UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 19th Jul 2019, 7:33 pm   #1
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 10,001
Default Balanced audio O/P from 3-terminal electret capsule.

Backstory: I have an old "Shure" microphone whose insert has gone open-circuit.

I want to replace this with a modern electret capsule-mic.

The input circuit to the modulator is a balanced stage, floating above earth.

The electret capsule I have has 3 terminals: Bias, Earth, and Output.

How to wire this? I'm wondering if I ignored the 'output' terminal and then used two equal-value resistors to connect the battery + and - to the 'bias' and 'earth' terminals, could I then use 2 capacitors from the electret's terminals to the modulator's balanced input?

[Essentially this would be building a 'split load' for the electret in the same style as a popular phase-splitter design]
G6Tanuki is offline  
Old 19th Jul 2019, 10:14 pm   #2
Refugee
Dekatron
 
Refugee's Avatar
 
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 5,029
Default Re: Balanced audio O/P from 3-terminal electret capsule.

The new insert may well be one with a FET wired as a source follower in it.
The original will most likely be a "posh" one for balanced feed.
Refugee is online now  
Old 19th Jul 2019, 10:22 pm   #3
Biggles
Rest in Peace
 
Join Date: Feb 2004
Location: Hexham, Northumberland, UK.
Posts: 2,234
Default Re: Balanced audio O/P from 3-terminal electret capsule.

Could you connect the unbalanced output of the electret module to a small audio transformer (LT700 sort of thing) and then use the secondary winding to provide your true balanced output, maybe with the centre tap earthed? Not sure about audio quality though. You may need to play about with equalisation depending on your requirements.
Alan.
Biggles is offline  
Old 20th Jul 2019, 11:44 am   #4
Alistair D
Octode
 
Alistair D's Avatar
 
Join Date: Jan 2010
Location: Edinburgh, UK.
Posts: 1,833
Default Re: Balanced audio O/P from 3-terminal electret capsule.

When I've done this kind of thing in the past I simply connected the output of the unbalanced mic to the 2 signal wires from the XLR plug and used the cable screen to earth any exposed metalwork. In this case you would also need a coupling cap to block the DC from the insert. So far I have never needed any ground reference resistors. That said I have never tried connecting an AC coupled source before so I not 100% sure if it will need resistors.

Al
__________________
I won't tell you how I discovered that.
Alistair D is online now  
Old 20th Jul 2019, 1:05 pm   #5
gingpeakin
Pentode
 
Join Date: Mar 2009
Location: Worthing, Sussex, UK.
Posts: 206
Default Re: Balanced audio O/P from 3-terminal electret capsule.

Here's a thing I made earlier that runs off the phantom power from a mixing desk etc. Any old low noise transistor will do - the microphone capsule will have of course to be able to withstand the phantom power - not many will take the regular 48V - this design ran on a PP3.

Ging
Attached Files
File Type: pdf TL3509 Iss1.PDF (21.4 KB, 43 views)
gingpeakin is offline  
Old 20th Jul 2019, 4:20 pm   #6
julie_m
Dekatron
 
Join Date: May 2008
Location: Derby, UK.
Posts: 7,729
Default Re: Balanced audio O/P from 3-terminal electret capsule.

The usual type of electret mic insert uses a FET as an internal preamp, in a circuit that certainly does not look hugely different from a phase splitter if you connect a resistor in series with the source and drain ..... It's got to be worth a try! You might need a high-value capacitor from one end of the battery to the outer shielding of the mic cable.
__________________
If I have seen further than others, it is because I was standing on a pile of failed experiments.
julie_m is offline  
Closed Thread

Thread Tools



All times are GMT. The time now is 11:42 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Copyright ©2002 - 2021, Paul Stenning.