What are the advantages and drawbacks of each op-amp attenuator topology ? Which one would you use ?
Resistor divider followed by a buffer :
Inverting op amp attenuator :
Electronic – Best topology for active attenuator
attenuatoroperational-amplifier
Related Solutions
Two power resistors can do this. For example, 8 Ω across the speaker and another 4 Ω in series with both would put only 1/4 of the output power into the speaker while still presenting a 8 Ω load to the amp.
Consider the power requirements of the resistors. In the example above, only 1/4 of the output power will go into the speaker, so 3/4 will heat up the resistors. Half the total power will be dissipated by the 4 Ω resistor, 1/4 in the 8 Ω resistor, and 1/4 in the speaker. If the amp dumps 40 watts, for example, then you need 20 W and 10 W resistors minimum, respsectively. In practise you want to get resistors rated for somewhat more than the maximum power you plan to have them dissipate. Those big ceramic wire wound types are probably what you will end up with.
To make life a bit easier use the switch wiper connected to the first resistor and the other switch contacts to individual resistors that connect to ground. Only one resistor gets grounded therefore there is no interdependence: -
simulate this circuit – Schematic created using CircuitLab
Best Answer
As the comments say, it depends on what you want to do. But I want to point out a dangerous thing about your second circuit: if you run with a gain of less than unity, the op-amp may oscillate.
Many op-amps are specified as "unity-gain stable" but they will indeed oscillate if your gain is less than unity. TL074 / 084 is an example.
Other op-amps such as the NE5534 are stable only with gain greater than 5. If you run a NE5534 with gain less than 5, it will oscillate.
There is a cure for this oscillation problem - you have to add noise gain to the circuit. You do that simply by adding a resistor from the op-amp (-) input to ground such that the gain of that resistor and feedback resistor is at least unity. Then you can have a larger input resistor (R2) for gain less than unity.
I ran into this exact problem almost 30 years ago and either EDN or Electronic Design did a good article on the problem some time after that. By then, I had already figured out what the problem was and fixed it but the magazine article went into a lot of detail that I hadn't thought about.