On this page I will feature some useful
circuits that I've used over the years.
.50A trip . .136 preamp . .LF loop preamp . .LF preselctor..50A keyer.
Audio level control device.
Starting with an audio
compressor ( well you've got to start somewhere! ). There have been umpteen
designs for compressors published over the years but many of them are difficult
to set up or cause distortion and others use expensive chips.
This one updates an old
principal I used years ago: a light bulb driven by audio illuminates an LDR in
the feedback loop. The brighter the light tends to glow, the lower the
resistance of the LDR becomes and the more negative feedback gets applied,
reducing the gain. The LDR is purely resistive so the amplifier maintains good
The problem with a light bulb
is its' thermal inertia, the attack time of the AGC circuit is too slow and you
get distortion on peaks. What we needed was a fast light bulb.... We now have
one, the light emitting diode!
The circuit is
shown above and should cost about £10 if you have to buy all the bits
The NORP12 L.D.R has a dark
resistance of around 1MegOhm but will come down to a few hundred Ohms in bright
light. It's peak sensitivity is to yellowish light at a wavelength of around
580nM. The best match is, of course, a yellow LED. Toshiba have some nice
bright ones with a peak output at 590nM which is close enough!
The crux of the thing is to
mount the LDR and LED in close optical contact whilst being careful to exclude
all stray light from the assembly. I mounted mine in a piece of black plastic
tubing and sealed it with silicone rubber and black end-caps.
The op-amps are two halves of
a TL072 and have enough gain to bring a mike insert up to a good loud line
level. Performance has been measured as below.
Performance with gain set
|theshold of compression
|maximum input level
40dB input range
is compressed to 3dB output change.
Distortion is less than 0.2%
within normal operating range.
Frequency response is 30Hz to
8kHz within 3dB at max gain.
The current drawn at 12V is
only about 4mA.
||Farnell part number
||Cost ( ex VAT and carriage )
|Op amp Chip TL072
|Light dependant resistor NORP12
|Toshiba yellow ultra-high-bright LED
The resistors are
nothing special, 0.4W metal film are OK; the diodes are small signal diodes
such as 1N4148 and the capacitors are at least 12V working.
Do not attempt to operate the
circuit below 12V as there won't be enough drive to the LED and clipping will
result. Using germanium diodes instead of silicon ones would give a little more
headroom ( and slightly reduce the output level ).
Take a high-current trip.
This one is useful if you are
experimenting with high current circuits (like mosfet PAs!). It is a trip which
will quickly break the circuit when the current exceeds about 50A. The pass
transistor is a power fet with an on-resistance of 0.007 Ohms. This means that
at 50A it will drop about 0.35V, just enough to turn a germanium transistor
on.... Yes that's the snag, you need a good junkbox with some OC140s or
something similar in it! As the transistor starts to conduct, it will reduce
the gate Voltage on the fet and begin to turn it off, this causes the Volt drop
to increase and it avalanches itself into the tripped state with the LED
glowing merrily. Pressing the reset button shorts out the base bias of the
transistor allowing the fet to turn on again. Only press this when you're sure
you know why it tripped in the first place!
The 9V battery will last for
ages in the untripped state because the only current drawn is the leakage of
the transistor which should be a few microamps.
||Cost ( ex VAT and carriage )
||£5.30 + VAT
|Ge NPN transistor
||see list below
||free from your junk box?
If you want it to
trip at a lower current you can add a bit of resistance at X, it's best to do
this by trial and error, just a few inches of reasonably thick wire should do.
Do not use it at more than 12V or so, or the current through the 4k7 resistor
will exceed the base current rating of the transistor when tripped.
It's all a bit experimental
but it works pretty well, you made need to add some Cs here and there to
prevent unwanted trips.
Possible germanium NPN
- NKT713 to NKT781
Have a good root through your
tat box and you may find one of these, it's a pretty comprehensive list! I'm
not guaranteeing that all of these will work, most of them will be thirty years
old and may be a bit leaky.... best to check 'em first. I used an OC140 in
Pre-amp / filter for 136kHz.
Most modern tranceivers cover
LF but the performance is not always very good. All you can hear are strange
whistles and burbles which aren't really there! You need this little circuit.
It has a gain of about 10dB and a nice sharp band-pass response about 3kHz
wide, enough to cover the 136 band.
The coils are Toko
type 719VXA-A017A0 and are available from BEC on 01753 549 502 at about 90p
each. If they are out of stock try type 719VXA-A018AO which has a slightly
lower inductance. Note the different pinouts!
More gain can be had by
connecting the fet gate to the top of the second tuned-circuit, instead of the
overwind. Done this way, you could use any variable inductor with an inductance
of around 680uH.
Loop Pre-amp for LF.
If you want to use a a small
multi-turn loop on LF, you will find that the sensitivity is not good enough to
receive the weaker signals. A good preamp for a loop needs to have a high input
impedance and good signal handling characteristics (to prevent
cross-modulation). The circuit shown below is the best I tried and has enabled
me to hear Italian amateur signals on the 136kHz band with a 1m diameter tuned
loop. The preamp is meant to be mounted outside, near the loop, and is powered
down the coax from a 12V supply in the shack. The +12V is applied via another
choke at the RX end. A blocking capacitor is also required to prevent 12V being
applied to the RX input.
The loop I used
for 136kHz had about 30turns of 22g wire wound round a 1mtr square former and
tuned with a 1000pF variable C. The preamp is fed from a two-turn coupling loop
just inside the main coil. The preamp also seems to work quite well with a
tuned whip aerial.
A preselector for LF.
Here is a preselector circuit
designed by Wolf DL4YHF as part of his transverter. It uses two pot-core
filters with FETs and feedback. By adjusting the controls, the bandwith can be
set to less than 200Hz. This is the preamp which Wolf uses at the club station
Keyed PSU for class D transmitters.
Keying the drive to a class-D
mosfet PA can create key-clicks due to the non-linear characteristic of the PA.
A nice clean keying shape can be achieved by keying the supply with appropriate
rise and fall times. In this design the series regulator is keyed thus
"killing two birds with one stone", regulating and smoothing the
supply and keying the PA. Two seperate rectifiers and smoothing Cs are combined
to give adequate capacity.
need to be mounted on a large heatsink and may need force-cooling. I mounted
the 40A bridge rectifiers on the heatsink too. This is big stuff, engineer it
I'll try and add
some more useful circuits over the next few months. If you have any that you
think should be seen on here, e-mail me!
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