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Old 18th January 2006, 04:27 PM   #11
kevinkr is offline kevinkr  United States
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I currently design semiconductor ATE for a living, and due to space and power consumption constraints mostly use SMPS for my raw dc supplies. Simple external filters using a single common mode choke, a couple of inductors and some low ESR capacitors can knock down conducted noise by several orders of magnitude or more.

For suitable chokes and inductors take a look at http://www.coilcraft.com

You can also use high performance linears after the SMPS for somewhat improved noise performance and to define a low source impedance at audio frequencies. (Note most linears do not have enough loop bandwidth to effectively remove noise above 100kHz. Linear Tech has several specified at 100kHz that do work well.)

Noise levels of much less than 50uVrms over a 100kHz bw are achievable with some care using the above techniques.

Shielding to reduce electrostatic coupling is also very important. Look at some of the other posts for more on this.

ATE applications often require measurement accuracies of 10ppm or better, including the noise contribution. No audio device is even close.

Kevin

Edit: Link is correct URL but does not seem to work correctly in firefox, copy and paste into your browser address bar if the link is misdirected. Might be some problem with the webserver as well.
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Old 18th January 2006, 05:11 PM   #12
EC8010 is offline EC8010  United Kingdom
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kevinkr: An interesting post - thanks. (The link worked fine with my Firefox.)
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Old 18th January 2006, 10:07 PM   #13
cerrem is offline cerrem  United States
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I really don't won't to give my opinions on VICOR and such....
Tell me what your voltage, power requirements are and then I can recommend something or I can suggest specs you should be looking for...

Chris
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Old 18th January 2006, 10:25 PM   #14
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by kevinkr
[B]I currently design semiconductor ATE for a living, and due to space and power consumption constraints mostly use SMPS for my raw dc supplies... ATE applications often require measurement accuracies of 10ppm or better, including the noise contribution. No audio device is even close.
Great post. I once designed a preamp for a capacitor microphone (Bruel & Kjaer) which was portable, so necessitated the use of SMPS to get all the required voltages. An ordinary flyback was used, the real stuff is in careful control of all current paths, and proper magnetic shielding - very much like what you describe. It proved much more troublesome to provide amp noise performance that was lower than the SMPS injected noise. Anyone who knows what these microphones are capable of, knows what I mean.
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Old 18th January 2006, 10:29 PM   #15
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Quote:
Originally posted by cerrem
I really don't won't to give my opinions on VICOR and such....
Tell me what your voltage, power requirements are and then I can recommend something or I can suggest specs you should be looking for...

Chris
Hi Chris, I have use for two voltages:

* 12VDC in the range of 5A to 10A

* 90VDC @ 5A (or larger, but that's getting big).

Thanks!

Tom
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Old 20th January 2006, 02:02 AM   #16
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I didn't make a sweeping statement when I said that SMPS do not have a home in high end audio. This is due to my experience working with switchers for the past 8 years. Ripple, & Noise and conducted/radiated emi (not audible yes but imagine the noise from the switcher coupling to the audio electronics - you need very good shielding).

Have also done auditions on vintage amplifiers using switchers.
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Old 20th January 2006, 02:47 AM   #17
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I'm not sure ripple and EMI are the most important factors for judging the sonic worth of a power supply. I might speculate that a power supply's ability to track changes in load current without generating spurious voltages in the process + a power supply's ability to reject mains grunge might be more important. I'm running a pair of Charlize amps via a cheap $25 switcher, and it sounds very good. Makes me wonder what a good switcher could do.

The highly rated and lovely sounding Emm Labs digital gear uses switchers.
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Old 20th January 2006, 03:42 AM   #18
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I believe you are referring to transient response. SMPS should have better transient response than linear power supplies.

Speaking of ripple and noise I still have to see ripple and noise figures in the 1mV range for SMPS.

Speaking of sound, this is purely subjective. If you don't hear it then SMPS is good for you.
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Old 20th January 2006, 03:45 AM   #19
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Quote:
Originally posted by rascal101

I believe you are referring to transient response.
Yes, and what might be called dynamic noise, ie, non-signal voltages created in response to amplifying the signal. One example of this, in a linear supply, is voltage variations created by current spikes filling capacitor banks.
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Old 20th January 2006, 04:52 AM   #20
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Sir Serengetiplains,

Transient response refers to the response of the SMPS PSU to load changes eg from 10% load to 50% load or from 90% load to 40% load. Typically SMPS companies design power supplies for a step load of 50%. Power supply must maintain regulation and get back to nominal output within a specified settling time.

Transient response is a measure of the responsiveness of the feedback loop and also gives you an idea of the phase margin - stability.

What we do to minimize noise from the FET switch is minimize the loop area, use a star connection for the ground, avoid low voltage PCB tracks passing through high voltage tracks, capacitor layout at edge of tracks to ensure filter action, use copper shield at transformer etc. Despite this you still have EMI (albeit passing CISPR or FCC requirements).

To offset this for audio application is to isolate SMPS from the audio electronics itself and to use a lot of low esr e-caps to minimize ripple & noise.

Despite this I still do not want any SMPS for audio application. The effort and time required to equal the quietness of a linear power supply is not worth it. It is quite possible but it will very very difficult using an ordinary SMPS topology (eg flyback, forward, 2 FET forward - you would have to go half bridge or full bridge).
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