Help with an aux input for a Dansette Chorister radio

[dcselec]

Hi. I have had a look through the forum and found most of the information I need. I have identified the wire going to the volume control. For my input I have two 120 ohm resistors for the stereo signal to mono but when I connect to the volume control there is talk of a voltage that may blow the MP3 player and to use a capacitor, Some say 1, some say 3. This is were I get lost. Could I not just use a diode to block this voltage? Could somebody please clarify what I need to do in simple terms please.

[Boater Sam]

If this player is a live chassis ie no isolating transformer, it is inadvisable to fit any extra connections. Have you the model number?

[dcselec]

Hi, thanks for the reply, it's a Dansette Chorister MW/LW 9 volt.

[crackle]

I would fit a 0.47uf polyester type capacitor between the volume control non earthy end and the 2 resistors you have.
The 120 ohm resistors you mention will probably be enough but personally I would have chosen 2 off 1k or 2k resistors.
Mike

[dcselec]

Thanks crackle
Just what I was after
I will take your advice on the resistors as well, better safe than sorry.

['LIVEWIRE?']

Once, when fitting an 'aux' input to a vintage car radio, I used 2 x 5k pre-set pots., and adjusted them until the external signal produced the same output, measured across the loudspeaker, as the radio, at the same volume setting. This is best done with a signal generator, which produces a steady audio tone at a known level, but can be done with spech/music sources.

[dcselec]

Thanks all for the reply's it works fine but I need a 3.5mm
jack socket with a set of contact that will disconnect the radio
tuner supply basically have a set of aux contacts does such a thing
exist a seen some post were there called normalizing contact but
can not find any for sale

Thanks

[Lucien Nunes]

You can definitely get 1/4" types with aux contacts that are electrically isolated from the jack contacts, single or double pole changeover. E.g. Stereo 1/4" DPCO jack at CPC But I don't think there's a 3.5mm equivalent, it's just too small to provide the necessary movement.

'Normalling contact' is just studio-speak for the normally-closed contacts on a regular jack socket, that open when you insert the plug. In studio patch panel applications they provide a 'normal' routing in the absence of a patch cord.

[Herald1360]

http://uk.farnell.com/cliff-electron...ack/dp/8044961

and several others from Farnell will do what you need.

http://cpc.farnell.com/cliff-electro...l%2BCategories

is same part at CPC

[Lucien Nunes]

I understood that what the OP was after was a socket where the switch contacts were electrically unconnected to an inserted plug. This allows an attenuator to be inserted between the incoming source and the signal line that is being interrupted.

['LIVEWIRE?']

I have sometimes used 3.5mm stereo headphone sockets when fitting 'aux-in' connectors to car radios. These always incorporate a switch, operated by inserting the jack plug, which can be used to disconnect the internal audio (i.e. radio) signal, and the two series resistors which, in the original application, are used to attenuate the signal from the amp. can be used as attenuating/combining resistors for a stereo to mono source, although higher values than those originally fitted will almost certainly be needed.
I salvage these from scrap radios, etc. To the best of my knowledge none of the usual suppliers stocks 3.5mm stereo headphone sockets, unless anyone knows differently!

[Nuvistor]

Possibility?

http://www.brimal.co.uk/plugs-socket...-mounting.html

['LIVEWIRE?']

That socket appears to be the same as those stocked by CPC and others. If it is it won't have the switch, electrically independant of the switched conacts in the socket proper, which is used to switch signal sources by insertion of a plug. When I don't have a switched earphone socket, I fit a 'Radio/aux' changeover switch to allow the user to manually select signal sources.

[Herald1360]

To disconnect the audio feed from the detector, you don't need isolated contacts. Disconnect detector source from volume control and route to fixed contact on socket "tip" connector. Connect socket "tip" spring contact back to volume control. Connect socket "sleeve" spring contact to radio 0V ground.

Put the combining resistors inside the cover of the 3.5mm mono jack plug on the lead from the external source. Connect both resistors to tip and other ends of resistors to the L & R signal leads. Connect signal lead shield to sleeve of jack plug.

[dcselec]

Hi, thanks for all the replies. I don't think its going to work as I need it to, without an isolation switch, which is a shame.
Herald1360, yep I understand what you're saying, but then if the input lead is substituted with a pre-made one we have a big problem, as the resistors are inside the lead. Thanks, will keep trying.

[Herald1360]

I've been fretting about this...

How about:

Get a 3 pole switched jack socket and fit to radio. Connect tip and ring fixed contacts together and reroute the output from the IF module to them instead of volume control. Connect each tip and ring moving contact to the top of the volume control via a 120R resistor. Connect sleeve moving contact to set 0V.

Now the player can be connected via a standard TRS stereo jack cable. When player is plugged in, L&R are combined by resistors and fed to volume control and signal from IF module is disconnected by the moving contacts. When the plug is removed, IF module audio output is fed to the volume control via two 120R in parallel. This extra resistance is too low compared with the 5K of the volume control to have any noticeable effect.

That's an engineer's solution rather than a scientist's- good enough for all practical purposes even if technically iffy.

[dcselec]

Thanks Herald1360
Yep that would work. The person that owns the radio has asked for a switch and jack socket to be placed inside the radio so there is no damage to the case. He is happy to remove the back to use this function, Thanks all for your input, problem sorted