Electro Voice FM12-2 crossover schematic

[tgmartin]

First time poster hoping that I'm not inappropriately busting in. I couldn't find this particular issue anywhere on the site or the greater interwebs so ...

I have a number of EV FM12-2 PA floor monitors (2 way, EVM12L woof and T35 tweet) that sound very different from each other. They are from various years and were acquired at different times. In trying to research these speakers I've been unable to find a schematic for the crossovers so I drew the one below from an example that I have. I haven't taken them ALL apart but the ones I have that work and that I've disassembled appear to be like the drawing. I do have a single uninstalled crossover that has a different "protection circuit" but otherwise appears to have the same parts and construction.

The parts from one example as found, measured with Atlas LCR40 tester:
C1 = 16uF (actually 2 parallel caps, 10uF and 6uF)
L1 = 1.3mH
C2 = 1.5uF
L2 = .44mH

I'm not absolutely sure that these measurements are correct as I don't have much experience with this tester. The inductance figures are indicated as measured at 15KHz. I was led to believe that inductance was relatively independent of frequency. Is that correct? More to the point, does anyone have experience with the Peak Electronic Design Atlas LCR40 and inductor measurement accuracy?

Here's the real problem - these numbers appear to produce a very wrong result when applied to various online 2nd Order crossover simulations. Wrong in that the Electro Voice docs mention a crossover frequency of 5KHz. The parts values above seem to generate a crossover of approximately 2.5KHz and it "looks" very uncharacteristic. See included pics. I understand that the caps may have drifted but I'm not sure how far to expect the inductors to have possibly wandered.

Does anyone have a real EV schematic for these FM12-2 or FM12-2A speakers or the at least the original parts values? Any other guidance?

I hope I haven't committed any unforgivable sins here. Thanks and, Help?

-Tim

 

[tgmartin]

I finally succeeded in finding an "official" schematic for the EV FM12-2A model which is essentially the same system as the FM12-2. The schematic above, with as found values pretty much agrees with the factory drawing below with the exception of the LF section cap and polarity of HF section. The factory says 9uF but my example reads 16uF. The EVM12L LF driver is rated 8 Ohms and the T35 HF driver is 16 Ohms.

Using these Factory values, the speakerwizard.co.uk crossover calculator shows crossover at 2920Hz with a serious dip at center point. Anybody know if this calculator is factual? If so, the EV published crossover point of 5000Hz would be wrong and the tweeter would be operating very close to its minimum LF point.

Any expert observations? Wild ass guesses?

 

[RTally]

The problem with relying upon crossover calculators is that many use nominal impedance instead of the actual impedance at the crossover frequency. What really matters is the driver's impedance at the crossover frequency, which is often different than the nominal impedance. Using a driver's nominal impedance for these calculations results in inaccurate results.

Ideally, you want to use the impedance vs. frequency graph for each driver. REW is a software package that can do this by connecting the driver to a computer's soundcard. Once you determine the actual impedance at the crossover frequency, you can plug that into the calculator and get a more reasonable result.

One benefit to REW is you can use it to measure the result of the actual crossover on the drivers. Crossover design is often an iterative process of tweaking and testing and tweaking and . . .

 

[tgmartin]

Thanks for the great info. I'll see what I can dig up. I know there are published Impedance/Freq charts for the woof. Not so sure about the T35 tweet. I've already learned so much! Thanks again, RTally!

 

[ldjohn]

its a little late... but don't suppose anyone has component values for the inductor on an SX100 horn... they're pretty old so its not the + version?
thanks in advance