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Power supply options for tube preamp
Power supply options for tube preamp
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Old 2nd December 2017, 11:06 AM   #1
Rootz is offline Rootz
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Default Power supply options for tube preamp

For most of my audio projects Ive been working with linear supplies. Convenient, easy to calculate and regulate. Never cheap or flexible though.

For a new preamp project Im looking for a power supply that is able to deliver 20 mA at 420V to 5 12ax7s. Two triodes will be cathode followers with the cathodes at 180 V. The heaters probably need to be elevated. The best transformer I could find in The Netherlands is a Hammond 369bx. 300 Vct and enough heater current. Antek is much cheaper, but shipping is around 40 bucks.

So I started looking for cheaper options. Found some China made dc-dc converters capable of 450V and looked at some offerings of LT (lt3757 as flyback controller). I could drive them of a 12 V transformer, maybe regulated. Problem is: how do I elevate the heaters in that case? Not possible I think. I could use a separate fly back converter for the heaters, but then which one and how do I regulate the output of the 12 V converter while floating?

All in all a linear supply looks like a much easier option, but maybe you guys see other options I just totally miss?
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Old 2nd December 2017, 01:55 PM   #2
kodabmx is offline kodabmx  Canada
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I use these... DC-AC Converter 12V to 110V 200V 220V 280V 150W Inverter Boost Board Transformer | eBay

For 420V use the 220V AC output and a voltage doubler using 10u-22u caps and HER208 (or equivelent) for the diodes. The output is isolated from the input, and you should still be able to elevate the heaters. I power it from a 12V SMPS but you could use a linear 12V supply. You could also use 2 SMPS 12V supplies, one for the heaters, and one for the boost converter.

A pair of these would work: 6A-8A Unit For 12V 100W Switching Power Supply Board AC-DC Circuit Module | eBay

Last edited by kodabmx; 2nd December 2017 at 01:57 PM.
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Old 2nd December 2017, 07:20 PM   #3
Rootz is offline Rootz
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That's one of those China smps modules I was thinking about. Point is: I can't see a way to elevate a linear supply and simultaneously let that supply feed the HV smps. So how do I elevate the heaters??
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Old 3rd December 2017, 12:10 AM   #4
MorbidFractal is offline MorbidFractal
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Quote:
Originally Posted by Rootz View Post
For most of my audio projects I’ve been working with linear supplies. Convenient, easy to calculate and regulate. Never cheap or flexible though.<snip>
How hard do you want to play? Google "Current Fed Buck Converter". For example...

UC3827-1 Buck Current/Voltage Fed Push-Pull PWM Controllers | TI.com

If you want multiple isolated outputs, high and low voltage, that track each other then the above is the topology to go for.
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Old 3rd December 2017, 12:13 AM   #5
nigelwright7557 is offline nigelwright7557  United Kingdom
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I always use 2 cheap transformers back to back.
First transformer drops to 12 VAC for heaters then 12 VAC is stepped back up using a 18:240 volts transformer to get 160VAC for B+.
Use transformers to suit the voltages you need.
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Old 3rd December 2017, 12:32 PM   #6
Rootz is offline Rootz
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Quote:
Originally Posted by MorbidFractal View Post
How hard do you want to play? Google "Current Fed Buck Converter". For example...
Not that hard haha. However, you are correct. This is perhaps the way to go when using one controller. But why not use two, e.g. EC3843's. One for HV, one for LV. There are loads of information to be found on the PWM controllers, not so much for the UC3827. I mean, I can read app notes and data sheets, but I'm not an EE. Working with a UC3827 probably requires a bit more than my basic SMPS knowledge.

Quote:
Originally Posted by nigelwright7557 View Post
I always use 2 cheap transformers back to back.
First transformer drops to 12 VAC for heaters then 12 VAC is stepped back up using a 18:240 volts transformer to get 160VAC for B+.
Use transformers to suit the voltages you need.
12V 30 VA into reversed 6V 15VA would do the job quite well. Buying new trannies (toroids) would probably set me back 50 to 55 Euros ex. shipping though. Will have to see what I've got lying around in the house.

EDIT: Got a 2x12V 50VA toroid around!

Still I wonder, where are the people that actually build a switching power supply for tube amps. Should be doable right? Wrth offers some nice offline flyback transformers with a HV winding. 90-265 VAC to 285 V @ 70 kHz for example.
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Old 3rd December 2017, 01:59 PM   #7
MorbidFractal is offline MorbidFractal
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Quote:
Originally Posted by Rootz View Post
Not that hard haha. However, you are correct. This is perhaps the way to go when using one controller. But why not use two, e.g. EC3843's. One for HV, one for LV. There are loads of information to be found on the PWM controllers, not so much for the UC3827. I mean, I can read app notes and data sheets, but I'm not an EE. Working with a UC3827 probably requires a bit more than my basic SMPS knowledge.
Yes... Not for the faint hearted. I would not suggest you try to implement an off-line power supply using this method but it might fall into your comfort zone if you consider using a 50Hz lump of iron to give you an isolated 15V supply with which to feed the chip.

http://www.farnell.com/datasheets/312001.pdf
http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf

Second link is to SLUP117.pdf which TI do not appear to wish to give you unless you create an account with them. Unfortunately it is a bit overboard in that it goes into a great level of detail for most available topologies before settling on the UC3827. Then it still tries too hard...

If you have basic SMPS knowledge, you mention the peak current mode UC3843, then getting your head around the UC3827 should not cause too much stress. Using a lump of iron to supply things simplifies your feedback, no need for opto-couplers or similar.

Once you get the idea behind average current mode control and slope matching then design of the internal current loop and external voltage loop is a veritable pleasure. None of this slope compensation to avoid subharmonic oscillation as with peak current mode control.... not strictly true because you still need it for a stable peak current limit.

If you want I can slap together an idealised LTSPice model of the beast so you can have a practice. How many 6V3 Heater Supplies do you need and at what current? How many High Voltage Supplies do you need and at what voltages and currents?

I can take you through the compensation of the thing and you might be pleasantly at the level of performance that should be possible. <50uS[20uS] response times to step levels in load and other nice numbers.

Biggest fear would be that you probably should not knock one up on a bit of Vero Board. It might work but a PCB is recommended. Depends on your resources.
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Old 3rd December 2017, 03:36 PM   #8
Rootz is offline Rootz
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Iron to a dc-dc converter was my first idea actually. Some nice converters on fleabay. 12V 24V to DC 200-450V 70W High Voltage Boost Converter Step Up converter New | eBay

Good for up to 40W continuously, although I have no idea what the resulting dc looks like. I also don't know if different filtering with an additional LC would improve matters if needed. It looks like a flyback converter, so diode -> C.

Then again, how can I elevate the preceding 12V supply without affecting the DC-Dc converter? Can that even be done?

Now if I could find me a 12V to 12V isolated DC converter, the problem would be solved. Power the whole thing - separate DC-DC converters for LV and HV that is - with the 12V toroid I was mentioning earlier and elevate the output of the 12V converter via a divider form the HV. Done.

Or power two offline flyback converters and use off the shelf Wrth transformers. Will post a schematic next, to clarify.

I'm just looking for a single 12.6V @ 750ma heater supply. Or 6.3V but double the current. I'm not sure how much current the tubes draw when cold. Cold resistance is 14 Ohms for 12.6V.

It would be great if you could make an idealised model, but please remember that it might just be for educational purposes.

I use vero just for low frequency setups and even then I almost never use it. I can build single sided photo's, although i generally prefer to outsource for more permanent setups/work. For small boards I send my stuff to OSHpark for example.
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Old 3rd December 2017, 03:39 PM   #9
Rootz is offline Rootz
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450 V with an LT1245 offline. At least it works in sim
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Old 3rd December 2017, 03:45 PM   #10
korpberget is offline korpberget  Norway
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Iron to a dc-dc converter was my first idea actually. Some nice converters on fleabay. 12V 24V to DC 200-450V 70W High Voltage Boost Converter Step Up converter New | eBay

I used one of those to power B+ to a se EL84 stereo amp. Tranny for heaters. Worked like a charm!
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