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.

Reply
 
Thread Tools
Old 18th Apr 2024, 10:39 pm   #21
ScottBouch
Octode
 
ScottBouch's Avatar
 
Join Date: Feb 2013
Location: Leicestershire, UK.
Posts: 1,138
Default Re: Dynamic microphone to phantom powered electret input

James - thank you s much for the guidance there over the difference between phantom power and "plug-in" power, this is a very useful distinction.

Cheers, Scott
__________________
www.scottbouch.com
ScottBouch is offline   Reply With Quote
Old 18th Apr 2024, 11:03 pm   #22
ScottBouch
Octode
 
ScottBouch's Avatar
 
Join Date: Feb 2013
Location: Leicestershire, UK.
Posts: 1,138
Default Re: Dynamic microphone to phantom powered electret input

SUCCESS!

I now have my Beyerdynamics DT109 headset with 200R electret mic working with a Sound Blaster X-Fi Go Pro USB sound card.

It's just a veroboard prototype at the minute, I'll build it properly with an enclosure soon, but this will work for tomorrow's Teams calls!

I followed the idea presented by Radio Wrangler, and built a very simple NPN/PNP Sziklai pair amplifier, using just some general purpose transistors that literally were just knocking around on my bench. So I'm using a BC635 (NPN) and a BC636 (PNP).

It was necessary to bias the pair's Base connection upward using two resistors as a potential divider to turn on the first transistor (the NPN), utilising power from the sound card mic input, which as pointed out earlier also added a little negative feedback.

As the base of the pair now has a DC bias applied, I also added two electrolytic capacitors back-to-back between this point and the dynamic microphone to block the DC. I did a quick check on the capacitor value used (4.7 uF back to back), and the high pass filter it forms with the lower resistor (10 K) of the potential divider has a cut-off frequency down at 3.4 Hz, so no loss of audible range.

After fiddling around a little with resistor values, it worked like a charm!

No magic smoke was released in the making of this little circuit!

Thanks to all who have thrown ideas into the mix and provided good advice, it's nice to have been able to talk this idea through and kick it around a bit before starting to prototype, it gave me some idea of a direction to go in. Thanks Mods to for running this great forum!

Cheers, Scott.

(PS: please don't close yet, I'll add some photos over the coming days)
__________________
www.scottbouch.com

Last edited by ScottBouch; 18th Apr 2024 at 11:14 pm.
ScottBouch is offline   Reply With Quote
Old 19th Apr 2024, 12:04 pm   #23
ScottBouch
Octode
 
ScottBouch's Avatar
 
Join Date: Feb 2013
Location: Leicestershire, UK.
Posts: 1,138
Default Re: Dynamic microphone to phantom powered electret input

The prototype amplifier attached, it has worked well in meetings today, there is a nice deep sound to this microphone, not tinny like some smaller ones.
Attached Thumbnails
Click image for larger version

Name:	1713524307912.jpg
Views:	91
Size:	80.1 KB
ID:	296554   Click image for larger version

Name:	1713524358216.jpg
Views:	87
Size:	80.3 KB
ID:	296555  
__________________
www.scottbouch.com
ScottBouch is offline   Reply With Quote
Old 19th Apr 2024, 12:09 pm   #24
ScottBouch
Octode
 
ScottBouch's Avatar
 
Join Date: Feb 2013
Location: Leicestershire, UK.
Posts: 1,138
Default Re: Dynamic microphone to phantom powered electret input

I alo just tried this with an Racal Atlantic aircraft headset with back to back noise cancelling dynamic microphone, and it was surprisingly good too!

The RAF headset microphone lost some of the deep rich tone, but was still very clear.
__________________
www.scottbouch.com
ScottBouch is offline   Reply With Quote
Old 19th Apr 2024, 12:39 pm   #25
Uncle Bulgaria
Nonode
 
Uncle Bulgaria's Avatar
 
Join Date: May 2016
Location: Cornwall, UK.
Posts: 2,363
Default Re: Dynamic microphone to phantom powered electret input

Well done Scott! That looks very neat and workmanlike. Now if you want, you've got the fun of trying to shoehorn it into a little box, or mounting it on the headphone strap perhaps. There's always another project...!

I always get comments on my 'air traffic control' headset on videocalls as it looks not dissimilar to yours, with Diabolical Artificer manufactured bracket to attach the period mic to the 'phones.

An advantage I've found with the USB audio interface, which your soundcard might be able to do as well, is the knob that controls the mix between the inputs. It's easy to balance microphone gain, other participants' voice through the headphones, and my own microphone input into the headphones for monitoring. I find it's a bit weird talking with over-ear 'phones on as without feeding some microphone back into the ears it's very easy to start shouting as I can't hear myself!
Attached Thumbnails
Click image for larger version

Name:	IMG_20240419_123225.jpg
Views:	73
Size:	76.2 KB
ID:	296560  
Uncle Bulgaria is offline   Reply With Quote
Old 19th Apr 2024, 1:11 pm   #26
ScottBouch
Octode
 
ScottBouch's Avatar
 
Join Date: Feb 2013
Location: Leicestershire, UK.
Posts: 1,138
Default Re: Dynamic microphone to phantom powered electret input

Hi Uncle Bulgaria, You have a very classic looking headset there!

Teams over the last few years has taught me to speak without any side-tone. I see that Windows offers an option to add side-tone with "listen to this device", I'll give it a try to see how it goes.

The USB sound card settings also offer an option apply side-tone. I tried this, but the sound reproduction in the headset is slightly delayed after my own voice, so I found it very confusing to speak with this tuned on! I ended up trying to shout above my own delayed voice! Perhaps the Windows function noted above may work better, I shall try.

I suppose the ideal solution for side-tone would be to remove all computing from the loop, and use a small dedicated headphone amplifier that can mix the headset mic signal with the computer output, no delays or echoes then! Maybe that's a project for another day!

For now I need to look into packaging this up neatly. I want to explore non-polarised capacitors for decoupling to save a little space (as I'm using two presently), I'm wondering if I could re-build this circuit to fit inside the back-shell of an XLR plug! I have the original 4-way XLR socket from this headset lead, and an XLR corresponding free plug to make an adaptor lead. I'd like to re-fit the original socket to the headset lead, removing the 3.5mm plugs (photographed) from the headset lead. I'd then use the 3.5mm plugs on a short lead from the 4-way XLR joint. Fitting the components into the rear of the connector would be brilliant! I may look into SMD transistors, but the decoupling capacitor may still end up being the biggest component, hence exploring non-polarised options for audio.

Cheers, Scott.
__________________
www.scottbouch.com
ScottBouch is offline   Reply With Quote
Old 19th Apr 2024, 1:16 pm   #27
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 23,009
Default Re: Dynamic microphone to phantom powered electret input

In our aircraft radios, we had a diode detector monitoring the modulation on the transmitter RF output, and it was this which we fed back as sidetone to the headphones. It creates the sensation you get talking into a live telephone compared to talking into a dead line. Working, it gives you confidence that what you are saying is really going out. I used a directional coupler so that the effect would be diminished even if some ******** had snapped the antenna off.

Agreed... you don't want the latency of software/dsp in a sidetone loop. Applies to speech or Morse.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 19th Apr 2024, 1:49 pm   #28
ScottBouch
Octode
 
ScottBouch's Avatar
 
Join Date: Feb 2013
Location: Leicestershire, UK.
Posts: 1,138
Default Re: Dynamic microphone to phantom powered electret input

Hi Radio Wrangler,

I've also seen a similar concept used in the PTR-175 and ARC-52 radios. In multi-seat (mid century) aircraft there is usually an intercom amplifier (typically an A1961), but in an emergency situation if the intercom amp fails, the radio can be switched in to circuit (often by a JB154) to provide the intercom function using the radio side-tone.

But interesting in your GA radios the side-tone was available when transmitting (this makes a lot of sense how you described it) but not in receiving operation? Would a multi seat GA aircraft use a separate amplifier for intercom purposes?

Cheers, Scott
__________________
www.scottbouch.com
ScottBouch is offline   Reply With Quote
Old 19th Apr 2024, 1:55 pm   #29
Uncle Bulgaria
Nonode
 
Uncle Bulgaria's Avatar
 
Join Date: May 2016
Location: Cornwall, UK.
Posts: 2,363
Default Re: Dynamic microphone to phantom powered electret input

Ah, that's known as 'side tone', is it? With this USB thing it seems to be doing what you're suggesting, in that it's hardware not software dealing with the audio. Twiddling the 'Input - DAW' knob changes the mix, and a switch on the back does +48V phantom power. As it lives on the desk by the PC, its bulk is not a problem and it's made a lot of Audacity and in/out audio things a lot easier than before, when I was using Windows's built-in adjustments.

Here's my very messy little box, but as it works I've never bothered to neaten the prototype!
Attached Thumbnails
Click image for larger version

Name:	IMG_20240419_134419.jpg
Views:	75
Size:	63.8 KB
ID:	296567   Click image for larger version

Name:	IMG_20240419_134513.jpg
Views:	80
Size:	58.1 KB
ID:	296568  
Uncle Bulgaria is offline   Reply With Quote
Old 19th Apr 2024, 3:22 pm   #30
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 23,009
Default Re: Dynamic microphone to phantom powered electret input

Both of the above, Scott.

Some people with a two-seater and a small panel just have the radio set-up to perform intercom duties when PTT isn't pressed with a vox-like mute stopping unnecessary noise.

With a larger aircraft a separate audio panel is common. It routes audio, drives more headsets, mixes in music as required and the radio does the comms. With the audio panel out of action, it connects things to the radio and you get basic intercom for the pilots' seats and sidetone when someone presses PTT. There are so many varieties of situations in GA planes and you have to cover just about them all... as well as restored warbirds.

We had a bit of fun trying to get people talking with different amounts of delay in sidetone, and in the end we had to do very little DSP speech-processing on Tx and the sidetone was all analogue from the RF output to the phones.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 21st Apr 2024, 9:36 am   #31
ScottBouch
Octode
 
ScottBouch's Avatar
 
Join Date: Feb 2013
Location: Leicestershire, UK.
Posts: 1,138
Default Re: Dynamic microphone to phantom powered electret input

Thanks David.

When you look at sidetone of traditional telephone handsets, they are elgantly simple, a carbon mic in series with earpice speaker, no chance of any delay there!

Thanks for your summary on GA aircraft, its always interesting to learn sonething new, as most of my experience is on Lightning, with a bit of Vulcan, Jet Provost, Hunter etc..

So, I have now had a couple of days using this headset at work in Teams calls, with super great success!

The noise cancelling microphoneis actually very effective too - I have been working in my garage, where I have a 19"rack mount server, ethernet swutch, etc.. running 24-7, the fans are quite noisy, but the microphone really cuts this noise out. I did some test recordings with Audacity, just to be sure it wasn't Teams filtering out the background noise, and was really inpressed with the clarity of voice (even with the mic pushed away from my face) and absence of background noise.

Top quality kit, its nice to have used this DT109 headset in original form, as I was tempted to change the microphone element in the boom for a modern electret type to solve the compatibility issue, but I'm so glad now that I made this amplifier instead.

Next step I think will be designing a miniature SMD PCB for the amp.

Many thaks again to all, Scott
__________________
www.scottbouch.com
ScottBouch is offline   Reply With Quote
Old 22nd Apr 2024, 2:06 pm   #32
m0cemdave
Octode
 
Join Date: Feb 2009
Location: Bletchley, Buckinghamshire, UK.
Posts: 1,228
Default Re: Dynamic microphone to phantom powered electret input

Congratualtions on getting it working.

DT109, and the single ear DT108, have been widely used for decades, for crew communications in the event production and entertainment industry.
One big advantage is that all parts are available as spares.
m0cemdave is offline   Reply With Quote
Old 22nd Apr 2024, 4:39 pm   #33
paulsherwin
Moderator
 
paulsherwin's Avatar
 
Join Date: Jun 2003
Location: Oxford, UK
Posts: 28,114
Default Re: Dynamic microphone to phantom powered electret input

It would be helpful if you could sketch your circuit out.
paulsherwin is offline   Reply With Quote
Old 23rd Apr 2024, 8:14 am   #34
ScottBouch
Octode
 
ScottBouch's Avatar
 
Join Date: Feb 2013
Location: Leicestershire, UK.
Posts: 1,138
Default Re: Dynamic microphone to phantom powered electret input

Hi Paul, yes, I roughly drew it up in KiCad last night, I need to smarten it up a touch, and I will share.

Thanks m0cemdave, yes, lots of availability, and still available new. They are expensive (even second hand), but I suppose that goes with the territory of professional gear. Sound quality-wise (speakers), they knock the socks off my Bose noise cancelling headphones! I was listening to Jefferson Airplane yesterday, and its like being in the same room as the musicians. I need to check the microphone sound quality against the Bose for comparison.

Cheers, Scott
__________________
www.scottbouch.com
ScottBouch is offline   Reply With Quote
Old 24th Apr 2024, 10:14 am   #35
ScottBouch
Octode
 
ScottBouch's Avatar
 
Join Date: Feb 2013
Location: Leicestershire, UK.
Posts: 1,138
Default Re: Dynamic microphone to phantom powered electret input

Here is the schematic of the little mic amplifier, with annotations describing parts of the design process.

Cheers, Scott
Attached Files
File Type: pdf prototype-2024-04-18.pdf (99.7 KB, 78 views)
__________________
www.scottbouch.com
ScottBouch is offline   Reply With Quote
Old 25th Apr 2024, 4:54 pm   #36
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 23,009
Default Re: Dynamic microphone to phantom powered electret input

It might benefit from a little decoupled resistance in Q1 emitter to improve the stability of DC conditions and make the output DC a bit more predictable.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 25th Apr 2024, 10:30 pm   #37
paulsherwin
Moderator
 
paulsherwin's Avatar
 
Join Date: Jun 2003
Location: Oxford, UK
Posts: 28,114
Default Re: Dynamic microphone to phantom powered electret input

Nice simple design, thanks for posting.
paulsherwin is offline   Reply With Quote
Old 26th Apr 2024, 11:06 am   #38
ScottBouch
Octode
 
ScottBouch's Avatar
 
Join Date: Feb 2013
Location: Leicestershire, UK.
Posts: 1,138
Default Re: Dynamic microphone to phantom powered electret input

Thanks guys for the positive comments.

David, thanks for the tip about Q1 emitter resistance - I'll be honest, I just referenced Wikipedia for the Sziklai pair diagram, which featured no Q1 resistance.

The circuit was just put together with what was literally on the bench at the time, and resistors selected via decade box and experimentation. I did do one little calculation for the capacitors to ensure that the high pass filter cut-off frequency wasn't in the audible range.

I've so far not had the time to poke my oscilloscope round it, only my DVM to set the voltage about right. It's functional, simple, but not glamorous.

In the next version, I shall try adding the resistance as you suggest. Thank you for the steer toward some elegance. I have found and wiil read this guide on emitter resistors in amplifier applications: https://www.electronics-tutorials.ws...esistance.html

Cheers, Scott
__________________
www.scottbouch.com
ScottBouch is offline   Reply With Quote
Old 26th Apr 2024, 11:22 am   #39
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 23,009
Default Re: Dynamic microphone to phantom powered electret input

If you've already been deep enough to know of the Sziklai, you ought to be able to handle Horowitz and Hill "The art of electronics" whichstarts off on the basics and builds gently. They are teaching how to do useful and effective electronics, not how to do full analyses. They also have a sense of humour.

Also for little audio amps withdiscrete transistors, there are a couple of articles on a stereo mixer by Hugh Walker in wireless world in the early 1970s. Hugh taught me a lot. He's an artful designer and you can see a lot of interesting things in his stereo mixer building blocks. He lives about 1 mile due east of me these days. He's one of the low frequency low noise gurus and he wrote the seminal paper on distortion in diode ring mixers.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 26th Apr 2024, 8:30 pm   #40
ScottBouch
Octode
 
ScottBouch's Avatar
 
Join Date: Feb 2013
Location: Leicestershire, UK.
Posts: 1,138
Default Re: Dynamic microphone to phantom powered electret input

Thanks David, that looks like an excellent book!

I'm keen to brush up on my analogue electronics as its been too may years since I carried out any decent projects. Free time is my biggest issue these days.. Young kids, wife house, full time job...you know how it goes.

One technical question about my circuilt you may be able to help me with: do I really need the back-to-back capacitors? Most similar decoupling colircuits I have seen just use one elctrolytic. I assume this answer may be related to the DC bias?

Many thanks again, Scott
__________________
www.scottbouch.com
ScottBouch is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 5:32 pm.


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 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.