Enter the H Model…
In early 2004 I acquired a pair of 604-8Hs, the last ALNICO magnet duplex model Altec produced before they switched to ferrite magnets. It was also the first Altec duplex model that combined the Tangerine phase plug with a constant directivity high frequency horn and a crossover that utilized dual equalization of the compression driver output. I swapped out my 604Bs for these Hs using the same cabinets, and was immediately impressed by the differences in output – well controlled upper mids, significantly more high frequency extension, improved imaging detail and more apparent bass & mid bass. What was in that Dual EQ crossover? Could it be adapted to older 604s as well?
Crossover Topology
I spent a few hours tracing out the crossover circuit, and enlisted Jim Easley’s help to organize it into a usable and readable format. Jim did an excellent job.
At the basic level the 604-8H filter is a parallel 3rd order HP and 2nd order LP filter, with L pad shelving of all horn frequencies when the EQ button on the front panel is in the released position. Pretty much like a stock 604-8G filter. However, when the EQ button is clicked inward to the depressed position, the similarity ends. The older style shelving circuit is disabled, an adjustable, frequency dependent, contour circuit is pulled in for the upper mids, a high frequency trim circuit becomes active, and a driver-side series resistor further shapes the rising HF slope. At the recommended dial settings the upper mids are appropriately depressed relative to the high frequencies, and results similar to the famous Mastering Labs and UREI 604 filter implementations are achieved. Unfortunately, this Altec filter is not time domain corrected like the others – so I naturally wondered just how would it sound if applied to my phase corrected 604B filters?
MF EQ on a 16 ohm 604B?
Adding the mid frequency EQ function to my 604B circuit was pretty straight forward. The old style shelving circuit was deleted and a tweaked contour circuit was substituted in its place. I used a 7uf capacitor instead of Altec’s 6uf, and modified the resistor values slightly to achieve a -3dB to -9dB MF EQ normal operating range with a 16 ohm speaker load. I chose not to add the high frequency trim circuit or driver side resistor as I had found myself running the 604-8H HF control wide open.  Yeah, yeah, I know what you’re asking…..but how did it sound?!? To make a long story short, I put the 604Bs back into the cabinets and immediately heard all the improvements listed above for the 604H, plus razor-sharp transient attacks from rim shots and piano. The improved transients were probably due to the phase correction through the crossover region. Don’t get me wrong – I’m not saying the B was now as good as the H! – but it was much improved over its standard shelving circuit/L-pad equipped brethren. It was still a bit too hot above 4kHz for my taste however, so I added a 2 ohm series resistor after the contour circuit to shape the rising HF slope a bit, and also lowered the resistance value between P1 and ground to achieve a wider -5 to -15dB MF EQ normal operating range. These two changes put the MF EQ’s pot dial near its midpoint for normal listening, and really dialed in the compression driver balance above 4kHz to my ears. Feel free to vary the series resistor following the contour circuit between 0 and 3 ohms to find the the high frequency slope that suits your ears and room.
The previous paragraph applies as well to the Altec 605A or B, although I didn’t have either of these drivers on hand to audition. I believe you can build it with confidence however, as feedback from the first MF EQ equipped 605 phase-correct crossover constructor has been positive. Or, if you have already built the original 605 crossover contained in these pages, I urge you to remove the L-pad and substitute the 605 EQ circuit components in its place.
Follow the Altec Lansing 605A/B Phase Correct Crossover with MF EQ link to view the circuit schematic.
I have also added a note to this schematic concerning differing component values for users of the early High Frequency voice coil/diaphragm. See the Recommended Parts – Substitutions – Construction Hints page for a bit more detail on this, and other topics, that you may find helpful.
How about MF EQ on a 604-8G, or an 8-H, but simplified and with better parts and no switch?
I’ve had several requests from 604-8G users for an 8 ohm version of the MF EQ crossover. My solution was to simplify the 604-8H circuit by eliminating the switch and Hi Freq EQ, substituting premium parts, and drawing it up. Follow the Simplified 604-8H EQ Crossover link to view an easy-to-build crossover for your 604-8G that includes the Altec MF EQ circuit.
And finally, a phase-correct crossover for the 604-8H!
This one has it all for the 604-8H – follow the Altec Lansing Phase-correct 604-8H Crossover with MF EQ link to view this circuit.
