Scrap Box DC Darling! - by L. D. Moore

Initial Release: 6/25/99

Having heard various incarnations of the now infamous Darling at NY Noise,
I set about building one. My intention was, and is, twofold:

First, to have an amp that I can grab as I head out the door to gatherings in my effort to convert the masses to the wonders of low power SE triodes.

Second, to build an amp that communicates "music" as cost effectively as possible.

I feel that while these two seldom go together, they are, in fact, uniquely united by Bob
in the Darling. Most of the prospective converts are fellow law students who, without two
pennies to rub together, long for what they hear in my Ultra-Fi system.

Boy, will they be surprised this fall when classes begin and they get a taste of Ultra-Fi
from the Scrap Box DC Darling! (see bob.d.'s "DC Darling" schematic)

I decided to build a "DC (direct-coupled) Darling" since I favored this topology over the RC coupled verision at NY Noise. Knowing that I had a pair of SE 6BQ5 output transformers in my junk box, yanked from what I can't remember, I began the search for the power supply magnetics
one rainy morning in the damp basement of Fair Radio. I selected a power transformer
which had primary taps of 103.5, 115, and 126 VAC. The secondary B+ winding was
rated for 480VCT@60mA and I figured since I had several primary taps to choose from,
how could I miss on the B+ voltage?
As fate would have it, I used the 103.5VAC primary to derive a B+ voltage of 385VDC.
The were also two 6.3VAC filament windings.
On one, I used a full wave bridge and CRC filtration to develop 6.3VDC for the 8532s
which were wired in parallel and referenced to ground.
On the other, I implemented a voltage doubler and dropping resistor to yield 12.6VDC for
the 1626s which are also wired in parallel. This filament supply was then referenced to
+150VDC using the cathode resistor of the 1626s.
You see, the maximum heater to cathode voltage of the 1626 is +/-100 volts.
Therefore, since Bob has driven the cost the 1626 up with this wonderful little Darling
(Wink! Wink! Bob!!!), I figured I had better protect my 1626s by not exceeding the recommend rating.

The power transfomer is mounted at the back of the chassis.
The choke, mounted just in front of the power transformer and behind the 1626s, is the last of a bunch which I also bought surplus. It is a 2H@325mA brute with a DCR of 50 ohms!
This, along with the direct coupling, really makes this little Darling fast under transient conditions.

I also used common cathode resistors and bypass capacitors for both the 8532s and 1626s, respectively. I feel this really adds to the Darling's charm and have even implemented the techique in several designs of my own.

Final touches which set this Scrap Box DC Darling apart are:

(i) a 50k carbon comp audio taper volume control,
for those fast "direct-in" CD hookups on the road

(ii) a power switch, an IEC power socket, a fuse holder,
and five way binding posts on the back panel

(iii) mounting the output transformers under the chassis,
since I hate the look of open frame transformers and chokes

(iv) piece of aluminum channel stock which runs across the chassis
to reinforce the 2"x8"x12" Hammond box which the choke, in turn bolts, to

(v) aluminum gussets in the corner of the chassis which the feet mount to.

Please do yourself a favor! Use some of the tips and assembly techniques
on this page and become a member of the Darling Club!

New Photos of a pair of cap-coupled "Darlings":

e-mail L.D Moore

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