Power supply question

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Power supply question

Postby dannyr » Sat Jun 24, 2017 3:05 pm

Hi, I have a preamp that I want to update the line stage in. Currently both the phono and linestage sections run on 300v but the changes would need to add both 275v and 200v. To get the right voltage would I run a dropping resistor into a new capacitor then repeat for the two new lower voltage outputs? If so would you happen to know what value of capacitor to use and the range of resistors that I'd need? Thanks.
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Re: Power supply question

Postby Geek » Sat Jun 24, 2017 9:23 pm

Hi,

Need more info. Make and model? That way we can find a schematic and deternine a suitable course of action.

Sometimes a dropping resistor and cap is OK, but we need to calculate the size of the resistor. Then there's the option of using a zener or regulator and decoupling that for noise, especially if it's in the phono stage.

Cheers!
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Re: Power supply question

Postby dannyr » Sat Jun 24, 2017 9:54 pm

Hi Geek. Thank you for taking the time to reply. I guess nothing is as simple as I'd like it to be:). Unfortunately, I don't have the schematic. It's a Van Alstine Fet Valve preamp. I have the changes to upgrade the line stage to a later model that uses the same circuit board but needs different voltages. I could change it over to a regulated power supply from tubecad though but I'm stuck once again with needing three different outputs from a supply that only has one.

I don't know if this helps at all but the output of the original power supply would go the phono stage and the reduced would then go to the updated line stage.
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Re: Power supply question

Postby Geek » Sun Jun 25, 2017 12:20 am

dannyr wrote:.... the original power supply would go the phono stage and the reduced would then go to the updated line stage.


Then it is simple! An R-C circuit should be fine.

Do you have any idea of the new line stage draw? You can calculate from that, or you'll have to just trial-and-error resistor values.

Cheers!
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Re: Power supply question

Postby dannyr » Sun Jun 25, 2017 11:43 am

Unfortunately I don't know the current draw. Each channel has a mosfet and a 12at7 if that helps at all. What values would you use for the added two capacitors? Would you bypass them with a film cap?

Thanks again Geek.
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Re: Power supply question

Postby Geek » Mon Jun 26, 2017 11:31 am

The capacitor will basically have to decouple, rather than filter. So I'd use 22uF rated for the maximum voltage it'll see (usually at power-on before the tubes light-up). If you have larger in your box, no problem, it's not critical here.

So, a pair of 12AT7's in there, eh? Let's assume 10mA to begin with. So you need to drop 25V and 100V to get your 275 and 200.

Here's some ballpark values and topology to begin with. Play with the first resistor to get your 200V when warmed up, then adjust the second for your 200V.

They will interact a wee bit, but I'd not worry about 10 or 15 volts either way.

Cheers!
Attachments
R-C-R-C_decoupling.png
R-C-R-C_decoupling.png (2.59 KiB) Viewed 4908 times
-= Gregg =-
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Re: Power supply question

Postby dannyr » Mon Jun 26, 2017 7:23 pm

Thanks Geek. I really learned a lot. One more basic question, what wattage resistors should I use?
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Re: Power supply question

Postby Geek » Wed Jun 28, 2017 12:22 pm

The first resistor can be a regular 1/2W.

The second resistor I use 1 or 2W weather it needs it or not, mostly for voltage handling.

Cheers!
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Re: Power supply question

Postby dannyr » Sat Jul 01, 2017 1:32 am

Thanks again for all your help. You've taught me a lot. Now I have to make a parts order to get this done.
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