I received an unexpected opportunity to perform electro-acoustic measurements on a pair of 604E Super Duplex speakers in early 2008 thanks to a long-time owner seeking improvement in their sound. He had been using the original Altec Lansing N-1500A series type crossovers, and wondered if an improved crossover implementation might make them sound better. I had wondered that too. The drivers had original factory cones and voice coils, as well as identical production date codes of 391611, signifying the 11th week of 1966. My initial measurements suggested weakened magnets, so the drivers were sent to Great Plains Audio in Oklahoma City, OK for a recharge.
The N-1500A Crossover
The first order of business after the recharge was to re-measure the drivers for impedance, output magnitude and phase. They looked much healthier. I next modeled the Altec Lansing N-1500A crossover in CALSOD. The N-1500A is a series-connected type crossover that utilizes a second order high-pass filter for the compression driver and a first order low-pass for the woofer. Series crossovers are a rarity among speaker manufacturers these days, as they have been abandoned for parallel designs which are easier to design and implement. Follow the link in the left margin to see the N-1500A schematic. I was very curious to see how CALSOD would predict the driver/filter combination’s behavior.
The CALSOD screen shot above shows the 604E’s predicted on-axis response using a stock, series type N-1500A filter. Keep in mind that CALSOD uses the driver’s actual measured performance imported directly from CLIO and then applied through a model of the desired crossover filter. The compression driver response is shown in blue, the woofer in green and the total Sonic Pressure Level in red. The compression driver was measured with its loading cap removed. As you can see, the combined output varies +/- 3dB above 1kHz and drops rapidly after 9kHz. The two drivers are approximately 90 degrees out of phase at 1500Hz.
The CALSOD screen shot below shows the input impedance of the 604E/N-1500A combination. The phase angle is generally low and the impedance varies between 10 and 18 ohms.
The N-1500A with MF EQ?
I next wondered what the addition of Mid Freq EQ would do to the behavior of the N-1500A filter, so I modified the CALSOD filter model with a contour circuit consisting of a 6uf cap paralleled with an 8 ohm resistor, which was then placed in series with the compression driver voice coil. The mid-point of the resistor was tied to ground with a variable resistance of 10-50 ohms. See Fig. 3 below:
Compared to the stock circuit in Fig. 1, the EQ circuit depresses the output in the 1.8kHz to 5kHz region slightly, and then lifts the remainder of the HF output several dB. The input impedance of the circuit was unchanged.
I then moved on from series topology and tried several promising parallel configuration models with CALSOD. They were promising in the sense that they could result in a filter with the desired driver acoustic center phase correction at crossover, and hopefully an improved overall sound. I was specifically interested in 3rd order high/low pass Butterworth and 4th order high/low pass Butterworth filters for this driver.
A strange thing happened though – when I modeled each parallel filter its input impedance rose dramatically at an intermediate frequency. For the 3rd order BW the impedance rose to 42 ohms at 650Hz; for the 4th order BW it was 45 ohms at 500Hz. The only way to approach the input impedance of the series filter was to add more components to these parallel filters in the form of conjugate networks for control of the woofer’s impedance and to counter the rising inductance of the compression driver’s voice coil. The conjugate networks, when added, were successful in restoring reasonable input impedance to the filters, but now I had networks with three to four times as many components in them as the N-1500A, and yet had very similar performance.
The N-1500A filter is hard to equal, much less beat, when using competing parallel type designs with the 604E. An N-1500A filter with upgraded components and an MF EQ circuit added is probably the most cost effective crossover improvement one can make for this classic duplex.
Getting the magnets re-charged on 42 year old Alnico drivers really helps too!