11-04-2015, 02:09 PM
The LM386 is a reasonable solution, but do be aware that many of these eBay amplifier modules using other ICs - including the last one linked - are class D. This means they are essentially a switched-mode power supply, and will generate a lot of high frequency noise. Not the best plan for radio reception, so make doubly sure it's switched off when you're not using the line input. The outputs are often "bridged", which means that there are two amplifiers per channel, and the loudspeaker is driven differentially to double the available voltage swing. Very commonly done with conventional non-switching amplifiers, of course - it's been the norm in car audio for decades. But that is why the negative output pins can't be connected together.
Of course, there is no need to connect the outputs of a module together to make mono - simply do this ahead of a single LM386 module. Option 1 in the first post of this thread. Simple!
It's interesting that these cheap modules are using ICs intended to drive loudspeakers. This requires considerable current, so the output stages of chips like the LM386 are designed with this in mind. But clearly the input of a radio does not require any current to speak of - all it cares about is the voltage is sees at the input terminals. So the current capability of an LM386 is in effect "wasted" - it doesn't do any harm to have the ability, but it'll never use it. It's worth being aware that some audio power amplifiers can misbehave mildly when working into a high impedance, so something like a 100 ohm resistor connected where the loudspeaker would go might be worth trying in the event of strange symptoms.
Personally, I would just mount an op-amp on a scrap of Veroboard for this. Something like a 741 would be perfectly adequate - it's not a dedicated audio op-amp by any means, but it'll still be better than the audio systems contained within any valve radio. It's interesting to note that simple "pre-amp" modules aren't so commonplace on eBay - clearly the sellers are responding to market need, and the market appears to require more power amplifiers than pre-amplifiers. There is one module that caught my eye, but it's more expensive than the LM386 modules - yes, the objective performance will be much better, but that's irrelevant in the context of a valve radio - even a good one. Also, the module is designed to run off the split voltage rails that exist within a hi-fi amplifier - it has on-board voltage regulators to take the split supplies and drop them down to something the op-amp is happy with - convenient for the target user, but not so helpful for us. Anyway, for interest, here it is: http://www.ebay.co.uk/itm/NE5532-Amplifi...ltDomain_0&hash=item3367a893a0 - and another: http://www.ebay.co.uk/itm/NE5532-Amplifi...ltDomain_0&hash=item3372d9ab8d
Of course, don't forget our friends at Velleman. Obviously not as stupidly cheap as the eBay offerings, but this is much more appropriate to the task, and you have the satisfaction of assembling it yourself: http://www.theelectronicsshop.co.uk/cont...uk/p7.html - I can't see what the op-amp is, but although they say 10V is the minimum working voltage, I bet it'd be fine on a PP3.
Of course, there is no need to connect the outputs of a module together to make mono - simply do this ahead of a single LM386 module. Option 1 in the first post of this thread. Simple!
It's interesting that these cheap modules are using ICs intended to drive loudspeakers. This requires considerable current, so the output stages of chips like the LM386 are designed with this in mind. But clearly the input of a radio does not require any current to speak of - all it cares about is the voltage is sees at the input terminals. So the current capability of an LM386 is in effect "wasted" - it doesn't do any harm to have the ability, but it'll never use it. It's worth being aware that some audio power amplifiers can misbehave mildly when working into a high impedance, so something like a 100 ohm resistor connected where the loudspeaker would go might be worth trying in the event of strange symptoms.
Personally, I would just mount an op-amp on a scrap of Veroboard for this. Something like a 741 would be perfectly adequate - it's not a dedicated audio op-amp by any means, but it'll still be better than the audio systems contained within any valve radio. It's interesting to note that simple "pre-amp" modules aren't so commonplace on eBay - clearly the sellers are responding to market need, and the market appears to require more power amplifiers than pre-amplifiers. There is one module that caught my eye, but it's more expensive than the LM386 modules - yes, the objective performance will be much better, but that's irrelevant in the context of a valve radio - even a good one. Also, the module is designed to run off the split voltage rails that exist within a hi-fi amplifier - it has on-board voltage regulators to take the split supplies and drop them down to something the op-amp is happy with - convenient for the target user, but not so helpful for us. Anyway, for interest, here it is: http://www.ebay.co.uk/itm/NE5532-Amplifi...ltDomain_0&hash=item3367a893a0 - and another: http://www.ebay.co.uk/itm/NE5532-Amplifi...ltDomain_0&hash=item3372d9ab8d
Of course, don't forget our friends at Velleman. Obviously not as stupidly cheap as the eBay offerings, but this is much more appropriate to the task, and you have the satisfaction of assembling it yourself: http://www.theelectronicsshop.co.uk/cont...uk/p7.html - I can't see what the op-amp is, but although they say 10V is the minimum working voltage, I bet it'd be fine on a PP3.
