[Elecraft] OT - WSJT-X & K3s

Tue Sep 5 16:21:46 EDT 2017

Jim,

Get some real world experience with typical amateur transceivers.
Dig out the service manuals and run their mic circuits through SPICE
or one of the other software programs that will show you where they
clip.  You will find almost without fail, the common emitter transistor
preamp with unbypassed emitter bias resistor is, in fact, clipping well
below 1V.  Those preamps invariably have between 26 and 30 dB of gain
and the outputs are connected in parallel (with or without switching) 
with the "Line In" (old Patch jack which was typically rated for 100 -
500 mV) which then drove the audio stage which included the mic gain
control.

Remember, we're talking about *AMATEUR* equipment - not professional
and broadcast equipment with its 1V or 4V P-P standard "Line" levels
and wide range mic preamps that provided standard level outputs

Some newer rigs do better - high impedance, low noise op amp preamps
that are much less sensitive to overload.  TenTec had a "line" input
with a 4V P-P tolerance but those "features" are far from the norm,
particularly considering the number of 25 and 30 year old rigs still
on the air.

In any case, a typical dynamic microphone - professional or amateur -
is rated around -55 dBV/Pascal (check Shure, EV, etc.) some may be a
more sensitive, some less.  For those who do not know, 1 Pascal is a
94 dB Sound Pressure Level (think traffic on a busy roadway) - nearly
25 dB above the EPA threshold for hearing protection!  -55dBV is in
*the millivolt range*.  5 mV is a decent level for normal conversation
with a dynamic microphone within 6" of the lips.

All of this information is available from public sources and a little
study.

73,

    ... Joe, W4TV

On 9/5/2017 1:17 PM, Jim Brown wrote:
> On 9/5/2017 6:24 AM, Joe Subich, W4TV wrote:
>>
>> A transceiver designed for use with a dynamic mic (Kenwood, TenTec,
>> Kenwood, etc.) typically expects 5 mV (or less) on the mic input. The 
>> attenuation needed to reduce 1 V to 50 mV is 46 dB (20log(.005/1)).
> 
> Joe,
> 
> While I greatly respect your engineering, the output of a dynamic mic 
> varies widely depending on what it's hearing. The classic "little old 
> lady on a lavalier" in a TV studio may produce only 5-10mV, but when 
> worked close to the mouth, as most ham mics should be used, the output 
> is more like 100 mV. The fictional lead vocalist Arthur Leatherlungs can 
> easily drive a dynamic mic to 1 V. Not only that, many ham mics are 
> electrets, which tend to have output levels equal to or greater than 
> dynamic mics. The mic input stage must accommodate all of these mics. 
> Although I've not measured input clip levels of ham gear, I'd be quite 
> surprised if they clipped below about 100 mV. And clip level is what 
> matters.
> 
> As to computer output levels -- while RATED output level for clip is 
> typically in the range of 1V RMS, some I've measured suffer from 
> increasing distortion 6 dB below their actual clip level. I've measured 
> -30 dB harmonics just below clip, and -40dB at 6 dB below maximum 
> output. Further, while pro systems are correctly adjusted so that all 
> stages in a signal chain clip at the same level to maximize signal to 
> noise, those systems are going for 100 dB or better. Few ham systems 
> need better than 50 dB SNR in the audio spectrum, but distortion 
> products can be critical. SO -- the output of a computer audio interface 
> should not be run at its maximum level, certainly not hotter than about 
> 0.5V RMS.
> 
> Years ago, the venerable Shure M67 and M68 were designed so that they 
> clipped in the range of 50 mV, and Shure had to sell inline pads (10-15 
> dB attenuators) to prevent overload. A new company, Tapco, led by Greg 
> Mackie developed new inexpensive small mixers to compete with Shure, and 
> ended up eating Shure's lunch with an input circuit with feedback 
> control of input gain that handled much higher signals without overload. 
> For at least 40 years, any pro mic input will handle signals up to at 
> least 3V RMS through the combined use of switchable pads and feedback 
> gain control.
> 
> 73, Jim K9YC
> 
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