Obsolete parts in older
projects
Could you please tell me whether
any hard-to-get parts are required
to build the Constant High-Current
Source from June 2002 (siliconchip.
com.au/Article/4065) or the 50W DC
Electronic Load from September 2002
(siliconchip.com.au/Article/4029)? I
realise that you probably don’t have
PCBs for these projects. (R. M., Melville, WA)
● For the Constant High-Current
Source from June 2002, the heatsinking arrangements might need to be
changed to suit available heatsinks.
The remaining parts are commonly
available.
For the 50W DC Electronic Load
Advertising Index
Altronics.................................37-40
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..................OBC
Ocean Controls............................. 7
PMD Way................................... 111
SC RTV&H on USB...................... 75
SC USB Cable Tester.................. 91
SC Vintage Radio Collection...... 10
Silicon Chip Subscriptions.......... 6
Silicon Chip Shop.................... 101
The Loudspeaker Kit.com............ 9
Tronixlabs.................................. 111
Vintage Radio Repairs.............. 111
Wagner Electronics..................... 87
112
Silicon Chip
from September 2002, the STW34NB20
200V, 34A N-channel Mosfet is obsolete, so an alternative will be required.
Suitable parts that are currently available include the IRFP240PBF, IRFP250(N)PBF, IRFP260(N/M)PBF and
IXTH26P20P.
Searching for another
discontinued part
I am trying to build the Sound
Level Meter from your Electronics
Test Bench book but I am having
difficulty finding a three-position,
two-pole switch with the correct pin
placement. This project is probably
over 20 years old. Is there some way I
can mimic what the switch does with
jumper pins, perhaps? Failing that,
where would I get such a switch? (S.
N., Clayton North, Vic)
● You are right that switches with the
contact arrangement used in that project are no longer available. Switches
are available with a similar layout, but
you will have to wire it to the board
using flying leads. You could use a
DP4T slide switch from Altronics (Cat
S2040) and wire the switch terminals
to the PCB, with the third and fourth
positions wired in parallel.
You could also use the Altronics
S2033 (4P3T) slide switch and ignore
the third pole. It would also be possible to wire up a rotary switch like
Altronics S3008 or S3022, or Jaycar
SR1212.
The PreChamp is an
old design
I am building several PreChamp
pre-amplifiers (July 1994; siliconchip.
com.au/Article/5252) to increase the
signal output from the line output jack
(not the headphone jack) on a TV, and
plugging the resulting increased signal into a Bluetooth transmitter then
to Bluetooth headphones.
It works OK, but I’m not happy with
the resulting audio quality when compared to another pair of wireless headphones that I have.
Using a signal generator and a
Hantek USB scope, I have discovered
that the frequency response of the
PreChamp is not flat. With a constant
input level at all frequencies, I found
that at 100Hz the output level was
85mV but at 10kHz, the output level
climbed to 200mV, and at 15kHz, the
level was 225mV.
Australia's electronics magazine
I have altered the Preamp’s gain by
changing the two resistors to 1500W
and 150W using the formula printed in
the magazine, giving a gain of approximately 11 times, which is around
21dB.
Would this have altered the frequency response of the PreChamp? I
suspect not. Can you suggest any components that I can change the value of
to get the frequency response flatter?
(N. L., Christchurch, NZ)
● The PreChamp is quite an old design
and we would not design something
like that today.
As a result, it has relatively poor
frequency response flatness. Still, it
should not be behaving in the manner
you have described.
Our circuit analysis of the original
design shows that it has a plateau-type
response with -3dB points at around
60Hz and 100kHz, and -1dB points at
around 115Hz and 37kHz. So it suffers a fair bit at the lower frequency
end, but should be pretty flat at the
high end, up to about 20kHz. Changing the gain-setting resistors doesn’t
have much effect on the calculated
response.
Note that we published a new
design in January 2013 – the Champion (and Pre-Champion). That circuit has a much flatter frequency
response. We even published frequency response and distortion
graphs in that article, unlike the original Champ/PreChamp.
Still, we aren’t sure why you are
getting an increased response at
higher frequencies. That points to an
increase in feedback impedance with
frequency, but the only non-resistive
element in the feedback network is the
1.5nF capacitor, which should have
the opposite effect.
The only explanations we can come
up with are that your input coupling
capacitor is too low in value or faulty,
which would cause lower frequencies to have more attenuation and
thus give you a rising response with
frequency. It could also be a similar
problem with the output coupling
capacitor.
SC
The March 2022 issue is due on
sale in newsagents by Monday,
February 28th. Expect postal
delivery of subscription copies in
Australia between February 28th
and March 11th.
siliconchip.com.au
