Altec 604-8KIn early 2008 I had the opportunity to evaluate a loaned Altec Lansing 604-8K utilizing the CLIO system. The driver was an early unit (serial # 3403, date stamped DEC 1982) and was equipped with the original factory low frequency cone and high frequency diaphragm. This model was manufactured by Altec from the early 1980s thru the company’s demise in the late 1990s and was marketed for use as a studio monitor, musician’s stage monitor and a premium ceiling speaker. The 8K had 8 ohm impedance voice coils and was supplied with a dual EQ passive crossover that had first been introduced for use with its ALNICO magnet predecessor, the 604-8H.

CLIO Testing and results

Testing of the 8K revealed frequency response very similar to the 604-16X that I had tested in 2006. The 8K however, had a slightly greater and smoother sonic pressure level (SPL) overlap between the two drivers in the 1kHz to 2.5kHz region and a somewhat lessened compression driver peak around 2.8kHz. Because these two models both shared the same horizontal voice coil spacing, it seemed logical that the same topology I had utilized for the 604-16X crossover might be the optimum choice for the 8K as well.

CALSOD Modeling

I initially worked in CALSOD with the Butterworth 2nd order/ 30% spread models I had built for the 16X, and it came as no surprise that the greater SPL overlap of the 8K in the 1- 2.5kHz region showed up as a broad peak in output using the original 1650Hz crossover point.  The center point of the crossover was then adjusted downward from 1650Hz in 50Hz increments and re-calculated in an effort to achieve a flatter sonic pressure level in this overlap area. When I reached 1500Hz the model’s combined output looked nicely flattened and I was happy… least until I looked at the filter’s input impedance curve which had risen to over 20 Ohms from 500Hz to 1kHz. It was certainly a usable filter, just not optimum. I substituted several sets of SPL optimized component values in an attempt to reduce this impedance peak with no improvement. I then substituted a model of the 604-8H/8K OEM filter, a 3rd order HP and 2nd order LP design centered at 1500Hz, and to my pleasant surprise found that it had reasonably flat combined output in the crossover region and also had the drivers virtually in-phase at 1500Hz. A check of its input impedance curve showed both impedance and phase quite flat and controlled across the peaked region seen in the 2nd order filter. Keep in mind I used the exact same circuit values found in a 604-8H crossover for this. Follow the link for the Simplified 604-8H EQ Crossover to view, download or print a copy. I made a few attempts to optimize the SPL level a bit more with CALSOD, but finally concluded that Altec had already pretty much done that work.

Conclusion and Recommendation

It appears that the Altec engineers had their stock crossover filter nicely adapted for the 8K. I therefore recommend my Simplified 604-8H EQ Crossover for use on the 604-8K by DIY constructors. Unfortunately, this filter does not get the driver’s outputs quite as flat as the Butterworth 2nd order 30% spread filter does, but it is nicely balanced overall and presents a much more consistent impedance load for your amplifier to drive. For those of you that have stock 8K crossovers, I have no doubt that they would benefit from an upgrade in the quality (and cost!) of their parts as I recommend in my Recommended Parts – Substitutions – Construction Hints page.

GPA's Vintage Altec E-Zine Jeff Markwart's Corner