# Design a audio amplifier for different load impedance

#### ZakVak

Joined Feb 21, 2023
1
Hello, In short I need some formula that adjust the gain according to different output load.
I am trying to design an audio amplifier. I need to adjust the gain for both an output impedance of 8 ohms and 150 ohms.
The sound is mono. The signal at my input is 315mV rms and at the output 3.1V rms
I'm using the TPA6120A2DWPR IC (I'd love to get better alternative suggestions)
The component is suitable for working with the 2 types of output load.
Where in the calculations do I need to consider the load at the exit?

Thanks for the help!

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#### Ian0

Joined Aug 7, 2020
10,276
An audio amplifier should have an output impedance as close as possible to zero.
Are you referring to the load impedance?
By how much do you wish to change the gain?
Also, don’t forget the the impedance of any loudspeaker changes with frequency. The “nominal” impedance is not something that can be measured. For instance, an 8Ω speaker may have an impedance of 80Ω at 20kHz.

#### BobTPH

Joined Jun 5, 2013
9,265
What do you mean by adjusting the gain? Do you want the power output to be the same for any load? If that is the case:

P = V^2 / R

So you would have to make the gain proportional to the square root of the load impedance. I.e. the gain at 150Ω would need to be sqrt(150/8) = 4.33 times the gain at 8Ω.

Is that what you want?

#### Ian0

Joined Aug 7, 2020
10,276
The TPA6120 seems like rather a pointless product to me. The op-amp could do the differential to single ended conversion, then all that would be needed to drive a 16Ω sub 1W load would be a complementary emitter follower on the output of the op-amp. That’s four transistors for stereo, and that’s a lot cheaper than the £3 for the TPA6120.
Am I missing something here? Is it the oxygen free copper?

#### BobTPH

Joined Jun 5, 2013
9,265
Re-reading the original post, I see that the TS specified an input voltage and an output voltage. That is all you need to compute the gain. The load impedance has no effect on the gain unless it s too low for the amp to supply the current.m, in which case you want a different amplifier design.

#### crutschow

Joined Mar 14, 2008
34,840
I need to adjust the gain for both an output impedance of 8 ohms and 150 ohms.
Why do you think you need to adjust the gain?
Won't the amp have a volume adjust pot?

#### Audioguru again

Joined Oct 21, 2019
6,782
3.1V RMS into 8 ohms is 1.2W. The IC will get too hot even when only one of its stereo channels is used. The datasheet shows a max output of 1W (when cooled properly) into a 32 ohms load with a +15V and -15V supply. The datasheet recommends a 16 ohms minimum supply.

The ICs are tiny. How will you solder their tiny pins? How will you solder the heatsink pad on the bottom of one of them?

3.1V RMS into a 150 ohms headphone is only 64mW which is probably not loud enough and will cause clipping distortion.

#### jlm1948

Joined May 19, 2014
19
If the goal is constant power, not only the gain must be adjusted, but also the power rails.
For example, 1W into 4 ohms means 2V rms, or 5.6V peak-to-peak, so the power voltage must be more than that.
The same power into 150 ohms means 12.25 V rms, or about 35V. Even if the topology uses a balanced output stage, that's minimum 17.5 V rail. All this supposing perfect lossless output devices, which is never the case.