This is where we go back to an oscilloscope. A first AF will likely need 0.03V or more, depending on the tube gain.

I opened up a Samsung telephone earpiece and its 30 Ohms & looks like a mini loudspeaker: That could mean that they are part of a collector circuit. If so with the circuit not being completed by the earpiece the transistor may not work?

Yes unfortunately I don't possess such equipment yet!

Hmmm I wonder if the headphones operate that way with this mp3....although with ground of mp3 connected to radio chassis would that complete the circuit anyway?

Or perhaps I should put a say 1m resistor from the combined mono output of the mp3 to ground would that perhaps 'trick' it to work?

It does seem to work going to the volume control though - just needs to be cranked up fully....would need a seperate transformer to get more volume I think....which means it probably isn't worth pursuing with this mp3.

Incidentally I just grabbed my iphone with an audio adapter and tried that - it's a pretty similar volume to the mp3 so I guess the old HMV just doesn't have enough amplification.

FYI,
At the Community Radio Station that I do the vollie techy work we run laptops and iPads and even iPhones directly into our mixing consoles.
Now these are NOT valve beasties but solid state desks that are Balanced inputs nominally at 600 ohms (Op amp input).

You can run Unbalanced stuff quite easily by simply ensuring that the + side or (HOT side) of the input goes to the audio source and the
-ve or COLD side is grounded.
I have at times taken the output from a small stereo radio that has a USB replay function and sent that into the L & R inputs directly from the earphone socket on the radio.
To block any DC from the radio upsetting the desk Op amps I terminated the two outputs with a 47K ohm resistor and then fed that into the L & R inputs via 0.01 μF capacitor and the desk was happy and the resulting sound that went to air sounded about as good as the CD's we also use.
In your case feeding into a high impedance valve input I would try using the 47K resistors across the applicable L & R outputs from the MP3 player with each going from them to the common ground (the sleeve connection on the 3 way plug).
Then take each through a 0.01μF cap and connect them BOTH to the valve set you are trying to play the thing through as you described above using the DPDT switch with each of the 0.01caps going to the switch connections such that as you flick the switch over BOTH caps then get commoned together and feed into the volume control.

BTW: The headphone O/P of most MP3 type players has heaps of signal level so you should have NO trouble getting the thing to work as you desire.

I do not have a circuit of the radio so cannot help there....must have a look at the PDF link as posted previously.

Having no circuit is always a pain and time consuming if it is to be reverse engineered. Which a lot of modern stuff can't be.

Based on pulling an earpiece open and finding it has DC resistance, we have a DC resistance of 30 Ohms which will actually be an inductive load in practice, as its a mini speaker; Virtually a modern version of a classic headphone driver. On that basis I am assuming that it is part of a collector, or it driven via a large cap.

If it requires a loading on the output to run then there is no alternative but to provide one. Theoretically you could tap an existing cable.

However, I will send Brad a drawing of a setup that may work.

Using the right 3.5mm or 1/4" plug. It will do the switching.

Working on just such a headphone driver at work this week.
This codec chip (about the size of a medium sized ant) is intended for mobile phones and wearable devices.
We are using the headphone driver in the chip to instead drive an amplifier with a differential input.

I've researched a lot of these devices and they all tend to run in what is known as Class H. This is a bit like like Class AB with added smarts to bump up the supply rail using a charge pump when needed to handle loud signal peaks.
We turn this feature off, we don't need it.

Anyway, the headphone outputs can happily drive a high impedance load, either balanced or unbalanced. So you don't need to load it with 30 ohms.

Downloaded the circuit from Kevin Chant's site and to my mind IF you have that DPDT switch inserted into the Non earthy side of the 0.5M Volume control when its set to the MP3 player it should work OK.

One position is radio and the other is MP3 player.

I would note that many American couplings are done with small audio transformers. They are capable of recombining the stereo, while offering isolation and providing more amplitude to drive the tube.


Marc

Document uploaded to Post 34.

Brad thanks.

Hi Guys thanks for so much great feedback!

I hadn't noticed - is there a way to subscribe for post updates?

As it stands I just left it connected to the volume pot through a dpdt switch which has mp3 input via a dc blocking cap (after the r & L channels were joined with 600ohm resistors).

When the switch is set to radio it reconnects the audio line to volume pot directly without the cap on that line anymore as radio wouldn't work with the cap on it).

Volume is still a long way down on mp3 though but I plan to sub out the resistors for 20ohm resistors next (but have to splice open my nice heatshrunk cable so haven't done that yet.....).

Overall the quality of sound is very average on mp3 so I haven't really persisted - my old Kriesler sounds so good that I find myself listening to that all the time over the HMV!

It's such a shame there is distortion on the local station but I have noticed that all radios that I tune into 1116 have a similar distortion (although hardly noticeable on the Kriesler mind you) so I think it is just the station that has the issues as opposed to the radio. From googling it seems the higher stations are more susceptible to interference so it's probably as good as it will get!

The fact that the distortion is only on one strong station, on one radio does suggests that its time to get the oscilloscope.

It looks to me like there is distortion in the audio, however if the speakers have been by substituted and that failed to eliminate the issue, then a pure tone is the only way it is going to be seen. If there is a wrong cap value other than looking. The Oscilloscope will show over coupling As those sets are normally popular, someone could have tampered with the IF can settings top & bottom, or the injection cap and its over modulating.

Hi Monaro,

Where are those 600 ohm resistors???

IF you have then ACROSS the outputs from the MP3 player then try taking them up to 22K or even 47K that I have used to feed MP3 players into the studio desks. 600 ohms will drag the volume down a LOT.

Lindsay

Thanks Lindsay, I will try that - I haven’t revisited the issue for a bit because the mp3 input sounded so ordinary!

I went with lower resistors thinking it would keep more of the signal…the resistors connect to each channel and then join (after the resistors).

I may try upping them next time I have it apart!

Hi Kakadumh can you explain to me how putting in higher value resistors will increase the volume. It intrigues me. I have heard this before but I don't understand it.
Carl.