Linear Power Supplies

[Phil Leigh]

I know about these outrageously priced cables but as of yet there are no "audiophile" routers on the market nor is there an air purification system for improving wi-fi signals available. So there still remains several good money making opportunities available

On a (slightly) serious note regarding hi-fi air, it is worth getting an ioniser or two in the room...

[ralphpnj]

On a (slightly) serious note regarding hi-fi air, it is worth getting an ioniser or two in the room...

Phil, since I highly value your pragmatic, fact based approach, could you please briefly explain why one might need an ionizer?

[praganj]

Hi
I know how to build power supplies, which is I why I also know that you don't need over-sized transformers on line level circuits and why you won't get full-wave rectification from just 2 diodes...unless you use an (expensive) centre-tap transformer.

The performance of the PSU is predominantly determined by the smoothing caps and the voltage regulator. Using an oversize transformer to support a mere 3 amps at 5v is just a waste of money. There's also transformer ringing to consider...

Dear Phil,

I believe that you know how to build ps.

If i want to build the best possible ps using Superteddyreg than i am trying to use best possible components around it and a oversized high quality transformator with low impedance is IMHO a good choice, or is there something wrong about it ?

The main cap after rectifier is 6800uF. Most people will also say that it is too much, 2000 will be enough.

For me the price difference using better oversized components is maybe 20€. So what ?

[m1abrams]

Dear Phil,

I believe that you know how to build ps.

If i want to build the best possible ps using Superteddyreg than i am trying to use best possible components around it and a oversized high quality transformator with low impedance is IMHO a good choice, or is there something wrong about it ?

The main cap after rectifier is 6800uF. Most people will also say that it is too much, 2000 will be enough.

For me the price difference using better oversized components is maybe 20€. So what ?

Using an oversized transformer for the demands of the circuit is not just a waste of money but also BAD for your circuit. The larger the transformer the MORE emf it generates which is one of the big noise generators in audio.

Also an oversized transformer will produce higher voltage than needed for the circuit and cause your voltage regulators to work harder and produce more heat.

Unless you are connecting your amps directly to the transformers secondaries they they will NEVER see the "lower impedance" of the transformer. That should be handle by the PSU not the transformer.

[JohnSwenson]

OK, now that actual PS design is being talked about I'll jump into this thread. (I actually have a couple hours not at work today!!)

I've done a huge amount of PS testing, design and building in regards to the Touch and other devices, what I'm presenting here is not just conjecture, its been tested many times.

My conclusions are that a large percentage of improvements with PS design for the Touch are related to high frequency noise sent BACK into the mains and picked up by other components. power sent into the Touch itself has little impact on what goes on inside the Touch (not zero, but quite small). The differences in mains injected noise is vastly greater than any changes in the power actually delivered to the components inside the Touch.

Given this the focus of PS design should be on decreasing what gets sent back down the AC line, not making the absolute lowest noise, lowest impedance feed to the Touch. Many of the attempts at getting the "best" power to the Touch increase the noise sent down the line.

For a linear supply there are primarily three things that contribute to noise sent back down the line: reverse recovery noise when the diodes switch, transformer ringing and current spikes when the diodes conduct.

Using Schottky diodes can get rid of the reverse recovery noise, transformer ringing can be dealt with with an RC network across the secondary which damps the resonance. This damping of the transformer ringing is very effective but almost never done. If you go and look at 100 different linear PSs there will be a very high probability that NONE of them have damped the transformer resonance. Its intriguing that the higher the "quality" of the transformer the worse the resonance is, thus using expensive "high end" transformers is one of the worst things you can do. (unless you damp it)

The current spikes are the hard part. These are caused by the traditional PS design of transformer, diodes and a big cap. The diodes only conduct when the voltage from the transformer is greater than the voltage on the cap. Thus the power coming from the transformer is in short high current spikes. For example a common design for the 5V 2A for the Touch will actually have 20A spikes coming from the transformer. These 20A spikes have all kinds of high frequency components which get sent right back through the transformer and into the mains. They ALSO excite the above mentioned transformer resonance causing the transformer to ring like a bell, even if you use Schottky diodes.

The DC signal coming out of such a design is a sawtooth wave. This also has large amounts of high frequency components. Unfortunately most rectifier designs have very little input rejection at high frequencies, they do great at low frequencies, but at high frequencies not so good. This is one reason that discrete regulators have been used in audiophile designs, they can have much better high frequency input rejection, which is needed to correctly handle the sawtooth.

There is a solution, but its hardly ever used in low voltage PS designs. Its called a choke. Not the little high frequency things designed to run at 50KHz and up, but big heavy things which have significant inductance at 120Hz. If properly designed they allow continuous conduction through the transformer and rectifier, eliminating the high current spikes. Another advantage is that the output waveform is a pure sine wave, no high frequency harmonics.

Put the choke filter, transformer damping and Schottky diodes together and you have a supply that injects almost nothing back into the AC line and delivers a clean pure sine wave to the rectifier. With this you don't NEED a complicated expensive regulator.

I've put together a design using these principles, the schematic is at:

http://home.comcast.net/~johnswenson1/stereo/SB_5V.GIF

All the parts should be available at Mouser or Digi-Key. I'm sure there are other distributors in other parts of the world where you can get these parts.

You should be able to get all the parts for $75 or so.

Give it a try I think you will like this design.

John S.

[praganj]

Wow ! Great post and a nice schematics.

Thanks John

It looks i have to rebuild a little my ps - i have already schottkys, but two things - a coil and C+R before rectifier are missing.

[JohnSwenson]

Wow ! Great post and a nice schematics.

Thanks John

It looks i have to rebuild a little my ps - i have already schottkys, but two things - a coil and C+R before rectifier are missing.

Just be sure to use that choke and cap values, they have been carefully tuned to work properly, changing any of the values will give suboptimal results.

Have fun!

John S.

[Phil Leigh]

Interesting John.
My (modded) DAC already has Schottky's in place and there is a snubber across the transformer... plus my "mains filter" uses a choke (actually 2 chokes)...

Of course the fact that the parts are there doesn't guarantee it all works properly.
How would you measure the re-injected artifacts?
thx
Phil

[praganj]

Just be sure to use that choke and cap values, they have been carefully tuned to work properly, changing any of the values will give suboptimal results.

Have fun!

John S.

Hi John,

I have few questions:

1. What AC voltage should deliver the transformator and how big should it be ?
2. R1 - 1W is enough ?
3. Can i use instead of LT1048 one superteddyreg and if yes, is C5 and C6 necessary ? Or should i just forget str and use LT1048 ?
4. When i am building it and measuring, what voltage exactly should be after choke and after reg ?

Thanks, p.

[Phil Leigh]

Phil, since I highly value your pragmatic, fact based approach, could you please briefly explain why one might need an ionizer?

All the electrical kit in your room gives off (low-level) ionic stuff - ionisers help to counteract that and rebalance the ionic levels in the air - makes you feel "more relaxed" etc - I'm not an expert on this stuff, but I do feel it is worth experimenting with these devices to create an atmosphere that is more conducive to various things, including listening to music. You need one per approx 15 sq meters of room. I've got 3 in my main listening room.
Obviously if you live in the countryside in the middle of nowhere it may not make much difference - you could just open a window...

However, I find (in a semi-rural setting) that they help. YMMV of course.