[AMRadio] Mic opinion (Jim Candela)

[Donald Chester]

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From: amradio-bounces at mailman.qth.net <amradio-bounces at mailman.qth.net> on behalf of amradio-request at mailman.qth.net <amradio-request at mailman.qth.net>
Sent: 27 July 2021 15:44
To: amradio at mailman.qth.net
Subject: AMRadio Digest, Vol 206, Issue 2

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Today's Topics:

   1. Re: Mic opinion (Jim Candela)
   2. Mics for vintage (CL in NC)
   3. Another question, this time Desk KW (CL in NC)
   4. Re: Another question, this time Desk KW (Robert Nickels)


 Jim Candela <jcandela at prodigy.net> wrote:

" I can only make one point, and that is the position of the volume control knob can have an effect on the linearity of the next stage. This might sound crazy, so let me splain. The audio drive has a source impedance, and the next stage has a load impedance. When the audio gain pot is at the limits (0 or 100%), the source impedance is zero ohms. With the pot in the middle, the source impedance is no less than 1/2 the resistance of the pot end to end. If the pot is 500K, then think like 250K source impedance plus the plate resistance of the driving stage. Then consider the next stage is a triode, and it might have 2-5pf grid to plate capacitance, and something similar from grid to ground. That 250K + source impedance will draw down the signal as the peak approaches the point where the control grid draws grid current. For some tubes, this happens several volts before the actual knee of grid current is achieved. This means one half of the signal gets attenuated more than the o
 ther. Then consider what happens at 100hz versus 3Khz were the capacitance of the load causes a low pass where the Fc point is lowest when the source impedance is highest (mid way on the volume control)."

In mine, I circumvent that problem by interconnecting each of my audio units with 500/600 balanced line: the mic preamp, line amplifier, compressor/limiter and transmitter.  Levels are adjusted using Daven attenuators appropriately inserted in the lines between units. The first stage of the mic pre-amp is designed so that the microphone output feeds it at optimum level, and the "mic gain" control is the attenuator between the mic pre-amp and the following line amplifier that also contains the selectable low-pass filter.

The  signal in a multi-stage amplifier should never reach peaks high enough to drive the next stage control grid(s) into grid current, but the level at each stage should be high enough to render hum and noise negligible.

Another factor to consider with triode amplifiers is the Miller Effect.  Essentially, the tube amplifies the effect of the grid-plate capacitance and effectively shunts this phantom capacitance across the input, thereby attenuating the high frequencies. It is less a problem with tetrode and pentode amplifiers because the grid-plate capacitance is minuscule in the first place. 

There is scant mention of this in the amateur radio handbooks, but it is given more attention in guitar amplifier and audiophile circles.  (Remember, ham radio audio is "supposed" to sound scratchy and distorted, with limited high and low frequency response.)

https://www.aikenamps.com/index.php/what-is-miller-capacitance