Dynaco A25 clone

[JKent]

"Dynaclones"

A friend gave me a pair of crappy no-name speakers with woofer rot but they have really nice walnut veneer cabinets (photo 1). So after considering building an E-wave I consulted RoyC, who suggested using the GRS Dynaco A25 replacement woofers and tweeters sold by PE. I've never owned or even heard Dynaco speakers so I thought why not?

The cabinet size is close: 12-7/8"w x 22-1/8h x 9-1/2d. The real 25s are 11-1/2"w x 19-3/4h x 9-7/8d.

The first challenge: My speakers were 3-way acoustic suspension type. So I had to close up one hole and make a port. Looking around the interweb I saw that the tweeter is close to the woofer and although most A25s have the aperiodic slot on the bottom, some have it on top (photo 2, from web). So I decided to put the new tweeter where the mid had been, close up the tweeter hole and cut a slot. I made a port out of pine and covered the back with crinoline to hold the stuffing in the port. Roy says the amount of compression of that stuffing affects the sound and will have to be "tuned". Added a front-to-back brace made from a scrap piece of closet pole. I found some metal grilles in my basement. Don't know what they were from but they fit the port nearly perfectly (Photo 3). Once the new configuration of the baffle was finished I filled and sanded, then coated it with some DuraTex I had on hand, also from PE (Photo 4). The round grilles over the tweeters are some $1.50 PE buyouts I had on hand. The mounting holes weren't perfect but a Forstner bit and some washers fixed that.

My timing was off for the crossovers. Some sold on ebay for a fair price a couple months ago. Oh well. So I built my own, using a schematic and photo on updatemydynaco.com as a reference (Photo 5). My local electronics store had a 2 pole 5-position switch. Only needed 1 pole. The other components are one 5uF cap, five 2.2 Ohm resistors and one 10 Ohm. I ordered the 2.2 Rs from Mouser, everything else I had on hand. The cap is 4.7uF so I played with "cascading"--something North Creek used to advocate. The 5th photo shows the xo. I don't know where I got the 4.7uF 400v cap. The green one is a 0.1uF Paper-in-Oil and the small one is a 0.47 poly. The bundle measures 4.95uF. Photo 6 shows the new label and crossover panel from the back. 5-way binding posts and aluminum knob were parts I had on hand.

The next step will be making new grilles. The cutout for the woofer in the old ones is a little too small and touches the gasket on the new woofer. I'll make new frames and cover them with light linen (another suggestion from Roy).

The resistors arrived about 6PM today but I had to rush to finish putting the speakers together 😄 First impression: I'm very happy with them. I haven't fiddled with the internal stuffing (poly from some AR-11s that were beyond repair) nor the stuffing in the port. I think I like the tweeter switch in the middle or "normal" position. Placement in my shop is far from ideal. BUT, these sound great to my ears. Clear and detailed are the first words that come to mind. They have some stiff competition in my workshop: AR-4x's, AR-91s and AR-3a's along with a couple of others that aren't hooked up. I'll post further impressions and photos of the new grilles when finished.

Kent

 

 

 

 

 

[JKent]

Finished the grilles. Instead of making new frames I just ran strips of 1/8" Masonite around the perimeter. This not only allowed the board to clear the woofer gasket, it also strengthened it and gave enough thickness to staple the linen.

Linen is some Zweigart 28 count Cashel Raw Linen from Michael's that I had on hand. After attaching the cloth to the board I spritzed it with water and dried with a hair drier. Nice 'n' taut. Finished the grilles off with some Dynaco "d" badges from ebay seller audiotagsiowa. I didn't want to use the full dynaco name, since these aren't real dynacos.

Still lovin' the sound.

 

Edited April 24 by JKent