Dynaco A-25 crossover network

[Guest happyman]

Hi,

i recently bought a pair second hand loudspeakers. They seemed to be Dynaco A-25, but with modifications (no front, no terminal, xo glued to woofer). I was wondering whether someone can tell what the original crossover is, or/and your modifications. I found a scheme on the internet, but after more readings on internet it seems not to be correct also.

here follows a description of mine

-----------------------------------------------------

My A-25 consists of:

SEAS TYPE H-087 1½ inch cloth dome, 4/8 Ohm (787 32775)

SEAS TYPE 25 TV-EW 10 inch paper cone, 4 Ohm (4 5/76)

Specially the 4 Ohm woofer looks weird, I read their prototype was 4 Ohm, but the "real" version was 8 ohm i believe.

My woofer has 8 holes, i've seen versions of 6 holes too (in triangular "bridges")

my cabinet has metal/aluminium strips (at the inside of the outer side of the front), which i've seen with some picture on internet, so

it seems to be a real dynaco, though there is no terminal present. The width of the sides at the front, are thinner than the sides

itself.

The dimension of the box are exactly the same as far as i can see, except for the height of the vent-port. Mine is 4cm, description on

internet says 5 cm.

my X-over:

10 ohm parallel to tweeter, outside of that, 8uF capacitor at +side of tweeter, 2.2 Ohm resistance at -side

-----------------------------------------------------

X-over found on internet (actually tweeter part of A-50):

5uF capacitor at +side of tweeter

10 Ohm at -side of tweeter, with 0 to 4 switchable resistors of 2.2 ohm in series (so 10 to 18.8 ohm possible)

-----------------------------------------------------

i've read somewhere that the 4 switchable resistors are in fact not all switched in series, but partly in parallel, is it?

-----------------------------------------------------

I hope someone could give me some hints about the crossover, or my 4 ohm woofer.

thanks in advance

http://www.classicspeakerpages.net/dc/user_files/1434.gif

[dynaco_dan]

Hi there;

You have probably have Scan/Seas 4 ohm drivers.

They were used in a few other brands.

Not meant for the 8 ohm Dynaco A-25's.

Could you please post a few photos of them and their labels?

Will be back later when you re-post.

[Guest happyman]

they are real seas units, guess the prototype would have been scanspeak?

attached also the front and back, and the vent coverage (notice the metal strips at the inner sides)

http://www.classicspeakerpages.net/dc/user_files/1440.jpg

http://www.classicspeakerpages.net/dc/user_files/1441.jpg

http://www.classicspeakerpages.net/dc/user_files/1442.jpg

http://www.classicspeakerpages.net/dc/user_files/1443.jpg

http://www.classicspeakerpages.net/dc/user_files/1444.jpg

http://www.classicspeakerpages.net/dc/user_files/1445.jpg

[dynaco_dan]

Hi there;

I read a small story about the first or chosen or designed Dynaco A-25 speaker system, I don't know much more than that.

Anyways, the prototype or existing design speaker system, which was presented to or designed by, Dynaco, or Scanspeak, in Denmark, was a 4 ohm version.

The name was given of the man who designed or developed the A-25 and the Dynaco man who requested that they be 8 ohm impedance instead.

There is a brief story at Greg Dunn's web site, under speakers.

Scan-speak, historically is a brand name that pops up, but the actual relationship to Dynaco, is yet to be told, as well as the Seas relationship.

When I worked at the local Dynaco warantee depot here, some, actually lot's, way too many, 100's, A-25's came in with destroyed, burnt woofers.

I believe these woofers had some round perimeters but also Seas, horned woofers, and the magnet was bolted together like AR alnico woofers, these were Scan woofers and they also used Scan tweeters.

They had a Scan sticker on the rear of the magnets.

The tweeter was different in number of screws and physical appearance and protection.

These also were the elusive, I saw one pair in a stereo store and never read or saw anything about these for another 30+ years until a few years ago on ebay, the A-25's with the vents above the tweeters.

There never were any other brand woofers other than the, Seas, horned woofer replacements.

Where the screw holes are, there is a projection on the Seas woofers.

To replace the round woofer with the horned versions, a wood chisel was used to knotch for the projections.

There appears to be, and I have never seen any other data as to when they started using Scan or Seas drivers, Seas or Scan tweeters or changing the vent locations, records are not forthcoming yet.

I am not 100% certain, that just the Scan speaker systems, had a screwdriver adjustment and not a shaft and knob for the switch.

Your enclosure from the front appears to be a Dynaco A-25.

The rear of the enclosure, I have never seen any like it before.

The rear mounting plugs? are really unique in my experience.

The cables out and connector look like a homemade job.

There is no recess or fibre connection plate, switch or banana jacks.

The tweeter is 4/8 ohm which is, confusing, but correct, the 4 ohm woofer is not the correct 8 ohm version and may not have been factory installed originally in that enclosure.

The mounting of those crossover components are not a Dynaco original design.

It lacks the other resistors etc,.

Now that I have said all of the above, perhaps you have an original prototype speaker system, that somehow slipped out of the factory or was discarded and or taken home by an employee.

There will be more to this story, as the world turns.

[Guest happyman]

hi,

thanks for the extensive replies!

i assume the prototype version had the vent above the tweeter with scanspeak units, as they changed it later. And so i guess the 4 ohms prototype woofer version should be from scanspeak.

i read some ad somewhere:

"DYNACO A25 Special. The last version of this classic bookshelf speaker with a much more modern looking cabinet and grill. SEAS 10 inch woofer, 1 inch tweeter. Real walnut cabinets. One of the best bookshelf speakers ever...."

with the first picture, which has the metal strips, like mine, but with the "horned" woofer, as you call it. (and my tweeterdome is at least 1½ inch).

so i'm afraid I have a collection of parts the simulate the a-25 :-(.

But where does the woofer come from. I've read the woofers in the a-50 were 2 parallel 16 ohms versions, so it doesnt come from that one.

Well than my question could be, does the crossover makes sense with this 4 ohms woofer (as long as the soundbalance between high and low sounds good). Is mine "a-25" a regular 4 ohms speaker?

Does the parallel 10 Ohms R, makes the tweeter look like a 4 Ohms tweeter, and therefore the 5uF C, is changed to 8uF?

But still I would like to know exactly what the original XO is like. Maybe I could find the 8 Ohms woofers once ...

I added photos where you can see the tweeter hole, and the inner damping, which i think is too much for the a-25 (?).

I checked to see the woofer holes, there are just 4 holes, 4 screws, woofer is also glued with a "keep-sticky" glue.

The sidepanels seems to be medium dark fine-chipboard. The back and front are very light more rough chipboard.

greetings

Wiebe

http://www.classicspeakerpages.net/dc/user_files/1452.jpg

(found at www.commonsenseaudio.com/gearpage.html)

http://www.classicspeakerpages.net/dc/user_files/1455.jpg

http://www.classicspeakerpages.net/dc/user_files/1456.jpg

[dynaco_dan]

>hi,

>

>thanks for the extensive replies!

>

>i assume the prototype version had the vent above the tweeter

>with scanspeak units, as they changed it later. And so i guess

>the 4 ohms prototype woofer version should be from scanspeak.

Hi again;

Other than the mention in the speaker section of Greg Dunn's web site I have read nothing else.

If the model presented to Dynaco for approval was Scan, then we can assume that the vent was above the tweeter.

There it was mentioned about the decision to use 8 ohm rather than the 4 ohm model drivers.

There was no mention as to where the vent opening was located, or even that there was one.

>

>i read some ad somewhere:

>"DYNACO A25 Special. The last version of this classic

>bookshelf speaker with a much more modern looking cabinet and

>grill. SEAS 10 inch woofer, 1 inch tweeter. Real walnut

>cabinets. One of the best bookshelf speakers ever...."

>

>with the first picture, which has the metal strips, like mine,

>but with the "horned" woofer, as you call it. (and

>my tweeterdome is at least 1½ inch).

>

>so i'm afraid I have a collection of parts the simulate the

>a-25 :-(.

>But where does the woofer come from. I've read the woofers in

>the a-50 were 2 parallel 16 ohms versions, so it doesnt come

>from that one.

The 4 ohm woofer you have, was manufactured and sold to other speaker manufacturers for their product line.

The A-50's used 2 - 16 ohm woofers in parallel and there again I have seen round and horned woofers versions on ebay.

There was only 8 ohm drivers used by Dynaco for their classic A-10's, A-25's, A-35's and the only exception was the 16 ohm woofers in parallel for the A-50's.

A few years ago I would have carved in stone that the round woofers were Scan woofers.

It is possible that Seas owned or bought Scanspeak and used their remaining stock, embossing Seas name on the frames, switching to the horned baskets at the end.

This will invite comments from other members.

>

>Well than my question could be, does the crossover makes sense

>with this 4 ohms woofer (as long as the soundbalance between

>high and low sounds good). Is mine "a-25" a regular

>4 ohms speaker?

Because of the 4 rear mounting posts and what I feel is a homemade electrical connection, it is possible that this is a homebrew cabinet using Seas brand drivers, but not necessarily Dynaco parts.

Actualy, this just came to my mind, why are we calling them Dynaco speakers at all.

Other that the fact that the driver brand, and vent opening is in common with Dynaco A-25's, there is no reason to even call them Dynaco speakers even.

Even the A-10, at least, had a 2 screw terminal strip.

>Does the parallel 10 Ohms R, makes the tweeter look like a 4

>Ohms tweeter, and therefore the 5uF C, is changed to 8uF?

I do not have that information, my memory is going and I didn't have a technical background.

>But still I would like to know exactly what the original XO is

>like. Maybe I could find the 8 Ohms woofers once ...

If you are enjoying these speakers, then by all means continue to.

As is, if you are wanting to create Dynaco A-25's from what you have, I would say don't.

The vent may or may not be correctly tuned, the interior dimensions, fibreglass, you would need a 5 postion MBB switch, resistors and the 8 ohm woofer.

When I first saw the vent insulation, it did look not like a factory placed piece, rather like a do-it-yourselfer did it.

You and I cannot, without factory equipment, adjust the vent insulation.

They probably adjusted their prototype and just went and cut the balance to fit.

It would have been expensive to test and adjust each one, they may have tested at random during their production life.

I have only seen a few hundred A-25's so I base my opinion on that.

>

>I added photos where you can see the tweeter hole, and the

>inner damping, which i think is too much for the a-25 (?).

>I checked to see the woofer holes, there are just 4 holes, 4

>screws, woofer is also glued with a "keep-sticky"

>glue.

It looks like the tweeter hole is cut like a water pump gasket rather than round.

>The sidepanels seems to be medium dark fine-chipboard. The

>back and front are very light more rough chipboard.

>

>greetings

>

>Wiebe

>

[Pete B]

Those are some interesting pictures. I might finish this page someday, it has the schematic for now:

http://members.aol.com/basconsultants/DYNACO-A25-XO.htm

Pete B.

[dynaco_dan]

Hi Pete;

Excellent reference, very well done.

[Guest happyman]

Hi, thanks, super!

I only wonder whether its 100 % original, because I thought the A-25 had only 4 resistors of 2.2 (and 1 of 10).

Look at this photo of a A-50 XO. Tweeter part is the same for A-25 I've read. So are u sure your R1 resistor is original? Or are there variations of the XO maybe?

But yours does look more like the photo than the other scheme I've found, and is more similar to my "A-25 look-a-like".

thanks

http://www.classicspeakerpages.net/dc/user_files/1457.jpg

[dynaco_dan]

Hi again Pete;

You certainly did a very nice job in drawing that up.

I would expect the speaker manufacturers to do similar work.

Your comment on the different sound of the new, versus the older driver.

Would you comment on that sound please?

What capacitor brand, model and voltage rating for the 1uf cap would you suggest, please.

Could you do a listeners review with your modification, please?

Thank you again, Pete.

[Pete B]

Yes I'm sure that it is correct and it makes sense because the A-50 is going to require more tweeter output to match the higher efficiency of the dual woofers. I'm positive that the schematic is correct. Your pictures of the A-50 XO confirm that the schematic you posted above is incorrect. The 5 uF tweeter cap obviously goes to the switch not directly to the tweeter. I don't know what you have there, if it is some sort of prototype you might want to try to preserve it. There is a gentleman from SEAS that posts and reads at the Madisound board, you might want to try there.

Pete B.

[Pete B]

Hi Vern,

Thanks again very much, nice of you to comment.

I mentioned about the sound differences in the other thread, about 3 dB more output 200 to 500 Hz, and even more above, about 5 dB more above 2 kHz. Magnet is stronger so the Q is lower resulting in less low bass.

Any film cap will do just fine, I didn't have a 1 uF so I used a pair of .47 uF Panasonic Polys in parallel.

I used Axons to replace the 5 uF, and would have used them for the 1 uF if I had them. Anything over a 100V rating is fine.

The cap just adds some "air" to the sound, not like turning up the treble or the switch on back, because it's only lifting the very high end. More like moving forward in your seating location. It is subtle but important for realism.

Pete B.

>Hi again Pete;

>

>You certainly did a very nice job in drawing that up.

>

>I would expect the speaker manufacturers to do similar work.

>

>Your comment on the different sound of the new, versus the

>older driver.

>

>Would you comment on that sound please?

>

>What capacitor brand, model and voltage rating for the 1uf cap

>would you suggest, please.

>

>Could you do a listeners review with your modification,

>please?

>

>Thank you again, Pete.

[dynaco_dan]

>Hi Vern,

>

>Thanks again very much, nice of you to comment.

>I mentioned about the sound differences in the other thread,

>about 3 dB more output 200 to 500 Hz, and even more above,

>about 5 dB more above 2 kHz.

Hi Pete;

Will this cause a boomier bass or more mid range presence?

Original Dynaco A-25's were noted as having a extremely smooth response in this area.

Is 3db double the output in this regard?

In my reading of old Dynaco literature yesterday, I came across a report of a 9khz and a 10khz resonance, one each from two different sources.

One source still tested and claimed outstanding transient response at the 10Khz, at the same time as it also showed a 10khz resonance photo.

Can we hear this resonance as a screech, buzz or rattle?

As always, Pete, a very informative write-up from you, thank you.

Magnet is stronger so the Q is

>lower resulting in less low bass.

>Any film cap will do just fine, I didn't have a 1 uF so I used

>a pair of .47 uF Panasonic Polys in parallel.

>

>I used Axons to replace the 5 uF, and would have used them for

>the 1 uF if I had them. Anything over a 100V rating is fine.

>

>The cap just adds some "air" to the sound, not like

>turning up the treble or the switch on back, because it's only

>lifting the very high end. More like moving forward in your

>seating location. It is subtle but important for realism.

>

>Pete B.

>

[Pete B]

Hi Vern,

I copied this to the other thread and replied there:

http://www.classicspeakerpages.net/dc/dcbo...id=17&page=#116

Pete B.

[Pete B]

It is interesting that your woofer picture shows a stamped steel frame rather than cast as with the original. These are probably a cost reduced version for the end of the product life which would explain the lack of input posts.

[Pete B]

The 4 ohm woofer will provide more acoustical output for the same voltage input as the 8 ohm version. I'd suggest you simply use the A-50 tweeter crossover, which eliminates the one 2.2 ohm resistor in series with the tweeter. The cone looks the same, and it should work just fine.

But are both of your woofers the same 4 ohm units?

Pete B.

[Pete B]

Something interesting about the A-50 crossover is that the two 16 ohm woofers should behave nearly the same in parallel as the 8 ohm in the A-25/35, however there is a woofer crossover in the A-50. Which suggests that the two systems were voiced differently, or that the A-50 was more refined being the more expensive model.

Looks like a series inductor, and a shunt R-C network. Can you read the R and C values, and measure the inductor, or do you have the values?

Pete B.

[dynaco_dan]

Hi pete;

I see a cast aluminum frame on that woofer.

Maybe I better clean my glasses again.

[Pete B]

Hi Vern,

Yes I had to look at it for a while, the picture with the cast frame is from another site, he's just showing the modern looking boxes, those are not his as I understand it.

This is his woofer:

http://www.classicspeakerpages.net/dc/user_files/1442.jpg

[Guest happyman]

Hi Pete,

thanks for your comments.

But I believe mine is not a prototype, especially because of the metal strips, which I've read is only used in the latest versions.

[Guest happyman]

The photo of the A-50 crossover is coming from the internet. I don't have it myself unfortunately. The characters are just a little too small to read it.

But the possible scheme (of the A-25) that I've posted with my first post here, is an edit of a scheme I've found of the A-50. (where they said the tweeter parts were equal).

The scheme is not 100% correct and complete though (the resistor switch is no L-pad there), but 2 values (resistor and capacitor) of the woofer part are given. The coil seems unknown.

http://www.classicspeakerpages.net/dc/user_files/1462.gif

[dynaco_dan]

Hi Pete;

You say this is his woofer and show a photo.

This is still a cast aluminum frame, or do I need to get new glasses. lol

[Pete B]

>Hi Pete;

>

>You say this is his woofer and show a photo.

>

>This is still a cast aluminum frame, or do I need to get new

>glasses. lol

OK, I see now, thanks Vern, maybe I need new glasses. The first spoke in that picture, has a reflection that makes it look wide like a stamped frame driver, even how smooth the metal is, and it is thin. But yeah looking again, I see that it is a reflection and they did use a thin casting so I agree it is the normal casting, so much for that theory.

Pete B.

[dynaco_dan]

Hi Pete;

Have you continued with your A-25 investigation?

How are you making out?

I kept a printout of your great schematic, 2/24/06 Rev 1.0 and started to look from where I copied it from.

Thank goodness the Dynaco forum doesn't have thousands of files, it wasn't too difficult to find.

I wrote elsewhere, without seeing your comment, about that A-50 schematic not being in my mind, correct as well.

I feel the 10 ohm w-w resistor in series at the lowest setting, is incorrect.

This would place the tweeter down at least 10 db +/- for starters.

It would be appropriate to have it at the end of the switch to common ground, such as with the A-25 crossover.

[dynaco_dan]

Hi there

I feel this is the correct topic for these uploads.

I photographed an unusual A-25 crossover that I bought off ebuy a few years ago.

It has the 5 position screwdriver switch shaft, not the usual shaft with a knob attached.

Switch brand unknown at this time but appears to be the same manufacturer of the regular shafted switch.

It does not have the serial number label to at least try to pin point the production date period.

As a side note the date would read something like 19-952XXXX.

Product is number 19 for A-25 speaker and -9 would be year of manufacture 1969 and 52nd week of production, xxxx would be the serial number.

The capacitor is a small black IEC brand NP 50 volt 4.7 uf.

The smaller 5 resistors are Noble RGB3 3 watt 2.2 ohm sandcast type.

The larger one is a Noble RGB5 5 watt 10 ohm sandcast type.

MOD 20110829 10:00 AM