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Old 20th October 2010, 10:48 PM   #11
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Yes, Unitrode stuff, which is now TI, who makes a useful range of products themselves.

Flyback is a bit nicer for HV stuff because you don't have to fight parasitics as much. The tubescope has a 100W forward converter, and because I was unable to wind the output transformer ideally (no split primary), the HV secondary has about 100% overshoot (500V spikes, requiring >1kV rectifiers -- I ended up with doubled-up UF4007's). That one's kind of a wash, because it needs 10A of heaters, which would be annoying to provide from a flyback (40A peak!). Something lighter, with more B-supply than A-supply, would be a fine fit.

Flyback is also good at wide input ranges. TOPSwitch datasheets often show 85-265V circuits. There's not really any point in PFC at low power, but if you want to be 'nice', you'll need the additional hardware. Of course, then you can use whatever converter works best. You could even use a buck or boost converter, since the PFC doesn't have to be boost, it can be flyback, providing isolation instead.

For magnetics, besides the junk box, there are parts available from Newark and Mouser (ferrite beads, toroids, and I think I saw E cores too). Amidon has online ordering. And there are a few distributors as well.

And of course for designing them,
Magnetics

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Old 21st October 2010, 12:29 AM   #12
tomchr is offline tomchr  United States
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Was just looking at some of Fairchild's products... SG6901 for example. PFC, PWM in one chip. Handy... I'm sure there are others out there.

A good write-up on flyback design.

Maybe, just maybe... it's time to dust off my copy of Erickson, "Fundamentals of Power Electronics". First edition...

I'm thinking that a boost regulator to 400 V followed by a floating LM317 post-regulator to get to the 375 V I need might just be the ticket...

~Tom
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Old 21st October 2010, 12:34 AM   #13
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If you want to play with SMPS then try the software from

PI Expert Design Software | Power Integrations

You give it the basic parameters and it gives you a parts list and even details on winding the bobbins. Pretty slick.
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Old 21st October 2010, 01:24 AM   #14
tomchr is offline tomchr  United States
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Quote:
Originally Posted by astouffer View Post
If you want to play with SMPS then try the software from

PI Expert Design Software | Power Integrations

You give it the basic parameters and it gives you a parts list and even details on winding the bobbins. Pretty slick.
Nifty. The main issue with switchmode supplies is still sourcing the magnetics, though. Newark has some stuff but generally you have to buy in quantity and accept long lead times. I've always liked the RM cores, but they're hard to find.

I might be able to get a toroid that I like though.

~Tom

Last edited by tomchr; 21st October 2010 at 01:26 AM.
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Old 21st October 2010, 04:34 PM   #15
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Originally Posted by tomchr View Post
Nifty. The main issue with switchmode supplies is still sourcing the magnetics, though. Newark has some stuff but generally you have to buy in quantity and accept long lead times. I've always liked the RM cores, but they're hard to find.

I might be able to get a toroid that I like though.

~Tom

CWS ByteMark, largest supplier of toroids, ferrite cores, iron powder cores, MPP cores and RF cores

I had good luck purchasing some 3C8 cores for a HV switcher project a few years back. They have just about anything you need and an online shopping cart.

I would think the "E-E" core style would be suitable for ease of isolating windings and sections.

Opto feedback doesn't need to be linearized; depends on where the comparator is.

I wonder if an unregulated "PFC-only" topology would be a decent approach to powering a tube amp?

A really good primer is "Power Supply Cookbook" by Marty Brown, Newnes press, 0-7506-7329-X

Last edited by Michael Koster; 21st October 2010 at 04:52 PM.
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Old 21st October 2010, 05:59 PM   #16
tomchr is offline tomchr  United States
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Awesome. Good to know.

Quote:
Originally Posted by Michael Koster View Post
Opto feedback doesn't need to be linearized; depends on where the comparator is.
Comparator? To me comparator implies on/off control. Do you mean error amplifier?
I suppose you could power the error amp on the secondary side and drive the switch via an opto coupler. But powering anything from the secondary makes start-up a bit tricky.

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Originally Posted by Michael Koster View Post
I wonder if an unregulated "PFC-only" topology would be a decent approach to powering a tube amp?
I'd prefer it to be regulated so the linear post-regulator doesn't fry, and it'd need to have galvanic isolation.

~Tom
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Old 22nd October 2010, 08:18 AM   #17
tomchr is offline tomchr  United States
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Originally Posted by analog_sa View Post
Very cool thread. However, it will be more beneficial to compare noise and sound to a decent heater supply and not to 'garbage'. I have never seen any decent linear regulator generate such noise.
Good point. Very good in fact. Tonight I rigged a 12 V transformer to a 25 A diode bridge, 22000 uF reservoir cap, and used it as the raw supply in place of the Sorensen supply. As I wipe the egg off my face, let me describe the two heater supplies under test:

Linear: LM317 fed by above raw supply supplying 5 V to the 300B filament.

Switched: LMZ12002 fed by above raw supply supplying 5 V to the 300B filament.

For both the Linear and Switched supplies, the LM2734 switchmode supply was used to supply 6.3 V for the indirectly heated 6J5 tubes. Hence, this comparison only compares the heater supply on the 300B.

In terms of sound quality, I cannot distinguish between the two. They both sound good. There's good detail on human voices, guitar string buzzing on frets, difference between "up" strumming and "down" strumming on guitars, etc. The mid seems very open and revealing. The bass is tight. The highs on cymbals are metallic and clear. I like it...

The measurements show virtually no difference between the two supply types. The 60 Hz ripple is nearly identical for the two - except the 2nd and 4th harmonics are lower with the switchmode filament supply. I expect the 60 Hz to go down further once I get my rat's nest amplifier prototype transferred to a PCB and into a metal chassis.

Attached images show the noise floor (20 kHz BW, 1 kHz BW) and the spectrum with a 1 kHz, 1 W into 8 ohm signal.

Oh... And by the way, I changed the soft-start cap, C5, to 57 uF (47 uF || 10 uF). This provides about 5.7 seconds of start-up time so the LMZ doesn't current limit during start-up. Nice, gentle on the 300B filament too.
Attached Images
File Type: gif 0W_8R_Overlay_Trafo_1kHz.gif (11.9 KB, 530 views)
File Type: gif 0W_8R_Overlay_Trafo.gif (11.7 KB, 525 views)
File Type: gif 1W_8R_Overlay_Trafo.gif (11.2 KB, 522 views)

Last edited by tomchr; 22nd October 2010 at 08:22 AM.
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Old 22nd October 2010, 09:47 AM   #18
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Quote:
Originally Posted by tomchr View Post
The measurements show virtually no difference between the two supply types.

Thanks for taking the trouble to measure this. Not sure i understand why the measurments are so similar but it's enough to persuade me to test the switchers myself. Great.
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Old 22nd October 2010, 07:17 PM   #19
tomchr is offline tomchr  United States
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Quote:
Originally Posted by analog_sa View Post
Thanks for taking the trouble to measure this. Not sure i understand why the measurments are so similar but it's enough to persuade me to test the switchers myself. Great.
Hey... Thanks for sticking with me here. I'm pretty thrilled about the performance of the switchmode supplies. I was a bit skeptical to start with but they turned out to be really good parts and easy to work with. If you decide to go the switcher route using National's parts, I suggest that you look at the evaluation boards also. The board layout is shown in the eval board instructions (download as .pdf from national.com). I practically copied those for my design. Also read the PCB layout instructions in the eval board instructions and application section of the data sheet.


I should clarify something:
Quote:
Originally Posted by tomchr View Post
I expect the 60 Hz to go down further once I get my rat's nest amplifier prototype transferred to a PCB and into a metal chassis.
What I meant to say is that I expect the 60 Hz and its harmonics to go down significantly once I clean up the layout of the amp. Specifically, getting a good star ground scheme going will make a world of difference.

~Tom
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Old 22nd October 2010, 09:48 PM   #20
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Default SMPS works

I use switch mode supplys on my 300B heaters and the amp sounds (and tests) great!

I was unsure how well they would work on a directly heated cathode tube, but after building and testing the amp I have to say I'm thrilled with the results.

I was admonished for designing the amp this way, and I almost changed the design based on some very negative comments about SMPS, but I decided to try it anyway. I'm glad I did. It works and it works extremely well, at least for me. I used "cheapies" at only $20 each and simply put filtering caps across the output to help remove noise and ripple. One way to tell how much noise is being generated by the SMPS and leaking into the signal path is to measure the output noise of the amp with the SMPS on, then dissconnect it from the heaters leaving just the caps across the filaments. For a short time, even if only for a second or two, the amp will continue to operate on the thermal inertia of the cathode and the energy stored in the caps. Any reduction in noise immediately after power is cut to the heater but while it's still orange hot and the amp is still working is due to the heater supply. Don't leave the filaments off for more than a second or two though - it's not good for the tubes.
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