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kevin gilmore

Megatron Electrostatic Headphone Amplifier

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please bear with me but I try to understand the trafo/cap thing on the GRHV supplies. 

By reading this thread and the Carbon one from which the GR supplies emerged as far as I can tell we have to consider

- 1.414 factor for rectified DC

- minimum of 20V drop voltage across the regulator

- 20% headroom on caps rating to assure long life 

- account for 10% max fluctuation of land line (230V here; governmental docs here confirm this)

 

For 450V output I calculate 450(output) + 20(drop) = 470VDC prereg minimum / 1.414 = 332VAC minimum + 10%(max line flux) = 365VAC nominal output from trafo @ 120mA (which is the current the Megatron draws I read correctly). 

For the prereg cap we should use the max values. So 365VAC nominal + 10%(max line flux) = 402VAC max * 1.414 = 568VDC max prereg DC. Accounting for 20% overhead on caps to assure long life we come to 680V rating on the prereg cap. 

Am I way too conservative here, missing something or clearly not knowing what I am talking about?

Steef

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You would want your trafo to be capable of delivering more than 120mA current per rail. I would personally spec at least 250mA per rail for Megatron use. I am sure there are others that would prefer going much higher.

Also, base on my own experience, 20V drop across the regulator may not be sufficient for 450V regulated output under load.

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Too conservative and you counted 10% line variation twice. Stick with 350-360vac transformer with 550V caps and you will be fine, as 600V+ caps are hard to source and they're probably very large. You can always measure your mains voltage to take some guess work out of it (be careful). 

YMMV but personally I find it unnecessary to go above 400V rails for any amp unless you are like pabbi1 and want to blow out your eardrums, or just like overkill. Going from 400 to 450V only increases headroom by 1dB anyways, and as you can see makes life harder. 

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Thanks for your feedback. I agree to use a lot of trafo overhead but I just stated the trafo should be able to deliver 120mA @ 365VAC output (because higher current == lower voltage).

If 20V drop is too low how much should we account for?

I used the 10% variation twice because the land line (theoretically) can go as low as 10% below nominal and 10% above nominal. Maybe account for both 10% line overload AND 20% caps overhead is a bit over the top though.

So, if we get a 350VAC trafo and use 25V dropout (for example) we can handle max 4% lower land line before PS falls out of regulation (350V ×,96 (4% lower land line) ×1,414 (rectifier) −25V (dropout) = 450VDC). Right?

I reckon our land line is pretty stable so indeed should not be a problem.

About the cap rating: with nominal output of 350V on trafo we have nominal unregulated 494VDC on first cap. That is at 89% of its max voltage rating of 550V. 'The internet' states larger margins - 20-30% - for this but the common sense here is that's overkill?

With these numbers a 550V cap accounts for a max of 11% higher landline before it gets out of spec DC from the rectifiers. That indeed seems pretty unlikely.

 

If we can feed the Megatron with +-400VDC without problems that indeed may be preferable. Are there other changes necessary besides the PS resistors and the 21k voltage drop resistor on the Megatron board to get 300VDC for the little tubes (should be lowered from 21k to about 14k i guess)?

Steef 

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I use 325V/250mA secondary for 400V@20mA*4 for carbon. Megatron should be almost the same.

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Megatron is probably 120 mA, that's plus 50 % compared to your Carbon.

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About load resistors to test the HV and LV sections of the PS. @kevin gilmore has stated somewhere in the Carbon thread

  • 16K / 20W for the HV (per rail)
  • 500R / 2W for the LV

Can someone explain (or are these only valid for Carbon loads)?

If I use 450V @ 150mA (to test with slightly higher load) I get values of 3K / 70W per rail (or two of 1.5K / 35W in series).

If I use 12V @ 800mA (somewhat higher load) I get values of 15R / 10W per rail

What am I missing?

Tia

Steef

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Probably not necessary to stress test the power supply, so some medium-ish load will do. Having massive heatsinks just for burning off heat from load resistors is also a PITA. As has been mentioned before, the current limiter prevents the PS from doing 150mA with the stock 5 ohm resistor. Also, for the Carbon the LV just powers the solid state front end. No tube heaters so much lower power.

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Ha! I knew that formula but didn't know if and how it could be put to use here :-)

3.9R gives me 166mA then. Should be enough for Megatron

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How to get a Megatron:
1) find Gerber files from Stax mafia site
2) download said Gerber file .zip file
3) send said Gerber .zip file to your favorite PCB manufacturer, ensuring correct specs (copper weight) and finish of your choice
4) find suggested BOM by reading this thread (also necessary for item 3, above)
5) order correct qty of parts determined in step 4 above from your favorite supplier(s)
Nothing to it...



Sent from my iPhone using Tapatalk

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