R:190807/0902Z 22384@OK2PEN.SP.BRA.SOAM [Sao Jose dos Campos] $:15025_GB7CIP R:190807/0902Z @:CX2SA.SAL.URY.SOAM #:11592 [Salto] FBB7.00e $:15025_GB7CIP R:190807/0900Z @:GB7CIP.#32.GBR.EURO #:15025 [Caterham Surrey GBR]

From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW

By G8MNY (Correction Apr 17)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

Here is my design using fairly modern parts. With it I came 1st in my local radio club home constructions contest a few years ago, mainly because it was so LOUD.

It gives VERY good sensitivity & selectivity. With 10mV RMS carrier @ 1MHz modulated 100% @ 1kHz on the low Z aerial input, gave at best match 5mV RMS into the 80Ω headphones. In the high Z mode, 10mV gave 1.5mV to the headphones, but the input circuit is untuned (no gain) & is just tightly transformer coupled. The selectivity was measured at 1MHz for -3dB was ±25kHz & at -20dB ±190kHz or a loaded Q of 20.

In South London with 40M of aerial wire up @ 8M it can pick up 7 MW & 4 LW stations (3 French) during the day. The stronger ones can even just about drive a good small 8Ω LS.

SCHEAMAIC
=========
┌───────┐ ┌────────┐┌────────┐┌──────┐┌─────────┐ _
\│/ │Aerial │Variable│Detector││Crystal ││RF / ││Impedance│ / \
│_/─HighZ┤Tunned │Magnetic│Tunned ├┤Detector├┤Audio ├┤matching ├─() ()
─Low Z┤Circuit│Coupling│Circuit ││ ││Filter││& DC Path│ Efficient
┌────────┴───────┘ └────────┘└────────┘└──────┘└─────────┘Headphones _│_
///

FEATURES
========
1/ DUAL TUNED CIRCUITS for Selectivity.
2/ VARIABLE COUPLING for Matching/Sensitivity/Selectivity.
3/ VARIABLE INDUCTANCE also widens the tuning range.
4/ DUAL BAND, MW & LW, plug in capacitors extend range to LW.
5/ HIGH & LOW AERIAL IMPEDANCE option, for best matching for short & long ant. 6/ AERIAL NEON gives visible protection against High Aerial Voltages.
7/ LENS to see "CAT's WHISKER on the CRYSTAL".
8/ R.F.C. to improve diode linearity & sensitivity performance (RF/AF Filter). 9/ TAPPED AUDIO MATCHING TRANSFORMER for optimum loudness.
10/ HIGH EFFICIENCY HEADPHONES home made.

CIRCUIT
======= GERMANIUM
AUDIO AUTO CRYSTAL ADJUSTABLE AERIAL &
MATCHING VIEW __ DETECTOR FERRITE ROD AERIAL EARTH
TRANSFORMER LENS<__> COIL <COUPLING> COIL CONNECTIONS
= = = = = = = = = = = = = = =
SENSITIVE ┌────┬──((()───┤<│──┬─((((()─┐ ┌─((((()─┐
HEAD- 7_│ │ ==== │ │ │ │ HIGH
PHONES 6_)║ │ RFC _ ├───┐ │ _ ├───┬────)──────O Z
@-\ 5_)║ │ |\ │ │ │ |\ │ │ │
│ 4_)║ │ 2x 300pF === /LW │ 2x 300pF=== /LW │ LOW Z @─/.....3_)║ │ variable │\ │ │ variable│\ │ ├──────O
│ TAP 2_)║ │ │ │ │ │ │ (:) / Earth for
│ 1_)║ === │ === │ │ === │NEON │ High Z
│ )_)║ │ 10nF RF │ │1n5 │ │ │1n5 │ │
│ │ │ decoupling │ │ │ │ │ │ │ └────────┴─────┴──────────────┴───┴────┴──────────┴───┴────┴─────┴─O EARTH

PARTS & TIPS
============
THE BASE
Use a wooden base board about 1x 25 x 10cm. Fit rubber feet so that underside protrusions don't scratch.

CIRCUIT WIRING
Use tin plate strips tacked down, this is easy to solder to & support the smaller components.

HEADPHONES
Use a coat hanger wire head band to connect 2x 40Ω 1970s telephone inserts (type 4T) rocking armature type. These are the most sensitive transducers made & peak @ 3kHz. They are around 10dB louder once matched than any high Z moving iron diaphram, moving coil, or crystal types.
_..--~~~~--.._
Make a headband from a single piece of / Headband \
coat hanger wire looped at each end & | | flattened (hammered) as the common __│__ ___│__ connection under 2 of the terminal nuts. _/ \ /(o)(o)\
Bend the wire to angle the inserts to │| .'. | │ │~~~~~~|
sit on your ears comfortably. They can \\_____/ \_\____/
be connected permanently to the set with '. .'
flexible leads or with a plug & socket '. wires .'
for comparing with other headphones. '. .'
Wind a couple of turns of the insulated 80Ω
wire around the headband as strain releaf.

AF TRANSFORMER
Acts as a large AF inductive load to the _____
detector's DC, maintaining conduction ___│ │___
over > 50% of the RF cycle! For best load /o _│__ │ \ 0V
matching & volume, I used 6 equal winding 4 │,_' │ | │ ,('│
of an old telecomms line transformer that 5 │,_' │ | │(,_'│1
had 2x 600Ω windings in 4 sections each. 6 │,_' │ | │ ,_'│2
The first 2 windings are paralleled up 7 │, ' │ |__│_, '│3
for low Z loads to reduce the winding \___│ │__o/
resistance loss for low Z loads. └─────┘

Adjustable connection is made with a small wander crocodile clip to the linking straps. Typically 2-3 of the 7 taps gives the best for the 80Ω headphones. Mount it well to the base board (e.g. 2 screws tapped into the 4 plastic corners not near the winding!) as it is heavy component.

If a suitable comms transformer is not available
try some small mains transformers that can give ───┬────┐
7:1-2:1 e.g. 110V to 15-0-15V, try fine ratio 15V )||(
adjustment by 1st using it as a straight ___)||(110V
transformer then as an auto transformer with the 0V )||(
windings in series, & try adding or subtracting 15V )||(___
the secondary (reverse connections) to alter the │
ratio & use which ever gives the loudest volume. ───┴───────
Also try paralleling or seriesing up the headphone
earpieces to change the load.

10nF CAPACITOR
This is used for blocking RF from the AF circuit. Theory says yes it is needed, but it is not really necessary component as it works without it.

R.F. Choke
Approx 20 turns of enamelled copper wire (0.5mm) on small polo mint sized (12mm dia) toroidal RF ferrite core. Again theory says yes it is needed as it will keep the diode conduction angle high & hence reduce the diode resistive losses, but it does work without it. It will also makes the load impedance on the tuned circuit 2x higher, as mean not peak voltage & currents are reduced by √2. Mount with an insulated screw (not to cut into the turns) through the middle.

GERMANIUM DIODE
Viewable old large glass bubble type e.g. an old OA91.

Mount a small plastic lens (from ,_______.
cardboard slide viewer) 2cm in // _ \\ OA91
front of the crystal with shaped =====│_│~\.=======
paper clip wire, so that the surface \\_______//
of the crystal & the internal ~~~~~~~~~
cat's whisker can clearly be seen.

GANGED 300pF VARIABLES
Two needed, air spaced ideally. Large value gives best tuning range, so the sections are paralleled up. Put knobs on the shafts & mount them well to the baseboard, as they get a lot of tweaking.

1n5 LW CAP
Two needed. Connected one side to ground & the other to a 2mm wander test plugs/sockets to give the Long Wave coverage.

FERRITE ROD
Standard radio type 14cm long, 8mm dia, but not used as the aerial! It gives plenty of L adjustment & coupling variation. With the coils the ferrite gives high Q tuned circuits, many times higher than lit's wire air space coils!
Mount with plastic cable clip in the middle (not shorting metal turn!) with high enough packing/washers to allow coils to be moved. The ferrite rod is quite fragile, if it does get broken just glue it back together with a 2 part epoxy resin, removing as much glue gap with presure while setting as possible.

COILS
Two standard MW radio type with tags, ┌─┬┬┬┬┬┬┬┬┬┬┬┬┬[]─┐Paper
e.g. about 30 turns of 0.5mm enamelled │ ||||||||||||| \ │tube
copper wire on a loose fitting paper └─┴┴┴┴┴┴┴┴┴┴┴┴┴[]│┘
tube with connection tags. 30 turns Tags \│____Twisted
~~~~Flex Wire Connect with twisted flexible leads as they will need sliding along the ferrite rod to vary the inductances & coupling.

NEON
Wire ended 80V bulb type. Used for safety, it gives a dramatic flash discharge indication when the aerial is in the low Z mode, when static is around. It also protects the Caps from much higher voltage flashovers!

CONNECTIONS
4mm socket with screw up type for Earth & the 2 Aerial options, with a swinging earth link plate, to make the high Z aerial configuration. For the headphone, I used 6mm (¼") Jacks, but screw up 4mm are fine also.

LAYOUT
====== wooden base with feet ┌────────────────────────────────────────────────────┐
│ Ferrite░░░░▒▒▒▒▒▒░░░░░█░░░░░▒▒▒▒▒▒░░░░░ │
│ rod coil ▀clip coil │
│ AF _LW Cap_ _LW Cap_ │
│ Transformer /________ /________ (o)│AE High Z
│ │~~░░~~│ RFC Diode │≡≡≡≡≡≡≡≡│ │≡≡≡≡≡≡≡≡│ │
│____│::░░::│ ,-. __ │≡≡≡≡≡≡≡≡│ │≡≡≡≡≡≡≡≡│ (o)│AE Low Z
│ LS │::░░::│ ( 0 )-(__)-│≡≡≡≡≡≡≡≡│ │≡≡≡≡≡≡≡≡│ │ ││Earth Link │Jack│__░░__│▐C`-' ____ │________│ │________│[] (o)│Earth └──────────────────<____>────││──────────││──────────┘
crock clip tap lens KNOB KNOB neon

TUNING IN
=========
1/ Connect an earth, mains earth can be noisy, old Gas & Water pipes are better
earths as long as they are not PLASTIC! Try a counterpoise along the ground
if there is too much mains QRM on water pipes etc.

2/ Connect the aerial to either the High Z or Low Z options. Short aerial wires
compared to a 1/8 wave length are always high Z. Higher & longer aerial
wires are always better, as they pick up more RF power.

3/ With the coils close together, adjust the detector & aerial variable
capacitors for a station of interest.

4/ If the station is loud enough, but can't be separated from another one try
adjustment of the 2 variable capacitors at once, also reduce the coupling by
moving the coils apart & try again. Note the affect of tracking both
variables, there is a best matching sweet spot for optimum loudness.

5/ Adjust the auto-transformer tapping for loudest output, note that there is
some treble difference as the match changes the Z on the RF decoupling Cap.

6/ Try the other aerial impedance option & see if other stations are heard.

7/ If the station is too HF or LF for the tuning range, try moving the coils
more to the middle for LF, or to the far ends for HF.

8/ With the LW capacitors connected the tuning is less effective & moving the
coils gives more frequency range.

DEMONSTRAITON
a) Scope & Spectrum analyse (hi Z probes) the aerial with xtal set in // tuned
input to show the broadband RF signals.
┌──────────────────┐ ┌──────────────────┐
│ SCOPE RF │ │ SPECTRUM │
│▒ ▒▒▒ ▒ ▒▒ ▒▒▒ ▒▒ │ │ │
│▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ │ │ │ │ │
│▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ │ │ | | │| │ | │
│▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ │ | ▌│ ▌ |▌│ |│ ▌ │
│ ▒ ▒▒▒▒▒ ▒▒ ▒▒▒ ▒▒│ │ ▌│▌|▐▌ ▌▌| ▌| ▌│ │
│ │ │~~~~~~~~~~~~~~~~~~│
└──────────────────┘ └──────────────────┘

b) Connect an AF amplifier & LS right across the full transformer, & move the
scope & analyser to the 2nd tuned circuit & tune across the band.
┌──────────────────┐ ┌──────────────────┐
│ TUNNED LF │ │ TUNNED HF │
│ │ │ │ │ │
│ | │ │ │ | │
│ │ ▌│ | │ │ │ │ | |│ | │
│ |│▌| ▌ || │ │ │ │ | | |▌│ ▌| ▌│ │
│ ▌|█▐▐▌ ▌▌│ |│ | .│ │ │.▌│ ▌ ▌▌| █▐ ▌| │
│~~~~~~~~~~~~~~~~~~│ │~~~~~~~~~~~~~~~~~~│
└──────────────────┘ └──────────────────┘

c) Tune into a strong station.
┌──────────────────┐ ┌──────────────────┐
│ SCOPE RF │ │ SPECTRUM │
│▒ ▒ ▒ ▒ ▒ │ │ . │
│▒ ▒▒▒ ▒▒ ▒▒ ▒▒│ │ | │
│▒▒▒▒▒▒-▒▒▒▒▒▒▒-▒▒▒│ │ │ | . │
│▒ ▒▒▒ ▒▒ ▒▒ ▒▒│ │ │ | ▌ |││ │ │ │
│▒ ▒ ▒ ▒ ▒ │ │ |.▌│▐█ ▌|| || |. │
│ │ │~~~~~~~~~~~~~~~~~~│
└──────────────────┘ └──────────────────┘

d) Scope the recovered AF on the transformer.
┌──────────────────┐
│ SCOPE AF │
│ │
│─┐ ┌┐ ┌┐ ┌┐ ┌┐│
│ │┌┘└┐ │└┐┌┘│ │└│
│ └┘ └┐┌┘ └┘ └┐┌┘ │
│ ~~ ~~ │
│ │
└──────────────────┘

See my TECH bul on "Spectrum IF Demo circuit"

Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP

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