Ultra Low Noise Linear AC to DC Regulator Design

Thread Starter

BjornMagnus

Joined Sep 4, 2022
5
Greetings,
I'm not a circuit designer. But I'm attempting to configure and assemble a DIY audio equipment design. I have looked all over for details on how to build a very low noise linear power supply that I would like to use with a SPDIF board which I intend to use in a music streamer. The regulated output of the supply should be 5VDC +10%/-2% and the load output current may vary from 75 to 120mA. I like the noise to be very low, say less than 200nV. The input voltage should be nom. 9VAC. I would be very pleased if someone could point me in the direction of a circuit diagram that I could use for this.
 

ronsimpson

Joined Oct 7, 2019
2,389
SPDIF board
I like the noise to be very low, say less than 200nV.
I have not done SPDIF recently but.... The SPDIF board must be poorly designed if it must have a very quiet supply. Same things goes for analog preamp that you might have. These audio circuits should have good power supply ripple/noise rejection.
Yes you should not have noise in the supply, but I think you don't need that good.
 

Thread Starter

BjornMagnus

Joined Sep 4, 2022
5
I have not done SPDIF recently but.... The SPDIF board must be poorly designed if it must have a very quiet supply. Same things goes for analog preamp that you might have. These audio circuits should have good power supply ripple/noise rejection.
Yes you should not have noise in the supply, but I think you don't need that good.
Thank you for your input. The supply is for powering the "clean side" of the Allo Digione Signature board for the RPi4B. I'm currently using batteries but wish to replace them with good circuit.
 

Externet

Joined Nov 29, 2005
1,932
If of any help, many years ago, broadcasting audio equipment I was in charge of manufactured at my workplace, used the PC supply it worked within. I was told its +/- 15V switching power supply frequency was synchronized with the sampling rate of the digital audio equipment in order to 'eliminate' noise. I have now, no access to details , schematics. Have the equipment in storage.
 

Ian0

Joined Aug 7, 2020
6,294
Thank you for your input. The supply is for powering the "clean side" of the Allo Digione Signature board for the RPi4B. I'm currently using batteries but wish to replace them with good circuit.
If its PSRR is that poor, throw it away and use something better.
 

MisterBill2

Joined Jan 23, 2018
13,128
Very low noise voltage regulation IS EASIEST DONE BY ADDING A BIT OF FILTER AFTER THE REGULATION. But what frequency range needs to be low noise?
 

WBahn

Joined Mar 31, 2012
27,395
Greetings,
I'm not a circuit designer. But I'm attempting to configure and assemble a DIY audio equipment design. I have looked all over for details on how to build a very low noise linear power supply that I would like to use with a SPDIF board which I intend to use in a music streamer. The regulated output of the supply should be 5VDC +10%/-2% and the load output current may vary from 75 to 120mA. I like the noise to be very low, say less than 200nV. The input voltage should be nom. 9VAC. I would be very pleased if someone could point me in the direction of a circuit diagram that I could use for this.
Where does the 200 nV figure come from? What is it based on? What bad thing happens if the noise is twice that?

What does that noise voltage refer to? The RMS noise over some frequency range? If so, what frequency range?

The supply you have only regulates to something that is somewhere between 4.9 V and 5.5 V. That's a range of 600,000,000 nV. With that kind of tolerance, I would seriously doubt that the output voltage isn't going to vary by a lot more than 200 nV as the current changes between 75 mA and 120 mA. So how are you going to deal with that?

To get the noise down that low, I would also suspect that you are going to have to deal with all kinds of other noise sources. You will need to identify them and quantify them and then look at how to reduce them to an acceptable level. That will likely involve a lot more than just a circuit schematic. It will need to go into things such as PCB layout and shielding.

Linear Technologies has some extremely low noise regulators that are in the 800 nV range. By paralleling them, you can improve the noise level, but every doubling of the number of regulators reduces the noise by about sqrt(2), to to get down to 200 nV you would need to parallel sixteen of them. But that only deals with the noise from the regulator itself in an otherwise noise-free environment. You would need to decrease it even further to allow for the residual noise from all those other sources that are injecting noise into your power rails even after you've mitigated them as much as you need to.
 

MisterBill2

Joined Jan 23, 2018
13,128
Consider that there are "audiophiles" who can easily hear the distortion contributed to a music signal by substituting a section of plain copper lamp cord for a similar length of expensive copper speaker lead. Now
Those folks can undoubtedly hear the additional noise added by 50 nanovolts of noise being present on any DC supply in their amplifier.
Their superior hearing ability is far more sensitive than even the very best laboratory equipment.
Just ask them.
 

Ian0

Joined Aug 7, 2020
6,294
Consider that there are "audiophiles" who can easily hear the distortion contributed to a music signal by substituting a section of plain copper lamp cord for a similar length of expensive copper speaker lead. Now
Those folks can undoubtedly hear the additional noise added by 50 nanovolts of noise being present on any DC supply in their amplifier.
Their superior hearing ability is far more sensitive than even the very best laboratory equipment.
Just ask them.
Their abilities seem even more remarkable when you realise that they can distinguish distortion from the non-oxygen-free copper cable when their £10,000 single-ended triode valve amplifiers and speakers are contributing >5% distortion.
 

Thread Starter

BjornMagnus

Joined Sep 4, 2022
5
Very low noise voltage regulati
Where does the 200 nV figure come from? What is it based on? What bad thing happens if the noise is twice that?

What does that noise voltage refer to? The RMS noise over some frequency range? If so, what frequency range?

The supply you have only regulates to something that is somewhere between 4.9 V and 5.5 V. That's a range of 600,000,000 nV. With that kind of tolerance, I would seriously doubt that the output voltage isn't going to vary by a lot more than 200 nV as the current changes between 75 mA and 120 mA. So how are you going to deal with that?

To get the noise down that low, I would also suspect that you are going to have to deal with all kinds of other noise sources. You will need to identify them and quantify them and then look at how to reduce them to an acceptable level. That will likely involve a lot more than just a circuit schematic. It will need to go into things such as PCB layout and shielding.

Linear Technologies has some extremely low noise regulators that are in the 800 nV range. By paralleling them, you can improve the noise level, but every doubling of the number of regulators reduces the noise by about sqrt(2), to to get down to 200 nV you would need to parallel sixteen of them. But that only deals with the noise from the regulator itself in an otherwise noise-free environment. You would need to decrease it even further to allow for the residual noise from all those other sources that are injecting noise into your power rails even after you've mitigated them as much as you need to.
There is a supply that meet my expectations, the Allo Shanti! But, it provides both 5V/3A + 5V/1A in one package and it's therefore quite bulky and expensive. I like to integrate a board into my streamer and the Shanty board will not fit. After some thought I now look for 5.2V/1A so I can power built-in DAC as well.

The reason for stating 200nV was simply to indicate that I want very low noise, but perhaps not quite so low as the Shanti unit which specifies around 80nV (if that makes the design any simpler). Also, I already have a 9VAC tap available on a toroidal transformer (which is already part of the project) that I can load with up to 1.5A.

Perhaps this makes more clear what I'm now looking for.



on IS EASIEST DONE BY ADDING A BIT OF FILTER AFTER THE REGULATION. But what frequency range needs to be low noise?
 
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