Power supply design sanity check please...

[MakerDP]

So I am building a London Power Z-Pre preamp kit and I am designing my own power supply for it since I have everything I need for that part of it on-hand.

Requirements:
12.6V elevated heaters (kit design)
12Vdc Relay switching (my choice since heaters are also 12V)

I am using an Antek power transformer with dual 6.3V secondaries wired in series for a single 12.6V heater/relay supply. I have never wired secondaries in series before, but I am OK with that. E-mailed Antek and they confirmed it was OK. Test wiring showed I know how to do it to get the required 12V.

Now here is where I am wanting to make sure this design is OK: I am going to use elevated heaters. Is it correct to reference the elevation off the center-tap created by the series connection as shown in the attached schematic? Or should I not have a center-tap and reference it from the bottom leg that feeds the bottom of the bridge rectifier?

I know that I must keep the ground reference on either side of the bridge rectifier (AC vs DC) completely isolated.

Thanks for looking!

[Malcolm Irving]

Using the junction of the series 6.3V windings as a ‘centre-tap’ is fine (as you show in the schematic).

There are two bridge rectifiers. The 0V at the output of the 12V regulator circuit must remain isolated. The 0V of the main B+ should be grounded.

[MakerDP]

Thanks for the confirmation... and yeah I just forgot to put the ground for the B+ supply on the schematic. I was only referring to isolating the 12V supply in my OP. Thanks for catching that.

[trobbins]

Perhaps try and use UF4007, and read up on how to wire a secondary to a diode bridge and first main filter cap - as that is where a hum/noise can originate.

[R.G.]

There is no such thing as sanity in power supply design.

[KevinOConnor]

Hi Guys

The 12Vdc will be floating and NOT tied to ground at either end. For an internal switching system this is no problem, but if you want to have a remote footswitch, you have to consider that BOTH end of the switch are elevated from ground by the DC-stand-off value.

It is also better to use a faux-CT to tie to the DC-stand-off rather than the real CT. The latter is not necessarily truly centred where the faux-CT can be, affording better hum rejection. Of course, the DC-stand-off swamps any of this nit-picking, but nits are there to be picked... hehe

If you are going to the trouble to make DC for the relay, you could easily apply this to the heaters, too and avoid the DC-stand-off, and thus allow a safe ground-reference remote switch as well.

Have fun

[MakerDP]

Yeah I may just go ahead and do it all DC. At the time I drew up the schematic, I did not have any high capacitance capacitors for proper heater filtering but I do have some now. It will probably save some headaches in the long run and I never considered that the remote footswitch would not have a proper earth-ground with that scheme... so thanks for pointing that out!

Also, I don't know why I said "Z-Pre" in my original post... it's actually the "D-Pre" that I am building.