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Thanks for the article by Chuck Gloman in the Sept. 11 issue that describes how to connect MP3 players to the turntable input of old tube radios (“Create an MP3-to-Phono Cable”).
As a collector and restorer of such devices, I really appreciate that this topic is being discussed. In countries like ours (Switzerland), for example, all AM stations have already been switched off, and the FM transmitters will soon be taken out of service, so that there will be no other solution for the layman to play his old radio than to connect a modern player.
It is of course true that many older tube radios are not stereophonic and therefore only have a turntable input for a mono sound source. In order to obtain such a one-channel sound signal, the author proposes a two-pole 1/8-inch jack plug (Tip = Audio, Ring = Ground) and plugs it into the stereo output socket of his MP3 player. This socket is for the exclusive use of three-pole jack plugs with tip (left audio), ring (right audio) and shaft (ground). If you insert a two-pole jack plug, its tip connects to the left output channel but the shaft connects to both the right channel and ground of the output jack.
This has the following disadvantages: First, the MP3 player’s built-in right channel output amplifier stage is shorted to ground. This can lead to higher power consumption and higher operating temperature of the player and increased distortion also in the left channel.
Second, the sound signal arriving at the record player input of the old radio consists only of the left channel of the sound source. The right channel and therefore practically half of the original event are missing. This results in an unsatisfactory reproduction.
The author rightly observes that most MP3 (and smartphones, I might add), when connecting the headphone output with a standard cable to the record player input of the old radio, produce no sound or only a very quiet one. He suspects the cause to be an “impedance mismatch.”
An impedance mismatch usually means that the output impedance is too high compared to the input impedance of the destination and that the output amplifier stage of the sound source is overloaded, distorted, automatically switched off or even destroyed.
In the present constellation, however, there is no such mismatch: The output impedance of the sound source is very low (a few ohms at most), and the input impedance of the record player input of the old radio is very high impedance (at least 100,000 ohms). As soon as the input impedance of a consumer is at least 10x higher than the output impedance of the sound source, there is no impedance mismatch.
So in our case an impedance mismatch cannot be the reason why no sound can be heard.
The actual reason is that in many mobile devices the headphone amplifier output stages detect whether a load is connected. Headphones have a low impedance (often 32 ohms). Only when such a low-resistance load is detected at the headphone output does it even switch on. However, the turntable input of an old tube radio has such a high impedance that the load is not recognized by the player and the player therefore does not emit the sound signal but remains silent or very quiet.
From the above it follows that a connecting cable between the headphone output of a modern mobile device and the mono turntable input of a tube radio must meet the following requirements:
- The left and right stereo channels of the source must be mixed together to mono.
- The mixing must be done in such a way that the two output channels do not interfere with each other (i.e. the channels must not simply be connected together).
- Each of the source’s two output channels of the source on the headphone jack must detect an impedance of around 32 ohms in order to be activated.
I have drawn a corresponding cable in the accompanying schematic.
The author responds
Chuck Gloman replies: Mr. Salis is very well informed, and his reasoning valid. I have since replaced the two-pole male 3.5 mm plug with a three-pole, for the exact reasons he stated. Adding two additional 33-ohm resistors to the mix is beneficial and well-illustrated in his drawing.
I’ve learned almost everything from professionals who are much more knowledgeable than I am, and I’m continuing to learn. Thank you, Mr. Salis, for pointing to an improvement to my initial concept. This is the first step in building a better mousetrap.
