by Alan R. Klase
The transducer that ultimately delivers the signal to the ear is an extremely important part of any crystal set. Really significant performance gains can be made by simply plugging in the right headset.
The standard headset for crystal sets since the dawn of time has been the 2000 ohm (DC resistance) ‘phones with steel diaphragms directly actuated by electromagnets. These are hard to purchase new, but there’s a lot of them around. Crystal earplug units are still available, they offer an excellent bang for the buck. (Actually two bucks.) Hi-fi headsets can be used with matching transformers, but most are pretty inefficient. There are some better, more exotic, solutions to the headset problem. Generally, higher impedance is advantageous. Lo-z phones mandate matching transformers. The following report should give you some ideas:
A report from the Stanton Signal Laboratory:
A recent discussion of headphones for crystal sets on the Boatanchors mailing list prompted me to go to the lab and make some, at least half-way, scientific comparisons. I had previously attempted to make measurements with a signal generator and a microphone, but peaks and dips in the responses make this a questionable practice.
This time I decided the criterion should be intelligibility of speech at low signal levels. I tuned my shop radio, a Collins 651S-1, to the local talk radio station, and adjusted the output for -20dBm into a 600 ohm load. This station is almost all talk and the audio levels are quite consistent. The receiver output was then applied to a stepped attenuator. At the output of the attenuator, three different audio transformers were available to provide impedances of approximately 50, 125, 300, 500, 1K, 2K, 4K, 8K, 12K, 50K, and 100K ohms.
The test procedure was to set the attenuator for an output of approximately -50dBm and choose the transformer tap that gave the loudest signal from the headset under test. The the signal level was then reduced to a point were I could still follow the conversation, and, presumably, ID the station. This signal value appears in the sensitivity column below. I also measured the DC resistance of each headset.
|TRIMM TYPE K||2K||12K||-70|
|TRIMM FEATHER WT.||3.8K||12K||-70|
|MOUSER XTAL PLUG||INFINITE||25K||-70|
|BALDWIN TYPE C||1300||8K||-76|
|SOUND POWERED #1||150||1K||-84|
|SOUND POWERED #2||30||300||-88|
NAVY LO-Z: WWII aviation style headset using ANB-H-1A elements, similar to HS-33.
TRIMM K: Trimm type K, from 1950’s. A better quality headset using steel diaphragms.
TRIMM FEATHER WEIGHT: Light-weight version of above.
MOUSER: Contemporary crystal (possibly ceramic?) ear plug from Mouser Electronics. An excellent buy at about $2.00. In actual use you’ll want a 100K resistor across it to provide a DC return for the detector.
BRUSH: The typical Brush crystal headset from the 1930’s-60’s.
BALDWIN TYPE C: The old standby “Baldys” with the balanced armature driver and mica diaphragms. They bear 1910 and 1915 patent dates, but were manufactured until, at least, 1941. These are typically very sensitive for CW, but roll off steeply above 1 KC. Even so they’re a good bet for crystal set use.
SOUND POWERED #1: Baldwin cases with elements from post WWII TA-1 P/T sound powered field phones.
SOUND POWERED #2: Elements form one of the clunky (Navy?) sound powered handsets cobbled into an H-161 VRC-12 headset.
One could probably do a lot of nit picking with my test methodology, but the results are consistent with my observations in actual crystal set use. The Baldwins are better than any of the standard headsets I’ve encountered. The sound powered elements, which share the same basic construction as the Baldys, are considerably more sensitive because they were designed to be peaky in the 2-3 KC range to optimize voice intelligibility.
An ohmmeter can check the DC resistance of headsets (should be in the 2000-4000 ohm range for use with crystal sets). The other quick and dirty test is to put the headset on, hold onto one terminal, and touch the other to a ground or large metal object. There should be a click or a hum with a headset that has enough impedance and sensitivity for a crystal set.
By the way, my current double-tuned loose-coupled killer crystal set, using the SP #2 headset, and a 100 ft flat-top antenna, hears 24 day-time stations in the NYC-Philadelphia-Scranton region from my western NJ QTH. The minimum input carrier lever for a intelligible AM signal is about -66dBm.