value, and the tolerance of these
inductors is probably ±20% anyway.
There might be slightly more midrange getting to the woofer (it has the
bandwidth to reproduce up to a few
108
Silicon Chip
kHz). In the unlikely event that you can
hear the difference, and it is bothersome, you could add 180µH or 220µH
air-cored inductors in series with the
2.5mH types. Just make sure they are
mounted at right-angles, so their magnetic fields don’t interact.
Increasing DC-DC
Converter soft-start time
I am building the DC-DC Converter
to power the CLASSiC-D Class-D amplifier (May 2013; siliconchip.com.au/
Article/3774).
Would increasing the value of the
47kW resistor connected to IC1’s pin
4 and the 10µF capacitor be a suitable
way to increase the circuit’s soft-start
time? I have found that connecting
the Mk.3 power supply board with six
4700µF capacitors is too much all at
once, resulting in blown STP60NF06
Mosfets, and I am hoping that a softstart modification would make the two
projects compatible.
Also, the TL494CDR switchmode
controller is out of stock at element14
and Mouser at the moment. Are the
TL494IDR or TL494CD (both from Texas Instruments) suitable alternatives?
(E. B., Viewbank, Vic)
• You can slow down the soft-start by
increasing the value of the capacitor
(originally 10µF) at pin 4 of IC1. 22µF
or 47µF capacitors would be suitable.
The 47kW resistor value should not be
changed.
As for the TL494CDR IC specified,
the following types are also suitable:
TL494CN, TL494CNE4, TL494IN or
TL494INE4. In fact, any 16-pin DIP
version of the TL494 should work.
CLASSiC-D overheating
and motorboating
I built two of your CLASSiC-D ClassD amp modules (November-December
2012; siliconchip.com.au/Series/17)
from Jaycar kits, Cat KC5514. My construction experience is extensive, having been employed by a competitor for
six years as the national production
manager for local manufacture and kit
assembly and tech support.
After many years of continual use,
my Series 5000 150W modules have
died with the 2SK49 and 2SK134 transistors failing, so I decided to upgrade
the modules to the cooler/more efficient Class-D type and get more power,
240W into 4W.
Australia’s electronics magazine
I used a multimeter to check all
resistor values and used my phone
camera to zoom in on the diode and
capacitor markings to make sure I had
the correct values in the correct locations on the board. The larger components are easily read, so I completed
the PCB assembly and also drilled the
heatsinks at the full 75mm height.
The steps to confirm the board setup
is correct worked as per the instructions, and I can get a clear sound from
the amp modules. I’m using a ±50V
supply rails from a 35-0-35V toroidal
transformer. The amplifiers have tested with the correct voltages, and when
I plug in an RCA male to 3.5mm jack
cable into my mobile phone, I get
clear sound.
However, the heatsink is very hot
with no signal applied and no speaker connected. When I connect a signal
and 8W speaker, the sound remains
clear for about one minute, but then I
can no longer touch the heatsink, and
the amp has distortion until it is turned
down and cools a bit.
Also, when I plug in a DJ mixer or
other preamp device, the amp gets a
low-frequency oscillation at full power and the speaker is thumping at full
volume.
Unfortunately, I lost a lot of gear in
a bushfire, so I currently do not have
an oscilloscope or signal generator. I’m
hoping you can provide some insight
into what steps I can take to resolve
these problems. (D. F., Perth, WA)
• The heatsinks are probably running
hot due to the dead time not being sufficient for the Mosfets being used. You
can initially lift the 5.6kW resistors between pin 9 and pin 12 of IC1 on each
amp board to get the maximum dead
time setting (DT4). If the heatsinks run
much cooler, that tells you that it was
definitely the dead time setting at fault.
Our original design uses the DT2
setting. If DT4 works OK, you might
like to try DT3, which will give lower
distortion. To test this, change the
5.6kW resistor to 8.2kW and the 4.7kW
resistor, from pin 9 to ground, to
3.3kW. You will need to verify that the
heatsink temperature is still OK with
this setting, but if so, it will give you
better performance.
As for the low-frequency oscillation, that’s possibly due to the power
supply cycling up and down in voltage when delivering a high power
output. This is explained on pages
21 and 22 of the IRAUDAMP5 Refersiliconchip.com.au
ence Design document (siliconchip.
com.au/link/ab2a).
The recommendation to solve this is
to reverse the input and output phases
of one of the amplifier modules. This
is catered for on our modules by op
amp IC2 and link LK2. Simply move
the LK2 shunt on one of the modules
to the alternative position, then swap
the speaker wires to CON3 on that
same module.
Using Bridge Adaptor
with Class-D amplifiers
I built a couple of your Bridge Adaptor For Stereo Power Amps (July 2008;
siliconchip.com.au/Article/1887) from
Altronics K5566 kits, and they work
perfectly. Can this adaptor be used
with a Class-D amplifier? (P. N., via
email)
• It depends on the amplifier but probably not, because most Class-D amplifiers already run in bridge mode. If you
can’t tell from the amplifier specs/data,
check to see if there is continuity between either of the output terminals
and ground (generally if there is continuity, it will be with the black/negative output).
Continuity to ground suggests that
the output is not bridged and you
could use a bridge adaptor. Lack of
continuity suggests that it is already
bridged. You can also tell looking inside the amplifier as a bridged ClassD amplifier usually has two filter inductors per output (ie, four for a stereo amplifier).
12V to 15-35V Inverter
output dropping
I have just built the 12V 100W Converter With Adjustable 15-35V DC
Output (May 2011; siliconchip.com.
au/Article/1009).
At the top of page 79, there is a
graph which shows at 25V you should
get 3A. I have hooked up a 12W LED
floodlight (Jaycar SL3931), tested on
my bench supply at 25V DC as drawing 600mA. But the inverter output
drops from 25V to 9V and the current
increases to 1.5A, which is not good.
I can adjust the output voltage from
12V to 30V. The pin 5 voltage is 1.25V
but does change on varying the output. The voltage at the gate of Q1 is
very low, less than 1V. I cannot get a
steady 10V reading. Reading the project notes, it says I should get 10V at
siliconchip.com.au
the gate of Q1. I do not understand
whether this is with the circuit under load or not under load. At pin 2, I
measure a 32kHz signal.
Do you know why it can’t drive the
floodlight with 24V DC at 600mA? (M.
T., Upper Swan, WA)
• The lack of output power can be
due either to the input supply not
being able to deliver the required
current and so dropping the voltage,
or the current detection resistance is
high (R1 on the circuit).
Check the input supply and note
that it will need to provide over twice
the output current when delivering a
25V output with a 12V input.
If the input supply is holding up,
possibly R1 (the 0.025W resistor) is
the wrong value or the connections
to the PCB are high resistance. Check
the value and also the soldering of this
component to the PCB. You might have
a dry joint.
Power factor correction
and mains-borne noise
Leo Simpson’s March 2011 editorial (siliconchip.com.au/Article/921)
claimed that power factor correction
circuitry won’t reduce your energy usage or save money.
Yet in this IEEE article, they point
out that smart meters can misread
when dirty power is fed into them:
siliconchip.com.au/link/ab70
As power factor correction reduces
noise (aka dirty power), why wouldn’t
it reduce your power bill? (M. C., via
email)
• Power factor correction (PFC)
doesn’t usually reduce mains-borne
noise. In fact, it can increase noise on
the mains supply.
Capacitive PFC shifts the current
phase to be closer to the voltage waveform, to compensate for inductive
loads. It might provide some noise filtering, but that is mostly incidental to
how it works.
On the other hand, active PFC,
which improves the power factor of
switching supplies using switching
techniques, can inject more noise due
to its switching action.
Also, power factor correction would
typically be applied on the load side of
the meter. It’s unlikely to do anything
to affect incoming noise from external
sources, which must be significant if it
is passing through the low-impedance
mains distribution network.
Australia’s electronics magazine
The primary way to reduce noise is
filtering. Mains filters are simple and
readily available. If smart meters are
misreading, that suggests they do not
have adequate filtering on the input
side and their metering circuitry.
Freq/voltage converter
for RPM counter
Have you published a project or
projects that shows how to create a
DC voltage directly proportional to
frequency, for example, using the
LM2917 IC? I want to make an RPM
counter for the tail shaft of an irrigation engine. (P. H., Gunnedah, NSW)
• Try the Twin-Engine Speed Match
Indicator for Boats from the November 2009 issue (siliconchip.com.au/
Article/1622).
It could be used for a single engine
by tying the pin 10 non-inverting input of IC3c to ground and deleting IC2
(LM2917) and its associated components. There are kits available for this
project from Jaycar (Cat KC5488) and
Altronics (Cat K6220).
Amplifier and power
supply kits wanted
Do you happen to sell a kit for the
20W Stereo Class-A Power Amplifier
(September 2007; siliconchip.com.
au/Article/2341), including the chassis? If not, do you have the PCBs and
the chassis?
Also, do you have a linear DC power
supply kit that is not a bench type? I
want a supply with 5V, 9V, 12V and
15V outputs, either variable/switchable or a single output with 2A capability. Preferably with chassis. (D. S.,
via email)
• The only kit available for the 20W
Class-A amplifier with a chassis was
Altronics Cat K5125, but unfortunately, it has been discontinued. We believe that the case is no longer available. You would need to make your
own chassis from a standard vented
rack case or similar. We do have the
PCBs for that project, which you can
purchase via this link: siliconchip.
com.au/Shop/?article=2283
As for the power supply, we don’t
have a non-bench supply that meets
your requirements. However, you
might want to take a look at the
4-Output Universal Voltage Regulator (May 2015; siliconchip.com.au/
Article/8562).
March 2021 109
This has 5V and 3.3V fixed outputs
and adjustable positive and negative
outputs up to 22V. It does not have
2A capability, however, replacing the
LM317 with an LD1085 would likely
mean that you can draw over 2A (and
possibly as much as 3A) from the positive adjustable output, given a sufficiently beefy DC input supply and
enough heatsinking.
Failed LC Meter from
2008
I built the LC Meter described in
your May 2008 issue (siliconchip.com.
au/Article/1822) from an Altronics kit
that same year. I was so happy with the
result that I have not bothered to build
the updated versions described since.
However, when I went to use the
unit the other day, I noticed the
capacitance reading was high. I
checked the readings against several capacitors of known value and
found that all readings were out by
the same amount.
I could get useful results from the
readings by measuring a known capacitor first, calculating a fudge factor to correct the error in the readings
and then applying that factor to the
unknown capacitor’s reading. While
this allowed me to get on with the
work I was doing, I feared it might be
the start of bigger problems.
I went back to the instructions and
re-ran the calibration procedure and
found that it gave 0.00pF when started and 49435 with the jumper shunt
in LK2, but the display vanished with
the jumper in LK1.
I checked all solder joints and reflowed a couple of suspect ones without any change in the performance. I
cannot see any solder bridges. Any
suggestions of what I should check
next? (C. K., Parkhurst, Qld)
• There isn’t a whole lot to go wrong
in that circuit. The lack of display
suggests that the microcontroller isn’t
running. First, check that the output
of REG1 is a steady 5V (4.75-5.25V).
The fact that your readings shifted by a consistent amount before it
failed completely suggests that there
may be a problem with crystal X1. If
its frequency changed then that could
throw the calibration out, and if it
failed entirely then the micro would
not run. Check for a 4MHz signal at
pin 15 of IC1 (eg, using a scope or frequency meter).
110
Silicon Chip
If there is no oscillation then there
is something wrong with either crystal X1 or microcontroller IC1. If you
have a PIC programmer, it would be
a good idea to attempt to reprogram
IC1. While we find PICs very reliable,
there is a slight possibility that your
IC1 chip has failed. In that case, you
can order a replacement programmed
PIC from us; see siliconchip.com.au/
Shop/9/1277
If the voltage across the electro is
low, as is in many coupling circuits
where both ends of the capacitor are
nominally at ground potential, the orientation doesn’t matter. Typical electrolytic capacitors can tolerate a small
DC voltage of either polarity (up to say
±500mV) indefinitely.
Modifying the Four
Input Mixer
I have a question about the FM Wireless Microphone project from your
October 1993 issue (siliconchip.com.
au/Article/5343). I have been trying to
work out how the RF oscillator based
around NPN transistor Q3 works, but
I have not been able to.
There needs to be capacitance across
inductor L1 to form a resonant circuit.
Is this the Miller capacitance between
the transistor base and emitter? I cannot see where the feedback path is for
the oscillator, either. Sadly, the article
does not specify a value for L1.
If you could help me understand
how the oscillator works, I would be
most grateful. (A. C., Gembrook, Vic)
• The 1pF capacitor across inductor
L1 forms part of the capacitance necessary for oscillation to occur, but is
only a small contributor. Q3’s Miller
capacitance would also make a small
contribution.
The rest is via the 15pF coupling capacitor, which is in series with 33pF
and 15pF capacitors to ground. That
combination has a total capacitance
of around 6pF, and is effectively in
parallel with the 1pF directly across
L1. You also have to consider trace inductance etc which will significantly
reduce the effectiveness of that extra
capacitance at 95MHz.
As for feedback to make Q3 oscillate, that would be the 33pF capacitor
between its base and emitter. The base
and emitter are effectively 180° out of
phase, so that plus the phase shift introduced by that capacitor should be
enough to sustain oscillation.
Making RF oscillators work reliably
and at a particular frequency is a bit of
a black art. It must have taken quite a
bit of tweaking for Oatley to come up
with the circuit as presented.
We could be accused of going into
too much detail in our circuit descriptions these days, but your question
makes it clear that there was far too little detail in these early articles. There’s
continued on page 112
I want to build a variant of the
Versatile Four Input Mixer from
the June 2007 issue of Silicon Chip
(siliconchip.com.au/Article/2256). I
only want two inputs, one for a guitar
and the other for a CD player. Can I
delete the master volume control
(VR8) and only use the headphone
volume control (VR9)?
I want this project to be heard on
headphones only. Could you please
tell me what other components need to
be deleted or added, especially around
the master volume control.
Also, in this design, you have some
coupling electrolytic capacitors that
the input goes into the positive side,
yet there are some where the negative
side is fed a signal. How do you determine which way the cap is supposed
to go in these cases? (J. R., Hoppers
Crossing, Vic)
• You could take the connection that
goes to the top of the master volume
pot (VR8) and connect this to the top
of the headphones volume control VR9
instead. Remove the original connection from the main output. Then the
output socket and master volume control can be removed.
Electrolytic capacitors are orientated based on the expected DC voltage at either end, ie, with the positive
lead to the more positive side. You
need to do some circuit analysis to
determine the DC operating conditions at either end, or run a simulation, or just build the device with a
non-polarised capacitor and measure the voltage before substituting
an electrolytic capacitor.
One of the trickier aspects of this
sort of calculation is taking into account op amp or amplifier input bias
currents; analog IC inputs can source
or sink current, or do neither, and
sometimes that changes depending on
certain factors.
Australia’s electronics magazine
Help to figure out how
an oscillator works
siliconchip.com.au
Notes & Errata
USB SuperCodec, August-October 2020: in the Fig.13 circuit diagram on page
88 of the September 2020 issue, pin 12 of IC7 (SDOUT) should not be shown
connected to pin 9 of IC6. Instead, it goes to the I2S_ADC1 connection at the right
edge of Fig.12 on p86.
Car Altimeter, May 2020: the design is missing one schottky diode (D8) which
connects from the cathode of ZD1 (schottky anode) to the positive terminal of the
battery (schottky cathode). This is needed to charge the battery. It can be added
to the underside of the PCB, as shown in the accompanying photograph.
Advertising Index
Altronics..................17, CATALOG
Ampec Technologies................. 20
Analog Devices..................... OBC
Dave Thompson...................... 111
Digi-Key Electronics.................... 3
Emona Instruments................. IBC
Jaycar............................ IFC,53-60
Keith Rippon Kit Assembly...... 111
LD Electronics......................... 111
LEDsales................................. 111
Microchip Technology.................. 5
Mouser Electronics...................... 7
Ocean Controls......................... 19
SC Colour Maximite 2............... 71
Silicon Chip Binders............... 111
Silicon Chip Shop...............98-99
6GHz Touchscreen Frequency Counter, October-December 2017: in the
circuit diagram on pages 30 & 31 of the October 2017 issue, a 1µF bypass
capacitor is missing between the anode and cathode of REF1. Also, in the
overlay diagram (Fig.3) on p86 of the November 2017 issue, the board shown
is RevA; the final (RevB) board adds a 100µF capacitor just to the left of REG2,
with its positive lead towards the regulator.
The April 2021 issue is due on sale in newsagents by Thursday, March
25th. Expect postal delivery of subscription copies in Australia between
March 23rd and April 9th.
hardly any mention in that article of
how the circuit works!
Disconnecting the
charger on full battery
Some time ago, you advised me how
to modify the “Add-On Regulator for
12 Volt Battery Chargers” published
in Electronics Australia, June 1997,
to charge a 24V SLA battery. I have
used it for several years, but I am now
planning to upgrade to a 24V Lithiumion battery.
If I set the charge voltage to 28.5V,
will I need to add extra circuitry to
disconnect the battery at this point?
Will one of the cut-out modules, as
available from eBay, be suitable for
this? (B. C., Dungog, NSW)
• Yes, you would need to switch off
the charger when the Lithium-ion
112
Silicon Chip
battery is charged. You could use our
Threshold Voltage Switch (July 2014;
siliconchip.com.au/Article/7924),
sold as a kit by Altronics (Cat K4005)
and Jaycar (Cat KC5528). Any other
similar device should also work.
Graphic Equaliser level
matching problem
I have been using an Electronics
Australia Graphic Analyser for many
years, even though it spends most of
its life in the cupboard. I drive it with
an electret mic which is switchable
between 600W and 50kW.
The problem is that to get a decent
level on the LED display, I must have
the sound level in the room extremely
high, to the point that I must wear ear
protection and only do it when nobody
else is at home.
Australia’s electronics magazine
Silicon Chip PDFs on USB....... 91
Switchmode Power Supplies..... 29
The Loudspeaker Kit.com........... 9
Tronixlabs................................ 111
Vintage Radio Repairs............ 111
Wagner Electronics................... 64
It has always been that way, but it
seems that it would be best to do the
process at a normal listening level.
Is there something I can do to increase the mic preamp gain, or might
there be some other problem? The
original build did have problems with
many dead or partially-dead quad op
amps. Might there be more remaining undetected? (R. A., Hunter’s Hill,
NSW)
• We suggest that you use a preamplifier to boost the microphone signal. Then you won’t need to have the
volume so loud.
You could use our Multi-Role Champion Preamplifier published in the
June 2015 issue (siliconchip.com.au/
Article/8609). It is inexpensive and
easy to build, and its gain can be adjusted to suit your needs. We can supply the PCB for that project.
SC
siliconchip.com.au
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