Daisy-chaining transformers

[Wingfeather]

Is it possible to connect two toroidal transformers in series? I'm having trouble finding toroids with more than about +-55v, but some designs I see use much higher voltages. For instance, G. Randy Slone uses +-85v rails in his 400W/8Ohm design.
So in effect what I'm asking is could I connect 2 +- 40v (or whatever) units together in series? Since they'd be run from the same AC source they'd be in phase, and provided I get some high-current units their max current wouldn't be exceeded. Seems to me it'd just be like a toroid with 4 secondaries...

Anyone ever tried something like this?

Cheers,

[SY]

Yes, you can, but you must phase them properly- one connection will give you zero volts (if these are identical secondaries), the other will give you twice the voltage (ditto).

[Wingfeather]

Thanks. Another point is about maximum current limits. On the toroid specification pages the +-40v units are rated for, say, twice the current as the +-80v units (not that there are any of those...). However, is that still true when the 40v unit is part of an 80v overall unit, or will I have to limit my power output to that of one of the transformers (assuming they're identical)?

I hope I worded that question ok!

Thanks

[pinkmouse]

Just a quick point, a rectified 55v traffo will give you 55*1.41=77.5V.

Surely this is close enough to 80V for what you need?

[Aud_Mot]

Wing,

Remember, The voltage rating for a transformer are rated in Volts AC. The +/- rails are in Volts DC. When you rectify and use a capacitor loaded transformer, you should get about (but not exactly) 1.4 times the AC value for your rails.

In the real world: an amplifier with +/- 60 volts DC rail will not put out much more power or certainly not sound much louder than a +/- 80 Volts DC rail amplifier.

Calculate in much difference a greater rail voltage will yeild. 33% more power equals about 1-2 dB louder.

If it were my choice, I would choose a transformer with a greater current capacity, or spend my money elsewhere in the amp. Remember, the higher rails will require you to buy larger filter caps and larger case to contain the larger transformer. It gets expensive quickly.

IMHO.....
Unless you are have a specific need for that amount of power or you want to be able to say "My amplifier puts out 400 watts" - stacking or stringing transformers in series would not be worth the trouble.

Have fun, the Sloan book is very good. Make sure your read the chapter on power supply design.

Aud_Mot

[Wingfeather]

Thanks for the replies, guys. I did realise that both the primary and secondary ratings are RMS voltages, giving peaks of sqrt(2)*Vrms, but I seemed to have my facts wrong. I had it in my head that a +-50v secondary would give 70.7v only when unloaded, but that when the rails had a current flowing this would drop down to the RMS voltage...
I was actually wondering how that could be the case - I think either I read a badly-written explanation or I took the wrong meaning from a good one.
Your version makes a lot more sense!

So now, the real explanation: A +-50v transformer will give rails of +-70v...

*guesses*

This'll be because the filter caps get charged up to 70v when the secondary voltage peaks there, and when the secondary voltage drops away from peak the capacitors briefly take up the job of supplying power.

If that is about right, I'd still expect the power supply rails to be a volt or two below the 70v because the caps won't be able to supply the full 70v for more than an instant... is that true? Or is the difference so small as to be insignificant?

As you can see, I don't know a whole lot on this subject, so all help is much appreciated!

[Aud_Mot]

For Power Supply Help:

http://duncanamps.com/psud2/index.html

Good Man, that Duncan.

[gammelmalle]

if you still have problem (I think the answers were rather good) but want to get more voltage, heres what I would do:
Use 2 tranformers , rectify and smoote them off , the secondary and THEN put them in series.
The benefit ? Is that you can use lower voltage caps (MUCH cheeper) and tailor your output much more easily.

cheers,
björn

[Aud_Mot]

Warning:

If you are going to put DC power supplies in series or parallel, isolation diodes are strongly recomended.

Aud_Mot

[PassFan]

One thing to be aware of:

Transformer windings have a polarity, if you parallel a dual winding transformer backwards they will buck each other and the current draw will be massive. I have done this and the fuse blows quick. If you set two transformers side by side and get the polarity wrong it could cause premature heating, maybe? The windings wouldn't be sharing the same core but would the different phase directions cause overheating..... questions, questions.