Crossovers: who tried step-up transformer

[elac310]

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.

[DorinD]

Hi,
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.

[g3dahl]

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

[elac310]

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?

Thanks!