# Power Supply Noise

##### Member
Power Supply Noise (in series)

I have a question about multiple power supplies in series and their noise.

For example, a linear power supply's datasheet might have -40dB PSRR. Does that mean that it will always lower noise by that much, or is there a point where input noise is unchanged if it is low enough, and is raised if it is even lower?

To put it another way, can you put multiple power supplies in series to get lower noise, or is there a point of diminishing returns? One could imagine stacking linear power supplies or zener followers at 11V, 9V, 7V, 5V. The same could be done with cap multipliers, "super" regulators, or any combination of power supplies that you can think of.

Thanks,
v

Edit: It occurs to me that multiple zeners would probably not help. But maybe multiple zeners *and* a cap across each zener would be good.

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#### DF96

##### R.I.P.
In electronics there is always a point of diminishing returns. Cascading regulators etc. may be helpful for someone carefully building precision instrumentation, but won't help much for audio. Use one, or two if you think it will help. Much better to design the audio circuit with good PSRR.

##### Member
Put another way:
If noise is low enough due to whatever fancy regulator, will a linear PS actually increase noise if put after the reg?

The same could be asked of a zener follower or cap multiplier

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#### AndrewT

##### R.I.P.
PSRR = power supply rejection ratio.

You measure the change in input and measure the change in output and find the ratio.
If your ratio is better than 0dB, then your supply has attenuated the input VARIATION.

#### DF96

##### R.I.P.
If noise is low enough due to whatever fancy regulator, will a linear PS actually increase noise if put after the reg?
I don't know what you mean, unless by "linear PS" you actually mean extra smoothing i.e. a low pass filter. Everything contributes thermal noise, but whether this is significant depends on the circumstances, so you need to calculate. Zeners can add a lot of noise.

Don't design by combining recipes; instead, calculate what you need and calculate what you have and see how they compare.

##### Member
PSRR = power supply rejection ratio.

You measure the change in input and measure the change in output and find the ratio.
If your ratio is better than 0dB, then your supply has attenuated the input VARIATION.

I am aware of PSRR.

The question, which I don't have the equipment to answer, is if there is a point where input noise for a linear PS is low enough where it doesn't attenuate noise and starts introducing noise.

Or maybe it just neither adds nor reduces noise however low the input noise is.

##### Member
I don't know what you mean, unless by "linear PS" you actually mean extra smoothing i.e. a low pass filter. Everything contributes thermal noise, but whether this is significant depends on the circumstances, so you need to calculate. Zeners can add a lot of noise.

Don't design by combining recipes; instead, calculate what you need and calculate what you have and see how they compare.

Linear PS would be a linear regulator like the LM317.

#### DF96

##### R.I.P.
Every linear regulator circuit attenuates the input noise and adds some noise of its own. If you cascade regulators then the first one needs good attenuation and the last one needs to be low noise.

However, in most audio circuits most of the noise comes from the circuit, not the power supply. Worrying about power supply noise seems to be a modern fad.

#### Triodethom

##### Member
Every linear regulator circuit attenuates the input noise and adds some noise of its own. If you cascade regulators then the first one needs good attenuation and the last one needs to be low noise.

However, in most audio circuits most of the noise comes from the circuit, not the power supply. Worrying about power supply noise seems to be a modern fad.
Agreed. The fixation may come from having a poor psrr and changes in the power supply being heard in the circuit when it not the power supply noise being reduced but the circuit is less excited by the power supply . Like the old joke Doctor it hurt when I do this . Doctor's reply do not do that. And the big issue has not been addressed PSRR.