Comms headset bias power circuit help

[Brian]

1 hour ago, sunray said:

My apologies, I'd been posting on a similar subject on another site where we'd been discussing resistor values and confused the 2 threads, so didn't realise you haven' got this far. The pic is typical of the circuit used:

A google search will throw up loads of ideas/solutions.

That's exactly the circuit I doodled out.

If you find the mic overloads the pack then a resistor between the capacitor -ve and the input to the pack will attenuate the signal BUT the value will depend on the input of the pack so it'll be trial and error, but start with 10k.

[sunray]

9 hours ago, Brian said:

That's exactly the circuit I doodled out.

If you find the mic overloads the pack then a resistor between the capacitor -ve and the input to the pack will attenuate the signal BUT the value will depend on the input of the pack so it'll be trial and error, but start with 10k.

It is certainly a standard arrangement, and totally usable. I copied it from a suppliers site, (I think Cricklewoods)

Habitualy I add a 0.1uF capacitor across the battery but only because I found that in a microphone once upon a time. Also electret mics don't work very well on voltages much below 1.5V so don't be tempted to use a single AAA/AA rechargeable.

I find it's quite rare to require any attenuation in my experience however when trying to make something emulate another or keep the levels constant across several devices it may be requied, When it is the case I usually use 2 resistors and aim for the total impedance of R1 + R2//input impedance to be something in the order of 2 to 5kΩ IE R1= 1kΩ, R2= 2.7KΩ gives about 3dB attenuation into a high impedance input (I haven't done this calculation for a lot of years so apologies if incorrect). I do this as I've found a large single resistor can introduce some unexpected frequency tailoring, although it's far from certain. Say in the order of 50KΩ as could easily be found when feeding an electret into a high impedance input, although this is not generally an issue when feeding into balanced inputs which tend to be lower impedance (typically ~1-4KΩ) where  R1=820Ω into 2KΩ input should do the same 3dB without R2. A problem then could be when using the mic for different applications.

However I normally add R2 regardless, either where shown or at point 'A' and even if R1 is not fitted but something like 100kΩ  so it has little effect but will reduce the chance of voltage buildup by leakage through cap and creating a big thump when plugging in etc.



.

 

None of this is particularly critical (other than voltage across element) so feel free to try values that you have to hand.

Edited August 18, 2022 by sunray

[thepurser]

Thanks Sunray!

Have spent hours on Google trying to find solutions but as a lady I worked with once said,  if you can't read music it's just fly ###### on paper!!
Sadly my lack of knowledge meant I couldn't weed out the relevant info from all that was available.

The heads up and help on here has been very useful.

I can't claim to fully understand the schematic but at least I know that once I have translated it the info is relevant.
That will be my Sunday morning sorted.

[sandall]

Next - do you have easy access to the bits, & a soldering iron? None of the component values are critical. I would suggest starting with R1 as about 1k & R2 as about 2k. The capacitor can be anything from 1 to 10 uF (NB: electrolytic capacitors need to be the right way round), with voltage rating at least twice the battery voltage. The resistor between battery & capsule will depend a bit on the battery voltage (up to 10k for 9V). For assembly you could use a bit of project-board, a terminal strip or even make your own spider's web (making sure wires are sleeved or taped, to save shorting things out). Good luck.

[thepurser]

Possibly is the answer to bits access!  Between a rummage through my Dads garage, colleagues in the theatre and Ebay I'm sure I can rustle them up now I know what I am looking for.
Have a shiny new soldering iron in my Stage Management kit, (for small things like fairy light repairs) which can get exercised.
HAve a shift on Sunday morning babysitting a church group so will have four hours to chat the circuit through with my boss.

Will report back when I have something physical!

 

[thepurser]

So I had a hunch and unscrewed the XLR connector on the headset.
Turns out there is already a capacitor and resister on the mic pins.
Can't quite get a good enough pg=hoto with my phone and my eyes are not good enough to read the figures on the cap.

Would I have to inject the 1.5v above the capacitor, (between the mic and the capacitor)?
Would I need anything else in the circuit?
I could put a battery holder inline with the cable if that would work which mightl be  simpler and much more streamlined.

 

 

[sandall]

Capacitor value is fine  (2.2uF, 16V)), not sure what the 200 Ohm resistor is doing. Can you do a sketch showing all the connections?

[thepurser]

Please excuse the layout. Hope its clear enough.

 

This is what seems to be happening.

[thepurser]

Had a better look at it today at work under an eye loupe. The resistor actually bridges the legs of the capacitor legs rather than being inline.

Have redone the illustration to correct it. 

[sunray]

4 hours ago, thepurser said:

Had a better look at it today at work under an eye loupe. The resistor actually bridges the legs of the capacitor legs rather than being inline.

Have redone the illustration to correct it. 

I thought I posted to that effect yesterday but I assume I didn't pres submit.

From your photo I struggle to tell if the resistor is 200 (red,black,brown) or 20K (red,black,orange) or possibly even 2K (red,black,red).

I can understand the resistor providing some attenuation and the capacitor providing some frequency tailoring.

Equally I can understand the resistor providing a DC path and the capacitor providing an AC path.

 

Only for curiosity can you tell the value of the resistor?

[thepurser]

Looks to be red, black, orange and possibly gold.

[sunray]

2 minutes ago, thepurser said:

Looks to be red, black, orange and possibly gold.

Yes I thought is might be 20KΩ, in which case I'm inclined to think these are intended to use several volts.

[thepurser]

Any ideas on the battery placement considering the location of the cap and resister?

 

[thepurser]

Got a newbie question.
My boss dug up a mini jack to 1/4 inch adapter for a lav mic, (very similar to the Eagle mic that Sunray posted). Takes an LR44 button cell and has a switch for the power.

Had a root inside and it has a Capacitor and Resister circuit buried behind the switch.

Capacitor is 225C 2.2UF 16V Type A 3216 1206 Piezoelectric and the resister is 3 red bands so 2.2k.

Can I simply splice that into the cable above the existing capacitor and resister, (those shown in the photo directly attached to the pin of the xlr connecter) or will that cause me problems.

Working out the easiest and least potentially distructive way of testing this out.

[sunray]

38 minutes ago, thepurser said:

Got a newbie question.
My boss dug up a mini jack to 1/4 inch adapter for a lav mic, (very similar to the Eagle mic that Sunray posted). Takes an LR44 button cell and has a switch for the power.

Had a root inside and it has a Capacitor and Resister circuit buried behind the switch.

Capacitor is 225C 2.2UF 16V Type A 3216 1206 Piezoelectric and the resister is 3 red bands so 2.2k.

Can I simply splice that into the cable above the existing capacitor and resister, (those shown in the photo directly attached to the pin of the xlr connecter) or will that cause me problems.

Working out the easiest and least potentially distructive way of testing this out.

The answer is "Possibly".

Is the 1/4" socket mono or stereo? the eagle mic I linked to uses a 2 core cable for signal on one wire and power on the other, it was the standard way of doing things for a long timeIgnore old values from cut and paste.

If that is the case it will mean linking both poles of the socket together.