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Old 21st October 2003, 07:13 PM   #21
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Default flyback smps & noise

Quote:
I hope no one seriously considers a flyback SMPS for audio purposes, just too noisy.
Pre- and post-regulation L-C filters will suppress noise substantially. Otherwise, the choices narrow down to a forward converter, or a push-pull converter, both requiring more parts and complexity. They are, however, quieter topologies.
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Old 21st October 2003, 08:36 PM   #22
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Quote:
Originally posted by Sch3mat1c
What is this noise everyone's talking about anyway? As-is it operates above the audio range, something easily filtered. After that, some hefty electrolytics could be used anyway, to add general stiffness to the line.

I don't see any problems a bit of shielding, L's and C's can't fix.

Tim
A few months back I did some tests with opamps, chip amps and a basic discrete design -- the flyback converter generally into another order of magnitude.

The noise isn't easily filtered -- it's not ripple, it's noise.

Further -- don't forget that when you combine the switcher frequency with the audio signal (however they are coupled) you get positive and negative sidebands, and the lower sideband can easily reach into the audio range. The lower sideband can mix again etc., etc.

well, everyone who likes to listen to music is going to get a rude awakening when the power companies put "internet over power line".
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Old 21st October 2003, 10:38 PM   #23
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Default flyback smps and the noise issue

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A few months back I did some tests with opamps, chip amps and a basic discrete design -- the flyback converter generally into another order of magnitude.
I don't dispute the validity of your tests. An SMPS is noisy by nature, and flybacks are noisier than forward topologies, making them the worst.
Remember, with discrete parts, the layout on the board is very critical. The highly integrated TOP Switch, or any equivalent (such as ST Micro's "Viper") is much less noisy due to the small area of the circuit, particularly the parts connected to the switching node. In addition, these IC controllers (TOP Switch) use frequency modulation on the switching frequency, resulting in a spread spectrum noise distribution, which is much easier to filter. Post L-C filters should feature small-valued inductors (around 1.0 uh) with high self-resonant freq, and low dcr. Caps should be X7R ceramic, or low esr electrolytics bypassed with X7R ceramics. With ceramic caps only, an additional R-C snubber may be needed to dampen the high-Q formed by "L" & "C". In addition, these chips give you the option of connecting them for forward converter operation, resulting in even lower noise than the flyback. Power Integrations has an application note on their site, for forward converter operation.
I was only trying to make the poster aware of the options available in this day and age. The forward converter would definitely be a better choice, if noise is still a problem, but requires a filter inductor for each output, plus an additional rectifier for each output, as well as an additional primary winding to reset the core flux. The cross-regulation on forward topologies (including push-pull, half- and full-bridge) is not as good as the flyback. It can be improved by magnetically coupling the output inductors. Just thought I'd share this to give an idea of what's involved. Best regards.
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Old 21st October 2003, 11:17 PM   #24
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Quote:
Originally posted by jackinnj
The noise isn't easily filtered -- it's not ripple, it's noise.
The only noise that isn't easy to filter is slow variation in the DC, which can (and will) be regulated out. Everything else, 1Hz to microwave and up, can be filtered out with Ls and Cs. Could someone please tell me how this noise could possibly defy the laws of physics?

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Old 22nd October 2003, 07:23 AM   #25
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I would try something based on the 74C14. It can use feedback through an optocoupler for regulation. The part values would need to be adjusted.
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Old 22nd October 2003, 11:42 AM   #26
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Default this is the noise I meant,

the periodic voltage swing isn't "noise" in the sense of random events, so it will go away with L+C, it's the 100uV which doesn't go away. This is off the PDF for the Linear Tech LT1533:
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Old 22nd October 2003, 05:31 PM   #27
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Well gee, 100uV is too low to care about unless maybe you're powering a phono amp.

Still, can you explain to me what generates it, and why it's impossible to remove?

I like that 7414-based circuit, short and sweet... I'll check my IC pile (gasp!) to see if I have any in there, might have to build that one. Er.. will 200V work for the MOSFET? That's the best I seem to have right now...
OBTW half-wave choke input won't work. It needs a second diode to clamp the inductor, IIRC. Cap input would be easier.

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Old 22nd October 2003, 05:51 PM   #28
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The 100uV is from the LT1533 ultra low noise switcher. Most SMPS are a 100 times worse, at least.

Here's another simple idea if efficiency is not a great concern, use a LM1875/LM3875/.... type power amplifier chip with sine wave signal to drive your ferrite xfmr. No noise except sine wave then.
(Well, you could use a P-P tube amplifier too, but kind of defeats the purpose then.)
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Old 22nd October 2003, 06:42 PM   #29
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Default if you can't solve a problem - make it bigger!

Perhaps enlarging the scope of the problem will ultimately provide an even better solution. Since you want the high voltage for a tube amplifier anyway, try the Berning output stage. Glass Audio No 1, 2000. Solves the tube output xfmr and the HV supply all in one shot. Saves a lot of weight too.

I would suggest modifying the Berning design to use slew rate limited switching to avoid noise. Even better, use a two phase setup instead of the original single phase switching. By using two ferrite xfmrs, two switch pairs, and two sets of commutating diodes for the tubes, each phase only need cover 90+ degrees instead of 180 degree switching. That way, you can really slew rate limit the switching (and without any efficiency penulty either) and the audio signal always has a solid path thru the network, no need to filter switching gaps out of the audio output then. Its an ideal solution. If you have qualms about the SS devices in the audio path, the two phase design is formally equivalent to a diode and an always turned on Mosfet in series with the tube plate, not much sonic effect from that.

By the way, the Berning type output is easily adaptable to exotic tube output topologies, like partial cathode feedback, separate screen windings, or ultralinear, by simply partitioning the HV windings and using commutating diodes on each winding. And the # of turns required on the ferrite cores is no more than on a SMPS. Once you have one of these set up, its easy to try all sorts of tube output topolgies with minimal effort.
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Old 22nd October 2003, 09:55 PM   #30
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Quote:
Originally posted by Sch3mat1c
Well gee, 100uV is too low to care about unless maybe you're powering a phono amp.
they get the noise low enough to use in ultrasonic medical equipment, hydrophones etc.

Still, can you explain to me what generates it, and why it's impossible to remove?
Thermionics

I like that 7414-based circuit, short and sweet... I'll check my IC pile (gasp!) to see if I have any in there, might have to build that one. Er.. will 200V work for the MOSFET? That's the best I seem to have right now...
OBTW half-wave choke input won't work. It needs a second diode to clamp the inductor, IIRC. Cap input would be easier.

Tim
If you the switching spike is 10mV on the edges of the square wave and you run it through a transformer, you probably multiply the spike by the turns ratio and "gremlinitize" the thing.

The LM3875 needs its own supply unfortunately.
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