[K3] Additional Gain Stage for FP Mic?

[Robert Wright]

In the K3 manual (version D10) under the MAIN:MIC SEL menu is the following note:

        "For the front-panel mic only, tap 3 to turn on an additional gain stage. Use this only
        with very low-output mics. An apostrophe will appear after the H, e.g. H' ."

(At first it didn't work, but I discovered by searching this list that tapping "7" instead is required)

Anyways, when I am monitoring my transmit audio, I can get a much better S/N by using the additional gain stage and changing the main gain from H to L.  So instead of FP.H, using FP.L'.  Mostly, the reduction in noise seems to be to the K3's internally generated noise.

(I'm still studying the schematics, but the pertinent gain stage seems to be U5A and U4 on the KS DSP:CODECS page (29 of 56).  I see three different gains available at this stage.)

Is there any drawback to setting up my gains using the additional gain stage?

And this leads to the question of where is the switchable gain for the rear panel mic connector.  I see no hardware to support RP.L vs RP.H.  So is the only gain change for the RP in the U7 codec?  If so, my tendency would be to drop the value of R15 on the KIO3 board (KIO3 Main:Sheet 2, page 12 of 56) to improve the S/N from the rear panel mic (assuming most of the noise is introduced on the lines going into the codec or by the codec itself).

Of course, one wonders whether any of this can be heard over normal HF propagation noise.  But it could make a difference on the higher bands and with FM.

73, Bob, N7ZO


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[Jim Brown-10]

On 9/27/2012 9:37 AM, Wright, Robert wrote:
> Of course, one wonders whether any of this can be heard over normal HF propagation noise.

ALL pro audio products that accept mic inputs have controls to set input
gain over a nearly 60 dB range. There are several reasons. First, some
microphone types are more sensitive than others -- that is, they produce
more voltage for the same sound level.  Second, the voltage produced by
a mic depends on the loudness of the sound source and how close the mic
is to the source.

The high and low settings in the K3 are simply two values of input gain
that are suitable for the range of mics used with ham gear, and for
sound levels of a voice at 3 -12 inches from the mic.  This "high-low"
setting combines with the Mic Gain (the front panel pot) to set the
total gain of the system. When the setting is too high, the audio chain
in the radio is amplifying room noise, and can sound really awful on the
air.

The "additional gain stage" should NEVER be used if it is not needed.  A
good rule of thumb is that it should be used ONLY if you can't turn the
Mic Gain pot up enough to get good audio levels as described in the K3
manual.

73, Jim K9YC
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[Robert Wright]

Hi Jim,

A good generic textbook explanation, but the feature of interest here is that using the "apostrophe" setting REDUCES the noise floor.  I am not changing the overall gain.

The overall gain of FP.H at a mic gain setting of 35 is about the same at using FP.L'  (note the apostrophe) at a mic gain setting of 15 or so.  But the second combination produces a significantly lower noise floor and so looks to be a more optimum setting of the various gain stages (better gain staging).  I have noted no problems with overhead or dynamic range.

73, Bob, N7ZO


-----Original Message-----
From: [hidden email] [mailto:[hidden email]] On Behalf Of Jim Brown
Sent: Thursday, September 27, 2012 10:14 AM
To: [hidden email]
Subject: Re: [Elecraft] [K3] Additional Gain Stage for FP Mic?

On 9/27/2012 9:37 AM, Wright, Robert wrote:
> Of course, one wonders whether any of this can be heard over normal HF propagation noise.

ALL pro audio products that accept mic inputs have controls to set input gain over a nearly 60 dB range. There are several reasons. First, some microphone types are more sensitive than others -- that is, they produce more voltage for the same sound level.  Second, the voltage produced by a mic depends on the loudness of the sound source and how close the mic is to the source.

The high and low settings in the K3 are simply two values of input gain that are suitable for the range of mics used with ham gear, and for sound levels of a voice at 3 -12 inches from the mic.  This "high-low"
setting combines with the Mic Gain (the front panel pot) to set the total gain of the system. When the setting is too high, the audio chain in the radio is amplifying room noise, and can sound really awful on the air.

The "additional gain stage" should NEVER be used if it is not needed.  A good rule of thumb is that it should be used ONLY if you can't turn the Mic Gain pot up enough to get good audio levels as described in the K3 manual.

73, Jim K9YC
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