I'd like to use my beautiful old tube radio with my iphone, cd-player etc. There are gramophone connections behind the radio for simple banana jack mono connectors (mono). Any idea what do I need to do to get my source connected in mono? Do I need to worry about impedances or such? Do I need a preamp in between the source and radio?
Cheers, Matti
This is a bigger can of worms that you might think!
First off, is that "lovely old radio" earthed? I am not familiar with the mains supply in Finland. If it is NOT earthed, go no further! If it is, I would suggest a PAT test but so long as you have a good mains
earth fine.
Now "Gram" connectors normally disconnects the radio section from the audio amp, just the opposite of what you want! The solution, if you are able is to connect to the outer tags of the volume
control which will give you a "pre-fade" feed. Another snag however!
That V pot will be 1meg Ohm and so you will need a very high impedance buffer. You could use a single JFET source follower but easier is an op amp such as the TL071. You could put the buffer in a tin and battery power it (the lead from the radio must not be more than about 1/2mtr) or, again if you are 'tronics cute, power it from the HT supply in the radio with a resistor chain and a zener diode.
There is another way to skin this cat. A resistive load to replace the speaker and then an attenuated, transformer isolated feed from that.
I can do you a couple of scrawls if you like?
Dave.