Need Technical Assistance

[Deang]

If I replace a 4 ohm driver with a 8 ohm driver, can I put a 8 ohm resistor across the leads of the new driver so the crossover "sees" 4 ohms?

Or, do I have to change cap values (and wouldn't that just suck).

[Malcolm]

If a speaker's actual impedance equalled its nominal impedance at all frequencies, your approach would work. You would just waste half the power fed to the speaker. The new speaker would have to be twice as sensitive as the old one to get the same volume. And your amp would have to deliver twice the power.

Whether your approach will actually work in your intended application is another matter. Actual impedance of real speakers varies with frequency. It is almost never equal to the nominal impedance. There is a bump or peak at the resonant frequency that can be several times nominal impedance. And there is typically a gradual rise in impedance in the higher frequencies. The impedance curve typically looks more like a Nike logo than a straight line.

So, if the speaker is being driven only in a relatively flat part of its impedance curve, your approach will work with the appropriate resistor value. If the speaker is being driven over most of its useful range, like most woofers, ouput will be considerably less than expected near the resonant frequency, and less at higher frequencies.

[Deang]

I had a feeling it wouldn't be that simple - nothing ever is.

Here is the graph for the tweeter. The Dahlquist DQ-10 utilizes 1st order crossovers, and is a 5-way. The tweeter runs from 6K to 12K -- where the piezo takes over.

What do you think?

I would imagine the line at the bottom of the graph is the impedence.

If so, would a 12ohm resistor be better?

I've attached the .jpg of the crossover. It's a bear. If I can't use the resistor, I'm thinking I could lower C3 to 4uF and C6 to 6.5uF. Those values should be lower -- but since it's a parallel crossover, the values effect things down the line. The tweeter is far superior to the one it's replacing -- so I figure I can stretch it some.

This message has been edited by deang on 09-13-2002 at 01:28 PM

[Bill Campbell]

Thanks Dean for the welcome. In the pre "hornie" days I had a pair of DQ-10's. Used lots of power as I remember. What are you planning on feeding them with?

[Deang]

Well Bill -- I'll be using LOT'S of power

I picked up a used Aragon 4004 MKII for a reasonable price. If 75 lbs. of solid state won't drive them, then I'm in trouble.

Even with that, I'll be surprised if they can reach the SPL's of my RF7's with 20 watts of tube push-pull.

[Bill Campbell]

I had 140w/side and that was far from enough, but that was a while ago and liked my tunes a ear bleeding levels. I have heard them with a McIntosh MC500 in a showroom a few years back, and that was very pleasing to the ear. Had plenty of headroom.

[Malcolm]

Yep, the bottom line on the graph is the impedance curve. Resonant frequency is around 800 Hz. Impedance is rising in the frequency range in which you intend to apply the tweeter, from roughly 6.5 ohms to 8 ohms.

The engineers at Dahlquist obviously put a lot of thought and effort into the crossover design. It is way beyond my ability to recommend the appropriate crossover changes when you swap tweeters in this design. You are basically going to have to re-engineer the system.

You might try posting what you are trying to do at www.audioasylum.com. Someone there might have gone through a similar exercise already.

How does it feel to be a speaker engineer?