I'm restoring an old tube radio from the 1940s, and wanted to add line inputs to it. It already has a gramophone input, that seemed to work very well with line level audio. I assume this is meant for a very different type of pickup than what is typically used today. There also doesn't seem to be any special equalization for that input.
So far, I've used my signal generator, which is happy with a wide range of loads. When having a look at where to put the new connectors, I measured the input impedance of the gramophone connection - and it was about 60 ohm! That seems very low to me. Is it common? How would modern gear act if connected to it?
So far, I've used my signal generator, which is happy with a wide range of loads. When having a look at where to put the new connectors, I measured the input impedance of the gramophone connection - and it was about 60 ohm! That seems very low to me. Is it common? How would modern gear act if connected to it?
What is the make and model of the radio?
I presume it is an AC only set and there are no safety issues in using the gram input?
I presume it is an AC only set and there are no safety issues in using the gram input?
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Many radios of that era had a short to ground switch on the phono input, to reduce hum when phono plug was not used or selected. To remove the short, a plug must be inserted all the way into the socket and phono input selected. Common impedances were in the megaohm range.
I read that some gram inputs were low impedance to match pre-war, high output, magnetic cartridges.
Such an input may be used directly with a line level input such as from a CD player.
Alternatively, as pcan has said, if the input was intended for a crystal cartridge then it would have a very high impedance in the megaohm range.
Such an input may be used directly with a line level input such as from a CD player.
Alternatively, as pcan has said, if the input was intended for a crystal cartridge then it would have a very high impedance in the megaohm range.
Before Pickering-type "moving magnet" and crystal pickups were a form of "magnetic" which was more akin to telephone ear-piece technology. It was not real high impedance but not normally near 60r. It was universally driven into the 500k volume control in place of the AM diode detector.
So study your setup.
So study your setup.