The Extra Stimulus Twin Iso 8

By James Lehman
11 Feb 1996

The crossovers

This Fig. 7. Picture of the crossover is a picture of both of the crossovers Fig. 8. The crossover circuit, side by side. Yes, they really are 30" by 8" each, and probably weigh about 35 lbs. each.

They are 12 dB per octave, fully cascaded networks. Each high pass filter section (from the bottom up) dumps into the next one above it. The center frequencies are: F1 = 250 Hz, F2 = 1080 Hz, and F3 = 4224 Hz. This design offers tweeter protection that is 12 dB/oct from F3 to F2, 24 dB/oct from F2 to F1, and 36 dB/oct from F1 down. The midrange also has the advantage of 12 dB/oct protection from F2 to F1 and 24 dB/oct from F1 down.

The coils were wound using AWG 16 magnet wire and a hand cranked machine that I made, with a turns counter. The coil formers Fig. 9. Coil formers are 1 1/4" diameter wood dowels with 3/4" thick particle board sidewalls glued together with epoxy. Epoxy was spread over the entire surface of the former to prevent the wood from absorbing moisture from the air.

All of the coils and capacitors were carefully checked for value accuracy. This was done by placing the part in series with a precision power resistor of a known value (in this case, 24.9 ohms). A sine wave, of the frequency that would make the part (L or C) of the correct value impose the same impedance on the signal as the constant resistor (R), was applied to the circuit. Then, a voltage reading was taken (with a digital AC volt meter) from across both the part in question, and the resistor, individually. When the voltage drops read the same, the part was the right value. The sine wave signals came from my computer. I wrote the sin("time") function at the needed frequencies to WAV files, and played them back through my sound card.

All of the coils that I made for these crossovers were tuned to within less than a half of a turn of wire (out of about 100 to 400 turns, depending on the value of the inductor). All of the capacitors in this crossover are rated at 100 volts and are groups of at least three. I used the values printed on the caps only as a guideline, and picked combinations of caps, that I had, to get as near to the needed values as I could (within about 2 or 3%).

The isobaric coupled woofers are wired in series with each other, and the two isobaric pairs are wired in parallel. This has the effect of joining the voice coils of the isobaric pairs end to end. Any bucking voltage (reverse damping factor), coming back off of the voice coils, is distributed across the pair, instead of to each woofer separately.

I put a switch in the line leading to the woofers' section of the crossover to be able to kill the woofers' output. I used L-pads to accommodate balancing the outputs of all of the drivers, of higher efficiencies, above the woofers and included SPDT switched non-inductive 8 ohm loads that can be switched in to replace the load and kill the output of any one of the drivers above the woofers. This makes it possible to hear the output of each stage of the system individually, with the effect of the L-pad settings, and without any interaction of the lack of the other drivers loading the crossover.

At last--the sound
The overall sound of this system is remarkable. There is no doubt that they really do put out at 20 Hz. There is very little noticeable intermodulation distortion, because of the separation of the audio spectrum into 4 nearly equal bandwidths. The average, slow response SPL reading in a room is somewhere around 106 to 110 dB at about 100 watts - very clean. I can't really comment on how much power they will handle, because my amp is only 150 watts / ch. and it breaks up way before the speakers do - not good for my tweeters!

My next project is going to be The "ES Twin Iso 10". Basically, it will be a very similar design, using the 10" version of this style of woofer (MCM 55-1215), an 8" carbon fiber mid bass (MCM 55-1550), and probable the same midrange and tweeter. Of course, I'll have to get a better, more powerful amp.

Thanks for checking this out.

James Lehman

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