Andrew Cooke: C[omp]ute

Bottlehead Crack, AKG K701

From: andrew cooke <andrew@...>

Date: Fri, 22 Nov 2013 00:31:26 -0300

After a series of misunderstandings, mostly on my part, I ended up with a
Bottlehead transformer that I had no use for.  Returning it (from Chile)
seemed like a waste of money, so I bought the kit that went with it.  Which
turned out to be the Crack headphone amplifier.

The kit was great.  A real pleasure to build.  Everything included and
excellent instructions.  I don't think there was a single error in the
instructions, although I would suggest:

  * Reading ahead and changing soldering order if it makes things simpler
    (nothing critical, perhaps just my taste)

  * Measuring wire lengths yourself rather than using the lengths given (you
    can't for the twisted pair; the value they give there is very generous)

  * Thinking about wire colours yourself.  The colours chose were strangely
    inconsistent at times.  This doesn't change the sound, but makes it harder
    to "read" and check the circuit.

I must admit I had a bad solder joint, but once I fixed that it worked
perfectly.


So I have a valve headphone amp.  Unfortunately it's intended for high
impedance headphones and the highest I have are the not-very-high AKG K701s.
So I tried those.  And actually, they don't sound that bad.  There's
significantly more bass (and low mids) than there should be, but since the
K701s have quite a lean sound that isn't altogether unwelcome.

On a first listen, what struck me most, after the bass, was the stereo
imaging.  I haven't made a direct comparison, but it seemed significantly
better than what I am used to (through either Music Fidelity V-Can or a Roksan
K2).  In fact, I don't normally pay much attention to stereo positioning - I
noticed it here because it "jumped out" (in fact, I was listening to a live
Paul Motian recording and thought someone was talking behind me; very
disturbing)


Next I am thinking of connecting it as a pre-amp.  As far as I can see, if I
feed each channel to a 220 / 22 ohm divider, and take the signal from across
the 22 ohms, it should (1) be about the right level and (2) have about the
right output impedance.  It's kind of a weird way to do things (a lot of extra
work), but since it's there anyway...

Andrew

Ooops K702!

From: andrew cooke <andrew@...>

Date: Fri, 22 Nov 2013 00:39:42 -0300

Actually they're AKG K702s (which I think are identical apart from colour?)

Andrew

Bottlehead Crack as Pre-amp

From: andrew cooke <andrew@...>

Date: Sat, 23 Nov 2013 14:36:57 -0300

Well, it works!  I used a 1k/100 ohm divider and the amplitude is slightly
lower than a direct feed at max volume, so if I were to do it again I'd
probably used 1k/220.

The sound seems fine (it's difficult to A/B because of the volume drop).  The
bloated bass from the headphones has gone; there may be better stereo imaging;
the high end may be softer (compared to pass-through from source, an Audio-GD
NFB3).

Constructing the bridge within the cable was a little tricky; I'm quite happy
with the final approach I used which I'll try to draw here


                           ,--/\/\/\-------------------
                          /               /
                         /    ,--/\/\/\--'   ,---------
  headphone   ----------'    /              /
  socket      -----------------------------o
              ----------`    \    100       \
                         \    `--/\/\/\--.   `----------
                          \               \
                           `--/\/\/\-------------------
                                1k

It's probably not clear from that, but when you actually lay it out with
wires, all the resistor bodies can be at the same height, resistor "legs" can
be soldered "along" cables, and the Y can be made fairly rigid, separating the
RCA plugs by the "right" distance.

Andrew

Little Gain...

From: andrew cooke <andrew@...>

Date: Sun, 24 Nov 2013 17:33:46 -0300

OK, so having a divider is kinda pointless.  Using a direct connection from the
output requires the volume to be at about half-way, which is just fine.

And if I do that, and have the Crack as a kind of "signal processor" with net
zero gain, I hear nothing.  A/B testing between the Crack and a direct path
show no differences (once volume levels are matched) that I can consistently
recognise.

That's both good and bad.  The good is that Crack is therefore nicely linear.
The bad is that there's nothing much to the idea of a "tube sound" - at least,
as something that can be heard in this scenario.

Andrew

Crack as pramp

From: Daniel Samanez <danielsamanez@...>

Date: Thu, 20 Mar 2014 00:13:55 -0400

Hi. Nice blog and posts. What amplifier did you use the crack.as preamp?
Are still thinking it isn't a.good idea? Thanks.

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