Hi, this might be a bit strange but I sometimes put a audio input jack on restored radios to make them a bit more useful. I just feed the signal into the high side of the volume pot via a switch. The necessary resistors and a DC blocker cap are also fitted.

Question is... Is there a simple way to boost the signal from the MP3 player/phone as it requires the radio volume to be turned up a fair bit to compensate and when switched back to radio the volume is way too high.

Thanks

Is there a simple way to boost the signal from the MP3 player/phone

There are commercial mini amps for MP3 players (Google), or you could buy something in kit form such as:

https://www.jaycar.com.au/the-champ-0-5-watt-audio-amplifier-kit/p/KC5152

It actually is preferable to have the signal go into the grid, or cathode of the first AF: Where the volume control is in the circuit matters. Generalising: It takes around 1.5V to drive the OP valve.

It is hard to find out what the OP impedance is or how the earphones are driven? I have heard 33 Ohms mentioned, but the question is are they in the collector circuit of a transistor and doing things different changes the load?

MP3 is going to be stereo, how are you re-combining it to mono & perhaps your method is the problem.

You may need to use two transistor radio OP transformers In reverse to recombine the stereo & up the voltage. Phasing is important. That will provide isolation & you will likely still need blocking caps to stop bias issues. Especially if the set is back biased.

More info needed.

GTC... Using a mini amp was what I was thinking. There are mini amp boards from china for under $10, the Jaycar one looks good though, food for thought, thanks.

Marcc... I combine the stereo into mono via 2 1k resistors then to ground via a 10k resistor mimicking the load the headphones would normally present to the player. The signal then goes through a 0.1 μF cap to the volume pot or if fitted, into the PU sockets. If I feed the signal into the OP grid the volume control will be bypassed.

I was just wondering if there was a simple way to passively increase the gain from the player. Connecting via Bluetooth helps.

In many cases the signal is fed as said into the grid of the 1st AF to get it to amplify the signal before it gets to the volume control. Several low impedance magnetic pickups went into the cathode of the first AF (cathode follower). Using a transformer/s the right way gives a voltage gain. That is still being done by the Americans & was done in coupling valves in the early days of radio. It cannot be overlooked as a solution & is cheap. You don't need megawatts to drive a valve, you just need volts.

WiFi has been used but that does not avoid impedance matching. It would be interesting to see if you get a bigger voltage swing if you raised the 10K. As noted one needs to see the circuits and actually know the OP circuit of the MP3 player & circuit of the Radio. An Oscilloscope may be useful in seeing what the signal amplitude is and if it is distorting.

On does not use a PP transformer. The normal thing is to arrange earth to be in the middle with each transformer correctly phased. There are circuits kicking around on the American Forum as they're constantly doing it.

Hi Marcc, sorry been away.

You are talking way above my level of competence but the radio I'm looking at at this time is a Philips Jubilee model 123.

https://www.radiomuseum.org/r/philips_123.html

I have injected the audio between the second IF and the high side of the volume control. I can't see how it can be input any earlier than that can it?

I have two of these Jubilee radios and the volume is low on both so it's set on about 80 to 90% volume when the MP3 input is used. Both radios are the same volume wise even using them as receivers although the volume setting is more like 50%.

Lifting the 10k has no effect at all.

Looking at the circuit I have in AORSM That circuit is fraught with danger. The volume actually feeds the first AF, so something is seriously wrong with your method.

Are you using a chassis mounted metal plug like a 3.5mm phono? Stereo or mono? I need to see preferably on a circuit just what the wires of the MP3 are connected too. There is a reason as to why .............. I can send a copy of my circuit.

If you look at your circuit C25 should be 10μF 40V. It may have a different voltage if its been replaced. It goes from the transformers High voltage centre tap to chassis. Check that the positive end of that one only actually goes to chassis.

Tippy: Another option you might like to consider is one that some of us here employ. We use ultra low power AM transmitters to broadcast MP3, etc, sound to all AM receivers in the household. It involves tuning the transmitter to a spot on the dial not occupied by a local AM station.

Mine is a valve based transmitter, but there are plenty of solid state designs out there.

The problem as far as I see it is probably related to the way it is coupled: This is why I asked for more info.

If you get it wrong, you can actually wipe out the bias. Not knowing the OP circuit of the MP3 is also not helpful but we can work around that. No matter how we get signal to it, it still has to be coupled the right way.

GTC, I've tried AM transmitters but my house is steel framed and transmitter and receiver need to be in close proximity wiping out any advantage. It could be that I need a better quality transmitter, do you have any suggestions?

Marcc, The centre tap cap is 10μF 60V and installed correctly, positive to ground. Having the 10k resistor grounded or not is not affecting the bias it stays the same regardless. I have - 5 on the 6M5 and -1 on the two 6N8s. I have no data on what it's supposed to be.

I will send you a copy of the circuit for the input jack to your email.

You can see the stereo is mixed together through 2 1k resistors then to ground via the 10k.

Yes, please send me your circuit, my email is open.

My circuit has voltages its a case of extracting it later. The bias voltage of that set like many Philips sets is developed across the resistors that C25 is across. If the set is not running properly that voltage will be wrong. I have an old Philips with a Magnetic PU

It may be a case of looking at a similar Philips set with a turntable & see how they fed it , often that involves a switch. Its the horrid 6N8 that we need to match to. However check the grid resistors on the 6M5. For some reason on many valves especially 6V6 & tuning indicators, These are rarely good.

Marc

I did compare other Philips circuits with PU inputs and they connect to the pot high side as I have done. The Philips 132 is basically the same circuit as the 123 with the addition of SW.

The only difference I can see is they appear to ground the output from the second IF coil through the GND contact at the PU input. Normally I would expect it has a link across the PU inputs if not connected to a phono but that my not be the case with this design???

I am going to re-check and change the resistors as required.

Thanks

Tippy: to be legal, these transmitters must be very low power so that they don't affect listeners outside of your property. I think your steel frame will provide too much of a challenge to the power limitation.

GTC, yes, that has been my experience. I can't even get a broadcast signal in the house

On the positive side, you appear to have a very effective Faraday cage