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April 20, 2024. The Ubiqutous Mic Amp and Balanced Modulator

 Following our template of small circuits and easily digested (almost) presented forthwith is a circuit module for a Microphone Amplifier and Balanced Modulator. 

This will terribly disappoint those home brewers who must see all and every component in a build and whose tenet is everything is scratch built. We used a packaged Double Balanced Mixer (SBL-1) and a Low Noise Op-Amp NE5534.

This circuit board is out of a project by me published in QRP Quarterly entitled JABOM (2011). Oh, you ask, JABOM is an acronym for Just A Bunch Of Modules. Of note the use of single sided copper vector board versus the Manhattan glue down, as the construction medium.




 
Later a Copper Box made from Double Sided PC Board was built and soldered to this board. The box had feed through caps and RCA Connectors. This added to the shielding of various circuits and made this "bullet proof". 
 
The 50K Trim Pot sets the Microphone Gain Level. But for those who are anal retentive there was also a panel mounted gain control that enabled mic gain level adjustment.
 
Now, full disclosure you could build this circuit with a homebrew DBM and use a single 2N3904 as the microphone amp. But one of the key advantages of this topology is lost if you use an ADE-1 an ADE-6 or a TUF-1 or TUF-3.

 



 

 


So, you ask what is so special about this circuit module. The answer is two components: the 100 Ohm Trim Pot and the 22 Ohm resistor. While the SBL-1 is a Double Balanced Mixer, these two components make it possible to UNBALANCE the Balanced Mixer. Why? The answer is to provide a "Tune Up" signal. This obviates the need to shout HOLA into the Microphone!
 
With the 22 Ohm Resistor un-grounded the 100 Ohm Trim Pot is balanced so no Carrier is present. The small relay when engaged, grounds the 22 Ohm resistor which unbalances the SBL-1, and the carrier is passed on through the circuitry. The microphone is not keyed as the signal to trip the relay also trips the PTT.
 
I did contact Mini-Circuits about the feasibility of doing this and while they thought I was nuts did say it would work. This bit of a tip now is forever ensconced in EMRFD. 
 
Hopefully this added to your dining and dancing pleasure for today.
 
 

 
73's
Pete N6QW

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