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Dynaco's A25 loudspeaker was amongst the most popular loudspeakers ever produced. More than 600,000 speakers were sold. They provided amazingly good sound for about $80 per speaker. This web page takes a look inside the A25, and speaks in particular about trouble-shooting.
The A25 is a two-way speaker with a 10" woofer and a 2.25" mid-tweeter. The crossover is quite simple, just a 5 uF capacitor in series with a variable resistor arrangement that feeds the tweeter. Please also note that according to some recent information from an Updatemydynaco customer, Ed Cameron, some of these speakers were built with 8 uF capacitors, rather than 5 uF. So, you may want to check to see which you have. The woofer connects directly to the binding posts, and the crossover relies on the natural roll-off of the 10" woofer. Here's a picture of the inside of the speaker showing the crossover and level control:
Here's that same information in schematic form (values were double checked with one of my A25's):
My right-hand A25 was making splattery sounds in response to the Hal Galper's piano comping on a Jamey Aebersold play-along. That wasn't right...neither Hal nor the piano should make splattering noises. I ran some test tones (sine waves) through the speaker at various levels to confirm my hunch about a tweeter problem.
On a further hunch, I put 3 volts RMS at 60 Hz on the binding posts, and measured the voltage on the tweeter terminals. It was almost exactly the same 3 volts as I saw on the binding posts!!! The crossover cap (5 uF) had to be shorted! Sure enough, it was a dead short, verified by an ohm meter when I pulled the cap.
The shorted cap had let a bunch of bass hit the tweeter. I replaced the cap with a 3.3 uF film cap in parallel with a 2.2 uF film cap for a total of 5.5 uF. I figured that should be close enough, given that the original cap probably had 10 or 20% tolerance. I then crossed my fingers, hoping to find that the tweeter survived its abuse.
The sound was a lot better, but still not right...the splatters were better, but the piano would still excite the splatters. Oh nubbins! I guess the bass in the tweeter caused some damage.
I went back to sine-wave test tones, and found the right around 1 kHz, the tweeter made really funny sounds at some sine wave amplitudes. That was the funny stuff I was still hearing. I would have to find a new tweeter!
The way to the insides of the A25 is by removing the drivers. Remove the front grille-cloth plus holder (I can't say how they're legitimately fastened...my pair had them loose). Each driver is held in by multiple phillips head wood screws and a sticky sealant. The stick sealant is marvelous stuff...when I pulled my (probably) 45 year old speakers apart, it was still appropriately sticky.
The secret to removing the drivers is to work your way around the driver with a screw driver, prying up a little at each point. With some patience and determination, you can break the driver free of the sticky stuff without causing much damage to the cabinet.
At that point, you'll see the driver wires descend through two rectangular blocks of dense fiberglass-looking stuff. I like to put on gardening gloves when I pull them out. They're held in by friction, and remove fairly easily. At that point, you'll see the crossover and level controls underneath a clear piece of flexible plastic. You can pull that to the side and get access to all the electronics.
The crossover frequency is about 3600 Hz. That's based on 5 uF seeing about 8.8 Ohms. We'd like any mid-tweeter to have a resonant frequency significantly below that to stay out of trouble. I made contact with Seas, the company that made the A25's. They provided some data on the original mid-tweeter, which they said was pretty much their H087 Midrange Tweeter Dome driver. The details can be found on this Data sheet of SEAS drivers from the 70's.
Here are some possible replacements recommended by Seas
In case those seem a bit expensive, Parts Express had this likely candidate
Being a connoisseur of fine values (ok, cheap), I ordered the $28.00 tweeter. Its sensitivity, at 87.4 dB SPL at 1 watt and 1 meter is a bit low. I also hedged my bets by ordering the 27TFFC. These tweeters have resonant frequencies below 1 kHz should work well with the simple crossover.
I substituted a Tymphany D27TG35-06. It pretty well dropped right in, though I had to finesse the terminals a bit as they are pretty much at the edge of the cut-out. The original 4 mounting holes were picked up perfectly with the replacement tweeter. The sound is very similar to the original tweeter as compared with the other speaker (which is still stock). The speaker sounds much better now, and I think the substitution worked out pretty well.
I did some frequency response tests using a calibrated microphone. It's hard to say for sure as my tests were not anechoic, but there seems to be a little hole between where the woofer gives up and the tweeter takes over. Both the new speaker and the original exhibited the same behavior, so I don't plan to mess with it (for now).
I've got eyes however on a gently sloped porch roof about 10 feet above the ground, and pretty far from the nearest trees...perhaps I can get about 20 ms of anechoic response before the first reflection. That would let me get a pretty accurate frequency response on the composite. Hopefully I can get this done without an ill effects due to gravity!
I was able to use the A25's for a while after the tweeter replacement. They sounded pretty good. Sadly, one of the speakers developed a rapidly worsening rub in the woofer. Certain piano and guitar chords would cause it to rub just diabolically, making a sound like someone was dropping a keg of nails. I looked around a bit for a woofer, and never came to a satisfactory (cost, similarity, and performance) solution. Woofers that are supposed to be quite close are made, but they are around $200. It just didn't seem worth it. I traded the A25's off to a local speaker afficianado, Keith, who has probably re-coned and sold them by now.
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