14-04-2015, 05:05 PM
Quick reply before setting off home 
Honestly, I really wouldn't bother with the regulator.
First, the regulator will add to the overall current draw, so will shorten the life of the batteries.
Next, the supply voltage has no effect whatsoever on the gain of the circuit. That is set by the resistances seen by the op-amp.
The supply voltage does affect the maximum output voltage before clipping, but I'm guessing that the signal needed by the radio will be only a volt or so to develop maximum output. Op-amps can typically get within a volt or two of their rails, so when powered from a single 12V rail, you'd get 8 to 10 volts peak to peak (2.8 to 3.5V RMS).
The op-amp will have excellent PSRR (power-supply rejection ratio), so you could feed it from a noisy, unregulated power supply (mains tx, rectifier, smoothing capacitor) with no problems. I'm not saying that you'd want to do that here, but I'm just making the point that an op-amp really is very good of working independently of the power supply. Unlike a simple valve amplifier, of course
Whether you need 1 or 2 batteries? Well, no problem with 2 (or 3) other than cost. How about 1? Well, that depends entirely on the op-amp in question, as I alluded to earlier. Different op-amps have different behaviour at low supply voltages, so we'd need to check the data sheet of the chip supplied. As mentioned, I couldn't see which op-amp was part of this kit - it's not specified in the construction guide, and the on-line photos were unclear. So, tell us which chip you've got, and we'll have a think.
Of course, a variable bench power supply is useful here. Apply a sine wave to the input, adjust the amplitude so that the tone is loud via the radio, and then reduce the supply voltage until the sine wave at the output of the preamp becomes clipped on a 'scope (though in practice you don't really need a 'scope, because 1kHz tone with even mild clipping is very audible indeed). Having done this experiment, you can measure the supply voltage and you'll know the point at which your battery will need changing. It might not be 8V - it might be as low as 6?
Shameless self-promotion: I have a basic op-amp primer on my site. It was written a long time ago, and needs to be replaced with a much better version that I wrote for work, but it might be of interest: http://www.markhennessy.co.uk/articles/op-amps.htm
Hope all that makes sense - now I really must dash!
Mark
Honestly, I really wouldn't bother with the regulator.
First, the regulator will add to the overall current draw, so will shorten the life of the batteries.
Next, the supply voltage has no effect whatsoever on the gain of the circuit. That is set by the resistances seen by the op-amp.
The supply voltage does affect the maximum output voltage before clipping, but I'm guessing that the signal needed by the radio will be only a volt or so to develop maximum output. Op-amps can typically get within a volt or two of their rails, so when powered from a single 12V rail, you'd get 8 to 10 volts peak to peak (2.8 to 3.5V RMS).
The op-amp will have excellent PSRR (power-supply rejection ratio), so you could feed it from a noisy, unregulated power supply (mains tx, rectifier, smoothing capacitor) with no problems. I'm not saying that you'd want to do that here, but I'm just making the point that an op-amp really is very good of working independently of the power supply. Unlike a simple valve amplifier, of course
Whether you need 1 or 2 batteries? Well, no problem with 2 (or 3) other than cost. How about 1? Well, that depends entirely on the op-amp in question, as I alluded to earlier. Different op-amps have different behaviour at low supply voltages, so we'd need to check the data sheet of the chip supplied. As mentioned, I couldn't see which op-amp was part of this kit - it's not specified in the construction guide, and the on-line photos were unclear. So, tell us which chip you've got, and we'll have a think.
Of course, a variable bench power supply is useful here. Apply a sine wave to the input, adjust the amplitude so that the tone is loud via the radio, and then reduce the supply voltage until the sine wave at the output of the preamp becomes clipped on a 'scope (though in practice you don't really need a 'scope, because 1kHz tone with even mild clipping is very audible indeed). Having done this experiment, you can measure the supply voltage and you'll know the point at which your battery will need changing. It might not be 8V - it might be as low as 6?
Shameless self-promotion: I have a basic op-amp primer on my site. It was written a long time ago, and needs to be replaced with a much better version that I wrote for work, but it might be of interest: http://www.markhennessy.co.uk/articles/op-amps.htm
Hope all that makes sense - now I really must dash!
Mark
