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tvrgeek 11th December 2011 11:56 AM

PS tests
 
I have been contemplating tests for power supply quality as used in Audio. Sure, measuring ripple, but what is it that really makes a sonic difference? What SHOULD we measure?

OK, Noise is pretty straight forward. I use TrueRTA software spectrum analyzer to look at the noise. This lets me see the effects of different diodes, suppression caps and regulator noise. Very insightful. Static resistive load, drive the probes through a simple voltage divider. If you have not done this, you may be very surprised at the level of harmonics produced.

What I was thinking about is how to measure the effect the supply has with transient loads. Basically, transient regulation. How much effect does one design have over another in resupplying the caps that supply the instantaneous current into the load? I wonder what the relationship between supply impedance and the size of the at-load capacitance needs to be, or if other parastatic effects nullify the extremes many have gone too.

I want to use some objective measures of expected quality.
If the latest fancy shunt regulator sounds better than a cheap three terminal, what measurement quantifies that?

What is the sonic difference between using a high efficiency switching supply as a pre-regulator rather than a linear supply and big caps? I want to quantify this.

Mooly 11th December 2011 12:07 PM

Might be something here to interest you on PSU's,
http://www.diyaudio.com/forums/power...ml#post1572320

Some real scope shots in the thread too a bit further on.

dhaen 11th December 2011 12:08 PM

Test to show effect of PS
 
On a stereo amp with shared power supply, drive one channel hard into a dummy load while listening to the undriven channel through a speaker. On many designs you may be horrified by the low level distorted audio audible. Much of this is leaking through the power supply.
OK, monoblocks solve the crosstalk, but distortion caused by "loss" into the PS is still happening. All designs I've seen use the power supply as a path to or from the load.

Mooly 11th December 2011 12:12 PM

Quote:

Originally Posted by dhaen (Post 2815419)
On a stereo amp with shared power supply, drive one channel hard into a dummy load while listening to the undriven channel through a speaker. On many designs you may be horrified by the low level distorted audio audible. Much of this is leaking through the power supply.
OK, monoblocks solve the crosstalk, but distortion caused by "loss" into the PS is still happening. All designs I've seen use the power supply as a path to or from the load.

:)

Read it all to get the context,
http://www.diyaudio.com/forums/solid...ml#post1624677

dhaen 11th December 2011 01:07 PM

Quote:

Originally Posted by Mooly (Post 2815425)

Very good thread and most enlightening. I shall bookmark it.
Of course the wiring to the PS including grounds are all part of the PS as far as the voltage/power amp is concerned. But I think we may have drifted slightly off-topic

gootee 11th December 2011 11:36 PM

Quote:

Originally Posted by tvrgeek (Post 2815406)
I have been contemplating tests for power supply quality as used in Audio. Sure, measuring ripple, but what is it that really makes a sonic difference? What SHOULD we measure?

OK, Noise is pretty straight forward. I use TrueRTA software spectrum analyzer to look at the noise. This lets me see the effects of different diodes, suppression caps and regulator noise. Very insightful. Static resistive load, drive the probes through a simple voltage divider. If you have not done this, you may be very surprised at the level of harmonics produced.

What I was thinking about is how to measure the effect the supply has with transient loads. Basically, transient regulation. How much effect does one design have over another in resupplying the caps that supply the instantaneous current into the load? I wonder what the relationship between supply impedance and the size of the at-load capacitance needs to be, or if other parastatic effects nullify the extremes many have gone too.

I want to use some objective measures of expected quality.
If the latest fancy shunt regulator sounds better than a cheap three terminal, what measurement quantifies that?

What is the sonic difference between using a high efficiency switching supply as a pre-regulator rather than a linear supply and big caps? I want to quantify this.

Great idea!

Transient response is very important and would be interesting to test, especially if you changed things and compared the results. But what to change? And what and how to measure?

I haven't thought any of this through but some things come to mind:

You might want to use a special test fixture, maybe using a MOSFET switch, maybe somewhat similarly to the way it was done with the one shown in Figure 11 at Capacitor Characteristics .

It would be interesting to see the rise time (and a plot vs time) of a transient current that was suddenly allowed to flow through a 4 Ohm or 8 Ohm load (and the voltage across the load), after switching (for example) the maximum available voltage across the load. It would be good to know the maximum available current slew rate, and how long it could be sustained, etc.

On a related note, it would also be interesting to measure the output impedance of the supply as a function of frequency, up to at least a few hundred kHz, possibly for different output-power levels. (The rise-time in the previous paragraph would be related to frequency by f = 1 / (Pi x trise). I guess you would want to measure impedance to a frequency somewhat beyond that.)

For all of those tests, it would be interesting to see how the results changed for changes in the power supply itself. But it would also be extremely intreresting, to me at least, and since you would already have most of the setup in place, to test different lengths and configurations of power supply rails and different types and sizes of decoupling capacitors at the load, with various current-pulse demands by the load. Maybe you could determine, as you mentioned, whether or not certain extremely good qualities in the power supply would be "wasted" to varying degrees if the power distribution were done in particular ways.

I guess some might think that you wouldn't need to bother with some of that, since we can calculate it all, off line. But it would still be very interesting to actually see what happened. And of course calculations or simulations are both only as good as the model being used. So there would be some real value in actual testing.

tvrgeek 12th December 2011 09:17 PM

Good thoughts. I'll chew on these for a while as I finish up a few other projects. I had not thought about cross channel effects. I was thinking more of low level power supplies, but I bet it applies there too. What got me back to this was building a buffer, aka CMoy head amp to drive the 600 Ohm input to my e-mu I use for speaker measurement. I have been using batteries, but was considering a line supply. It took me a long time to get the residual noise and distortion below 130dB of my test rig, so introducing a line supply became a topic for deeper consideration. So, simple 3 terminal? , External switcher and a rail splitter? Sulster ( my apologies, I bet I have that wrong) or Jung and so on.

Z out for supply over freq. Very good idea. We do it for power amps as a unit, we should do it for the supply. Construction can sure make a difference. Changing the power and ground system of my B&K st140 made obvious improvements. REALLY obvious. I think they did something themselves in later units.

The ability to have a PC generate odd but repeatable pulses has opened a few doors. I found how an amp deals with initial wave fronts to vary a lot. I suspect the power supply including the physical construction and treatment of source and return to be part of it. Why does my old Rotel sound so darn good? Or my Creek?, yet my Hafler measures far better.

I have followed cap development since Walt Jung's original article in Audio about ESR and DF helped me solve a mystery problem at my work. A multi-million dollar problem solved by changing from a ten cent electrolytic to a 50 cent foil-film in the right place. The Phd engineers did not believe me until I made the change and proved it. (It was a half nano-second bump in Dcc causing a Z80 to NMI).

tvrgeek 12th December 2011 09:45 PM

If nothing else, Jan pointed me to some publications I had not heard of: Linear Audio.

magnu 12th December 2011 09:57 PM

You have the regulated power supplies. These supplies have a feedback loop. This feedback loop can oscillate with some loads, if it is not properly designed.

The stability of the feedback loop is recommended to be measured.

! 12th December 2011 11:41 PM

The moment you write "sounds better", you are chasing a ghost, building a supply only suitable for a particular combination of listener preference and amp (and maybe source and speakers too).

Otherwise, objectively it would seem logical to reduce ripple at the amplification stage and respective output transistors so these are the appropriate points to measure ripple. Once you have these measurements you can either try to assign cause or blame, and/or simply build up or add additional filtration to provide more smoothing.

Point being, in a power amp the ripple caused by the amp itself greatly exceeds the ripple present from most other factors, assuming a certain level of diligence to follow typical, time tested designs.


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