Dynaco A-25 clones finished (again)

[lasse]

http://techtalk.parts-express.com/album.php?albumid=445

[OlderMongrel]

Re: Dynaco A-25 clones finished (again)

Let us know how they sound. I really used to crank mine, and they did very well. (Cranked the A-50's louder still, had to put damper cloths on the door coming into the room, it was buzzing. That was also just 1st order with resistors. Ah, college days...)

John

[lasse]

[moron#99]

Re: Dynaco A-25 clones finished (again)

One article mentions that A-25 were hand tuned by stuffing fill into the cabinet until they got the cleanest scope trace with a 5Hz square wave. In another place it was noted that an aperiodic speaker has an 18db per octave rolloff (a third order circuit). These two items taken together open the possibility that the A25 had less bass ringing than is possible in a purely passive bass reflex or acoustic suspension.
http://home.indy.net/~gregdunn/dynac...ents/speakers/

Another interesting item about the A25 is that the tweeter and crossover appeared to change over time with the Seas 1-1/2" being most common. Here is a thread in audiokarma with a copy of crossover drawing from Pete Basel
http://www.audiokarma.org/forums/sho...d.php?t=241374

I think that if you are going to use a modern 1" tweeter then you are forced to use a modern crossover. The woofer will have trouble going higher and modern tweeters will have trouble reaching down to the woofer. with a 6db slope you may be introducing a color that was not originally there.

[carlspeak]

Re: Dynaco A-25 clones finished (again)

Visit this link for some info. and measurements on a very vintage and somewhat unique A-25.

http://www.classicspeakerpages.net/I...showtopic=7091

The SS tweeter rolls off similar to your clone @~ 10 kHz. However, the nearfield woofer (also SS) measurement clearly shows a 12 dB/oct. roll off, typical of a closed box.

[moron#99]

Re: Dynaco A-25 clones finished (again)

Visit this link for some info. and measurements on a very vintage and somewhat unique A-25.

http://www.classicspeakerpages.net/I...showtopic=7091

The SS tweeter rolls off similar to your clone @~ 10 kHz. However, the nearfield woofer (also SS) measurement clearly shows a 12 dB/oct. roll off, typical of a closed box.

roll off is one consideration. My real thought was that the 1.5" Seas probably has a low Fs.


Wow. that's a nice impulse response you got. Did you change the fill a little to optimize? I wonder if when you add fill it affects the two poles inherent with the cabinet/speaker? It may be that an aperiodic also has an ideal cabinet size so that the cabinet/driver poles are fixed in a particular location. If that's the case then it would be impossible to have a perfect third order impulse without a perfect cabinet. Did you explore the effect of changing internal volume and +-filling?

[Pete Basel]

Re: Dynaco A-25 clones finished (again)

http://techtalk.parts-express.com/album.php?albumid=445

There were some revisions in the A-25 but the vast majority were built with SEAS drivers and per the crossover schematic that I provided.

I've not done extensive testing however it would be interesting to see the input impedance with the port unstuffed, and then with port stuffing that reduces the impedance peaks in half. I'm fairly certain that this is really just a very low Q 4th order system that looks 3 rd order due to the gradual rolloff.
It would be very interesting to see the driver acoustical output, port output and total summed response versus damping. And also to compare it to the sealed box with different stuffing densities.
The port is going to combine in a destructive manner at some point below the tuning frequency, for a high Q (infinite) ported system the tuning freq is that point, as the Q is lowered so too is the frequency where this happens.
The advantage of a sealed system is that this never happens obviously since there is no port output. I don't usually favor sealed systems, however sometimes they are best for smaller boxes.

[moron#99]

Re: Dynaco A-25 clones finished (again)

There were some revisions in the A-25 but the vast majorty were built with SEAS drivers and per the crossover schematic that I provided.

I've not done extensive testing however it would be interesting to see the input impedance with the port unstuffed, and then with port stuffing that reduces the impedance peaks in half. I'm fairly certaing that this is really just a very low Q 4th order system that looks 3 rd order due to the gradual rolloff.
It would be very interesting to see the driver acoustical output, port output and total summed response versus damping. And also to compare it to the sealed box with different stuffing densities.
The port is going to combine in a destructive manner at some point below the tuning frequency, for a high Q (infinite) ported system the tuning freq is that point, as the Q is lowered so too is the frequency where this happens.
The advantage of a sealed system is that this never happens obviously since there is no port output. I don't usually favor sealed systems, however sometimes they are best for smaller boxes.

You're a clever one Pete. Here I was trying to figure out how to modify the A/S formulas for an air leak. Your idea of just calling a spade a spade is so simple that it's brilliant. Small's model allows for air resistance in the port as a variable. I feel so stupid. All those years of admiring how much sound Dynaco got for so few dollars - and I never even tried to solve the A25 using Small's equations. now I just HAVE to try.

hmm ... what would you call the port length? I would guess the thickness of wood. Or would it be zero?

[carlspeak]

Re: Dynaco A-25 clones finished (again)

I dredged up the WT2 impedance/phase test I ran on that rare A-25. Here it is.
Not much damping on the woofer imp peak eh? :-) Fc was 65 hz with a Q of about 1.2.

[moron#99]

Re: Dynaco A-25 clones finished (again)

I dredged up the WT2 impedance/phase test I ran on that rare A-25. Here it is.
Not much damping on the woofer imp peak eh? :-) Fc was 65 hz with a Q of about 1.2

you wouldn't happen to have an frequency response plot in there would you? Jeff B has me wondering if it is a 12 or 18dB per octave rolloff

[carlspeak]

Re: Dynaco A-25 clones finished (again)

you wouldn't happen to have an frequency response plot in there would you? Jeff B has me wondering if it is a 12 or 18dB per octave rolloff

The FR is located at the link I gave you in post #5. IIRC, I said the rolloff was 12 dB/oct. in the woofer NF test.

[moron#99]

Re: Dynaco A-25 clones finished (again)

The FR is located at the link I gave you in post #5. IIRC, I said the rolloff was 12 dB/oct. in the woofer NF test.

thanks. well that settles it. 12/octave is second order. slope never lies. thanks a million.

[lasse]

Re: Dynaco A-25 clones finished (again)

Please, let me humbly direct you to the folder in which I have put some info on the subject...


http://techtalk.parts-express.com/album.php?albumid=445

Best regards//lasse

[lasse]

http://techtalk.parts-express.com/album.php?albumid=445

Best regards//lasse

[Pete Basel]

Re: Dynaco A-25 clones finished (again)

thanks. well that settles it. 12/octave is second order. slope never lies. thanks a million.

Hold on, the woofer near field measurement does not take into account the port output which will cancel and cause a higher slope. Carl is using a near field measurement method where normally the woofer and port output are measured independently then combined as vector sums. I don't think he summed the port output.