Dynaco A25 clone build thread

[lasse]

Hi all.

So now I´ve landed four SEAS 25 TV-EW from swedish ebay. Will hopefully be delivered this following week and be the basis for my long-planned project to build a pair of Dynaco A-25´s look-and-sound-alikes.

The "tesimonies" of the sound of the original A-25´s on the net all say that this is a very exceptional system in terms of SQ and my future decisions, of which you will hear a lot if you follow this thread, will deal with the reasons why the original system obviously sound so good.

I will house the clones in two (I have four that is two more apart from the ones in the pic´s) cabinets from Marantz described in this thread:

http://techtalk.parts-express.com/album.php?albumid=357

A few thoughts on the basic design to start with:
-Is this vent opening (aperiodic...) really necessary? If the reduction in impedance is the "thing", why not fix that in the XO?
-Is the very simple XO vital for the SQ? The Marantz system has a very simplistic XO and it has a very relaxed sound that I like (I use it like 8 hours /day at work on moderate level)
-Has anyone made a impedance measurement of a original A-25 to share with me? (Good to compare with the imp of my clone when that day comes)

More to come later.....

regards//lasse

[OlderMongrel]

Those are 15 ohm woofers, used in the A-50 (2 per side parallel for 8 nominal). I owned both the A-25 and A-50s (wish I had the 50s back). That's a good find. They made relatively few 50s. You really can't duplicate the 25 with that woofer. You would need the 8 ohm version. If you can't find used, MAD sells a similar one:

http://www.madisoundspeakerstore.com...10-paper-cone/

To me it would make a lot more sense to just build the A-50 clone, if the woofers test out OK. There's some info here:

http://www.t-linespeakers.org/classics/dynaco.html

It looks like the A-50 is the only model that does not show box dimensions, but they do have the XO. I would go aperiodic, it's easy.

Good luck, and keep us posted!

EDIT: If the woofers are in fact good, and you decide you don't want them, PM me!

John

[lasse]

Those are 15 ohm woofers, used in the A-50 (2 per side parallel for 8 nominal). I owned both the A-25 and A-50s (wish I had the 50s back). That's a good find. They made relatively few 50s. You really can't duplicate the 25 with that woofer. You would need the 8 ohm version. If you can't find used, MAD sells a similar one:

http://www.madisoundspeakerstore.com...10-paper-cone/

To me it would make a lot more sense to just build the A-50 clone, if the woofers test out OK. There's some info here:

http://www.t-linespeakers.org/classics/dynaco.html

It looks like the A-50 is the only model that does not show box dimensions, but they do have the XO. I would go aperiodic, it's easy.

Good luck, and keep us posted!

EDIT: If the woofers are in fact good, and you decide you don't want them, PM me!

John

Hi and thanks for the feedback.

The seller on ebay states that the drivers are 8 ohms and I hope that he is right on this. I have no idea where the drivers come from but the first thing I will do is to make a quick imp-sweep on each driver to see...

regards//lasse

[djg]

I had some A50s long ago. Ex gave them away.

[airtime]

There were several versions of that woofer in the A25 line. I believe that was one of the older ones?

The current replacement is the Seas A26ar4 (I think?) with the seas 27TDFC tweeter.

XO is basically nothing to a 0.8mH coil on the woofer and a 8ohm cap on the tweeter. The selector switch was just to pad the tweeter to suite the user


charles

[OlderMongrel]

I might be wrong about the woofer cited in the OP being 15 ohm. People seem to be using that part number for both the 8 and 16 ohm versions.

John

[JimHRB]

Box dimensions for the A50 were 28" x 21.5" x 10". That's per an old brochure I have from the 1970s. I built the SCA-80Q amp and FM-5 tuner kits back in the day.

[J.Lee]

djg - reason enough to become an ex.

[djg]

Box dimensions for the A50 were 28" x 21.5" x 10". That's per an old brochure I have from the 1970s. I built the SCA-80Q amp and FM-5 tuner kits back in the day.

Ha. Me too.

[stan]

I am replacing the woofers on my A25’s now, so this is timely. The attached photos may help you in your re-creation of the A25. Here is some info, in case you don’t already have it.

Inside dimensions of the cab are 8” deep by 10 “ wide by 18” long. Cab walls appear to be 5/8” thick particle board, with walnut veneer.

There are recesses cut into the cab sides where the woofers mount to allow the woofer frame to fit into the narrow cab, difficult to see in the photo. The woofer frame slightly overlaps the tweeter flange.

There is considerable rock wool insulation in the cab, about 3” thick on the back, and about 1” thick on the sides.

The crossover for the tweeter is 1st order with 5 step variable attenuation. Cap value is not visible, but s/b 10 uf IIRC. At the highest level, there is a 2.2 ohm 5 W resistor after the cap. Each step lower adds another 2.2 ohm 3W resistor. There is a 10 ohm 5W resistor in parallel with the string of resistors, forming somewhat of an L-pad.

More on the next post, only 5 pics allowed per post.

Stan

[stan]

OK, here is a little more. The aperiodic vent is 1.5” wide by 8” long on the front, but only about 1” wide on the inside. A heavy plastic grate compresses the insulation in the vent, and is stapled to the front. The woofer runs full range, with nothing between it and the binding posts.

IIRC, each speaker’s aperiodic vent was tuned at the factory by feeding it a 5 Hz square wave, observing the signal on an oscilloscope, and adjusting the stuffing for the best possible waveform, i.e. least amount of under/overshoot. This would help reduce the effect of variations in individual woofer Qtc.

Interesting technique, might be good to try with modern drivers. The woofers I used are the $5 buyouts. I am listening to them now, not nearly as good as the original SEAS woofers, but did not expect them to be. I have a pair of A35’s that I listen to frequently, so not to worry.

Stan

[PWR RYD]

What is the black material that is applied to the baffle? It looks more substantial than just paint. Is it for some sort of dampening or diffraction purpose?

[lasse]

OK, here is a little more. The aperiodic vent is 1.5” wide by 8” long on the front, but only about 1” wide on the inside. A heavy plastic grate compresses the insulation in the vent, and is stapled to the front. The woofer runs full range, with nothing between it and the binding posts.

IIRC, each speaker’s aperiodic vent was tuned at the factory by feeding it a 5 Hz square wave, observing the signal on an oscilloscope, and adjusting the stuffing for the best possible waveform, i.e. least amount of under/overshoot. This would help reduce the effect of variations in individual woofer Qtc.

Interesting technique, might be good to try with modern drivers. The woofers I used are the $5 buyouts. I am listening to them now, not nearly as good as the original SEAS woofers, but did not expect them to be. I have a pair of A35’s that I listen to frequently, so not to worry.

Stan

HI Stan,

And thanks a lot!!

This is the kind of info a guy like me really long for in this effort to build a clone- an "autopsy" of the original thing!

Now, there is one thing on my list to Santa- and that is a impedance plot of the original system. To compare my future clones with.

The very best regards//lasse

[stan]

The black material on the baffle is just semi-gloss black latex paint, applied heavy with a foam brush. The baffle surface was damaged when the grill was removed, and I just wanted it to look a little nicer.

If I get a chance, I will run an impedance curve on both the revamped A25's, and my A35's. Neither will be correct for an original A25, but might be interesting anyway. Will also measure FR at the same time.

Stan

[lasse]

HI,

somehow this thread seem to have dissappeared from the index ??

regards//lasse

[lasse]

HI,

This is my intended workflow for this project:

0. Build stands on the Marantz Imperial 5-boxes (2 units, for future XO)
1. Unpack woofers, visual inspection
2. Measure Re on all four as first check
3. 24 h break-in of two selected woofers
4. Measure T/S-parameters, check with data sheet for consistency
5. Mount in box including aperiodic vent ( 1 unit)(Leave old tweeter in place)
6. Measure woofer SPL and IMP ( 1 unit)
7. Select tweeter. Mount in box ( 1 unit)
8. Measure tweeter SPL and IMP ( 1 unit)
9. Measure and calculate Delta-AC ( 1 unit)
10. Simulate system with actual Delta-AC and original XO
11. Simulate system with Delta AC=0
12. Decide if worth lifting woofer out from tweeter level (Wood-work!)
13. Measure system SPL and IMP on one unit
14. Baffle, paint, frame and XO-work (on both units)
15. Measure system SPL and IMP on both units

This is if not one (or more) of you talks me into skipping the aperiodic vent...

Regards//lasse

[stan]

Hi Lasse!

I did a little more poking around on the internet after my last post. There is a fair amount of info about these out there, since so many were sold. One other thing I noted was that when the aperiodic vent was properly stuffed, the woofer impedance peak at resonance would be only 2:1 compared to the impedance above and below the peak.

Don't know what that relates to in terms of Q, but seems pretty well damped, compared to impedance curves for sealed systems in general. Might explain the comments about exceptional transient response for the A25. Probably produces a very gradual rolloff that complements room gain well.

My A25's were part of a system I bought in 1973, shortly after getting married. Driven by a Harmon Kardon 330B, with an AR-XA turntable spinning the vinyl. Many hours of musical enjoyment from that system. To be honest, I'm not sure any combination of stuff I've owned since then has produced overall better sound. Probably more accurate, but maybe not more enjoyable in a "just sit down and enjoy the music" kind of way.

I don't want to talk you out of the aperiodic vent, it will be the most interesting part of your build. Would be really interesting if you build it sealed first, take measurements, then install and tune the aperiodic vent, with additional measurements following. Plus your comments about how they sound, of course.

Cheers!

Stan

[greywarden]

I happened upon this the other day, forgot it existed.

http://www.usdaudio.com/products/ap.php

Aperiodic Mats, might help with the developement of the AP membrane, you can just cut the shape you need out.

[PWR RYD]

Hi Lasse!

I did a little more poking around on the internet after my last post. There is a fair amount of info about these out there, since so many were sold. One other thing I noted was that when the aperiodic vent was properly stuffed, the woofer impedance peak at resonance would be only 2:1 compared to the impedance above and below the peak.

Don't know what that relates to in terms of Q, but seems pretty well damped, compared to impedance curves for sealed systems in general. Might explain the comments about exceptional transient response for the A25. Probably produces a very gradual rolloff that complements room gain well.

My A25's were part of a system I bought in 1973, shortly after getting married. Driven by a Harmon Kardon 330B, with an AR-XA turntable spinning the vinyl. Many hours of musical enjoyment from that system. To be honest, I'm not sure any combination of stuff I've owned since then has produced overall better sound. Probably more accurate, but maybe not more enjoyable in a "just sit down and enjoy the music" kind of way.

I don't want to talk you out of the aperiodic vent, it will be the most interesting part of your build. Would be really interesting if you build it sealed first, take measurements, then install and tune the aperiodic vent, with additional measurements following. Plus your comments about how they sound, of course.

Cheers!

Stan

I can comment on part of this from personal measured and subjective experience. On my Trials project I did quite a bit of impedance measurement and listening of the bass bins regarding stuffing and the aperiodic vents. (Cliff Notes version) I started out sealed with no dampening material and no vent which resulted in a Qtc just over 1. In room it was definitely boomy. Adding 1 pound/ft^3 of fiberglass stuffing brought the system Q down to about 0.9. It sounded noticably better. I added the Scanspeak variovents (unmodified) and system Q dropped to 0.78. Wow, much better sounding!!! I peeled a thin layer of fiberglass mat out of the variovent, put it back together and Q dropped to 0.735. That's where they are at right now and I absolutely love the bass response. It is very natural sounding and certainly not weak. As Stan said, I think the bass response rolls off shallow and is complimented well by the room gain. I didn't do any in room FR measurements on the bass bins, just nearfield measurements w/ adjusted level for the initial XO simulation design.

I don't believe aperiodic vents are the necessarily the "best thing ever". Everything is a compromise of course. But they do seem to work well with the right woofers if box size is a constraint.

I hope you explore the aperiodic vents because I think you just might like what you hear. Best wishes Lasse.

[lasse]

I can comment on part of this from personal measured and subjective experience. On my Trials project I did quite a bit of impedance measurement and listening of the bass bins regarding stuffing and the aperiodic vents. (Cliff Notes version) I started out sealed with no dampening material and no vent which resulted in a Qtc just over 1. In room it was definitely boomy. Adding 1 pound/ft^3 of fiberglass stuffing brought the system Q down to about 0.9. It sounded noticably better. I added the Scanspeak variovents (unmodified) and system Q dropped to 0.78. Wow, much better sounding!!! I peeled a thin layer of fiberglass mat out of the variovent, put it back together and Q dropped to 0.735. That's where they are at right now and I absolutely love the bass response. It is very natural sounding and certainly not weak. As Stan said, I think the bass response rolls off shallow and is complimented well by the room gain. I didn't do any in room FR measurements on the bass bins, just nearfield measurements w/ adjusted level for the initial XO simulation design.

I don't believe aperiodic vents are the necessarily the "best thing ever". Everything is a compromise of course. But they do seem to work well with the right woofers if box size is a constraint.

I hope you explore the aperiodic vents because I think you just might like what you hear. Best wishes Lasse.

Hi and thanks for the feedback!

yes it would be interesting to be able to swich between closed box and aperiodic, and after this days work in the workshop, I have thought of a way to do it. I lay out the aperiodic vent as a long hole in the baffle with a piece of wood glued to the back of the baffle with the more narrow slot in it. In the rim around the slot, I arrange six T-nuts from the back so that alternately a piece of wood the size of the baffle-cutout can be fastened (Closed box) OR the aperiodic vent-foam with the grid held in place with the bolts/Tnuts (Aperiodic).

It seem to be quite simple if you arrange it from the start.

Todays work has been to strip the speakers from drivers and old XO-part and to build the stands. Tomorrow, I will glue the stands onto the cabinets.

By the way, I have to choose tweeter in a few days. What is your opinion on that?
I have two basic alternatives:

• SEAS (a few well regarded drop-ins for the original )
• Dynaudio D260

Regards//lasse