You are confusing terms. Maximum power is one thing and gain is another. Although they are relationed, gain can be limited ir fixed with external resistor, while power depends on the load resistance and the power supply. Obviously the active device must conduct the current demanded by the load safely, but the device doesn't create any power.
I've got a few LM12 and all I will say is that you can design in EXACTLY the same way as any other operational amplifier IC. You can change gain by altering the ratio of the input and feedback resistors. The protection diodes shown in the datasheet for the device should be used. It makes a pretty good regulator too! I.e. change the 3k3 to 12k and gain will increase accordingly.
From 4 times gain Volt for Volt, to 13 times. Say for 60 Watts at max signal input (when signal is 0.7 V RMS), then you need 22 Volts across 8 Ohm loudspeaker. 22/0.7 = 31.5 times. Increase the 3k3 resistor to 30k (nearest common values are 27k or 33k). Either should do it.
If your source is a mobile phone, or anything operating on 3.3V or 5V then you may need more gain. Too much gain and you'll start to have issues with the somewhat low GBP of this device.
From 4 times gain Volt for Volt, to 13 times. Say for 60 Watts at max signal input (when signal is 0.7 V RMS), then you need 22 Volts across 8 Ohm loudspeaker. 22/0.7 = 31.5 times. Increase the 3k3 resistor to 30k (nearest common values are 27k or 33k). Either should do it.
If your source is a mobile phone, or anything operating on 3.3V or 5V then you may need more gain. Too much gain and you'll start to have issues with the somewhat low GBP of this device.
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Gain and output power are two different things, and there are also different types of gain.
What you're talking about here is voltage gain, which is simply the ratio of the change in output voltage to the change in input voltage, dVout/dVin.
Likewise, there is also current gain, dIout/dIin, and power gain, dPout/dPin.
The output stages of most solid state power amps have negative voltage gain, because they are emitter or drain follower circuits which have a slight voltage drop between input and output. Most of the voltage gain in a solid state power amplifier is in the VAS circuit that precedes the output stage.
What the output stage does have though is a large current gain. The power gain is the product of voltage gain and current gain, so while they may have a voltage gain of something like 0.95, when you multiply this by the large current gain you get a large power gain.
The voltage gain will determine how much input voltage an amplifier will need to drive a given amount of power into a given load impedance, the power an amplifier is capable of is determined by the amount of voltage and current it can safely drive into a given load.
What you're talking about here is voltage gain, which is simply the ratio of the change in output voltage to the change in input voltage, dVout/dVin.
Likewise, there is also current gain, dIout/dIin, and power gain, dPout/dPin.
The output stages of most solid state power amps have negative voltage gain, because they are emitter or drain follower circuits which have a slight voltage drop between input and output. Most of the voltage gain in a solid state power amplifier is in the VAS circuit that precedes the output stage.
What the output stage does have though is a large current gain. The power gain is the product of voltage gain and current gain, so while they may have a voltage gain of something like 0.95, when you multiply this by the large current gain you get a large power gain.
The voltage gain will determine how much input voltage an amplifier will need to drive a given amount of power into a given load impedance, the power an amplifier is capable of is determined by the amount of voltage and current it can safely drive into a given load.
> Why is the amplification factor so low
The LM12 is NOT a really clean wide-band amplifier. Lower gain, more NFB, less THD, more bandwidth.
So how low can we go? A standard +/-15V op-amp in front can do 10V peak swings. The LM12 can approach 40V swing. A gain of 4 is about as low as we can go.
"Normal" sources and preamps can do say 2V swing. So this op-amp booster before the LM12 should have a gain of about 5. Total gain of both is 5*4 which is 20 which is about right for typical hi-fi systems.
Can you even buy a LM12 today?? They were hard to get in the 1980s. That package hasn't been made in a decade. There's some military and industrial need for LM12 and I would assume they sucked-up all the inventory.
The LM12 is NOT a really clean wide-band amplifier. Lower gain, more NFB, less THD, more bandwidth.
So how low can we go? A standard +/-15V op-amp in front can do 10V peak swings. The LM12 can approach 40V swing. A gain of 4 is about as low as we can go.
"Normal" sources and preamps can do say 2V swing. So this op-amp booster before the LM12 should have a gain of about 5. Total gain of both is 5*4 which is 20 which is about right for typical hi-fi systems.
Can you even buy a LM12 today?? They were hard to get in the 1980s. That package hasn't been made in a decade. There's some military and industrial need for LM12 and I would assume they sucked-up all the inventory.
If the GBW of the op amp is low you don’t want to use it in a high gain mode. That goes for any op amp, even 741’s and 358’s. Set it to a gain of 1000 and the HF roll off is quite noticeable (less than AM radio station). I think the idea with things like the LM12 is to boost the output of another op amp. Gain of 4 on a 40 volt supply, which is driven by something like a 5532 to get the rest of your gain.
Another issue with limited GBW amps is they are often quite slow. You may only get full power (voltage swing) up to a few kHz. I’ve heard op amp based phono preamps slew limit, and the result is not good. And that’s not a lot of voltage swing. Sounds like the stylus mistracking (seriously it does). You certainly don’t want that in your power amp.
Another issue with limited GBW amps is they are often quite slow. You may only get full power (voltage swing) up to a few kHz. I’ve heard op amp based phono preamps slew limit, and the result is not good. And that’s not a lot of voltage swing. Sounds like the stylus mistracking (seriously it does). You certainly don’t want that in your power amp.
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