Just a small test I did on power supplies

[ergo]

The following is a test I did to help me figure out a new supply for my power amp.

The page looks awful and is too big, so you have to wait while it downloads... sorry...

Still I wanted to publish it and as the time is in very short supply lately this is the best I managed.

http://f5.infonet.ee/ergo/ps/powersupply.html

Anyway as a teaser - this is the spectrum of an output of a type of power supply that is most commonly used - transformer -> diode bridge -> caps -> load



Ergo

[Jan Dupont]

However I don't see any test for a PSU with 100nF across the rectifying diodes, and with 1uF to 10 uF decoupling caps across the output..............

[millwood]

nothing unexpected, other than the >30K peak. What is that?

amps usually have pretty good psrr at low frequencies so nothing to worry about.

Also, have you tried the same test on a psu using those fancy soft recovery diodes?

[ergo]

Good point ACD. I actually plan to continue with this line of testing. For one I'm waiting my 100'000uF RIFA caps to arrive then I can test how low can I get the ripple....

Still at least for me this series of measurements was a bit of an eye opener. I had not done this kind of spectrum analysis on high current power supply before. It seems that there is a huge amount of crap totalling from the diode bridge + the stuff coming from the wall socket.

The goal is to get the spectrum as pure as possible and then connect the power supply to my amp and listen - at the moment it's only a suspicion but I'm not surprised if the change in sound will be huge.

One thing to test would be an active filter. I'm wondering how much of the high frequency crap evident with simple filters gets through active filter.

Ergo

[millwood]

Originally posted by ergo
The goal is to get the spectrum as pure as possible and then connect the power supply to my amp and listen - at the moment it's only a suspicion but I'm not surprised if the change in sound will be huge.
Ergo

probably not a whole lot of differernce but it is nice to try.

How about the idea of regen? what if for example you use the traditional PS to power an amp which generate either DC directly or high frequency AC to be rectified, to then power your real amp?

Wouldn't that help a lot from PSU specturm's point of view?

[janneman]

Hi Ergo,

Nice test. The crap you see are actually 50 and 100 Hz harmonics caused by the sawtooth-shape of the voltage on the last cap. The noise, which is at a much lower level, *may* be caused by the diodes, but it can also come from the mains.

You'll note that the harmonics of the 100 Hz rectified signal are higher than the 50 Hz harmonics. There are no surprises here, all is just as theory predicts. When you increase your filter cap, you will probably see that the lower harmonics are lower but that there are more and higher high-order harmonics.

You can get rid of those 50/100 Hz harmonics most effectively with a CLC filter. This will clean up the low level hash as well.

Jan Didden

[Jan Dupont]

Good point ACD. I actually plan to continue with this line of testing. For one I'm waiting my 100'000uF RIFA caps to arrive then I can test how low can I get the ripple....

Just adding bigger and bigger caps doesn't mean that you will get rid of the noise. If you take a look on other supplies in this forum, you'll see that they all have decoupling caps on the output (normally about one 1/100 the value of the main caps)

I'm looking very much forward to see your next test results

[Eva]

This 31Khz component looks like common mode EMI from the power supply of the computer. Most AT and ATX PSUs operate between 30Khz and 35Khz

It may also come from anywhere inside the computer since there are dozens of EMI sources affecting the soundcard

[slowhands]

You have 50 Hz harmonics galore. One very cheap way to reduce them is to put a small (0.22 5w) resistor between the caps, creating a CRC filter. Though it seems crude, it is very effective for low frequency filtering.

As Eva astutely noted, you have noise from your PC power supply too. You may be able to dramatically reduce this with caps across the transformer primary and secondary, typically 0.047 works quite well. This will also filter out power supply and power switch noise on the primary side, and diode noise on the secondary side.

At the load, I would suggest additional filters on each voltage. Conventional wisdom suggests a small electrolytic (100 mF) in parallel with a good film cap (0.1 mF).

[Eva]

The only thing that works in order to reduce common mode noise is common mode filtering

Take a big ferrite toroid and simply wind a dozen or so of turns of your signal cable on it before entering the sound card. Use the entire signal cable to do the winding

Look at this example : I have to do this kind of filtering in order to be able to get precise measurements when the ground of one probe is connected to the primary side of a SMPS and the ground of the other is connected to the secondary side. These grounds aren't at the same AC potential due to common mode EMI generated by the own SMPS prototype and the SMPS of the oscilloscope