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120studio.com
April 3, 2015



Intro

One aspect of stereos and pre-amps is the perennial subject of "low-noise components".   The same thing goes for vacuum tube pre-amps...  maybe especially so.

Most components, especially power supplies, have some tendency to introduce noise.  And at some time or another, most of us have heard that annoying hum or buzz that sometimes finds its way into the audio signal.

This article is sort of an ongoing notebook... I'm just going to ramble a bit about choosing (or modifying) a power supply for audio.  And hopefully, you'll find it useful too.


A Quick Note


This article is made possible only with the support of readers like you, when you use the links on here to purchase your gear.  The small commissions from sales are what allow me to keep this site going.   Thank you in advance for your help.


Phono Pre-Amp

The Source of Audio Noise


Clean Power Supply

Noise Reduction


Easy Solution

Easier Solution

Deluxe Solution

Conclusion



Phono Pre-Amp


Many people are wanting to get into tube audio nowadays, and that's a very good thing.

I was really hoping this cute little device would work as both a phono pre-amp and a headphone amp, but actually it's just a headphone amp.  It requires a separate phono pre-amp if you want to use a turntable with it.   That is, unless your turntable has a true "phono" output, which would mean the turntable has its own phono pre-amp built-in.  (This popular one has it.)  If you had something like that, you could pair this headphone amp directly with it.

Many turntables are just "line-out", though.  That means the signal is not pre-amplified.  These turntables are designed to output the signal into a receiver or integrated amp that has its own phono stage. 

Or, feed the signal into a separate phono pre-amp first.

Sooner or later you're going to want to use a turntable that has no phono stage of its own.  That's where you'll need a phono pre-amp which also acts as a headphone amp.  If you like the warm sound of vacuum tubes, the obvious choice would be this unit from Rolls;  it can take the line signal from a record player and turn it into a listenable audio signal in your headphones.  Or, pass that signal into a stereo receiver.  That would allow you to listen to it on big speakers. 

And it's all-analog, meaning there's no DSP (digital signal processing).  (Is it an all-tube amplifier circuit?  See this review for more details.)

Problem is, some people have noted this unit is kind of noisy.  Which, of course, is a drawback.   So, how to fix this?


The Source of Audio Noise

 
The power supply is not the only possible source of hum, but here's a simple way to test:   simply turn up the volume while listening to quiet music (or better yet, a completely silent passage in the music).  If the humming or buzzing sound gets louder-- much louder-- when the volume is turned past a certain point, then chances are there's noise originating from your power circuit.  

Amplifier circuits simply magnify the signal that's handed to them.  If there's noise in that signal, that's going to get louder, too.

So, what can you do about this? 


Clean Power Supply


The key in making a good power supply is to choose quality components.  That's a way-oversimplified view of the subject, but 90% of the time it works. (OK, I pulled that 90% number out of thin air).  It's true, though;  choose good components, and most of your work is already done.  (That assumes you already have the circuit designed properly.   But you knew that.)

If you don't believe this, simply consider the opposite case:  a bad or failing component.  That will produce noise, which may then get amplified and become part of the audio signal.

A too-small power supply can also introduce noise, by producing insufficient current. 

So, how to make a good power supply? 

Well, you don't have to make one from scratch.  It is much easier just to buy one (see below).  But if you wanted to make one and were experienced enough to work with mains power... you'd start with the transformer

Supposing you wanted to make a new power supply for a tube headphone amp... first you'd want to take 110/120 volts AC and step that down to 15 volts AC.  This transformer would probably be a good foundation for a custom-build power circuit.  It's got Nippon silicon-steel sheets and enameled O2-free copper wire, like a good hi-fi transformer should.  And it's rated for an output of 1000 milliamps (1 amp), which is enough to power something like the VP-130.

So then you would need four diodes to make a bridge rectifier.   For audio, it would be a good idea to use Hi-Q components.  So, you could grab a set of these and make a bridge rectifier out of that.

That's the core of your basic power supply:  a transformer and a bridge rectifier.  But that's going to have noise which has to be filtered out.


Noise Reduction


Put a well-made capacitor in parallel with the rectifier output, and you'll smooth out the pulsed DC output of your bridge rectifier.   Audio capacitors are going to work better than those basic, cheapie electrolytics... and they last longer.  

Are "audio grade" capacitors better than everything?  Well, maybe not everything.  I sort of go back and forth on this subject.  What makes a good capacitor is really a complex bunch of science from about three or four different disciplines.  There are things to consider, like ripple current rating, operating temperature, materials used, and that sort of thing.  I know a little about this, but never enough, and there are people who know a lot more about it than I do.   (Helpful, I know.)

Ultimately you probably do get what you pay for.  But when you buy audio grade capacitors, read the specs.   This is going to take some research.  Premium caps from a good brand are certainly better than generic caps from a cheapo brand.    Not all the best caps for the job are necessarily going to be "audio grade", but quite often they'll at least be the upper-tier ones from a good manufacturer.   (Again, compare the specs to prove this to yourself.)

Anyway, this is a 15V DC power supply, so the required capacitor is not going to be one that will fry you if you accidentally touch it.  However, you could always temporarily put a high-value resistor across the output, just ahead of the capacitor, to drain off the charge harmlessly if you want.  Just make sure the wattage rating is enough to handle the dissipated power.  Or, use something like a megohm resistor, which will drain off the charge only very slowly.


The Easy Solution


You could spend a long time messing around with making your own circuits. And some people will find this very enjoyable.  Sometimes, getting there is all the fun.

If you don't want to do that, you could always just buy a ready-made circuit to regulate the voltage.  With that one, you'd still have to run the output from your transformer to it. It's just that you'd already have the work done for you, as far as building a circuit to filter out or reduce the noise.  So, transformer plus that regulator, and you're almost ready to go.

Then all you need to do is find an old "wall wart", working or not, with the right connector on the end. Remove that connector, use it on your voltage regulator, and put the whole thing in a suitable project enclosure. Good to go!


The Easier Solution


If you have the Bellari preamp (or are planning to buy it), you might want to get yourself this power adapter. Be sure to get the "M" tip for it. Then, you're all set. Be advised that it has to be center-negative.  Just use this power adapter instead of the stock one, and you'll get the sound that the amp was intended to produce.  (Also see my new review.)


The Deluxe Solution


If you're looking to build a mad-scientist audio lab, it's a great idea to have a couple of good benchtop power supplies. 

Many benchtop supplies are switching power supplies, which tend to generate more noise.   That said, I'm tempted to get one of these.  It's way overkill for most little phono-preamp units, but here's the thing about amperage... the unit you're powering will draw only as much current as it needs.  So, a power supply that can do 50 amps will not really be putting out 50 amps most of the time.  If your phono stage draws only 1 amp, then that's all it will supply. 

Here's an even better solution.  This power supply is low-cost... and it's linear-mode.  The noise is low enough for something like a headphone amp or phono stage. 


Conclusion

Low-noise power is one of the most important things in any good audio setup. 

Many audio problems can be traced to bad power, faulty components, or just all-around poor design.  If you have audio gear that's powered by DC, get (or make) a supply that generates that voltage.  Or, get a variable-voltage power supply designed for low noise.  And if you're feeling adventurous, build your own. 

If you found this page useful, informative, or entertaining, please help me out by purchasing your stuff through these links.   Your help is greatly appreciated.

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