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.

Schematic for amplified crystal set

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 dimensions.  
  • 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 each other.
  • 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 each other.
  • 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: 02/25/2003

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