Comms headset bias power circuit help

[thepurser]

Hi all,
After struggling with lightweight headsets in noisy shows I have purchased a Point Source CMI5 headset.
My issue is it has an electret mic that needs 1.5v to drive it.
I would like to b able to use it with both wired and wireless systems and the idea is to build a small interface module that sits on the end of the cable and accepts an AA battery and injects the bias into the mic line.
This will give me the benefit of having adapter cables for different systems, (4pin xlr for wired comms and an HME mini din adapter for DX series wireless etc)
As far as I can make out it needs a cpacitor to block and DC power coming from the beltpack that is potentially damaging along with a resister to bring the impedance of teh mic from 1500 ohm to around 200 ohm?????
Sorry to say that beyond that my understanding of the electronics is not up to understanding what components are required.
Could anyone point me in the right direction or provide an idiot proof diagram with simpleton labels for those not electronic symbol literate.

Any help most gratefully received and will of course share files, results etc to anyone interested upon successful completion of the project.

For reference the headset I have is here      https://www.point-sourceaudio.com/products/intercom-headset/cm-i5/

scroll down to the bottom and select Specs for the full spec.

 

[sandall]

I would suggest using a transformer for your impedance matching. Electret mics are generally much higher level than dynamic mics, so a small 1.5k to 200R step-down transformer will solve both problems. A couple of AAs or a PP3 via a resistor (anything from 2k to 10k, chosen to give about 2v at the mic) & a small capacitor to isolate battery volts from the transformer.

[sunray]

Generally I've found the transformer is not required and just a resistor has worked, don't forget the series capacitor has an impedance too.

Are your inputs definitely 200Ω? How fussy are the mics?

Many years ago I purchased XLR plugs from CPC which had a removeable sleeve to accomodate a couple of button cells for the tie clips pictured below. These were only available as males which of course wouldn't be ideal for belt packs.

I can't find them in CPC now, I dare say they are not very popular due to everything having phantom for so long.

This sort of thing  but XLR

 

https://www.ebay.co.uk/itm/304388128284?hash=item46def23e1c:g:eR4AAOSwRiBiJ11T

[sandall]

It's for comms & emulating a 200R dynamic mic for level & impedance, so I think I'd stick with the transformer. I have some of those original Eagle mics somewhere, but not sure of their relevance here.

[sunray]

48 minutes ago, sandall said:

It's for comms & emulating a 200R dynamic mic for level & impedance, so I think I'd stick with the transformer. I have some of those original Eagle mics somewhere, but not sure of their relevance here.

Just trying to demonstrate the sort of device to hold a battery. As it happens my original task was to replace the jack versions with XLR type for a venue without phantom power.

 

Yes but is the impedance relevant? Think about the DIN standard, something like 470kΩ o/p and 10kΩ I/p IIRC

[thepurser]

Thanks for the info so far.
Not sure how sensitive the packs are.
Only figures I am sure about are those for the Pointsource headset and the 1.5v to drive the mic which came from their tech department.
Have a version of the Clearcom MDXLR4M adapter which takes a standard dynamic 4pin xlr headset and interfaces with a DX series wireless beltpack.
AS I mentioned the nuts and bolts theory of it I understand but my electronis understanding doesn't go much beyond not letting the magic smoke out!!

 

[sunray]

2 hours ago, thepurser said:

Thanks for the info so far.
Not sure how sensitive the packs are.
Only figures I am sure about are those for the Pointsource headset and the 1.5v to drive the mic which came from their tech department.
Have a version of the Clearcom MDXLR4M adapter which takes a standard dynamic 4pin xlr headset and interfaces with a DX series wireless beltpack.
AS I mentioned the nuts and bolts theory of it I understand but my electronis understanding doesn't go much beyond not letting the magic smoke out!!

 

Ah... I can offer lots of advice on magic smoke, the real skill is getting the colour correct😉

Over the years I've worked on kit of all sorts of standards and levels of professionalism. and interfacing with others kit on site is a regular feature. Realistically impedance is not usually the prime importance. I carry transformers of 1:1, 1:4 (~ 600Ω:10KΩ), 1:9 (~600Ω:50KΩ) & 33:1 (10KΩ:8Ω) but more often than not they are primarily used for ground loop isolation and gain/attenuation than impedance matching.

[sandall]

12 hours ago, sunray said:

I can offer lots of advice on magic smoke, the real skill is getting the colour correct

Returning to the original question -

If your comms set specifies a dynamic mic it won't have phantom power, so won't kill your mic, though putting even 1.5v DC across a belt-pack mic input MIGHT be problematic.

The quoted 1.5v mic power will be a minimum value. The capsule is likely to be generic Chinese, & will probably be happy with anything from 1.2v to at least 5v. I suggested using AA or PP3 batteries, but stand-alone miniature mics are more likely to come with alkaline or lithium button cells these days.

I'm curious as to how you make your distinction between "nuts and bolts theory" & "electronics understanding". 

If it helps, I can send you a simple suggested circuit, or maybe sunray can offer you a practical solution?

[Brian]

Don't get too hung up on impedance matching, the beltpack manufacturers don't!

When John (boatman) and I designed Bluecomm I reverse engineered just about every beltpack out there. All of them have a resistor strapped across the mic input, mostly to keep the input DC operating point stable, and those resistors vary between 3k3 and 47k. So no attempt6 to match 200R.

The ONLY times impedance matching is important is...

a) when you need maximum POWER transfer across an interface

b) when you need to avoid reflections and are terminating a transmission line

In audio work we are ALWAYS concerned with maximising VOLTAGE transfer, which means have a load impedance much higher than the source impedance.

[sandall]

Not really hung up on them - just old-fashioned Beebthink - if in doubt (or even if not) add a transformer (there's a LL/something for every occasion!)

[sunray]

10 hours ago, sandall said:

Not really hung up on them - just old-fashioned Beebthink - if in doubt (or even if not) add a transformer (there's a LL/something for every occasion!)

It was always funny providing private wires at events for the media guys. We'd do our level checks, BBC would turn up and check the impedance and match it, check level at a particular tone which was different to that in the spec then do a full frequency sweep.

In the mean time Itv guys would say "Hi, this one ours? Cheers mate." stick the pair in their interface and presenter would start describing the journey there or someones hair do until the far end said OK.  Do a report or introduction or trailer for the event then "Where can we get a cuppa?"

The BBC technicians would still be working beavering away before the presenter was even seen. We always reckoned the local radio guys were the most pedantic.

 

No criticism of either, we simply thought it was funny the 2 services achieved relatively similar results.

 

 

Edited August 16, 2022 by sunray

[thepurser]

11 hours ago, sandall said:

I'm curious as to how you make your distinction between "nuts and bolts theory" & "electronics understanding". 

Hi Sandall, I'd say that I can follow the logic of how something works if explained but don't have the technical knowledge .
In this situation for example I understand that the mic needs power to operate and that logically a small box injecting the 1.5v into the line should do the trick. I also understand from some research that the use of a capacitor to block unwanted phantom power entering the circuit and damaging the capsule. I also get that because of the way the electret generates the signal its is "quiet" and may need boosting before entering the comms in order for people to hear me.
Now how that all works actually works is a bit of wizardry involving the magic smoke.

If this all works I have an ultimate goal of getting a bone conduction headset with bone conduction mic working!! It is designed to work with a motorola or similar radio so in theory should be possible and would help prevent the show monitors injecting back into the comms which kills me!!!

Any suggested circuits or help/practical solutions always gratefully received!

[sandall]

Ok, a couple of points. Powering - Yes, for a comms mic 1.5v should be fine. The capacitor - No, it's there to keep your 1.5v out of the mic preamp (there shouldn't be any phantom power* on a comms unit). Signal level - No, electret mics are usually much "louder" than the dynamic ones used for comms, so a transformer, a couple of resistors or a potentiometer are needed to get your mic level down, to match other mics on the system.

(*connecting electret mics to mixers with phantom power is a whole other topic, which gets discussed here from time to time).

I have no experience of bone-conduction mics, so no idea of the level they produce, but this might be of interest - https://www.vocal.com/voice-quality-enhancement/utilizing-a-bone-conduction-microphone/

[thepurser]

Seems I overestimated my understanding!!!! Thanks for the corrections and the link to the article.
Interesting read.

[sunray]

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.