My experience with ‘Sakuma Darling’

I have heard a lot about the Sakuma Darling and
was eager to try it out. For the uninitiated, Sakuma San is a cool Japanese guy
whose amp philosophy is without peer. He believes in

• like drives like. If you like the sound of a
  300B, then use a 300B as driver. If you stick a 12AX7 in front, then you are
  no longer listening to the sound of a 300B.
• iron and lots of iron. Sakuma San believes in
  transformer coupling. Also, for 300B to drive 300B, you first need lots of
  voltage swing. This is where step-up interstage comes in. Unfortunately,
  interstage trans are REALLY expensive.
• mono is cool. Yeah, Sakuma San listens only in mono…

I can’t afford a 300B, lest a 300B driving another
300B. Nor can I afford those Tamuras Sakuma San uses, so the Sakuma Darling will have to do. I
hope it brings light on the Sakuma sound.

The above circuit is modified from Bob Danielak’s
Sakuma Darling. The cheap interstage transformers make it interesting and
affordable. Since make-or-break depends on the trans, they warrant a closer
look. Look here for my experiments with the AES 10k:90k
PT159 interstage. You’ll understand why there is a 47k on the secondary of
the PT159.

I spent some time testing the Allied 10k:10k
interstage as well. Unfortunately, it’s not as great as the AES interstage. In
fact, it SUCKS. With or without DC current flowing through it, it SUCKS.
There seems to be 2 peaks in the frequency response. A mild 6dB peak at ~30kHz
and a horrendous 12dB peak at ~55kHz. Worse thing is, there’s nothing much I
could do about it. Low frequency response isn’t great either. The spec says down
to 150Hz and you don’t have to doubt this. By the way, the interstage looks like
it’s made by Hammond. The box, the build and the labelling looks too similar to
Hammond. To improve the bass, the above schematics has a 30H choke in series
with the primary.

Anyway, I built it and… There’s really nothing
much on the lows. I don’t expect a lot of bass but it’s so rolled off that it
even struggles with some lower acoustic guitar notes. It does sound a lot more
transparent than the conventional Darling. it took out the fun and isn’t very
fun listening to.

A search on Audio Asylum suggests bridging a
0.22uF cap between primary and secondary of the Allied interstage. Said to
improve the frequency response from DC to light, but hey, this isn’t transformer
coupling anymore! Might as well take out the interstage and leave the cap there!
That’s what I did. I took out the weak link, the Allied and reverted to
capacitor coupling.

Now we have all the bass back! It’s still pretty
transparent but you could definitely change the sound with your choice of
coupling capacitor. I used Auricap here as it’s more neutral sounding. Image
sizes are a hell lot more realistic, more lifelike but it still retains its
ability to “boogie”. Unfortunately, gain is not that great. The 5965-1626 Darling has a gain of about
36x (so direct connection from CD player works just fine) while the above only
manages <15x. A preamp will be great but the above works just fine on my
Hammer Dynamics.  

There you go, the “Not-Quite-Sakuma
Darling”! This will have to do until I could afford some REALLY serious
interstage.

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Return of the Darling

Yeah folks, all this travelling from my day job has me listening more to elevator music than my humble setup. This is NOT good.

So the idea came to build a small amp, as compact as possible, so that I can bring it around. And what better than the humble little Darling amp? It’s been almost 2 years since I dismantled the previous “super mojo” Frankensteinian version.

OK, enough nostalgia. A Darling amp at 0.7W should be tiny, compact and cute right? The above, weighing more than 15kg for the power supply alone is more “Beast” then “Darling”. So the below transpired.

However, just building another Darling amp won’t be fun right? So there’s a twist. The output stage is no conventional SE but the Matrix topology. No, not that Oracle, The One, Zion blah blah nonsense but this topology rediscovered by the venerable Steve Bench. (Thanks Steve!) Since I’m using toroidal power trans as output trans, I adapted the wiring ala Steve’s 7695 version.

With 3 pieces of toroidal iron and the cool Matrix topology, we have a very small/compact/tiny/cute/darling Darling amp!

Uncommon signal will go through the first trans. If this doesn’t make sense, well, it works exactly like a center-tapped interstage transformer.

The common mode signal will be rejected by the top trans but fed to the bottom trans and mixed back at the secondaries to the speakers. So dissimilar trans won’t work!

Unfortunately the above configuration has unbalanced DC current on the bottom trans. This is where parallel feed comes in. Steve used a ridiculously small 1.5H as parallel feed choke! But this doesn’t matter to the Matrix topology as there’s still plenty of bass. Due to size considerations, I can’t fit in a choke (look at above pics). So I fitted in a 1kohm resistor. Works just as well. I can’t fit in a big 30uF DC blocking cap too so 5.6uF it is. The output trans are actually ILP power trans with primaries of 110-0V, 110-0V and secondaries of 9-0V, 9-0V.

The background is not as pitch dark as my reference system but then I economized so much on the power supply, so this could explain it. There is some channel crosstalk. It could be due to so many reasons I haven’t even started figuring out. Look at the wiring and tell me could there be crosstalk?! I think I’ll snip the wires after I’m done with modding it.

So there you have it. This is the cheapest/smallest tube amp I have ever built. Darling in all its glory!

www.diyparadise.com

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NOT “My Darling is a Monkey!” 

This is adapted from Thosten Loesch’s Full Monkey featuring the 5842 driving a 300B.

The simplicity and use of so few components is really inviting. Very very tempted to build… Beware though, unlike most tube designs, component values here is quite “tight” and may need some fine-tuning.

Circuit description is quite a mouthful…

“Ultrapath choke-loaded driver stage, direct reactance/’Free Lunched’, direct-coupled to output tube which is ‘Monkeyed’, ‘Ultrapathed’ and ‘Parafed’.”

Phew! Note that Ultrapath or Parafeed is of course, optional.

Most direct-coupled circuits commit suicide when the input tube goes bust, resulting in extensive damage to output tube and in some cases, even to power transformers. If you “forgot” to put a fuse in your amp, it might even bring down your house. Now your Monkey is a BIG MEAN KING KONG.

This Monkey Darling is safe though due to it’s “Monkey” design.

That said, bass performance might suffer due to insufficient loading on driver tube. Shouldn’t be a problem down to 40Hz though… Changing driver tube to lower rp types like 5842 will rectify this. Adjust resistors for 5842 operating points.

Read about “Free Lunch” here.

Read about “Direct Reactance” here

Read why this arrangment is safer than normal direct-coupled amps here.

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