(This is a new thread on an old subject, as I have updated the previous project. Also, I am aware that this circuit isn't intended to be a guitar amp and people will suggest not doing it and doing a different conversion etc. I'm doing it for my own reasons so bare with me please and thank you for the help).

Hey All,

I converted a Panasonic RE6137 AM/FM radio into a guitar amplifier by first feeding the guitar signal through a preamp(photo below), and then into the volume pot of the radio. (No Isolation from Mains) It sounds fantastic

I now need to add an isolation transformer and have a Shure 51A303 from a Shure Sm57. The preamp itself is separately powered from the radio's circuit, so I could add the transformer after the preamp and still be isolated, but wasn't sure which was a better option.

Any thoughts?

 

It is fine as long as you are not planning on using it with an electric guitar.
(For an AC powered guitar amp, it is critical that the amplifier is properly grounded to earth ground and that the guitar's ground is also connected to earth ground.)

For the pre-amp connected to the radio amplifier, you do not need a transformer. The output capacitor of the pre-amp is a DC blocking capacitor. Hence the signal is AC coupled to the radio.

If you wish galvanic isolation, you can use a linear opto-coupler or go RF.

 

I can barely see a 12V label on the output transistor's collector.
Why don't you use a 12VDC wall-wart supply, connect it across C25 (not sure exactly about the reference designator). Because this will be full-wave vs the actual half-wave, you will have lower hum and in addition it will be isolated from the line .
But I have a feeling that you would like to keep the original hot chassis and will dismiss the suggestion.

 

It is fine as long as you are not planning on using it with an electric guitar.
(For an AC powered guitar amp, it is critical that the amplifier is properly grounded to earth ground and that the guitar's ground is also connected to earth ground.)

For the pre-amp connected to the radio amplifier, you do not need a transformer. The output capacitor of the pre-amp is a DC blocking capacitor. Hence the signal is AC coupled to the radio.

If you wish galvanic isolation, you can use a linear opto-coupler or go RF.

Ok I get the capacitors blocking DC and understand that after the diode, it's only DC. But in the photo below, can you explain how AC can't follow the yellow highlighted path back to the input?

Is it because of that polarized electrolytic capacitor right before the first transistor or the resistors pull it to zero? Just want to understand why, thanks.

 

I can barely see a 12V label on the output transistor's collector.
Why don't you use a 12VDC wall-wart supply, connect it across C25 (not sure exactly about the reference designator). Because this will be full-wave vs the actual half-wave, you will have lower hum and in addition it will be isolated from the line .
But I have a feeling that you would like to keep the original hot chassis and will dismiss the suggestion.

Hey no, I appreciate that. Are you talking about powering the preamplifier with 12VDC? I am currently using a wall-wart for that. If that's what you mean

 

No he means powering the radio from a wall-wart.
But it would have to be about 18 V.

 

Oh that would be awesome. Let me take a look at that option.

And Mr Chips, for the sake of me understanding, can you explain that previous question I had? About the AC taking that path I highlighted and why that isn't an issue?

Thank you

 

Sorry, I cannot figure out the Panasonic radio schematics. It is not as simple as applying 18 VDC. Maybe it needs 18 VAC or something of that order. I just don't know.

One part of the circuit needs +12 VDC. Another part of the circuit needs -6 VDC.