Radio Digest, August 12, 1922, pages 1-2:
CRYSTAL SET AMPLIFIES
CHICAGO MAN USES GALENA BUT NO TUBES
C. E. Butterfield Discovers Method of Amplification Using Three Detectors
Boosts Signals Double
Inventor Tells of Circuit--Believes More to Be Accomplished by Experiment
Crystal detector amplification, of great importance to every amateur possessing the less expensive crystal set, has been accomplished successfully by C. E. Butterfield of Chicago who has been able to double the strength of received signals without recourse to three-electrode vacuum tube detectors or amplifiers.
Working on the circuit proposed by a South American amateur for crystal amplification, Mr. Butterfield discovered the new arrangement which proved so satisfactory. While not giving near the amplification possible with a tube set, the results were acclaimed very good considering the saving in material outlay. Another advantage of the crystal amplifier has been found in its minimum amount of distortion in voice and music reception.
The circuit uses three galena detectors, a tuning device such as a loose coupler, two audio frequency amplifying transformers, three fixed condensers, a high voltage or B battery and telephone receivers. A schematic diagram is given on page 2.
Battery Pushes Signals Through.
The battery was necessary to furnish currents to help the weak incoming Radio signals push their way through the amplifying transformers and detectors.
"While many Radio authorities contend that the circuit using only crystal detectors cannot be made to amplify as has been done in this case," Mr. Butterfield said. It is possible to do so. Whether a loud speaker could be used with this set I have not been able to determine, but I believe that this would be possible with a few more steps of amplification. However, as it stands the signal strength is at least doubled.
"While the range of the set is not greatly increased by the amplification, several miles are added. How much farther it is possible to receive can be determined only by experimentation.
Not Hard to Set Up.
"In connecting the various instruments together, very little trouble will be experienced. Any form of tuning inductance may be used, but better results will be obtained with a loose coupler, as it will permit closer tuning, an important factor.
"The most sensitive crystals must be used, galena being preferable. However, if a very sensitive crystal is used in the first detector, it will be found that the amplifying detectors will not require ones so sensitive. Crystals may be tested by a buzzer.
"In adjusting the detectors, the cat whiskers of the amplifying detectors should rest on their respective crystals. Then the first detector (D1 in diagram) is adjusted by means of the usual buzzer scheme. The adjustment of this detector is very important and should be made carefully.
"One point to bear in mind is this: The second detector must not be carried to the primary of the second transformer, but must go to the secondary. If this is not done, it will be found that the signals will not carry through the second transformer."
Circuit Diagram and Constants.
The circuit used by Mr. Butterfield is given on this page. In the diagram, D1, D2, and D3 are galena detectors. C1, C2, and C3 represent fixed condensers of 0.001 microfarad capacity. T, and T2 are both ordinary audio frequency amplifying transformers of the type found in tube sets. Their primaries are indicated by P and secondaries by S. B is a 22½-volt battery, connected as shown. This may be increased with somewhat better results. The best voltage must be determined by experiment.
The telephone receivers are no different from those possessed by every amateur. A loose coupler with adjustable primary and secondary inductance values was found best suited to the circuit.