I believe I have solved the schematic for the Dynaco A25XL.....

Wayner · « **on:** 2 May 2015, 05:13 pm »

The other day I bought an LC meter and some caps to redo my Dynaco A25XL speakers. I believe the riddle of the schematic is finally solved. The coil measures .5mh and the tweeter's resistance measures 5.3 ohms. In the full tweeter output position on the 3 position level switch, all 3 resistors are in put in parallel with the tweeter, making the total resistance of the circuit for the tweeter at apprx. 4 ohms. That fits perfectly with a Linkwitz-Riley circuit for a 2500 hz cross-over point. 8uf cap with a .5 mh coil for a 4 ohm load. The woofer has no cross-over and rolls off naturally.

Here is the math:

Wayner

JerryM · « **Reply #1 on:** 2 May 2015, 05:46 pm »

How does it sound?

Wayner · « **Reply #2 on:** 2 May 2015, 06:08 pm »

For some reason, this pair (I have 2 sets) had piss-poor bi-polars in them. I put in Solens, and I am listening to them as I type. I never thought that caps could change the sound like these did. My vinyl is now way over the top. Very detailed indeed.

'ner

JerryM · « **Reply #3 on:** 2 May 2015, 07:00 pm »

Sweet

jtwrace · « **Reply #4 on:** 2 May 2015, 08:06 pm »

Quote from: Wayner on 2 May 2015, 06:08 pm

I never thought that caps could change the sound like these did.

Uh, oh, this thread could take a very sharp turn if the cap fans see this.

Wayner · « **Reply #5 on:** 2 May 2015, 08:35 pm »

Remember, these were cheap bi-polar caps. And very old, slightly out of tolerance.

The change in sound was instant.

'ner

PMAT · « **Reply #6 on:** 3 May 2015, 02:13 am »

Have you tried the Black Duelund Gate Mundorp solid gold in oil and vinegar???

dB Cooper · « **Reply #7 on:** 3 May 2015, 03:46 am »

But weren't all Dyna speaks 8 ohms?

heiba · « **Reply #8 on:** 3 May 2015, 08:30 am »

Sometimes I'm thinking of building a clone, but finding a 10" woofer suitable for the job isn't
so easy.
http://www.t-linespeakers.org/classics/dynaco.html

Ola
Norway

Wayner · « **Reply #9 on:** 3 May 2015, 11:59 am »

Quote from: dB Cooper on 3 May 2015, 03:46 am

But weren't all Dyna speaks 8 ohms?

Yep, go measure any 8 ohm raw driver and its not going to measure 8 ohms. The electrical resistance of the tweeter is 5.3 ohms (at rest). The 8 ohm rating is an average impedance thru the driver's operating range. The impedance changes are dependent on the frequencies it is reproducing.

Also, there are a series of resistors that are paralleling the tweeter. (resistors in parallel....)

dlparker · « **Reply #10 on:** 4 May 2015, 01:30 am »

Quote from: PMAT on 3 May 2015, 02:13 am

Have you tried the Black Duelund Gate Mundorp solid gold in oil and vinegar???

Frozen or fried?

Mark Korda · « **Reply #11 on:** 4 May 2015, 05:37 am »

Hi Wayner, I worked on my A-25 Mark2's today They use the same tweeter as the A-25xls, and I remember that way back I had trouble identifying the coil and you helped me. My mission today was to install the tweeters. The original wires with spade lugs were right in front of me and not much play from the crossover to the tweeter posts was available so I tried the old crimp thing. Forget it, the connection at first seemed okay but then worsened. I knew I should have soldered the bare wire ends but I got lazy. Then I tried to solder the spade lugs to the tweeter terminals. My solder joints looked picture perfect but when I screwed the tweeters back in I gave a little stress test wiggle and all 4 solder joints failed so I had to start over yet again. The tweeter screw holes lost their gripping power so I have to fill them with bondo and relocate them. I guess what I'm trying to say is never use the spade lugs when installing tweeters. Maybe I didn't leave my iron on long enough but it seems any longer and your playing Russian Roulette with your tweeters. Thanks for sharing your info on the Dynas and new caps and such....Mark Korda.

dB Cooper · « **Reply #12 on:** 8 May 2015, 03:38 am »

Quote from: Wayner on 3 May 2015, 11:59 am

Yep, go measure any 8 ohm raw driver and its not going to measure 8 ohms. The electrical resistance of the tweeter is 5.3 ohms (at rest). The 8 ohm rating is an average impedance thru the driver's operating range. The impedance changes are dependent on the frequencies it is reproducing.

Also, there are a series of resistors that are paralleling the tweeter. (resistors in parallel....)

I misread your post. Didn't realize you were referring to dc resistance rather than impedance.
I also didn't realize that the crossover was almost an octave higher on the XL vs the original A25. Never heard the XL as far as I can recall but it would be interesting to.

G Georgopoulos · « **Reply #13 on:** 8 May 2015, 04:00 am »

Speaker impendance is quoted at around 1Khz...

Wayner · « **Reply #14 on:** 13 May 2015, 09:11 pm »

Oops! Slight drawing error (shame on me).

The resistor network is nothing more then an "L" pad, creating resistance in series and in parallel to reduce tweeter output. Hopefully tomorrow I can draw a real schematic and show the network with the tweeter level control in all 3 positions.

Sorry for the confusion, if you had any....

'ner

Wayner · « **Reply #15 on:** 14 May 2015, 03:40 pm »

Here it is:

JerryM · « **Reply #16 on:** 15 May 2015, 07:23 pm »

Quote from: Wayner on 14 May 2015, 03:40 pm

Here it is:

Awesome, Wayner. Just awesome.