WTD: Electrovoice 638 Microphone

[Paul]

I've edited out the earlier comments from this thread to maintain message
size compliance (no more than 150 lines)

You folks have hit on something DXers and Contesters have known for years. 

At the risk of stating the obvious (but frequently over looked)...

Radios behave differently with microphones types. Just because the
manufacturer provides a mic with the radio doesn't mean it's the right mic!

There are a number of variables in the transmit audio chain. These include
the impedance of the mic (cartridge itself or any built in amplifier),
impedance of the mic amp in the radio, gain of the mic amp in the radio,
output level of the mic itself (when properly terminated. You can actually
have a microphone who's cartridge has such a high output that it will
overdrive the mic amp in the radio - this becomes evident when the mic gain
setting of the radio is very sensitive (usually very low value)

Typically "hollow state" rigs have a high impedance mic input and solid
state rigs have a low impedance input. 

Next is the audio response of the mic (when properly terminated), audio
response of the mic amp of the radio and (finally) the users voice. This
topic leads to the "mission" of the radio. Are you a ragchewer who wants
good, clean audio, are you a DXer or Contester who wants punch and every bit
of advantage he can get, or are you one of those hi-fi audio types?

Most SSB radios are designed with transmit audio circuits that have a
response from 300 Hz to 3500Hz. Keep in mind that radios that can support AM
or FM (as well as SSB) will have mic audio circuits with frequency responses
that are wider than this.

There are two messages to take away from the above statements. 1) If
impedances don't match, the audio will be impacted. Typically you will get
reports of low or "muddy/muffled" sounding audio. 2) If the frequency
responses don't match up, your audio will have excessive high or low
frequencies or possibly even sound "constricted" (over filtered) and make
your voice less intelligible.

Do a little research. Read the manual to determine what the audio input if
your radio is. Some manuals may even indicate what the typical input level
needs to be (not many and most don't say anything about frequency response).
Read about the microphone you want to use to understand its impedance,
output level and frequency response. Be doing your homework, you can get a
good idea about what should work and what won't work (without a lot of extra
effort). Finally, when you hear someone with outstanding audio, ask them
about their mic/rig/accessories. What you learn may be worth applying to
your station!

Possibly this note will inspire someone to do some research into the various
microphone cartridge types and their characteristics and share that
information with all on this reflector. This would be good reference
material to have on hand.

If you want to experiment to determine what really works best for you, you
will have to try different mics. Does this mean you have to actually buy all
sorts of different mics? No it doesn't. You can borrow different mics from
your buddies! All you need is to invest in a male connector that fits your
radio and various female connectors (2 pin, 4 pin, 8 pin or what ever).
Change the adapter as required. The best suggestion is to have a buddy
listen to your station and make recordings of your audio. If you want to do
A/B/C type comparisons, you may need to fabricate two or three adapters. No
big deal, it's still cheaper than buying mics!

Also, there's one last set of variables that you need to be aware of, how
your radio behaves with speech processor on/off or any audio mixing devices
(W2IHY etc) will also be affected when changing microphones.

If you have access to lab grade equipment and have the time, you can really
"dial in" your transmit audio, but (IMHO), it's not worth it.

One last point, external audio devices (W2IYH equalizers or external speech
processors for example) can be a point for RF to enter the transmit audio
system. Pay careful attention to reports of screeching noises or VOX
circuits not releasing.

The definition of communication is to convey information between two or more
parties. Needless to say, you can't communicate effectively if your transmit
audio is poor.

VY 73
Paul

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