Bob Latino wrote:A few years ago I tried to find online the efficiency rating of the original Dynaco A-25's. I was unsuccessful ... If anyone does find out the efficiency rating of the A-25's, please post the link here in this thread. The aperiodic design of the Dynaco A-25 or Madisound A-26 has a damped port. This is not the most efficient design available but its efficiency should be better than an acoustic suspension design. I do remember one customer driving an original pair of A-25's with a 60 WPC VTA ST-120. He liked the sound and never complained about any lack of power. On some web sites, they mention that the A-25's have a "DIN power rating" of 35 watts. This leads me to believe that you could probably drive them fairly well with an ST-70 as long as your room is of average size and you are not interested in rock music at high volume levels.
Bob, I came across this, which I hope is helpful: http://www.audioasylum.com/forums/vintage/messages/19/199270.html
It lists the A-25s at 82 db @ 60 watts. The slightly larger, internally ported A-35 comes up with 85 dB at just 30 watts. The even larger dual-woofed A-50s will report 86 dB at a mere 25 watts. All the Dynaco speaks (save the A-10s) used identical components, so these figures, if accurate, illustrate the significance of cabinet design.
When the A-25s came out Dynaco was still building mostly tube kits, and the A-25s were paired with the SCA-35 which put out a mere 17.5 watts/channel, and it could make them howl in a pretty big room. At the Seattle hi-fi store I hung out in, I never heard any complaints about that combo, and their clientele were fairly well-heeled Magnolia Hill yuppies who could afford Macs but thought, even back then, that the blue meters were a con job.
Having had extensive experience with the -25s and -35s, you can't go wrong with either, but if you've got the patience, hang loose until you can locate a pair of A-35s with their original components and decent cabinets. I swear you could drive them with a crystal set. The only bitch is they're heavy to ship. The woof Madisound uses for its A-26 will not fit into the holes in the original Dyna speaker holes: the basket flange extends beyond the the face and into the trim of an A-25. There is no dimensional replacement for those old Seas woofs. The old ESS Heils used the same basket woofs and magnets, but with foam surrounds, which by now would be dry as dust. That, too, can be fixed with new rubber surrounds but it takes some serious surgery not to harm the cones.
To make a long story longer, wait for an acceptable pair of all-original A-35s to come along. Superb efficiency, they'll take a beating and deliver a very sweet sound. The only fix they'll need is maybe replacing the single crossover cap, and putting a little De-Oxit on the tweeter trim switch.
I have a 200-watt s/s Bedini hooked up to my A-35s in the secondary (read: man-cave) system. I've also run them on some flea-power home-brew amps. You can't hurt 'em.