|
My "Darling" Project by Jeffery Larson A Sweet 1626 Micro Power Amp |
| An amp a long time coming.
See here for the original concept and more than you ever want to know about these amps.  |
|
I didn't have the tweak 8532 tube used in the original. I guess the Darling's popularity has had its effect on the price as well. So what to do? Well, I didn't want to make an exact copy anyway. I reviewed what I did have, and was pleased to discover that one of my all time favorite tubes, the 5965, was going to work just great. (Later I discovered others have also chosen this tube.) Giving a roughly similar curve as the 8532, but at a little different operating point. The final OP has not been set yet, but I am currently running it at 4mA/side with 85V on the plate. I like driving the 5965 hard, as I think it sounds way better.
 I grabbed some parts outta my supplies and got ready to breadboard the amp. The nice little NOS power tranny has a 500 ct, 6.3v, and 24v (at 250mA) windings, so I thought: well the 24V can be used to power the 1626's in series so I don't have to add another tranny! The series arrangement works perfectly. I might add some balancing resistors, but the readings say I don't need them. This circuit is about as easy to breadboard as anything I've built It was up and running in under 2 hours, including gathering the parts. I had a lot of 6V windings on the power tranny, so I tried out a 6X5 because I thought it would look cool and give me a slower warmup. It's a big job for a 6X5, but I knew it could handle it. Unfortunately, I could not get enough output voltage at 60mA so I sighed and went with SS diodes. That's when I needed the choke. I like using a choke in the PS because they smooth the DC out so well. I also needed about 300 ohms of drop. In the end, I settled on a resistor. It actually sounded better. (Changed later - see below.) For breadboarding I opted for a pair of 1W 1K carbon comp's in parallel for the shared cathode resistor in the output stage. (Changed in final version.) It actually sounds pretty good. My parts bin turned out a nice 470uf cap for the bypass. Unlike the original Darling, I added an additional resistor drop and filtering for the driver/preamp. Dropping a mere 25V or so and quieting the supply further for this stage. So my input stage voltage is about 250V and is dead-bang quiet. There is only 5mV on the output. The first time I fired it up I knew it was going to be a sweet amp even though not yet operating at the best voltages. Now it was time to tune it up. Mostly this was power supply work. I swept the output trannies to be sure I wasn't wasting time. They tested great. Next I adjusted the power supply. I tried various chokes, resistor (better sound) and finally came up with using fast switching diodes and feeding the DC to the plates of the 6X5. This gave the 6X5 an easy time, provided the correct voltages and permits slow warm up to protect the output tubes. Looks cool too, and that counts, doesn't it? I added 1-ohm resistors to each output cathode so each output tubes current could be measured to be sure they were reasonably balanced. Got a nice vintage 500-ohm vitreous enamel resistor from Walter (thanks Walt!) for the main self bias resistor. This is bypassed with a vintage 470uf which in turn is bypassed with an NOS PIO cap. These caps are placed for easy upgrade if desired. For coupling I chose my favorite PIO caps and then added some great sounding hermetic paper foil caps from Russia. These are strategically placed so that one could upgrade them also. There's a lot of decoupling in the power supply. I used both PIO and film caps in various locations to bypass the aluminum electrolytics, to help ensure clean noise-free power, and speed up the supply a bit. The heaters are also bypassed with ceramics as a de-hashing method. I do this in almost all my amps and find it is worth while. Silver plate wire is used in most of the signal paths as well as silver bearing solder. The input wire is old milspec silver plate. Copper is used for the ground plane and tinned copper for the power distribution (such as it is; not much power to send around this amp). These wires are all oversize for the current they are carrying without getting weird about the size (too big) which seems to de-focus an amp. In fact, smaller wire seems to sound better generally, but my gut won't let me install tiny wire for power leads. The output trannies have multiple output taps. I didn't want to fill up the back panel with all the connectors, so I opted for an impedance switch. The input connectors are from Daytron and the speaker connetors will accept 8 ga. wire, a spade terminal or banana plug. |
|
I have this rather large collection of vintage lamp indicators (thanks again, Walt). I am saving most for guitar amps, but decided one would look good here. The red cut glass lens is lit by a standard #47 bulb with a dropping resistor to cut the brightness into something more suitable for a living room. I made a special chassis indentation to mount it in just to add some class and glow.
|
|
What can one say? (I think I always say that as I bow my head in acknowledgement of how impossible it is to convey sound quality in mere words.) How about THIS word: WOW. Does that cover it for ya? I've never been so impressed with an amp. Plays WAY louder than you would think for a mere 750mW/ch. Detail, space, depth: all these words work for this amp. I cannot hear it lacks anything including bass detail. My wife has the golden ear. We listened to her rock as well as my usual gamut of stuff. I'm into Native American music. With this amp the flutes have full air and the tom drums sound authentic with just the right decaying "ring" they have when heard live. The Emenem Show sounded great. Wife leaves the room but returns for The Wall which was outstanding in detail and depth. I listened to Mary Black (of course), Tanita, chorus, Eric Clapton the list goes on. Everything sounded wonderful except those CD's that already sounded bad (yes we play those too to be sure we're on track). Lately I've been listening to Los Lonely Boys and their rock/Santana style sound comes across well. This is a pretty good recording.
My wife gives the nod to the 807 amp for its more serious rockability. Personally, I prefer this one for its detailed imaging. It seems to handle complex music with ease and grace. Something very rare in a small power amp. And even tho I could see the peak clipping on the scope, it was very hard to tell it was going on in the music as it didn't become blurred together as most amps do running near their limits.
All in all an extremely satisfying amp. Although I built this one to sell, I will definitely build one of these for myelf and I doubt I will ever give it up. Sweet! Thanks for looking.
A beautiful, foggy fall morning. |
