I have this radio



which has been modified to take the audio signal from the input to the volume control.

My desire is to feed this radio into a solid state MPX decoder and drive a small T amp along with a pair of vintage Automatic Radio speakers.

The problem is I am pretty sure this radio has a de-emphasis circuit. I fed it into my Ampex 520 MPX decoder and got muffled highs. I will need the schematic to see if it has a de-emphasis circuit and then remove it.

All will be powered by a 12 volt rechargeable battery with a 9 volt regulator circuit for the radio. I will need to build a charging circuit for the battery though. I plan on possibly using an external 12 volt power supply and will need to make a circuit that will switch the battery to a charging circuit and still allow the radio to play whenever the unit is connected to the 12 volt power supply. I assume I can just use a resistor in series with the battery to allow the recommended charging current to the battery, but I will need an automatic switching circuit though which could be done via a diode in series with the power supply and circuit so that only the power supply will turn on the relay and switch the battery to the resistor. That would reduce the available voltage to the circuit by .7 volts or less while battery is charging, but that should be ok. I could always use a 16 volt battery and a 12 volt high current regulator circuit along with a 16 volt power supply. That way I would have 12 volts regulated and the radio could play longer between battery charges. I have no clue how much current a T amp would draw though. I suppose I would only need around 10 WPC as that is about what the speakers I have can handle.

Should be an interesting creation when I am done. Best part of this is I can even feed the AM audio signal through the MPX decoder and while not stereo it will play in both speakers.

I have built and used several T Amps. They have sound quality as good as a good vacuum tube amplifier. You need a quality tuner as well. How about getting an FM Stereo Walkman or similar unit?
There are many T amp kits available from 41Hz.com (I have no connection with them) I used the AMP6-BASIC-SNEAKY http://shop.41hz.com/shop/item.asp?itemid=23&catid= because I could mount it inside a diecast aluminum enclosure from AES and bolt the chip to the side (with heatsink grease) for a heatsink and it will never overheat. I used transformers to match the output to large outdoor horn speakers for speeches, and it can be heard for a block in each direction. I use 12 volt sealed lead acid batteries from old UPS units, and the current draw is very little, since average power is 10% of peak power. I charge it with the power supply from a discarded emergency light, although the old UPS that supplied the battery can also charge it. Leave the UPS turned off and it will still charge the battery if it is plugged in.
Don

I will post pics later.

I wound going with a Dayton Audio DTA-2 T amp and am satisfied. About the time the one amp came in I found an 8 ohm version of the same Automatic Radio speakers so I got another DTA-2 T amp to drive the other speakers.

I put both in one small project box from Radio Shack and just use the Astron power supply as the required battery would be rather large and heavy.

My plan when I have the cash is to get a Pro Ject tuner box http://box-designs.com/main.php?prod=tu ... ce&lang=en and use that for the tuner. They even make an iPod dock http://box-designs.com/main.php?prod=do ... ce&lang=en which I could eventually incorporate into the design. I could also get this preamp http://box-designs.com/inhalt/en/manual ... eboxse.pdf

Let me know how those amps work out, I have a pair of them I'm planning to use as small-time V/H amplifiers for modifying TV circuit deflection coils into audio scopes. I haven't tried them yet, though, been too busy.

I would recommend not using them for that purpose unless the impedance of the deflection coils does not go below 4 ohms.

I'd probably throw a 20W 10 Ohm resistor in there...this is more of an art project than anything functional. Lots of tech-art sites out there advocate just hooking up an external amp...my idea is to let it take a line in.

Anyway, thanks for the tip, didn't think of that before.

Maybe you should try a 15 watt 5 ohm resistor. That way you will have more voltage for the coils.