[ARC5] Crystal Headphones

[Richard Knoppow]

    The transformer makes all the difference and explains why low 
impedance phones, like sound powered phones, will work well with 
a crystal set. You would probably do best with a microphone input 
transformer meant to connect low impedance microphones to the 
grid of a vacuum tube. A typical transformer will have a primary 
tapped at about 50 ohms and 200 ohms and a secondary of around 
50K to 100K. The sound powered phones are very efficient so if 
the impedance is matched the output will be quite high. Not 
exactly high fidelity, RCA made their sound powered phones to be 
resonant in the middle of the speech range while Brush crystal 
phones are fairly flat from perhaps 100hz to maybe 10Khz.
      I know exactly what a Boonton Q-Meter  is and have a couple 
of them that are not in very good condition. A very useful 
instrument if you understand it. Its a forgotten gem like a grid 
dip meter; not understood by younger people because they don't do 
much RF work. I wouldn't mind finding another Q-Meter if in very 
good condition. Often the oscillators get intermittent as the 
result of the contacts getting dirty. Very easy to fix. I also 
had a Boonton RX Meter but it fell off a table and was damaged 
beyond repair. That's something else I would not mind finding a 
replacement for. Has the same oscillator problem as the Q-Meter, 
also easy to fix.
     I know where KNX is. They moved to that site in 1938 after 
the original high power site near the Sepulveda flood control 
basin was flooded out in the notorious flood of 1938. The 
original Torrance transmitter building was a beautiful Streamline 
Moderne (often confused with art deco) design, demolished when a 
new transmitter was installed sometime in the late 1960s.
     Brush crystal headphones, like crystal microphones, do look 
like capacitors. Conventional magnetic phones look like 
inductors. I was not aware the crystal phones needed a resistor, 
perhaps as a load to develop voltage from the diode. I don't 
remember hearing this before.
    BTW, I am in an area near the old KDAY transmitter in Echo 
Park. This is a 50KW daytime, lower power at night, I think the 
current call is KBLA. The six tower array has two other stations 
duplexed on it, KYPA (formerly KGFJ) and KHJ. Blankets the area. 
Diplexing or duplexing AM broadcast transmitters has become 
common since the transmitter sites have often become more 
valuable than the station.  Fairly recently KABC was diplexed 
onto the old KFAC (now KWKW) site. The coverage is sometimes not 
as good but the cost is less. There is an engineering company who 
specializes in this using something they call the method of 
moments to calculate the required phasing arrangements. I think 
the three main New York transmitters are now duplexed onto the 
same antennas.
    Well this is OT so enough.
    Thanks for the links, I will definitely look at them.

On 9/25/2018 2:23 PM, Jim Hill wrote:
> A little more on using sound powered phones (with crystal sets).
> 
> Back in the heyday of high-performance crystal sets (I think 
> around the early 2000's), sound powered phones were considered 
> the best, with the RCA "big cans" the ultimate.  A matching  
> transformer was required, and Fair Radio sold one that worked 
> well. I bought some, and they worked fine, Fair Radio ran out of 
> them, so started substituting others without mentioning that 
> fact.  I don't know how they worked, but listening tests with a 
> UTC multi-match transformer indicated that the ratio was not 
> critical.
> 
> I'm and off-and-on crystal set dxer, but have a severe problem.  
> I live very close to 50kw KNX 1070, so my crystal set DXing is 
> done on camping or RVing trips.  I remember that my best location 
> was at a campground near Grand Canyon.
> 
> A big aerospace crunch in that era caused crystal set on-line 
> activity to drop dramatically, I guess everyone was out 
> "looking", or at least spending more time doing other things.  
> For example, I had an extra Boonton Q-meter, a very desirable 
> item for crystal set builders when attempting to design the best 
> coil.  I am getting old and am downsizing my radio gear, so 
> decided to sell it.  It was really difficult.  Nobody knew what a 
> Q-meter was, and I finally found someone at an ARRL convention 
> swap meet who wanted to tinker with it.

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL