Bluecom and PC headsets

[boatman]

Bluecom is now out in the open and one of our first builders has tried to attach two 3.5mm jacks to the 8-pin Molex connector on the beltpack board. Unfortunately, the common grounding of this arrangement lets the magic smoke out of the headphone amp protection resistors (that's why they are there!), so we need a bit of help from those who are familiar with PC style headsets.

 

The 4-pin 3.5mm jack was added at the request of one of our members and the pin-out was described in this post. Since we didn't get any feedback about that pin-out we assumed it was OK.

 

Could anyone who has experience with PC style headsets please have a look at the Bluecom beltpack schematic and see if we need to make any changes to the connection arrangement for the front panel jack (currently 4-pin).

 

Incidentally, if anyone is needing a very cheap solution, I have had great success with this headset from CPC on Tecpro, Stonewood and of course Bluecom.

[timsabre]

I think we need to know how they have wired the 2 jacks. In theory there should be no problem - how is the common ground occurring?

 

However I think your 4 pin pinout is wrong, I think it is normally tip=headphone left, ring1=headphone right, ring2=mic, ring3=common ground

 

You can't do this with your bridge amp...you'll have to drive L+R with HP+, ignore HP- and lose some output drive.

[boatman]

I think we need to know how they have wired the 2 jacks. In theory there should be no problem - how is the common ground occurring?

 

However I think your 4 pin pinout is wrong, I think it is normally tip=headphone left, ring1=headphone right, ring2=mic, ring3=common ground

 

You can't do this with your bridge amp...you'll have to drive L+R with HP+, ignore HP- and lose some output drive.

 

The user has two jack sockets mounted on a metal front panel. The problem might be solved by using plastic sleeved sockets.

 

So, would the simplest solution be an adapter cable from a 4-pole jack plug to twin sockets?

 

I've searched for a twin stacked 3.5mm PCB mounting jack socket and didn't find anything so that's not an option as we don't have space for two side-by-side sockets.

 

It's a fairly minor routing change to connect both Tip & R1 to HP+ and leave the other pins as they are.

[timsabre]

Ah... the metal panel. So yes either plastic sleeved sockets, or wire the headphone socket to HP+ and GND rather than HP+ and HP-

 

Minijacks are unreliable enough without introducing an adapter cable, I wouldn't like to do that.

 

Mouser do a twin stacked 3.5mm socket: mouser part number 806-STX-4235-3/3-N, but it's £1.74 at 1 off

[cedd]

Would it be possible to make the existing socket so that it becomes just a headphone socket? Cut the mic track and short that part of the 4 pole socket to ground/sleeve with a jumper wire. Cut the headphone - track and short it to headphone + in a similar manner. Then imagining you plug a standard pair of headphones in, you've got headphone + on the tip and ring, with ground on the sleeve and second ring (which would be part of the sleeve on a normal 3 pole plug anyway).

 

Now straight away you've got a useable 3.5mm headphone socket meaning for users who aren't going to be wanting to use PC headsets you've got at least a socket where somebody sitting alongside you training or just "along for the ride" can plug in their headphones from their mp3 player and hear what you're hearing.

 

It also means that now you can safely wire to the mic pins on the Molex and mount a panel mount 3.5mm socket on the case or front panel without fear of toasting the headphone amp. This is a really simple job and allows you to mount the 3.5mm mic in wherever you like. Now you can use the split lead 3.5mm headsets perfectly happily.

 

 

 

 

I think that all makes sense! it did to me when I wrote it, but that doesn't stand for much these days!

 

 

 

[alistermorton]

I've been thinking along the same lines myself, so you're not the only one it appears to make sense to :-)

[boatman]

Ah... the metal panel. So yes either plastic sleeved sockets, or wire the headphone socket to HP+ and GND rather than HP+ and HP-

 

Minijacks are unreliable enough without introducing an adapter cable, I wouldn't like to do that.

 

Mouser do a twin stacked 3.5mm socket: mouser part number 806-STX-4235-3/3-N, but it's £1.74 at 1 off

That looks promising and £1.74 for a double jack socket doesn't seem excessive to me.

 

 

Would it be possible to make the existing socket so that it becomes just a headphone socket? Cut the mic track and short that part of the 4 pole socket to ground/sleeve with a jumper wire. Cut the headphone - track and short it to headphone + in a similar manner. Then imagining you plug a standard pair of headphones in, you've got headphone + on the tip and ring, with ground on the sleeve and second ring (which would be part of the sleeve on a normal 3 pole plug anyway).

Despite Tim's reservations about adapter cables, I think a short 4-pole jack plug to headphone socket adapter would be preferable to untrained hands applying scalpels to the PCB.

 

 

Now straight away you've got a useable 3.5mm headphone socket meaning for users who aren't going to be wanting to use PC headsets you've got at least a socket where somebody sitting alongside you training or just "along for the ride" can plug in their headphones from their mp3 player and hear what you're hearing.

That scenario hadn't occurred to me but sounds like a good idea. There's certainly enough wick in the headphone amp to drive two headsets.

 

 

It also means that now you can safely wire to the mic pins on the Molex and mount a panel mount 3.5mm socket on the case or front panel without fear of toasting the headphone amp. This is a really simple job and allows you to mount the 3.5mm mic in wherever you like. Now you can use the split lead 3.5mm headsets perfectly happily.

If custom panels are being built then two standard stereo jacks will do the trick. Mic & GND to one and HP+ (or HP-) & GND to both sides of the other one will do the trick.

 

 

I think that all makes sense! it did to me when I wrote it, but that doesn't stand for much these days!

 

Ah yes, the gradual encroachment of senior moments gets to us all in the end.

[chelgrian]

If custom panels are being built then two standard stereo jacks will do the trick. Mic & GND to one and HP+ (or HP-) & GND to both sides of the other one will do the trick.

 

Also note that many PC headsets will assume that there is a bias voltage on the mic socket. This is specified (go and read the Intel AC97 specifications...your head may explode) to be 5v and heavily current limited however the such is the drive to make things cheap that it won't be regulated and may dip significantly under load, 1.5v is not uncommon.

[boatman]

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This is specified (go and read the Intel AC97 specifications...your head may explode) to be 5v and heavily current limited however the such is the drive to make things cheap that it won't be regulated and may dip significantly under load, 1.5v is not uncommon.

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I located that document but didn't manage to find a reference to the mic biassing. A hint as to which section it's in would be good.

 

It looks like the 4-pole jack socket will need an adaptor cable and a 100uF capacitor to interface with PC headsets. If anyone isn't happy with that solution, please suggest how you think it should be connected. Input would be particularly helpful from those who already interfaced PC/Gaming style headsets to other devices. Alternatively, an obscure double 3.5mm jack (exclusive to Mouser) is a possibility, but will also need a mod to the front panel as well as the PCB. There is just enough room for a 100uF capacitor, a mic bias resistor and a second mic input capacitor on the PCB and I'm happy to make any requested changes to the layouts. The second prototype beltpacks, power unit and panels I have, all work perfectly with the 4-pin XLR headset connector and these very cheap headsets from CPC, so I won't be using PC style headsets.

 

Also, please note that the iPad/iPhone headsets follow the CTIA pin-out (see below), so the mic won't work in the current socket.

 

http://mydesk.myzen.co.uk/_Useful/BlueCom/130701-trrs.jpg

[chelgrian]

I located that document but didn't manage to find a reference to the mic biassing. A hint as to which section it's in would be good.

 

In AC97_r23 (http://download.intel.com/support/motherboards/desktop/sb/ac97_r23.pdf) it's in Table 6 in section 2.3.5 on page 18.

 

'Vrefout - Reference Voltage out 5mA drive (intended for mic bias)'

 

Also look on pages 20,21,23 and 24 of

 

http://www.formfactors.org/developer/specs/A2928604-005.pdf

[boatman]

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Also look on pages 20,21,23 and 24 of

 

http://www.formfactors.org/developer/specs/A2928604-005.pdf

 

Thanks, that's what I needed. It will require a change to the PCB and to the front panel to incorporate a double stereo jack socket and it might be easier to just change the schematic and PCB so that I can retain the 4-pole jack with the correct circuitry behind it. Do you have any idea how the iPhone/iPad headsets mic connection is biassed?

[chelgrian]

Thanks, that's what I needed. It will require a change to the PCB and to the front panel to incorporate a double stereo jack socket and it might be easier to just change the schematic and PCB so that I can retain the 4-pole jack with the correct circuitry behind it. Do you have any idea how the iPhone/iPad headsets mic connection is biassed?

 

Apple don't give you the specs unless you are an authorised supplier who have joined their program however FaberAcoustical have worked it out from opposite direction to the one you want, that is what specs a mic needs in order for an iDevice to recognise and blogged about it at

 

http://blog.faberacoustical.com/2009/ios/iphone/iphone-headset-input-options/

 

As far as I can tell Macs don't support mic level on the input port at all nor provide a bias voltage on it. However they do support the 4 pole connector on the headphone output and have characteristics such that any headset that works with an iDevice will work with the mac headphone jack. However I can't find any official documentation about this anywhere.