[Elecraft] Noise Cancelling Headphones

[Jim Brown]

On 12/25/2013 11:57 PM, Gary Smith wrote:
> I have an Electrovoice RE-10 which
> would probably be nice & needs no power

Being a retired audio pro who has done a lot of recording, I have a lot 
of pro mics, including several RE16s.  Before I discovered the CM500 
(thanks to W6XU, another audio pro), I used an RE16 in my ham station on 
a boom stand. It worked quite well.

The hookup of a pro dynamic mic to ham gear is very simple. Pro mics are 
wired balanced via shielded twist pair cable to a three-pin connector 
called an XL.  Pin 1 is the shield, Pin 2 is "high", Pin 3 is "return."  
To connect a dynamic mic like the RE10, wire Pin 2 to mic in on the rig, 
wire Pin 3 to mic return on the rig, and connect Pin 1 to the chassis. I 
made up some adapters for the various rigs that I used that mic with.

Nearly all ham mics have a peaked response in the range of 3 kHz to 
compensate for the rolloff of SSB filters, both on RX and TX. This was, 
I believe, first developed by Shure about 40 years ago, and you can see 
it in the response of their communications mics. Pro mics don't have 
this peak (or if they do, it's in the range of 8-10 kHz), so TXEQ must 
be adjusted to provide that peak.  I used the same low frequency rolloff 
for the RE10 that I use now for the CM500 -- the three lowest frequency 
bands max cut, the fourth cut by about half, tweaked by ear based on the 
reports of a known good listener.  For the high end, I boosted the top 
band by about 10 dB, the next highest band by a few dB, again tweaked on 
the basis of reports from a good listener.

With rigs other than a K3 that don't have TXEQ, I added a capacitor in 
series with the mic wiring to the rig input and a resistor between the 
rig mic input and mic signal return. The resistor was about 470 ohms, 
and the capacitor value was chosen so that the network produced a strong 
rolloff below 2 kHz -- in other words, a sloped response that peaked 
around 3 kHz.  I got very good audio reports from this simple setup with 
a TS850, an Omni V, an FT1000MP, and a K2.

I modified the K2 to add a low frequency rolloff around 400 Hz also to 
increase the mic gain by about 6 dB.  I did this simply by changing the 
values of a few resistors and capacitors early in the audio signal 
chain. The purpose of the mod was simple -- with most ham mics, the K2 
is a bit low on gain, so I wanted to hit the limiter harder. I also 
wanted to minimize the wasted LF energy, and also prevent that LF energy 
from activating the limiter.

73, Jim K9YC