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Updated Dynaco A25 schematic

markw300d

markw300d

Super Member
I had posted the schematic in another thread and while reading some other posts about tweeter replacements I thought about adding those notes to the schematic...so I did. Please look at Note no. 3 and let me know if this not correct. I also reduced the graphic just enough to print on a full page to add a hard copy to my docs and for anyone else who may want it.

EDIT: I found a link to a very nice A25 enclosure drawing that I put in my notes as well. This seemed like a good place to add it.
http://homepage.mac.com/planet10/TLS/classics/images/dynaA25-plan.gif


thanks, Mark

a25schematicjpg.jpg
 
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Mark -
FWIW, I replaced the original Seas domes on my A-25's with Vifa D27TG-05 silk domes from Parts Express. They fit right in without modification (other than minimal filing for the terminal clearance) and cross over at a relatively low point, I think it's something like 2500hz. They really improved the high end of the A-25 system and I can't detect any holes in the response, although I haven't done any critical testing with instruments. I did an A/B test with one original and one modified speaker and to my ears the midrange response was just the same with the Vifa, but the highs were outstanding with the Vifa whereas with the original Seas they rolled off far too early for my tastes (and I love New England sounding speakers). Just my 2 bits, I was pleasantly surprised with the Vifa D27TG-05 1" silk dome as a drop in replacement for the Seas and I listen to them all the time (they're right here in the den). I've never grown tired of them and I never yearn for the Seas tweeters (they're in the attic in a box just in case).:D

*EDIT* - I just found the specs for the Vifa D27TG-05. The frequency range is actually 2,500 to 30,000 HZ, it handles 100 watts rms and 140 max, it's a 6 ohm unit and it measures 4 11/16" diameter at the edge of the faceplate and 2 7/8" diameter at the back where it fits into the cabinet... it was around $20 when I bought it a few years ago. Vifa also offers the same basic unit except that it goes down to 2,000 HZ and it's around $26, however I don't know if they are still avaialable.
 
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Mark -
FWIW, I replaced the original Seas domes on my A-25's with Vifa D27TG-05 silk domes from Parts Express.
Click to expand...

It's worth a lot!...I appreciate all the info that I can gather right now on these. It's why I started this thread. These are popular speakers and I got VERY lucky finding mine. I will edit the graphic according to the info gathered at the end of this.

thanks,
mark
 
How timely (almost).
I got my (second) pair of A-25s just yesterday. :yes:

Actually they are A-25VWs (made in US)
- does this schematic still apply also, or is it just for the original (made in Denmark)? :scratch2:

TIA :thmbsp:
 
I never yearn for the Seas tweeters (they're in the attic in a box just in case).
Click to expand...

The replacement Seas tweeters are much better than the stock original from what I read. I believe this quote is from the same gentleman who kindly drew the schematic....

The 1.5" SEAS H-087 (ALNICO magnet) tweeter was the one most commonly
used in the A-25.

This large tweeter was probably in an attempt to provide better power
handling. However, this tweeter utilized a paper voice coil former
which does not function as much of a heat sink. Aluminium or
aluminized paper is much better.

The H-087 was a good tweeter for its day, but it was a dated design
even in the early 1970s. The SEAS 27TFFC suggested as a modern day
replacement is a far superior tweeter.

It does employ tinsel lead in wires which helps to avoid wire breaks
with first order crossovers.

It is a simple design with a felt pad under the dome, no pole vents
or chambers to add resonances to its response.
Click to expand...
 
I wonder where would you find a replacement Seas tweeter and how much it would be? I also wonder what the specs are, particularly the low frequency figure.

Intersting about the A25VW. I have A-25's made in Denmark with the real walnut veneer (not vinyl) but they have the dark brown / burnt orange grill covers. They are all original (I bought them from the original owner years ago and I know that she never changed the grill covers) and I even have the original cartons with the matching serial numbers on them. They don't say anything about being a VW version on the cartons or on the cabinets, just A-25 and Made in Denmark. Must be some kind of in-between model?
 
I'd do a recap before I swapped tweeters. It's dirt cheap and a fresh cap may have a much lower ESR than a 30+ year old one, which may lead to a more productive tweeter.
 
The specs look pretty similar to the $26 Vifa except that it goes down to 1,500 instead of 2,000 hz. Good info!
Click to expand...

1,500 Hz is the factory crossover spec for the A25 I believe. I will try the cap replacement before swapping tweeters but if I do ever swap, I will try to stick with the SEAS...especially since they already have a SEAS driver.
 
audiojones said:
I have A-25's made in Denmark with the real walnut veneer (not vinyl) but they have the dark brown / burnt orange grill covers... Must be some kind of in-between model?
Click to expand...
The A-25 underwent numerous design changes during it's tenure, so I wouldn't be surprised. Pics?
 
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Nice find Mark, enjoy them.

Just thought that I'd comment that note 1 is not exactly correct. I later learned that there are more than version, or H number of the 25F-EW, one of my speakers had the H250 version which is incorrect. Madisound sells/sold the H085 which is the correct version for the A-25XL:
http://madisound.com/catalog/advanc...ries_id=&search_in_description=1&inc_subcat=1

It has a stronger ceramic magnet that makes it more efficient.

Here is the original thread where I posted that schematic; the schematic link is dead due to AOL:
http://www.classicspeakerpages.net/IP.Board/index.php?showtopic=3901&st=0
 
Hi, Pete,

Of potential interest, I have a pair of 25F-EWR 4 Ohms that came out of BeoVox 275, I believe they were.

Any difference other than the impedance? :dunno:
 
Hi Pete,

I appreciate you responding to my note and stopping by. Thanks for the info.

Madisound sells/sold the H085 which is the correct version for the A-25XL:
Click to expand...

from Madisound link above...
This woofer is no longer made in this frame size. It is available as the new H1411 (A26RE4) in the 269mm frame. It probably won't fit an old Dynaco A25 without some changes to the cabinet.

This is the woofer that World Designs uses in their WD25A speaker kit.
http://wduk.worldomain.net/acatalog/info_WD25A.html
 
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Zilch said:
Hi, Pete,

Of potential interest, I have a pair of 25F-EWR 4 Ohms that came out of BeoVox 275, I believe they were.

Any difference other than the impedance? :dunno:
Click to expand...

Not sure, do you have the H number?

Does it have a ceramic or ALNICO magnet?
 
Pete B said:
Not sure, do you have the H number?

Does it have a ceramic or ALNICO magnet?
Click to expand...
They're ceramic. No H number, I don't believe.

I'll be doing a build with these soon, and will run the impedance, Fs, and Q, at least, and post them here. With some head scratching, I could likely get Vas, as well, since I have closed boxes for them....
 
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