604 crossovers

[jmarkwart]

Hello Altec Users,

Thought I’d share a few ideas I've had about 604 passive crossovers.

I acquired a pair of 604Bs in the 90's, and had them refurbished with 604E LF cones and new 16 ohm HF diaphragms at the Altec factory in OKC. I've used them with N1500A xovers for years in a secondary A/V system, but began pursuing a crossover upgrade over the last year. My main system has horn loaded 515-8Gs and 909-8As with custom xovers, and the 604s really seemed to lag behind them in a number of areas. Active xovers were rejected due to higher costs.

One of my major concerns in getting the 604 xover "right" was compensating for the phase shift between the acoustic centers of the LF and HF sections in the crossover region. All 604s have a horizontal separation between their voice coil centers; alnico motor 604s have 5.25 inches, 605A/Bs have 4.25 inches, ceramics are around 2.25 inches. At 1500Hz the phase shifts between the sections for the three are 210, 169 and 90 degrees. Depending on the 604 used, its filter needs to account for this phase shift to ensure reasonably flat SPL magnitude throughout the crossover region.

Another concern I had was the frequency of the xover itself. It was desirable to have a xover freq and filter order high enough to ensure adequate crossover region horn loading and driver protection for the HF section, but this needed to be traded off against the rapidly rising impedance and falling non-pistonic SPL output of the LF as F increased. Depending on the 604 horn used and the steepness of the xover slope, there is a limited range of frequencies to choose from. Once you choose the xover freq, you define the phase shift between sections and need to design a filter that will compensate for it.

So what did I come up with? Well, after measuring the impedance magnitude and SPL output of my 604Bs, modeling them in CALSOD, and optimizing several filter combinations, I settled on third order Butterworth HP and LP sections centered at 1500Hz with a 20% spread. I would have preferred to use second order filters, due to the lower parts count and complexity, but they just didn't attenuate enough of the out-of-band energy near resonance of the HF and roll-off of the LF to produce a flat enough combined response. Among the rejected filters were second order Butterworth filters at both 1000Hz and 1500Hz (using 30% spreads), and the 604-8G xover (2nd LP/3rd HP) that had been scaled up to 16 ohms.

I built it, tested it, and recently installed it. Compared to the N1500A, it images far better, is very clean and detailed, and overall the system sounds better balanced from top to bottom.

Jeff

[billfort]

Would you mind sharing your design in both 8 and 16ohm versions Jeff? I know there are a few people who would love to have a look at and try your approach. Did you try a compensation circuit on the horn?

Thanks, Billfort

[jmarkwart]

Billfort,

I would be glad to share, but keep in mind I only measured and modeled 16 ohm 604Bs fitted with 25hz 604E LF cones. I haven't had the opportunity to measure any other 604 types, and other variatons might not work as well with this filter, nor need as much filtering for that matter! You could, however, scale this design down to 8 ohms if you applied an 8 ohm 604's actual impedance at the F3 points in your calculations.

The LP is a 3rd order Butterworth calculated for an F3 of 1250Hz using 24.5 ohms impedance. The HP is a 3rd order Butterworth calculated for an F3 of 1800Hz using 16 ohms impedance. The HP circuit uses a 25 ohm pot standing on a 20 ohm resistor for adjustable shelving from 0 to -6db. I'm running 5 to 6dB down at present so this may not be enough in some rooms. Output to the HF driver is taken from the wiper. (You could use a 16 ohm L-pad here if desired) The LF is connected with normal polarity (positive to + or L1); the HF connection is reversed (positive to - or L1). HP values: C1=3.7uF, L1= 1.1mH, C2=11uF. LP values: L2=4.7mH, C3=7uF,L3=1.6mH. Inductor DCR values: L1=.235, L2=.52, L3=.3.

I didn't use any HF compensation circuits for the 604B horn as its output was pretty smooth above 2kHz. It did show a nasty dip at 1100Hz however (-10/12dB), but the 3rd order HP slope limits its effect on the combined output.

I used premium parts: CFAC inductors, Hovland caps and OFC hookup wire.

Haven't got around to generating a nice schematic yet.

Jeff

[ckdf]

Jeff,

thanks for sharing all that useful information. Would you mind to tell how your optimized 604s compare to your standard horn setup? I was always interested in both approaches, but so far have only experimented with a 515g/909 combo.

Usually people have either a horn combo or a duplex at a time (and in both camps many state they love their setup). It seems you own both at the same time and as such would be qualified to give a comparison stating the pro?s and con?s of either approach.

[jmarkwart]

Claus,
I've had the opportunity to compare my 604B system side by side with two large horn loaded systems.
The first system was a pair of A7s with 825 cabinets and 511B horns/802D drivers plus 416A woofers. My second, and current system, the Exemplar, utilizes 909-8As and 515-8Gs and tractrix horns. Both of these horn loaded systems (well, horn loaded down to 130-160Hz or so, then vented below that) have an effortless and seamless midrange compared to the 604, plus a larger soundstage and more dramatic midrange dynamics. They just have a big, open, relaxed sound compared to the 604, which sounds a bit constrained when side by side.
Don't get me wrong, unless they are side by side in the same room it's hard to hear the differences between them. If I had to, I could live with either one and be perfectly happy - they are that similar. I suspect the lower crossover point coupled with the horn loading and resultant lower distortion of the woofer in the mids is the key to the differences.
Jeff