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Input Impedance Tbp12 Beltpack When Pin 4 Tied To Pin 3

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Hi Everybody,



The manual of the TBP12 beltpack states that the input impedance is 1,3 MOhms. According to the same manual the impedance is lowered when pin 4 is tied to pin 3 in the TA4F connector. Does anyone know the value of this impedance?






Merlijn van Veen

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  • 2 weeks later...

Hi all,


I've been going through our mics soldering 3 and 4 together in the connector, which seems to help solve the distortion problem we were getting during loud scenes.


Just a thought though... seeing as we're never going to use the TBP12s for anything other than mics for theatre applications, is it feasible for us to open the packs and solder 3 and 4 together at that end instead? Saves having to do it every time we buy a new mic - none of the off-the-shelf TA4-terminated mics that I've come across so far have had 3 and 4 tied in the connector.

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It was actually a nice easy modification in the end, as you can just solder the two pins together on the reverse of the input assembly. As an aside, I also smeared non corrosive silicone-based sealant around the input assembly, and on the RF board (especially where it sits very close to the main PCB). End result... the high-pitched whine I was getting at mid-high gain from the beltpacks has now completely disappeared - they're finally completely silent!

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Did you get the Line 6 versions by Countryman? They add a small resistor across pins 3&4 which is similar to shorting the pins. You can check out their wiring scheme on their web page. Don't worry about the Zener diode. It's just an added protection ( but probably unnecessary)

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Hi Don,


We've got the SL (Shure) versions - Countryman said there was no difference between the SL and L7 (Line 6) wiring, and certainly on their website the wiring diagram is identical.


Are you not thinking about the 4.7k resistor across pins 2 and 3?


We've had an issue with the B3 & B6 capsules (red and blue-band) distorting during loud musical theatre scenes, especially when they're positioned on the cheek (although we've experienced it in the hairline position too). It sounds more like digital clipping or limiting than microphone distortion. I've not quite been able to get to bottom of it yet - it seems intermittent. The Line 6 specs say to tie 3 and 4 for lavalier use and leave them open for instrument use, which isn't how most TA4 shure-wired mics seem to be from the factory.


I'd been through all of our B3 mics and soldered 3 and 4 together, and the next show I did was perfect. However we had the problem again with 2 out of the 14 systems we were using on a show last week. They started off fine, but after the first few scenes the distortion appeared. The same thing happened on the second rehearsal with different capsules, so we ended up having to patch in some analogue Sennheiser systems for those cast members instead.


I'm not sure what to try next. Judging by the TBP12 specs, it should be impossible to overload the inputs no matter how loud the cast sing! The receiver audio LEDs never get much further than 2 or bars, and I've never seen the red clip light on the beltpacks either. Puzzling!

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That's not in the show yet as far as I know ;)


I'll pay closer attention to the mics during next week's show, and hopefully at least work out what factors change between the channels that do distort and the ones that don't. I've tied pins 3 to 4 on every beltpack now anyway to lower the resistance. Didn't notice any difference in level whether those two are connected or not though.


If nothing else, I'm at least now very familiar with the insides of those beltpacks, especially the little battery cover springs!

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  • 2 years later...

hi, i have nearly read everything i found about the pin connections and resistors. i use CO-8WD headset mics and sound quality is nearly perfect - but the output level of the mics is not very high and it would be great to boost the level for the tbp12 transmitters...

as far as i understood, there is no way to do this without an external batterypowered preamp - or can you adjust the gain of transmitter by changing some smd resistor or something?

thank you :-)


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