[mgalusha]

For some time I've been eyeing Decware's ZSLA-1 (positive feedback review here) as a DIY project since Steve Deckert publishes the schematic on his website.

After collecting all the parts I finally got started a couple of weeks ago. I used a chassis purchased from a friend who had too many. I don't know who made it originally. The chassis measures 8 x 6 inches and is about 3 inches high. The material is black anodized aluminum.



chassis machined and ready to assemble

The design called for 4 50uF/450V caps in the power supply. I choose some Sprague Atom axial units. The problem with these caps is mounting them. Since the inside of this unit would see virtually no heat, it seemed that a nice piece of maple would make a great mount for the power supply caps.



underside of chassis with caps and transformer mounted

For a volume control, I choose a TKD pot purchased from Michael Percy. I really like this pot. Channel tracking is excellent and the feel is very smooth. The Solen caps are on the power rails and not in the signal path. The diodes are Fairchild "stealth" fast recovery types rated at 1200 volts.



underside of chassis, partially wired



top of chassis before wiring, end plates not attached

The real ZSLA-1 has female RCA's on a pigtail connected a 4 terminal XLR plug. I wanted a more "normal" setup with chassis mounted RCA's. I picked up some nice rear mount jacks from Antique Electronic Supply. They appear to be Vampire clones. A friend said they may be an OEM version of the Vampire's. I don't know for sure but they are pretty nice and reasonably priced.



vampire jack look alike

Not shown are some RelCaps employed as coupling caps. Everything is wired point to point using terminal strips. Wire is silver plated copper with teflon insulation. I planned on using some lovely wire supplied by ABEX but it was just a bit too fragile for this task. The wire is quite small with thick walled teflon insulation. I had it break several times and a mental picture of the lead running to an input or output jack breaking off and touching the B+, sending 400 volts to the attached component was truly frightening. 



all done and ready to boogie, love the gold Svetlana logo. tube is a 6N1P dual triode.



the business end. transformer is mounted on some nylon standoffs to allow clearance for the wire grommets.

Of course the $64 question is how does it sound? I completed it last night and even with only an hour on it last night it was doing a lot of things right. Listening to some John Lee Hooker was startling, the sense of physical presence was very good. I listened to after work today and the sound is very nice. This is one of those pieces that I would describe as musical. Nothing that really stands out as far as tons of detail or slamming bass but it has a very non electronic, organic sound. At least to me and in my system.

It's a near certainty that the sound will change as additional hours are accumulated. I'll post a mini review in the Critics Circle after I have some more time with it.

At the moment I'm quite happy with the results, especially since I have only about $200 invested.

mike g

updated 7/25/2009 to fix gallery links due to inquiry about project, text unchanged

[mgalusha]

One thing I forgot to mention, the little sucker is too tall to fit into my rack! Even with the rubber feet removed it's about an inch too high. DOH!

Not sure what I am going to do but for now it's happy sitting on the floor next to the amps. Maybe it's time for a DIY rack as well.

[lonewolfny42]

mike g ,  Nice project , excellent pictures ! Happy that there's a circle like this now, even though all I've got is a hammer and screw driver, all thumbs too , I'll enjoy reading about the many members future products....with pictures,  GREAT !!  

[PeteG]

Mike
Good job here too, nice idea with the wood & caps. Let us know how it sounds after a few weeks.

[Rocket]

Hi Mike,

Thankx for sharing your project with us it was very informative.

please let us know how it sounds after break in.

regards

rocket

[JohnR]

Cool

I believe those boxes are made by a company called Context Engineering in Santa Clara. In the Bay Area some sizes are sold by Fry's, but if they don't have the size what you want, Context don't seem to mind you calling them and going over there to buy a couple

[mgalusha]

Update - I've decided the bass is too thin. I did a quick calculation I think it's about -3dB at 19Hz. I will put it on the bench and take some measurements to be sure. Since it uses no feedback I think the coupling caps are acting like 1st order high pass filters.

Since my amps only have 18KΩ Zin and the coupling caps are .47µF the -3dB point looks to be about 19Hz. I have a nice pair of Jensen copper foil in oil 1µF caps I will parallel with the Multicaps. This should put the knee at about 6Hz, so the output should be pretty flat to about 20Hz or so. At least I think so...

Stay tuned for the continuing saga...  

Mike

[mgalusha]

I decided to take some measurements prior to making any changes as I wanted to know if I was just imagining the thin bass. Nope, it was not my imagination. It wasn't as bad as I thought, down 1.6dB at 20Hz and down 6db at 14Hz. What really sucked was the frequency response was all over the place and anything but flat. Like +/- 3dB from 20 to 20k. Not very good. It really started rolling off at about 40kHz. Not that I can hear that high but I had hope for better performance.

After dinner and a movie (O Brother ) I installed the Jensen coupling caps. I decided not to parallel the Jensens with the Multicaps, thinking there would be less chance for smearing and other anomalies. I put it back on the bench and had not reset the test gear, so everything was in the same state. Wow, I could hardly believe how different it measured. It was now down .1dB at 20Hz and -2dB @ 10Hz. -3dB occurred at 4Hz near as I could tell. The rest of the frequency response was within about .05dB reaching -3dB at 70kHz. I had to run through the tests a couple of times to make sure I wasn't imagining things. Nope, it was for real.

I haven't gotten to listen to it yet as it's too late today so I left it on the FryKleaner for the night. Really looking forward to trying it in the system tomorrow.

One good thing about the measurements, I ran the function generator up all the way and could not make it overload or clip. The waveform on the scope of the output was virtually identical to the input. I was able to lay them on top of each other, sum them and invert one and had nearly a straight line. Very clean, at least on a sine wave.   That was with a 12V P-P input signal and the volume pot all the way up.

mike

[mgalusha]

I've been listening to the system with the Jensen's in the preamp since 8:00 this morning and haven't wanted to get up and do anything, so that is a good indicator of how it sounds. 

The bass is much better, deeper with a fuller sound. Not quite as tight as the Grounded Grid that is usually in the system but quite good for a zero feedback design. Dynamics seem very good and at the moment I am very happy with the overall sound of the system. Swapping the Jensens for the Multicaps was a very positive improvement.

The real test is to see how well I like it after a few weeks of living with it. It's easy to be excited about something new only to find it grating in the long term. But for now it's kickin' butt for only a couple hundred.  I'm also enjoying having a volume pot again. While a stepped attenuator sounds great, it's nice to be able to obtain exactly the volume level I want.

One last photo, inside with the Jensen caps installed. They are quite large compared to just about anything else in the circuit.



mike

Edit: Forgot to mention that the preamp is much queiter with the Jensens. Not sure why but before there was enough hiss to hear a foot or two away from the speakers. Now it's nearly dead quiet, with my ear right up against the tweeter there is a very faint tube rush hiss. Very nice indeed.

7/25/2009, gallery link fixed