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I'm not sure I understand the theory behind a transformer wired as an "autoformer." My very basic understanding of electrical theory seems to get in the way of me contemplating how a transformer whose primary and secondaries are wired together in series could "transform" one signal to another. I had always though this took coils that were not physically connected, but inductively coupled. So, what really happens when the secondary is physically wired to the primary? How does this that work? How does taking a signal from a smaller "tap" on a long run of wire (i.e., primary and secondary in series) change the impedance if the grounds are wired in common?
Follow Ups:
The grounds are not exactly "common" on an autoformer. Think of the secondary of your average run of the mill tube output transformer. You have 16, 8, 4, and common on the secondary. If, let's say, you had a solid state amp that worked its best at 16 ohms, you could drive the 16 ohm tap with the amplifier and connect your speakers where they need to go to properly load the amp.
Now if we look at applications in tube amps, the autoformer would be like an extended secondary, and in the case of the Quickie you have a winding with 8k, 500, 8, 0. (Initially I didn't wire in the secondary, so it was really 8k, 500, 0). The ground of the primary connects to the 8 ohm tap on the secondary, and the secondary common is now the ground for the autoformer.
The downside to all of this is that you cannot have any DC on the coil! If you have a SEX amp and have the manual handy, you will see that some effort has been made to supply the output grid with a negative bias and to leave the cathode grounded to get the most optimal performance from the autoformer. This is the main difference between an autoformer and an output transformer, the autoformer is devoid of isolation.
The other downside is that pulling a truly balanced output off an autoformer is quite the challenge.
> > The other downside is that pulling a truly balanced output off an
> > autoformer is quite the challenge.
I have always wondered how autoformers might work for 'single-ended'
push-pull. If you put a 10k:10k transformer with a centertap at the
input of a S.E.X. amp for phase splitting,and then connected your speaker
across the hot speaker posts, what problems would arise? That would be
a quick conversion to a (perhaps) 6 watt monoblock. The two autoformers
would be balanced across a common ground with DC blocked at both HV
ends.
Skip
I will try it and let you know!
(Building the SEX amp as a push-pull amp from the get go would lighten up the iron load quite a bit)
I appreciate your picking up on this. If I had a suitable
transformer, I'd have tried it already myself. I remember
Phil Sieg's Marcel and Clovis which sounded fabulous at
VSAC in 2001.
Skip
I'm certainly no expert but since none have answered yet, I'll take a stab at it.
Because the secondary is connected "electrically" and inductively, the voltages are combined. So if your running the primary off of 100v and the turns ratio is 1:4 you either end up with 140v or 60v depending on if the secondary is wired in phase or 180 degrees out of phase. Decreasing the gain comes in handy in those power adapters for use in other countries that have different standards.
One of the big drawbacks with having the primary and secondary wired together is it will not reduce hum as in a inductively connected transformer.
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