WD Phono III Clone Questions!

[gninnam]

OK - just been looking around and would like to attempt to build the WD Phono III Cole.

Last year I got to listen to Graeme's and really liked what I could hear.

So, like him, will be doing this on a budget and was wondering if I could re-use some old valve kit I had from many years ago.

Mike H will be able to help with this question - could I use the PSU from the Millennium power amps (already have the ECC83's) to power the phono III - obviously reduce the power output as I think it pumps out 450V and the clone only needs 250 ish?

Could I then use other parts from the power amp too.?

Thanks for any help you can give.

[pre65]

That's a lot of volts to drop !

Can be done with resistors and extra chokes for smoothing and perhaps a current tap to aid regulation as you won't be drawing anywhere near the same current as a power amp would.

Can you post the PSU circuit ?

EDIT.

Is the mains transformer secondary centre tapped ? And what sort of rectifier are you using ?

[simon]

How much current does the Pre 3 draw? Perhaps drop the voltage with a couple of VR tubes and has the added benefit of shunt regulation?

[gninnam]

Cheers chaps.
Yep it is centre tapped - here is the specc:

(taken from Mike H's website...)

[pre65]

Well it looks like a bridge rectifier is used, so if the secondary is centre tapped using a FW rectifier (valve or diode) would get you a lower HT voltage.

Best bet is to get to grips with the Duncan Amps Power supply designer software (free) and start playing with different designs and component values. It's quite easy once you get going.

[Nick]

If its a bridge, then its unlikely to be center tapped. I would do as Simon suggests, use a resistor and a VR tube to drop to 250v, the preIII only needs a few ma.

350v from the TX, will rectify to 490v (so you will need to double up the caps in the power supply). get one of Mikes little 7H chokes, 4 100uf 400v caps, 220k resistor across each cap to equalise the voltage.

Then you have about 490v after all that, you need 250v at say 6ma, use a couple of VR tubes, 0a2 and 0b2 in series will give you about 250v. 15ma through the VR tube, 6ma through the phono stage, so thats 21ma in total. to go from 490v to 250v with 21ma, you can use a 10k resistor. It will dissapate about 6w, so a 50W or 25W ali clad one will do nicely.

[gninnam]

THanks guys - will have another look at Mikes site for the chokes and what nots and start to bu some of the parts mentioned, plus have a look at the Duncan Amps Power supply designer software to.

Cheers

[Mike H]

Strewth that's a big picture

The original Millennium mains is 250VAC, 350mA (version 1) 200mA (revised version), single winding, after rectification 490 odd VDC without a load on. Simplest method is probably add a dropper resistor in front of the bridge.

[gninnam]

Tis a big picture (not 3D though...)

What rating resistor to get it down to 250V would you suggest then??

Cheers

[Mike H]

That's the $65,000 question! Offhand no idea, Duncan's PSU designer might give you a clue, otherwise means experimenting. Whatever it is will depend on the total current drain. Do we know what that will be?



 

[pre65]

I believe that Nick designed a "sooper dooper" PSU for Pre3 some time ago with VR tubes, can anyone remember where it might be on the forum ?

I would like to "improve" my Pre 3 and keep PSU3 for the phono.

Sorry to "butt in" but it has a relevance to the thread.

[Mike H]

That's a possibility mind you, start with high Volts and drop with VR's or Zeners.

Or even, if you're feeling adventurous, a valve Voltage regulator.



 

[pre65]

Is this Nick's last version ?

[Graeme]

whats the idea with the 2 transformers?

[Mike H]

Didn't have a 240 > 240?