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Call Sign : 26-CT-4145 Posts : 90 Times Thanked : 1 Join date : 2021-06-03 QTH or Location : Chatham, Kent Equipment Used : Includes :- AT-878UV, FT474Gx, AT-5555N, FT-818, DNT M40 (RT Factory, Manpack config), Midland Portapack, IC-211E, TS700G, TS2400, MAXCOM 16E, unbranded 40 channel Japanese 49 & 2.4Ghz transceivers (supposedly Uniden, but unlikely) Age : 53
Subject: Re: Mic Wiring Fri Jun 11, 2021 12:13 am
What you might want to add, is point out that there's two basic PTT switching types used over all CB, mostly one type on LMR stuff (including external mic and speaker mic socket equipped items).
There's the basic PTT -> GND aka classic relay switched although transistor switching in later years/gear. So the actual audio lines (Referred on pinouts as Tx and Rx connections referring to what audio routr is passed) are usually left permanent unswitched on relay switched 'two wire' PTT setups. Easy ID is leave radio on receive, squelch off and vol up and (given a suitable wired test mic) if you unplug the mic and Rx audio mutes from main speaker, it's not a '2 wire PTT' arrangement. 5 pin DIN was the more common connector but really you only used 3 connections :-
Tx/Mic Audio to same, ground to ground, PTT to PTT and the PTT switch on the mic grounded PTT line to Gnd at the mic end. So on this, and Vox mode radios used essentially the same breakout and wiring, all you needed was the most basic push to make switch for the PTT which could be a switch of a simple single bus bar type. These ones were idiot proof and you could easily put a PTT lock (lock on/off) or a locking latching PTT on any mic. So, properly set up mic end and with sensitivity set up vox side on rig, on vox capable gear you could forgoe the need for a switch, but it is advisable to at least have a PTT lock/inhibit or a mute switch override to break the mic audio out when not in active use.
The other main config was usually 4 pin type transistor switching, most CB's you'll come across have with arrangement. These used an Rx line, Tx line, PTT and Gnd lines.
So your switching worked as follows :-
When you pressed the PTT, Tx & PTT were grounded and Rx not. This shut of received audio, allowed mic audio to be fed and the PTT trigger state enabled. When you released the PTT, Rx was grounded (allowing the loop-through it creates to unmute Rx audio). As stated in the three wire 'relay' type setup, unplug the mic and you hear no Rx audio at all. That's why you have a loop through dummy plug on such sets where you setup for monitoring and swap with a mic for on-air use.
On most expanded pinouts, which often provided extra functional connections, the logic type lines were usually a pin to GND switch action to usually momentarily trigger (such as a CHNL-UP and it's reverse counter part).
Additionally, as it became common practise to use PEC inserts such as (electret/condensor/MEMS), there was an additional 3-5V (either extreme or possibility switchable) which fed a phantom supply for the PEC's internal preamp and secondary preamp that would deliver a mV level SIG to radio. So on older gear of any setup, you'd have to find a phantom supply to use such inserts - luckily, the common PEC transducers commonly used on consumer grade mics needed really barely 1V to energise most typical tiny inserts, so a single alkaline mounted mic side had a very long lifespan.
Finishing the mic element bit - the phantom will accommodate driving the PEC insert in most PC/Console headsets that normally drawn their current drive computer/console connection. What you'll get, however, if you try to re-purpose a condenser large element inset or equipped mic such as designed for broadcasting or serious recording setups is a near unusable or nil pickup from a mic test lash-up. This is due to such mics being designed to use 48V low current phantom supplies and 3-5v sod all current isn't suitable by a country mile at least.
So for those, either feed via a phantom supply equipped mixer and use a voltage dropped rec or master out 'line' level attenuated to a mV level range. Thankfully, since in the era of PEC mic use, the relative output (mixer or PEC output for consumer mics) is in the K's impedance wise as is the mic feed impedance on the mic in rig side.
On older gear built for dynamic mics, a simple resistive load added to the mic end would solve impedance matching. In fact, it was often practice to put that resistive load between PEC internal amped output and secondary main mic preamp so you fed the secondary amp with a slightly higher or equal impedance termination to what you terminated the final mic amp out at.
Finally, mic side - whilst you could get away with close-mic use on dynamic mics, don't do it on condenser types (unless it's specifically tailored for close mic use). I won't go into deep detail, but there's a very distinct reason why these condensors type mics aren't used as main pickups on drum kits, because like with close proximity misuse of condenser hand mics, close use creates a very high SPL and most condenser inserts weren't designed for it - similarly to with ribbon insert mics, you'll damage the insert in the long term as well as create an awful dual muffled and distorted compressed tone to the fed output audio.
On my original homebrew base mics, I used a dual pickup with a blended mix of moderately eq'd pickup from dynamic to cater for the low mids, and a lowish gained PEC feed to give mid /high presence. Just needed about two BC series transistors, and just enough basic components to build up the preset mixer channels with a trim for the OP.