A new approach to amplifying the output of a crystal radio, using energy extracted from the RF carrier to power a micro-power IC to drive headphones or a speaker
By Ben H. Tongue
Quick summary: This article describes an
amplifier that can be used to substantially increase the volume from
a crystal radio set when tuned to a weak signal when using headphones. It
can also be used to amplify the output of a crystal radio set, when tuned
to medium or strong stations, to drive a speaker. No battery for
powering is required. The amplifier can be added to most any crystal
radio set, provided access to a strong station is available. As shown
here, the amplifier is applied to Version 'B' of the "Benodyne",
a single-tuned four-band crystal radio set. See Article #22. It
has been also applied to version ""C" of the "Benodyne",
described in Article #26. A switch is provided so the crystal
radio set can perform as it normally does, or with about 20 dB of audio amplification
(+20 dB represents a large increase of volume.). This amplification
is provided by a micropower integrated circuit that does not use battery
power. Power to operate the integrated circuit is stored in an
electric double-layer "supercapacitor" that can be charged
overnight by leaving the crystal radio set on, tuned to a strong local station.
One charge can last for tens of hours when listening to weak stations.
For loudspeaker operation, a large reentrant horn type PA speaker is
best, for the highest volume, although other types may be used. Depending
upon volume, a full charge on the capacitor can last for about 5 hours
of low volume loudspeaker listening.
The Amplifier, applied to a crystal radio set:
This crystal radio set operates in the same manner as the ones described
in Articles #22 (Benodyne version B)and #26 (Benodyne version C) when
switches SW7 and SW 8 are in their 'up' positions and SW9 is to the
right. To operate the amplifier, first, supercapacitor C13 must
be charged up to at least 1.5 volts. The manufacturer of the IC
specifies a minimum of 1.8 volts, but so far, I have found that 1.5
volts to be sufficient. To charge C13, set SW7 and SW8 to their
'up' positions, SW9 to the right, and the wiper arm of R3 to the center
(see Fig. 1). Tune in a station that provides between 1.3
and 5 .5 volts DC at the 'Detector bias monitor' terminals. If
the voltage is too low, try changing the antenna impedance matching
by optimizing the settings of C7 and C8. Set SW7 to its down position
and C13 will start charging. If no station exists that is strong
enough to supply at least 1.5 volts, C13 may be charged by connecting
a series combination of a 4.5 volt battery and a 100 ohm current limiting
resistor across it for about 30 minutes. Make sure the + side
of C13 is charged positive. After C13 is charged, set SW7 to its
up position. The higher the final charged voltage on C13, the
higher the maximum volume will be.
Non-amplified operation with Sound Powered 1200 ohm headphones:
SW7 and SW8 are up and SW9 is to the right. R3 is used to
optimize DC current in the diode for minimum audio distortion.
Amplified operation with Sound Powered 1200 ohm headphones on very
weak signals: SW7 is up, SW8 down and SW9 is to the right.
Amplified operation, driving an 8 of 16 ohm speaker from medium
and strong stations: Operate SW7 to its up position, SW8 down
and SW9 to the left. The speaker will probably give forth with
some distorted audio. To reduce the distortion, try adjusting
R3. If this doesn't help enough, reduce the signal into the
amplifier. The attenuators, controlled by SW1 and/or SW2 can be
used to do this (see Fig. 1). If no SW1 or SW2 is present, reduce
the output of the detector by decoupling the antenna (reduce C7 and
restore tuning using C8).
Switch SW10 provides a tradeoff between maximum volume and current
drain. Switching to the white wire connection gives the, longest
listening time, but with a lower maximum volume. Each listener
must make his own choice here. The current drain from C13 and
the life of its charge are directly proportional to the strength of
the audio signal and the setting of SW10. Maximum low-distortion
volume is proportional to the voltage charge on C13.
For comparison purposes with receiving locations other than mine, there
are two 50 kW stations about 10.5 miles from my home. They are WOR and
WABC. My attic antenna is described in Article #20. Either station
can deliver about a 5.0 volt charge to C13.
This crystal radio set was constructed by modifying a Version 'b' crystal
radio set (See Article #22), using the air-mounted, flying joint method of
wiring the amplifier components. A convenient way to connect to
the tiny leads of IC1 is to first solder it to a surfboard such as one
manufactured by Capital Advanced Technologies (http://www.capitaladvanced.com).
Their models 9081 or 9082 are suitable and are available from
various distributors such as Alltronics, Digi-Key, etc. The amplifier
can be built in as an addition to any crystal radio set if proper allowance
is made for impedance matching considerations.
Charge/discharge considerations for C13: C13 (0.33 F) will charge
close to full capacity after about 24 hours of charging. The first
charge will not last as long as subsequent ones because of a phenomenon
known as "dielectric absorption". If C13 is reduced
to 0.1 F, about 8 hours are needed. Listening time when using
headphones should be greater than 24 hours when using 0.33 F, and 10
hours when using a 0.1 F value. There is a greater current drain
on C13 when using a speaker, and the listening time will depend upon
the volume setting. Listening times approximate 10 hours when
using a 0.33 F cap and 3 hours when using 0.1 F.
Parts List when the crystal radio set used is the that described in Article
#22. The amplifier is
easily adapted to the higher performance
crystal radio set described in Article #26 as well as others.
- C1, C3: 200 pF NPO ceramic caps.
- C2: 100 pF NPO ceramic cap.
- C4, C6: 270 pF NPO ceramic caps.
- C5: 18 pF NPO ceramic cap.
- ** C7, C8: 12-475 pF single section variable capacitors,
such as those that were mfg. by Radio Condenser Corp. (later TRW).
They use ceramic stator insulators and the plates are silver plated.
Purchased from Fair Radio Sales Co. as part # C123/URM25. Other
capacitors may be used, but some of those with phenolic stator insulators
probably will cause some reduction of tank Q. The variable capacitors
are fitted with 8:1 ratio vernier dials calibrated 0-100. These
are available from Ocean State Electronics as well as others. An insulating shaft coupler is used on C7 to eliminate hand-capacity
effects. It is essential, for maximum sensitivity, to
mount C7 in such a way that stray capacity from its stator to ground
is minimized. See Part 9 of Article #22 for info on mounting
C7. The variable capacitors used in this design may not be available
now. Most any other capacitor with silver plated plates and
ceramic insulation should do well.
- C9: 47 pF ceramic cap.
- C10: 100 nF cap.
- C11: 1.0 uF non-polarized cap. This is a good value
when using RCA, Western Electric or U. S. Instruments sound powered
phones, with their 600 ohm elements connected in series. The
best value should be determined by experiment. If 300 ohm sound
powered phones having their 600 ohm elements connected in parallel
are used, C11 should be about 4 uF, and a different transformer configuration
should be used.
- C12: 0.1 uF cap
- C13: 0.1 to 0.33 F electric double layer capacitor (supercap).
Elna's 0.33 F "Dynacap", available from Mouser as #555-DX5R5H334
or Panasonic's 0.033 F "Gold" capacitor #EEC-S0HD334H, available
from Digi-Key etc. are suitable. Do not use an ordinary electrolytic
cap in this application. Its leakage current will probably be
so great that C13 can only charge to a low voltage, and it won't be
able to hold a charge anywhere near as long as a supercap. A
0.33 F supercap will charge more slowly, but it will last longer than
on a 0.1 F supercap.
- C14: 1.0 nF ceramic cap
- C15: 470 nF plastic film cap. (polyester or mylar)
- C16: 10 nF ceramic cap (Connect with short leads across +
and - supply voltage terminals of IC1.)
- ** L1, L2, L3 and L4: Close coupled inductors wound with uniformly
spaced teflon insulated 18 Gage silver plated solid wire. This
wire is used only to gain the benefit of the 0.010" thick low-loss
insulation that assures that no wandering turns can become 'close-spaced'.
L1 has 12 turns, L2 has 8 turns, L3 has 6 turns and L4 has 14 turns.
The coil form is made of high-impact styrene. I used part #S40160
from Genova Products (http://genovaproducts.com/factory.htm). A
piece of plastic drain pipe of the same OD, made of ABS, can also
be used, with the same results. PVC pipe will result in somewhat
less selectivity and sensitivity. See Fig. 6 for hole drilling
- IC1: Texas Instruments micropower opamp OPA349UA (Formerly
a Burr-Brown product.) Here is a link to the data sheet for
this IC: http://www-s.ti.com/sc/ds/opa349.pdf A convenient way to connect to the tiny leads of IC1 is to
first solder it to a surfboard such as one manufactured by Capital
Advanced Technologies (http://www.capitaladvanced.com). Their
models 9081 or 9082 are suitable and are available from various distributors
such as Alltronics, Digi-Key, etc.
- SW1, 2, 7 and 9: DPDT general purpose slide switches.
- **SW3, 4, 5 and 6: Switchcraft #56206L1 DPDT mini Slide switches.
This switch has unusually low contact resistance and dielectric loss,
but is expensive. Other slide switches can be used, but may
cause some small reduction of tank Q.
- SW8: 3P2T slide or other type switch.
- SW10: 3 position rotary switch.
- T1, T2: Calrad #45-700 audio transformer. Available
from Ocean State Electronics, as well as others. If 300 ohm
phones are to be used, see the third paragraph after Table 1.
- T3: Bogen T725 4 watt P/A transformer. Available from
Lashen Electronics, Grainger or other sources (http://www.lashen.com/vendors/bogen/Speaker_Transformers.asp)
- R3: 1 Meg linear taper pot.
- R4: 10k resistor
- R6, R7: 10 Meg resistors. For minimum waveform clipping
in IC1, values of R5 and R7 should be selected to be within 5% of
- R8, R9: 2.2 Meg resistors. For minimum waveform clipping
in IC!, values of R8 and R9 should be selected to be within 5% of
- R10: 10 Meg resistor
- Baseboard: 12'' wide x 11 1/8 '' deep x 3/4" thick.
- Front panel: 0.125" thick high-impact styrene.
Other materials can be used. I was looking for the lowest loss,
practical material I could obtain.
** For lower cost, the following component substitutions may
be made: Together they cause a small reduction in performance
at the high end of the band (1.75 dB greater insertion power loss and
1.5 kHz greater -3 dB bandwidth). The performance reduction is
less at lower frequencies.
- Mini air-variable 365 pF caps sold by many distributors such
as The Crystal Set Society and Antique Electronic Supply may be
used in place of the ones specified for C7 and C8.
- 18 Ga. "bell wire" supplied by many distributors such as Home
Depot, Lowes and Sears may be used in place of the teflon insulated
wire specified. This vinyl insulated non-tinned copper
wire is sold in New Jersey in double or triple twisted strand form
for 8 and 10 cents per foot, respectively. The cost comes
out as low as 3 1/3 cents per foot for one strand. The main
catch is that one has to untangle and straighten the wires before
using them. I have used only the white colored wire but I
suppose the colored strands will work the same (re dielectric loss).
The measured OD of strands from various dealers varied from 0.065
to 0.079". The extra dielectric loss factor of the vinyl,
compared to the teflon will cause some reduction of sensitivity
and selectivity, more at the high end of the band than the low end.
- Radio Shack mini DPDT switches from the 275-327B assortment or
standard sized Switchcraft 46206LR switches work fine in place
of the specified Switchcraft 56206L1 and cost much less. See
Article #24 for comparison with other switches. Any switch
with over 4 Megohms Rp shown in Part 2 of Article #24 should work
well as far as loss is concerned. Overall, losses in the switches
have only a very small effect on overall performance.
#25 Published: 07/04/2002; Revised and renamed:
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