GE A1-400 & RCA 501S1 (aka SL123)

Safety First

Please note that the RCA SL123 was built on a more rugged frame for commercial
installations but uses exactly the same cones, voice coils and AlNiCo magnets
as the 501S1, which was intended for domestic hifi settings.

As the above specs show, these drivers were rated at 25W (peak music) or about 15W rms. These were conceived when the output of a typical hifi amp was only about 10 watts from a pair of push-pull 6V6s. IME, they sound best driven by SE-DHT amps but a pair of Grommes LJs, a dynakit stereo 35 or other classic PPEL84 amps will also produce some nice tunes. That said, a Dynakit ST70 or Marantz 8B operating in UL mode can easily fry the delicate voice coils on these drivers.

If your idea of hi-fi is high SPLs with pumping mid bass, stay away from these classics. Instead, get modern Eminence drivers + Class D amps from Parts Express. Satisfaction guaranteed and they'll even look better on the test bench.😉 

Interestingly, both drivers share the same simple crossover topology. A 4µf high pass capacitor is used to connect to their respective tweeters, while the 12" drivers are allowed to roll off mechanically. According to the GE brochure, its metal grill also serves as an acoustic balancer.

Given their 8Ω impedance rating, one can assume that the crossover point is a gentle 6dB/octave at around 5000 Hz. The GE was fitted with a wax paper type capacitor, whereas the RCA had an early type of electrolytic. As the LCR meter shows in the above picture, both caps have significantly drifted in value and needed replacement for sonic and safety reasons. 

After extensive listening, I settled for paper in oil caps for the GE and a mixture of Mylar + PIO for the RCA.

Altec 614 Box

I initially tried using the GE A1-400 and RCA 501S1 in the above Altec 614 boxes that date back to my Altec 2-way experiments. However, the results were quite disappointing. Both drivers sounded like they were being choked = muffled midrange! Nothing like the pleasant and natural midrange I've gotten accustomed to hearing when the GE was in a corner cabinet or the RCA in an open baffle.

Since my woodworking ability is rudimentary at best and computer simulated and optimized boxes for each of these drivers weren't within my budget, I had to make do with what I had ...🤔 

Cone suspension left to right: Altec 414A cloth accordion, GE and RCA with rolled paper surround

I surmised that the port of the Altec 614 was tuned for the more compliant accordion-type suspension of the Altec 414A woofer vs. the stiffer rolled paper surround shared by the GE and RCA 12" drivers. Having been a voracious reader of 1950s issues of Audio Magazine and Radio TV News since the 90s, I've come to associate stiffer cones with larger vents. After racking my memory bank, I remembered a post in the Lansing Heritage Forums about the older version of the 614 cabinet wherein the size of the port was stated as 13" x 3". 

With the vent enlarged to 13" x 3", the midrange issue was ameliorated. I started hearing the distinctive midrange character of the GE A1-400 when it was in its corner cabinet as well as the RCA 501S1 in an open baffle. This workaround would probably be unacceptable to those who rely on measuring T/S parameters and build cabinets based on computer simulations. But for my purpose, the low end was already more extended with good tonal definition than when the GE was in the Norman H. Bates corner cabinet and the RCA mounted in a G. A. Briggs derived open baffle. With the cabinet colorations in the all important midrange out of the way, I started listening to music!

GE A1- 400 

I've discussed the GE A1-400 driver and its vintage DIY corner cabinet in detail in a previous upload. As mentioned there, I found this setup while I was doing my final sweep at the Kutztown Radio Show in the Spring of 2021. I've been hunting for this driver since I saw its younger brother, the A1-401, featured in the Stereo Sound 2002 Special Issue on Vintage Speaker Units. Although I have no hands on experience with the A1-401, according to a trusted source, both are technically identical except for the grill color.

Thinking that I'd never find another, I had consigned it as an alternate speaker to the Altec 756B/32C/806/23744 for my mono hifi setup. Lo and behold, I was able to pair it up at the same venue four years later!😊

In stereo mode, the GE A1-400s are very exciting speakers! Very snappy with lots of sparkle, no pun intended!😉 Dynamics from about mezzo piano to fortissimo are well portrayed. However, micro dynamics and detail below mezzo piano tend to be obscured. Also, if I had to split hairs, I heard a slight nasal resonance in the upper midrange that might be emanating from the combination of the 12" driver cone break up mode interacting with the acoustic balancer/metal grill. Yes, as I often preach, I also spent considerable getting to know both of these drivers with their respective cone tweeters disconnected.😉

Enhancing the higher frequency harmonic energy of the A1-400 is the big chunk of AlNiCo magnet GE provided for the cone tweeter.

Even if to my ears the 501S1 sounded just a bit more subtle and nuanced, the A1-400 also performed remarkably well, reproducing music that required delicacy and finesse!

RCA 501S1

Without even trying too hard, it took merely four years for me to pair up my GE A1-400s. OTOH, the path towards listening to the RCA 501S1 in stereo proved to be more challenging and took more than two decades. Finally! I'm very grateful to a generous vintage audio/DIY enthusiast friend who gifted me with this single 501S1 pictured above!🙏

RCA tweeter with smaller AlNiCo magnet

The GE and RCA share a very similar design concept - a 12" wide range driver + 2.75" cone tweeter. Nevertheless, the A1-400 and 501S1 couldn't be more different in the way they present music. With the RCA's tweeter disconnected, the 12" wide range cone rolled off very gently, similar to the upper midrange behavior of the Altec 414A, 406Z, 406-8Y and 756B. Although the highs are silky, sweet and airy from the RCA's tweeter, it couldn't equal the high frequency energy of the GE due to its smaller AlNiCo magnet. 

The harmonically rich and smooth sounding RCA 12" contribute a great deal to convey a more 3D midrange compared to the GE, most noticeably on vocals. As mentioned earlier, the 501S1 edges the A1-400 in portraying micro dynamics and detail. It's only at the loudest end of the dynamic spectrum wherein the GE takes the lead again by going a couple of dB louder while the RCA loses composure. Again, the GE tweeter's bigger magnet helps.

Having been fortunate to reacquaint myself with a pair of RCA LC1As at an audio gathering last summer, I hear the trickle down effect tonally. But the 501S1 was definitely not an effort to create a 12" version of the LC1A. As the saying goes, "a picture's worth a thousand words," so please click to enlarge the picture above, which illustrates the LC1A as a much more complicated and sophisticatedly engineered transducer!

After praising the 501S1's sonic elegance and refinement, it can also let its hair down and boogie!😊

Impressions from our Japanese friends who rediscovered these gems decades ago!

From the Stereo Sound 2002 Special Issue on Vintage Speaker Units

Hmm, no mention of the RCA in the wrap up 🤔...seems like they prefer the GE.😉

乾杯! (Kanpai!) 🍻😊