Recently my brother wanted me to look at his Altecs and see if I could optimize the passive crossovers. I’m no expert but I wanted to look at it objectively and methodically and see if I could help within the limits of my own ability. I want to emphasize this is not my vocation but simply a hobby, however a very passionate one!
His speaker system consists of the following:
Altec 421-8C 15″ woofers with 416 cones. So I guess that makes them 416-8C?
The cabinets are an Onken style and are constructed from 24mm baltic birch plywood. The internal volume is 230 liters and is tuned to 40 Hz.
My measurement setup, which consists of the Dayton UMM-6 Reference measurement mic.
The high frequency is the 808-8A compression driver mounted to the 511B horn. The drivers have new diaphragms from Great Plains Audio part number 26420 typically used in the 802 driver.
I’ll start by measuring the impedance curve of each driver and then the frequency response. I’ll use these measurements to export to my passive crossover modelling program.
808-8A + 511B Driver 1 Impedance Curve
808-8A + 511B Driver 2 Impedance Curve
808-8A + 511B Driver 1 Frequency Response on axis
808-8A + 511B Driver 2 Frequency Response on-axis
808-8A + 511B Driver 2 Frequency Response 15 deg off-axis
808-8A + 511B Driver 2 Frequency Response 30 deg off-axis
808-8A + 511B Driver 2 Frequency Response 45 deg off-axis
808-8A + 511B Driver 2 Combined response, 0,15,30,45 off-axis
416-8C Woofer 1 Impedance Curve
416-8C Woofer 1 near field frequency response
So, starting from the top, I realized the two drivers do not look the same. The impedance and frequency response curves are completely different. I decided to take the back off driver 1 and see if there was anything amiss.
There was some black cotton stuffed in the rear chamber which is odd.
Taking the diaphragm off I could see a few issues. First there was some damage to the diaphram to what looks like an iron filing.
There was also an iron filing resting on the surround of the diaphragm.
The solder connections were also broke.
There was lots of possible causes. I decided to re-wire the diaphragm to the back cover. I found it odd that the wire had to be soldered to the diaphragm terminal tabs so I attempted to use spade lugs but they simply did not fit when I put the cover back on.
Original wiring was falling off because of bad soldef joints
I thought I would be smart and make these nice jumper wires.
Very beautiful however they don’t work! They simply don’t fit in the space allowed.
In the end I simply replaced the solid copper wire with stranded copper wire and did a proper soldering job. I also cleared out the voice coil gap with some wax paper just to make sure nothing was lodged in the gap.
Time to measure these drivers again and see if this fixed the problem.
The impedance curve measured exactly the same as originally. None of this was the problem!
I decided that it was time to examine driver 2 to see if I could spot something different.
Well what do we have here. There appears to be a black cover over top of the diaphragm that is missing from driver 1. This would certainly alter the response!
Before I can even begin looking at the crossover design these physical problems need to be addressed first. I tried wrapping the one horn in sticky back rubber but it did nothing the quell the issue. I think I will have to resort to something more extreme, but I’m not sure what at this point.