26 replies to this topic

[dynaco_dan]

Hi again;

I went back and the other parted speaker drivers are now listed.

A much better view of it's crossover.

It appears that there is a large cap, coil, large resistor, and a small cap, maybe, as well, on the other circuit board.

This is a much more complex crossover than the A-25 and A-35.

I believe that the A-10 will have a similar, if not identical crossover, composing of the right board contents only, that is.

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[dynaco_dan]

Hi there;

There is a parted Dynaco A-50 speaker system on ebay as I write.

The woofers and crossover is all that is being offered.

The crossover is very interesting.

I do not remember ever actually physically seeing one before.

It has the usual switch board on the left but I see another one on the right.

It appears to have a power resistor, cap and maybe 3 coils.

More to come up in the future about this.

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[jwjensen356]

>I am fortunate that I have an old bit of scientific gear
>called a power oscillator. This is an oscillator with range
>0-25kHz (in decade steps) in the same box as a DC-coupled
>amplifier. I can try the (Dynaco/SEAS (factory?) tuning
>technique that I read about where you apply a 5Hz sinewave and
>adjust the aperiodic damping for minimum cone displacement. I
>guess this is the critical damping point. The other method
>would involve measurement of speaker impedance around
>resonance which is a bit more involved. It is quite
>interesting to see a woofer operating at 5 Hz. I haven't seen
>something like that since I stopped playing warped vinyl ;-)

My recollection is that it was a 5 Hz square wave input used during testing. You can do a Google search to confirm this (the indy.net site and a diy site).

[Pete B]

Hi Vern, I don't recall where the picture is from either.
The difference in ports could be the natural perspective difference of the 3D image projecting onto a flat surface, I don't know for sure.

[dynaco_dan]

Hi Pete;

I even remember saving the photo before to my computer, somewhere.

I see Scan drivers also.

It appears that there are not just two vents but the one across appears smaller that the vertical one.

Or do I need to clean my glasses again?

[Pete B]

Here's a picture of a system that was referred to (on the web) as a Dynaco A-45 which appears to have two 8" woofers. Those look like Scan drivers if I'm not mistaken. I don't know anything more about it:

http://baselaudiolab...om/DYNA-A45.JPG

[dynaco_dan]

Hi there;

You may try to find older B&O speakers, they may have had that combination of 8" drivers.

[Pete B]

This is my schematic:
http://members.aol.c...NACO-A25-XO.htm

Yes you could do a mini A-50, that would be fine, or something
like a mini double A-35 with a single tweeter, similarly an
A-25 double would be fine for a smaller enclosure but
with less low bass. A TL should work fine also, even normal
vented which will provide the most bass.

You might want to consider an MTM tower layout if you prefer
something more modern.

The A-50 crossover is the way to go IMO to keep it simple, we
don't know the inductor value probably between 1 and 3 mH. You
can just purchase a 3mH and unwind/tap to get an adjustable L.
The crossover could be improved, but you would need the tools
to do it.

Remember that these have paper voice coil formers and so you
should take it easy on power. There are much better modern
tweeters also. But, give it a try if you want, you have the
Dyna designs as an example.

I do think there was a less well known Dyna with two 8", I don't think it was imported to the US.

A-50 photos:
http://www.classicsp..._files/1981.jpg
http://www.classicsp..._files/1885.jpg

[Pete B]

>Sorry Vern, just realised I messed up the pic of 21-TV-ew.
>Here it is again

21-TV-EW is an 8" woofer, A-50 and A-25 require the 10" 25-TV-EW.

My schematic would be correct for the tweeter section of the A-50 if you short R1. Your schematic and Dave's are wrong.


You can estimate driver positions from Advertising pictures, and there was one cut away design picture that showed the partition.

I've never worked on A-50s so I don't have any more to offer.

[dynaco_dan]

Hi Ian;

Do the woofers have a 16 ohm or 4 - 8 ohm stamping on the paper label?

Does the tweeter have a 4 - 8 ohm or 3 ohm stamping on their label?

Can you download a clear photo of each?

I have not seen the cabinet dimensions for the A-50 for about 30+ years.

The internal cabinet dimensions are quite critical, you should not change them.

The internal vent opening size and covering material is very, very important as well.

The fiberglass content is important.

In my honest opinion, I believe that the schematic has the 10 ohm resistor in the wrong location, in that particular drawing.

It is very kind of David to have those drawings and schemetic posted on his web site, he is a very nice man.

I may well be wrong about that issue.

The actual driver layout, with the front cabinet dimensions, can quite likely be moved about an inch or so without a great difference.

I have seen at least two different mountings.

One with the woofers on an angle and also woofers both at the horiziontal.

There was the Seas version and there may have also been a Scan version at the beginning of production.

Adding braces to the inside is a very good idea but the same internal volume must be kept.

Do not increase or decrease the designed internal measurements appreciably as this will alter the bass at least.

The 10 ohm resistor does not go across, or parallel the tweeter.

Using MDF is probably the best cabinet material today.

As far as punch goes, with the correct internal volume, drivers, amount of fiberglass, correct vent opening and covering, solid cabinetry, there will be bass, not necessarily boomy but there.

You should never use loudness or bass boost for increased bass output.

Fuse them when you are finished.

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[dynaco_dan]

Hi again;

I had misplaced my A-50 schematic, so now my question is answered.

The extra board has a coil, cap and w-w resistor to cutoff the upper woofers output, for some reason.

It was not done for the A-25 or A-35. MMMMMMM

It obviously must have been needed for this A-50 speaker combination.

Manufacturers usually don't throw in that many extras for nothing.

Here is the A-50 schematic from t-linespeakers.org.

Here is also a Dynaco artist's conception of the A-50, note the non-horizontal woofers and the horizontal baffle board inside.

I believe, and I don't think I am alone, there is a small error in the tweeter switching arrangement, in my opinion.

I believe that one of those 2.2 ohm resistors in the highest, or brighest switching position, is still in series with the tweeter.

If I am right, the removal of this resistor would give the tweeter about 2 db more output, but this may have been added for driver protection.

A photo of a parted A-50 from ebay has the 4 smaller resistors and 1 larger one for the switch, which conflicts with the A-25 switch I have in my hand.

It does agree with the t-linespeakers.org schematic, in numbers at least.

From attachment 8 above, the numbers of resistors on the switch is the same as the schematic.

My interest now is that the missing resistor, the 5th 2.2 ohm, is not in the A-50's schematic or board, but is in the A-25 tweeter circuit.

The schematic with the 10 ohm resistor in series with the tweeter would mean that the tweeter level would be about 5 db down, which will be at it's highest, brightest setting.

Switching to it's dullest setting adds another 8.8 ohms in series which means 18.8 ohms on series with the tweeter. MMMMMMM

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[dynaco_dan]

Hi again;

I wonder why the extra coil, w-w resistor and board?

I can understand it with the smaller woofer of the A-10 but not the A-50.

Same crossover frequency as the A-25 and A-35.

MMMMMMMMM

[jwjensen356]

Looks like the board renames attachments.

Attachment #1 2719.jpg
Attachment #2 Ignore
Attachment #3 2722.jpg
Attachment #4 2723.jpg
Attachment #5 2724.jpg
Attachment #6 2725.jpg
Attachment #7 2726.jpg
Attachment #8 2727.jpg

Attachment #9 (2728.jpg) somehow got attached. It is an angle view looking at the slanted cross-piece with the coupling slot (covered with felt)

Here are more:

2229.jpg (attachment #1?) The old capacitors have been clipped out and the tweeter cap has been soldered in.
2730.jpg (attachment #2?) Both new caps are installed.

OK. The speaker was rewired, the cabinet was closed up, and I fired up the amplifiers. From here on, as Carl says, "It's all about the music."

John








>OK, here goes with photos and comments. >Continued in next message.
>
>John
>
>
>
>>Hi John;
>>
>>Thanks for your write-up.
>>
>>If possible please take a before photo, or two, of the
>>crossover board and after mod.
>

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[jwjensen356]

OK, here goes with photos and comments. There will have to be another message after this with more photos (I hit the limit for the message).

2719.jpg The A-50 with custom cane grill.
2722.jpg Getting the patient ready for the operation.
2723.jpg Comparison of new and old caps (old caps were from the other system). Note how large the new ones are. The 24µF cap is really two 12µF capacitors soldered together
2724.jpg One woofer has been removed.
2725.jpg With one layer of sound absorbing material removed.
2726.jpg With two layers of sound absorbing material removed.
2727.jpg With vinyl shroud removed. Note old capacitors.

Continued in next message.

John



>Hi John;
>
>Thanks for your write-up.
>
>If possible please take a before photo, or two, of the
>crossover board and after mod.

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Attached Files

•  2720.txt   452.63K   131 downloads
•  2721.txt   420.45K   121 downloads

[jwjensen356]

I am doing some inside work on mine so I made a measurement. This is of the front panel (with the speaker cut-outs). I assumed the original was dimenstioned to metric measure. The thickness is 2 mm which is the equivalent of 0.787" (a bit greather than 3/4").

John

>
>I also need to know thickness of original cabinets (3/4
>chipboard??) or enclosure volume as i plan to make thicker
>walls and perhaps some bracing front-to-rear and
>side-to-side.