A brief visit from a very vintage A 25 and an A 10

Carlspeak · March 16, 2012

I've recently had the privilege of restoring a pair of each of these. The A 25 proved to be a particularly pleasant surprise in the form of a very early version with Scanspeak Drivers and the FG stuffed vent located ABOVE the tweeter (see pics). The A 25 had a conventional crossover with 6 resistors and one 8 uF Jensen NPE cap. I replaced the cap with a Sonicap 8 uF, cleaned the rotary switch and left the original resistors intact.

The rotary switch forms a conventional 2 resistor L-pad combining a varying series resistor (depending on switch position) with a 10 ohm parallel resistor. The resistors are 2.2, 4.4, 6.6, 8.8 and 11 ohms. The middle "normal" switch position utilizes the 6.6 and 10 ohm parallel resistor resulting in a 4 ohm resistance upstream of the tweeter. The other positions result in 1.8 ohm, 3.06 ohms, 4.7 ohms and 5.22 ohms. Each click of the rotary switch changes the SPL response about 2 dB.

The cabinet was lined with thick rock wool-like batting and two extra layers loose under the woofer the length of the cabinet. The woofers were hard to remove due to assembly with a black, tar-like substance that remains to this day extremely sticky.

When the work was completed, I took some measurements. The A 25's tweeter rolled off at about 15 kHz as shown below.

The A 10 also had Dynaco drivers. The cabinets had two batts of yellow FG. The crossover was more complex, consisting of an 8 uF cap, 5 ohm series and 8 ohm parallel resistor in the tweeter circuit. The woofer had a 4 mH series wired air core inductor and 24 uF parallel cap. The woofer cap was replaced with Dayton caps and the tweeter cap was another Sonicap cap. i have the speakers for a few more weeks. Let me know if you have any questions or need any further info.