Crossovers: who tried step-up transformer

The reason why I'm asking is that there can be circumstances in a DIYer's life where one would want to keep a speaker/driver although its sensitivity would be too low for a given or new combination.

It came to my mind that these transformers that are used to step down a signal for a mid or high frequ driver can also be reversed and used to, in fact, step up the signal. I just never have seen it applied by anyone so far.
You maybe are reffering at using public address step-up and step-down trafos of 25, 70 or 100V?! I'm using such trafos to negociate a better voltage/impedance range after my DCX/Amps to my high sensible horn drivers. Now, I don't need anymore those 20-30dB resistive attenuators on DCX outputs. Also, no more special loudspeaker cables; on a simple cat.5 UTP I am running 3 ways to my speakers.

I'm thinking that your intended use (crossovers for drivers with big difference in sensivity) make those PA trafos good candidates also.
If you were to find a transformer that had the desired ratio, just keep in mind that the amplifier and crossover would be driving a lower impedance, because the driver's impedance (as seen by the amp and crossover) would be stepped down by the transformer. This may or may not be a problem, depending upon the impedance curve of the driver and the ratio of the transformer. This approach would also result in the driver being driven by a higher impedance, reducing the damping factor.

You might consider using an autoformer, such as the Universal Transformer #3619, instead of a transformer. The issues of the above paragraph still would apply, but you might have an easier time obtaining the desired ratio. The #3619 has taps for several different attenuation settings, perhaps one would be right for you. To use it as a step up, connect the driver to the input terminal, and connect the signal input to one of the output taps. In my experience, it is necessary to use a swamping resistor in parallel with the autoformer.

Another approach, if you are using a tube amp, might be to feed the less-sensitive driver from a higher-impedance tap. In other words, you would simply be stepping down less to feed that driver.

Gary Dahl
Thanks Dorin and Gary for your thoughts and tips. Yes, it's acually an autoformer I was thinking of; I'm not familiar with the ELA 70/100V technology and will have a look at this too.

The driver I want to boost is the Stage Accompany 13ohms tweeter; I guess I should get 4-6dB level increase by the time I reach the 3ohms impedance that the amp would see.

Gary: can you elaborate more on the role of the parallel resistor you mention?