Wiring two SC480 modules for stereo
I am building a stereo amplifier using two SC480 amplifier
modules (plastic version) and was wondering what is the best way to position the
two modules and their associated wiring, so as to minimise interference,
distortion and radiated noise.
I am using a single heavy-duty power supply module and a larger
transformer. Clearly, both modules need to be more or less next to each other,
since they have to be attached to the heatsinks at the rear of the case. This
means that the power leads to the module further away from the power supply
board have to run past/over/under/around the first module and its audio input
leads. (J. S., via email).
You really have no
choice but to put the modules side by side with the output transistors mounted
to a common heatsink. You will have more problems with radiation from the power
supply than from each module. Keep the power transformer’s secondary leads as
far away as possible from the input leads to the modules.
Suppressed zero voltmeter needed
I am having a spot of bother in that I haven’t done any
electronics for about 20 years and now have the need to nut out a circuit. I
have a bank of batteries in my motor home (2 x 6V in series and two sets in
parallel giving 12V @ 175Ah) which run a 12V-to-240VAC inverter.
I have reclaimed a 250mA meter from an old cassette recorder
and placed a 62kW resistor in series with it to give a 0-16V FSD meter. But
measuring the batteries gives only about two degrees of deflection between 12V
and 14.6V which I am finding hard to observe.
Is there some way I can make a circuit that gives a full scale
deflection of 15V but which doesn’t kick in until it reaches 10V; ie, reading
from only 10V to 15V?
I have thought of using zeners and voltage dividers, etc but as
I don’t want to blow the guts out of my meter, I thought I’d ask for help. (P.
F., via email).
All you need to do
is to put a 10V zener in series with the meter and then calibrate the circuit
for full-scale deflection at say, 15V. You will need to adjust the series
resistor to achieve this.
The result will be that for voltages below 10V, there will be
no pointer deflection on your meter. As the voltage is increased above 10V, the
zener diode will conduct and the meter will indicate a linear range up to 15V or
whatever you set it to. This is called a "suppressed zero" meter.
Test circuit for a silicon bilateral
How do you test an SBS (silicon bilateral switch)? Do you have
a test circuit? (A. W., via email).
We do not have a
test circuit. All you need to test an SBS is a variable DC supply and a 4.7kW
resistor. At any voltage below its breakover rating, no current will flow
through the SBS. When you wind the supply above the breakover rating, the SBS
will "break down" to a low voltage with the current limited to a safe value by
the 4.7kW resistor.
Just connect up the suggested test circuit, wind up the voltage
and you will have a graphic demonstration of how an SBS works.
Energy Meter will not run at 9V
I have just completed the Energy Meter from the July &
August 2004 issues of SILICON CHIP and
while it seems to work satisfactorily there are a couple of problems. Also, to
make the unit more readable in some locations, I have replaced the supplied LCD
with one with a LED backlight, with a separate pushbutton to charge a capacitor
to drive a transistor to operate the LED for a short period after the button is
pressed, rather then drive the backlight all the time (and save the backup
battery) when not powered.
I cannot get the meter to zero regardless of OFFSET setting.
When mains-powered but with no load connected, the POWER reads between -13.7W
and -15.2W with a variety of OFFSET settings from 0 to 150. There appears no
correlation between OFFSET parameter and Watts.
Under battery power, the Watts reading does shift with OFFSET
as I would expect and a setting of 7 seems to zero the meter successfully. The
POWER parameter also works as expected, allowing me to calibrate the meter
according to your directions. Watts up?
When I first constructed the meter, I installed a partially
charged rechargeable 9V battery and when powered up, the lower line of the LCD
was not visible, nor could I get it to restore by cycling power or holding CLEAR
to reset the device. This is not the case when the battery was charged (by
external charger). I presume its charging current is preventing full voltage on
the 5V rail and/or somehow inhibiting the correct initialising of the LCD module
on power up.
Is this something I need not be con-
cerned about or is it a
symptom of some other problem? (B. G., via email).
The Energy Meter is
not designed to run on a 9V battery as a standard power source. This is because
the 9V battery cannot supply sufficient current at start-up.
This is why the LCD does not start up correctly on battery
power. It will operate correctly when powered via the mains power supply and
then by battery. The battery is there to maintain operation if power goes
The zeroing problem is probably due to signal entering via the
transformer. In other words, it is coupled in via the transformer laminations.
Make sure the metal case is earthed correctly and that the mains leads kept away
from the PC board.
A Studio 350 on steroids
I have a number of questions with regard to the Studio 350
amplifier module described in the January & February 2004 issues.
First, the power supply design shows a massive 6 x 8000mF 80V
capacitor bank just for one module. I assume the large capacitor bank is
intended so that two modules can be used from the one power supply with the
addition of an extra 500VA transformer. I just want to verify with you that this
I was very interested in modifying the amplifier for more
output power. I was interested firstly in the maths behind how you calculated
and plotted the power curves for a complex load, so I could do some plots of my
own with different speakers. I wanted to increase the number of output devices
from eight to 16 so an inductive 2-ohm load can be driven without damage.
The issues, as far as I know, are that the MJE15030/15031
driver transistors probably will not have enough grunt. If I modified the module
to have 16 output devices, changed Q8 and Q9 to MJW1302A/MJW3281A and provided
the power supply has double the 8000mF filter capacitors and a 1kVA transformer,
would 700W of power into a 2-ohm load be achievable at low distortion?
I was also interested in increasing the power rails from 70V to
95V so that 500W was available into a 4-ohm load. If all the filter capacitors
were 100V and I increased the number of output devices so the SOA (Safe
Operating Area) was not exceeded, including upgrading Q8 and Q9, and Q2 and Q3
(2SA1084 90V) were increased to 2SA1085 (120V devices) and Q1 (BC556) was also
upgraded to 2SA1085 due to the higher voltages, will this work? Perhaps my bias
current may be a bit high? Can you see any other issues?
Finally, if I went to the trouble of gain-matching Q2 and Q3
and all the output transistors, would it improve THD performance? Would you
agree that matching Q2 and Q3 (the long-tail differential input pair) will make
a bigger improvement with distortion figures than going to all the trouble of
matching the output devices? (B. T., via email).
The answers to your
questions are as follows:
(1). The amount of capacitance used for the supply is about
right if the module is to be operated at full power and with low distortion.
However, a stereo pair could be driven from the same supply with little
perceived reduction in output power.
(2). We assumed a "typical" reactance for our calculations,
allowing us to plot single load lines for the 4-ohm and 8-ohm cases. An easier
method is to plot the worst case for all reactive loads, which is just a
straight line. For the 4-ohm case shown in Fig.1 of the article, this line would
extend from the same maximum current point as the 4-ohm resistive plot to a
point at twice the rail voltage (ie, 140V). To understand how this works, we
recommend a good textbook such as Douglas Self’s "Audio Power Amplifier Design
Handbook" (see the review on page 65 of this issue).
(3). We cannot recommend that this amplifier be used to drive
2-ohm loads – even with modifications. At best, distortion will be higher due to
beta loss in the output stages and at worst, it may be unstable.
(4). Matching will not make a noticeable difference to
performance, especially considering that we’ve provided offset adjustment. Just
make sure that the output transistors are genuine On Semiconductor (Motorola)
We described a 500W amplifier in the August, September &
October 1997 issues of SILICON CHIP.
Running a 6V car radio on 12V
I am restoring my 1953 Pontiac convertible here in California.
I have converted the car to 12V with a GM alternator and have a perfectly good
6V radio that I want to keep as is.
I was told that if I change the vibrator to a Delco 12V unit,
then I can use the radio as is. Is this so? I did not try it but I have the
vibrator. I could not find a voltage step-down device to match the current
requirements of the radio.
Is this the right fix? (M. S., via email).
You cannot simply
change the vibrator as other parts of the radio circuit work at 6V as well,
principally the heaters of the valves. The safest way to run the radio is to use
a 12V to 6V regulator. You would need to find out the current requirement
Problem with Deluxe Battery Zapper
I have been struggling to get the Deluxe Battery Zapper
(SILICON CHIP, May 2006) to work. In the
condition checker side of things, I had to change the value of the two resistors
connected to Q7; the 10kW to 5.6kW and the 4.7kW to 22kW. This was done to make
Q7 function properly (before, it was on all the time). And since there was a
high amount of positive voltage at the gates of the Mosfets, why wasn’t there a
constant high current running through the 0.22W resistors?
I can now detect the pulses from Q7 at the gates with my logic
probe but my scope is not working properly, so I can’t test whether I’m getting
a pulse of current across the 0.22W resistors. The logic probe detects no pulse
at the drain or sources of the Mosfets. I am detecting a battery positive
voltage at the sources. Is this normal? Are my Mosfets shot?
On the Zapper side of things, although I have seen a spike on
my scope, the meter outputs only reveal the battery voltage. (C. B., via
It should not have
been necessary to change the resistor values in the base circuit of Q7. If this
transistor was "on" all the time, this must have been due to the output of IC2d
being low all the time (instead of high and low only during the pulses).
This suggests that there is a problem earlier in the pulse
generating circuit – ie, around D7-D9, IC3 or IC2, etc. You may have a faulty
component or one that is reversed, or perhaps even a solder bridge between pads
somewhere in this area.
If you can measure battery voltage at the sources of switching
FETs Q3-Q6, this does suggest that one or more of the FETs may have developed a
You don’t mention what kind of meter you are using to measure
the voltage at the "meter" terminals. In order to measure here properly you need
to use a DMM, or at least a 20,000 ohms per volt meter, to provide a
Touch switch for LED control
I am interested in a low voltage touch-sensing switch to turn
on a LED drawing 350mA. Is this possible and if so can you give me details on
how do it? (A. P., via email).
Have a look at the
Body Detector in the October 2001 issue and the Proximity Switch circuit in the
January 2002 issue.
Uprating the versatile electronic load
In your March 2006 issue in Circuit Notebook, Jim Rowe
contributed a circuit for a Versatile Electronic Load. Is it possible to replace
the Mosfet so as to increase the SOA (safe operating area) to 10A at 20-24V or
is there another alternative? I’ll appreciate very much your help. (R. R., via
If you don’t need to use the
electronic load at voltages above 100V, substituting an IRF540N device for the
STP6NK60Z MOSFET shown in the March 2006 issue will allow it to cope with
currents beyond 10A at 20-24V. On the other hand, if you still need to use the
unit at voltages up to about 400V, you would need to keep the STP6NK60Z and add
a second one connected in parallel with it, on a second heatsink.
Tacho For 2-Stroke Outboard Motors
I am looking for a portable tachometer/rev counter suitable for
2 & 4-stroke outboards. Your digital tacho (SILICON CHIP,
April 2000) as sold by Jaycar (Cat. KC5290) is claimed to do the job.
We have tried other tachometers whose makers claim that they
will do the job but upon testing, these claims have proved false. Can you give
me an assurance that this kit will, in fact, read revs from 2-stroke outboard
motors? (G. G. Taupo, NZ).
The Digital Tachometer is designed to
operate with 2 or 4-stroke engines with up to 12 cylinders.
There are two inputs – a digital input and an ignition coil
input. However, with some ignition systems, there is no connection that can be
used to trigger the tachometer since there is no digital tacho signal or
suitable high-voltage ignition coil signal. That’s because some outboard motors
use a capacitive discharge ignition with a magneto style trigger and a coil that
develops a high voltage via the spinning magneto magnets. This ignition system
requires no extra supply and so there is no 12V supply available. However, a 12V
supply is required for the tachometer.
Because of this we cannot guarantee that the tachometer will
work with all outboard motors. The tachometer will work on outboards that have a
12V supply and that operate with a standard Kettering ignition system.
Some readers have successfully used the tachometer on outboard
motors with capacitor discharge ignition by building an optical trigger using a
rotating vane and a photo interrupter such as the Jaycar
Battery Zapper Not Suitable For Gel Cell Batteries
I recently built the Deluxe Lead-Acid Battery Zapper &
Condition Checker (SILICON CHIP, May 2006) but have the following problems:
(1). The Zapper mode works for 6V, 12V and 24V batteries but
the Condition Checker only works on 12V and 24V units. When using the 6V mode,
all five of the LEDs light up and remain lit.
(2) When in the 6V position, putting the voltmeter on pins 14
& 1 of IC2 (4093B) shows that the voltage is down below the working voltage
of the IC (3-15V). The red high LED (pins 2 & 4) on the logic probe will not
light up because the voltage is not high enough. Pins 3 & 5 are lighting up,
indicating green on the logic probe.
I purchased this kit to use on SLA Gel Acid batteries (both 6V
and 12V) and also lead acid batteries in ride-on mowers and motorbikes. Could
you please tell me how to fix these problems? (G. K., via
First of all, you mention that you
purchased the kit for use on SLA gel cell batteries. We assume that you will
mainly be using the tester to check the condition of these batteries, because it
is generally accepted that SLA batteries do not respond to zapping and
apparently can sometimes explode if they are connected to a zapper for a
significant period of time.
If the condition-checking LEDs remain on when you are trying to
check 6V batteries, this may be because pulse oscillator IC2 is not functioning.
This in turn may be due to a very low supply voltage for IC2, which would
correspond to the very low reading you are apparently getting for the voltage
between pins 14 & 7 of that IC. The voltage at pin 1 is not relevant,
because this is not a supply voltage pin.
When the circuit is idling, pins 5 & 3 of IC2 should be at
logic low while pins 2 & 4 should be at logic high. These two sets of pins
only switch to their opposite logic levels briefly after you have pressed S4,
the Check button.
It’s not easy to suggest what may be causing the very low
supply voltage for IC2 and IC3 in the 6V position. We suggest that you check the
polarity of all diodes, including ZD4, in case you have fitted one of them with
Frequency Readout For Radios
(1) The October 2003 frequency counter was a great design but
many people would love to be able to use this unit as a digital dial attached to
older communications receivers, etc. This would involve IF frequency offsets,
both above and below the received frequency.
An obvious and popular choice would be the counter reading
455kHz low. Perhaps you could consider revealing the programming secrets of this
design to those of us who have yet to venture into PIC programming. A small
amplifier to interface between the counter and RX oscillator would also be a
useful addition. (M. K., Jandowae, SA).
(2) I wonder if would be possible to design a small indicator,
that when placed next to a radio, would indicate its tuned frequency, maybe via
induction with the local oscillator. I have a clock radio that only displays the
time – the dial indicator is not illuminated and just about impossible to see
with my aging eyes. So, I find myself tuning up and down to try to get a station
that I want to listen to. This takes a while as I invariably tune in the wrong
direction, then have to wait for a station ID to be broadcast, etc. Anyway, just
thought it may be a challenging but useful little project. (G. T., New Farm,
Both the above requests are looking for
the same solution – ie, frequency readout with an input offset to compensate for
the receiver’s intermediate frequency. In fact, the approach is to measure the
receiver’s local oscillator instead of the tuned frequency and then offset the
frequency reading by the value of the intermediate frequency.
SILICON CHIP has not produced a project along these lines and
we feel that few people would build one if we did. However, a suitable design
was published in the October 1982 issue of "Electronics Australia", entitled
"Digital Readout for Shortwave Receivers", by John Clarke. That design is still
valid although it uses a large number of 4000 series CMOS and 74LS series
We can provide a photocopy of the article for $8.80 including
Notes & Errata
StarPower Luxeon Star LED Power Supply, May 2004: several
constructors have reported that the sense voltage (set with VR1) could not be
adjusted high enough when driving 3W and 5W Stars, resulting in insufficient LED
This problem was resolved by
replacing the MC34063A
switchmode controller IC with an On Semiconductor (Motorola) branded part.
Smart Card Reader / Programmer, July 2003: the plastic
3.5-inch to 5.25-inch disk drive adapter shown in the photos is available from
PC Case Gear, on the web at www.pccasegear.com.au (look
in the "Accessories" section) or phone (03) 9584 7266.
Automatic Daytime Running Lights (Circuit Notebook), August
2006: on the circuit diagram, transistor Q2 should be identified as a BC327
not a BC337. Also, the resistor in the base circuit of Q1 and resistor R1 should
both be 4.7kW in value, not 10kW as shown.
Silicon Chip magazine regularly describes projects which employ a mains
power supply or produce high voltage. All such projects should be considered
dangerous or even lethal if not used safely.
Readers are warned that high voltage wiring should be carried
out according to the instructions in the articles. When working on these
projects use extreme care to ensure that you do not accidentally come into
contact with mains AC voltages or high voltage DC. If you are not confident
about working with projects employing mains voltages or other high voltages, you
are advised not to attempt work on them. Silicon Chip Publications Pty Ltd
disclaims any liability for damages should anyone be killed or injured while
working on a project or circuit described in any issue of SILICON CHIP magazine.
Devices or circuits described in SILICON CHIP may be covered by patents. SILICON
CHIP disclaims any liability for the infringement of such patents by the
manufacturing or selling of any such equipment. SILICON CHIP also disclaims any
liability for projects which are used in such a way as to infringe relevant
government regulations and by-laws.
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