Drive unit sensitivity matching

[alnewall]

Current speaker drive units are 96db full range with 91db bass.
Is it possible to use a transformer to boost the output of a bass unit rather than attenuating the full range with a series resistor? Which is how i'm matching the speakers at the moment.
There is no crossover as such, just a low pass filter for the bass.

[Scottmoose]

I doubt it, but there are others here who'll know better than me. What drive units are we talking about here BTW?

[Primalsea]

The transformer idea seems to me to be a bad one. To get the sound quality you probably want the transformer will have to be large and expensive. Not only that it will more than likely be a custom job (extra expense). Thats even if its possible to design one.

IMHO I would say your options are:

Active crossover for which you will need an extra stereo amp.

Drive the bass and full ranges with their own amps using the passive crossover. For this you will need one amp to be 5db louder than the other. An option would be to use any extra amp you like and use inline attenuators to match them if the amps dont have volume controls.

WARNING! Cheeky note follows;
I do make such attenuators and sell them on Ebay. Standard is -14db but I can make them to any rating.

However as this is a DIY forum I will be happy to give you the info to make a set of attenuating leads if you want to have a go yourself.

[Phil Y]

I would agree that a transformer is a no-go. Transformers do not give you something for nothing. If you use a step up transformer to increase the voltage you will have less current to drive the bass unit, the last thing you need. It would probobly work with a treble unit where the current is fairly small though.

Regards, Phil.

[alnewall]

Thanks.
I wasn't expecting it to be viable.
Just wanted to know why.

[petercom]

Thinking laterally here, why don't you parallel a second bass unit to give an extra 3dB (providing the amp can handle the lower impedance)? Output will be closer to the full range sensitivity then.

Otherwise I agree with Primalsea in using an active crossover for the bass - you'll get better performance from your amp and full range driver that way.

[alnewall]

Thanks Peter.

A couple of ideas there.
Might be time to dig the old Gainclone out and have a play with that driving the bass.

[Primalsea]

If you are now willing to experiment my may want to have a look at the frequency responces charts of the drivers. If you can't get them maybe you can do some rough measurements yourself.

Often the sensitivity is a nominal figure that does not hold across the full FR. The figure is usually an average which can be pretty useless.

I have a set of bass drivers that have 90db sens. The datasheet says 93db but this is a worthless figure. The driver actually has as sens. of 90db across the frequencies it will be used 42 to 300Hz. This sens. quickly rises to 95db at 3Khz before dropping sharply.

93db is just a nominal average.

It's could be possible that your drivers have a similar kind of responce and at their respective frequencies the drivers may actually have similar sensitivities.

Also if your full ranges have decent low end responce the bass drivers could be acoustically summing with them to give an increased bass output which could compensate for the difference

[petercom]

I entirely agree, Primalsea, but the sad fact is that he has quoted the sensitivity of both drive units as being miles apart. Drive unit sensitivity is usually taken at 1kHz. As you say this is a meaningless figure as the output of drive units rises considerably through the upper midrange compared to the bass. Yet the bass sensitivity is what we are really interested in!

Unfortunately unscrupulous speaker sellers take the highest figure and list it in specs. That's why you get ads for bass reflex speakers with a 6 inch bass unit and 96dB sensitivity - we know it's not possible from a sensibly equalised system, but these people rarely actually measure their designs!

[Scottmoose]

They'll be selling cables next...