How do I tame screwy voltages on an Xformer

[drdagor]

I have access to some nice transformers that are perfect for a good preamp or a baby gainclone, EXCEPT...

The voltages are 16.5 and 33 (essentially 50 volts with an "off-center" tap)... It's not something civil like 33 volts C.T. ARGH! What to do?

I could build a discrete component regulator for the 33v side that would get the voltage down, but thye regulator would have to deal with the big voltage drop.

I could build a regulator for 33v side using the "floating reference terminal" method outlined in LM340 App notes and use a pass device in front of the LM340 to get the higher voltage tamed.

I could use the 33 volt side to power op-amps single sided, and then build a synthesized ground like is called out in the Burr Brown single-supply op amp app note (AB-067) (or in the National Overture parts app notes). Not my cup of tea.

I could use the 33v winding as my secondary and then use the 16.5 volt winding inverted to provide a ground reference. (+16.5v - 16.5v = 0v) Wierd but possibly workable. I wonder what the current return path would be on such a hook-up.

I could start poking holes in the varnish and try to find a CT for the 33 volt winding.

Any recommendations or experience among the membership?

[head_spaz]

I don't quite understand what you are saying.
Are you sure you're measuring this right?
How many leads on the secondary?

Are you sure it's not 16.5 - 0 - 16.5 ?

16.5 volts is exactly half of 33.

If this is the case, ground the center and you have two symetrical 16.5v outputs.
This would be within the ballpark of running a gainclone driving 4
ohm speakers, if they provide enough current. (VA)

[FastEddy]

" ... How many leads on the secondary? ... Are you sure it's not 16.5 - 0 - 16.5 ? ... 16.5 volts is exactly half of 33. ..."

That would have been my Q&A as well ...

" ... within the ballpark of running a gainclone driving 4 ohm speakers, if they provide enough current. (VA) ..."

How much does each transformer weigh?? ... This is a poor man's way to guestimate the power available, the ballpark "VA".

[drdagor]

Nope... Unfortunately, it appears to be two secondaries with two different voltages -- not symmetrical... One is 16.5 volts, and the other is 33. With an ohm meter I don't find a center tap, and one of the two secondary windings has twice the DC resistance of the other.

Like THIS...
16v 33v
[---------------] [--------------------------]

NOT THIS...
16v 16v
[---------------] [---------------]

Four secondary leads. First pair DC isolated from second pair. First winding 16.5 volts and second winding is 33 volts.

Yes. I understand that 16.5 is half of 33 (except on Thursdays). And I bought the darn transformers thinking that the two secondaries were the same (as any sensible transformer maker would make them). But NOOO... This one is screwy (which is why it was cheap).

[seventenths]

Drd,
Where did you get the transformer? local?


7/10

[gmphadte]

Use two transformers for 1 voltage
2x16.5volts
2x33Volts

But check your cofigurations so that no winding shorts.

Gajanan phadte

[drdagor]

The best suggestion so far is to check to see if the 33v is on the outside of the toroid, and if it is, remove half the windings so there are two 16v secondaries.

Any other ideas?

[seventenths]

Do you only have one such transformer? If you'd like to build a GC, you should probably look into other trafo options. The 33V secondary will yield ~ 46VDC at the rail(s). the 16V will yield 22.5VDC, but why use half of the transformer?
All hail the experimenter, but I can point you toward a good deal locally if you so choose.


7/10