The "Ankoru Writings" of Andy Grove (its creator)
for Sound Practices magazine
information is a dynamic four dimensional continuum, like the one posed
by the relativity theory, consisting of three spatial dimensions and
time, all inseparably inter-related. A system for music recording and
reproduction must transfer this continuum and faithfully reconstruct
the original sonic performance in the listening room. Test instruments
are supposed to ensure that the transformation of the continuum is linear
and accurate to certain parameters.
Alas, these instruments and
the mathematical models that we use, such as the relativity theory,
lack the spontaneity and emotional content vital to music. Somehow our
primitive method of recording scratches into a vinyl disc captures some
of this emotion, and the lump of rock we call a stylus is able to extract
the information and convert the vibes into a signal ready for the amplification
chain. The amplifier, therefore, must not only perform well electrically,
it must also convey emotion in order to fully satisfy both the analytical
mind and the inspirational soul.
These days, science is beginning
to discover an essential, almost mystical, interconnectedness of everything.
It is intuitively obvious that the character of the universe on a macroscopic
scale should rely on the properties of the subatomic particles of which
it consists. At the same time, the character and properties of those
particles is defined by the universe at large, the whole system mysteriously
holding itself up by its own bootstraps, each piece of the giant jigsaw
fitting exactly into place without deficiencies or excesses.
It is only by virtue of an intellectual
gesture that we perceive a condensed, solid, and definable part of the
web of reality, yet we have deceived ourselves into thinking that our
mental creation is the be-all and end-all of existence.
Most of our old scientific "laws"
- including those currently used to judge sonic performance - are only
close-ups of the whole picture. I'm afraid we are not seeing the wood
for the trees.
Certainly these measured parameters
do have some relevance in terms of overall performance, but to recreate
a musical event, an amplifier must work on both a macroscopic scale
as a part of a communication system between the performance and listener,
and on a microscopic level as a collection of valves and parts which
must be tamed and optimized for the task at hand.
A magazine article can only skim the surface of any design philosophy and, of course, there will be shouts of "what the *@$! is this guy on?", but I hope my discussion of the Ankoru design will be interesting nonetheless.
The Schematics of the Ankoru
Starting with the basic precept
that each part of the amplifier should fit exactly into place, and have
a character defined by the overall requirements of the system, the validity
of feedback and push-pull operation, two pillars of
traditional amplifier design, are immediately called into question.
These concepts are purely intellectual constructions, created in laboratories
with no motivation from natural music, and I am convinced that they
detract from sound quality as a result.
In practice, the ultimate purpose
of feedback and push-pull operation is to make amplifiers easier to
make not better. In any event, reducing harmonic distortion to vanishingly
small levels and increasing bandwidth from DC to cosmic rays does not
make a more musically satisfying amplifier. Specs must give some satisfaction
though, 'cos we all know a guy who slinks off to the bathroom with a
copy of his tranny amp spec sheet!
I agree that limitations such
as distortion and bandwidth abhorations unquestionably colour the sound
and should be eliminated, but beyond that I maintain that there are
more important areas to be considered if musicality is the ultimate
According to my way of thinking,
all of the above leads to the assertion that the overall topology of
an amplifier must be single-ended and there must be no feedback.
Transistors and all things silicon sound unnatural ... put sand in the
signal path and you get gritty sound! So, let's proceed directly to
valves and, in particular, the simplest and purest amplifying
The materials used for the construction
of the passive elements of the amplifier are just as important since
the signal must pass through them. Every material has a tonal colouring
effect, so only highly-specified, high-purity, listening- testing materials
For example, in the Ankoru
we use only Black Gate and Cerafine electrolytic capacitors for the
audio circuitry. These caps eliminate the electrolytic mushiness without
going over to the brashness of certain plastic caps. The valve selection
was guided by the notion that the different sonic signatures of each
type should be complimentary, leading to the goal of a sound that possesses
both strength and finesse.
Before I go on to describe the
Ankoru circuit in detail, I would like to say a few words about
transformers and transformer coupling, since transformers play an important
role in the design.
In any valve, waveform distortion
is caused by the characteristic parameters of the valve changing in
sympathy with the applied signal. In a standard RC coupled triode circuit,
the valve is set up with a quiescent current (Iq) following through
it and the load resistor, yielding a particular quiescent voltage on
its anode (Vq).
With a negative-going input
signal, the current is reduced and the anode swings positive due to
the reduced voltage drop across the load resistor (R1 X Iq). The reverse
is true with a positive-going input signal, the valve's anode current
is increased so the voltage on it reduces due to increased drop across
There is a problem with this,
however, because as the anode swings positive and the current decreases,
the trans- conductance of a valve goes down due to the curvature of
its characteristic. Of course, the reverse is true with a positive-going
input signal, the transconductance goes up with the current.
This means that the positive
part of the anode swing is compressed and the negative part is expanded
- waveform distortion. Usually, this distortion only becomes serious
with very non-linear valves and/or large voltage swings. When we want
to drive a fairly meaty output valve, we need to swing a lot of volts
because the mu of these types is necessarily low to keep loudspeaker
damping up. In this circumstance, waveform distortion can easily rear
its rather ugly head.
We need a system for keeping
the current through the valve as constant as possible over the anode
swing, i.e. a high load impedance. Increasing the load resistor on an
RC coupled stage can only go so far, however. One soon runs into problems
of resistor dissipation and PSU voltage if the anode current is kept
at the optimum level.
The SRPP stage and his other
active loaded cousins, such as the mu follower, have never really delivered
the goods for me. Close listening reveals a lack of focus and immediacy
compared to even the humble RC coupled stage. Anyway, SRPP is a feedback
device and quite often that scheme doesn't work very well electrically
either, especially with the low impedance valves we would like to use
as drivers. Simply pretending that you've got a low output impedance
just doesn't cut any ice in the world of real audio.
For large power valves, a low
AC drive impedance is necessary because large valves have large and
therefore highly capacitive grids. Thankfully, the low gain keeps down
the Miller Effect, but it's still there, so for good HF response, there
is no getting around using a good low-impedance driver.
From the standpoint of sound
quality, for a strong sound we need a beefy, low impedance driver. Wimpy
driver equals wimpy sound. Drive two 845s with an ECC83 and it'll be
like putting a model aircraft engine in a Chevy Impala. Not exactly
The DC resistance of the grid
circuit must also be kept low to control the effect of another rather
annoying bugbear, grid current. Unfortunately, the vacuum in many modern
valves is far from perfect, so there are quite a few gas ions floating
around inside the bottle. Some of these ions will collide with the grid
and draw electrons from the grid circuit. If the grid resistance is
high, the grid bias will be modulated in tune with the signal, a real
no-no in my book.
Also the grid may occasionally
be driven positive on signal peaks, causing the grid cathode diode to
conduct, rectifying the input voltage in the manner of a shunt diode
supply with the decoupling cap as the reservoir. This action makes the
bias voltage more negative, reducing the quiescent current through the
tube, sometimes to the point where it will only conduct on peaks (Class
C). In fact, a severe peak can cause the amp to cut off altogether,
resulting in a total loss of output.
Worse still, the grid resistor/coupling
cap combination as an RC time constant, so the effect lasts for some
time after the overload has passed in sort of a time-delay distortion
Reducing the grid resistor to
combat these effects is no solution. We want a DC grid resistance similar
in magnitude to the impedance of the driver valve, i.e. a few hundred
ohms, not a few hundred kilohms.
Making your grid resistor 600
ohms will likely kill the driver stage and, anyway, would require a
coupling capacitor so big that the RC time constant would put us right
back where we started.
To cure the voltage swing problem
requires a circuit element which has low DC drop but a high AC impedance.
Plus, we need a low DC resistance in the grid of the output valve. And
the device should efficiently couple the driver valve to the output
The driver transformer is exactly
what we need for the job. Its primary inductance presents an extremely
high AC impedance to the driver valve and reflects the anode impedance
of the driver into the grid circuit of the output valve. A good driver
trans will have a primary and secondary DC resistance on the order of
300 ohms, so the problems associated with grid current are more or less
eliminated. This is a resistance 1000 times lower than I've seen in
Ideally, the transformer secondary
is left unloaded, i.e. there is no "damping resistor" put across it
to cut ringing. An unloaded transformer sounds better and it gives the
driver valve a higher impedance load.
There are two large-scale problems
with driver transformers; HF frequency response and LF frequency response.
These two requirements are mutually exclusive to a certain degree and
many commercially available transformers sacrifice one for the other.
The Tango transformers, for example, seem to go for impressive-looking
HF specs but they have diminutive primary inductances which limit the
The problem is compounded by
the unbalanced DC current imposed by SE operation, which requires that
the number of primary turns must be increased to counterbalance the
loss of permeability caused by the air gap in the core. Leakage inductance
is proportional to the square of the primary turns so it's a real pain
in the butt.
The driver transformer in the
Ankoru has to handle 45 mA and still have superb bass, so it
took some heavy calculator work and a few trees worth of paper to get
it all working! [The Ankoru interstage trans will be available
as a DIY part-ed.]
I love the sound of large triodes
like the 211 and 845. The 845 was used in this amp because if offers
greater power in Class A1. The 211 is a more voltage sensitive valve
than the 845, its mu is higher but then so is its internal
impedance. It can't swing a lot of current at the low voltage end of
the anode swing without having the grid driven positive into Class A2.
When pushing the grid above zero volts, it no longer reacts as a high-impedance
terminal. It starts to draw appreciable current, corrupting the input
signal in a most unattractive way unless the driver impedance is extremely
The grid-cathode diode impedance
of a 211 is about 2k, so we would need something around 100 times lower
or hideous distortion would result. The waveform distortion could be
corrected using feedback but why build an amplifier that is intrinsically
The 845 can sink a lot more
juice where the 211 starts wheezing, but since the mu is so
low, it requires a driver stage capable of considerable voltage swing.
The 845s in the Ankoru are biased at -100 to -200 volts for
an anode current of 75 mA at 1200V B+, they look into a load impedance
of around 6k, and put out a formidable 70 Watts. The output transformer
has to cope with 150 mA DC and hold its 6k impedance down at LF, requiring
a high primary inductance. This takes a serious hunk of iron, but the
Ankoru output is just such a beast and the bass is awesome,
if I do say so myself.
To keep the drive signal to
the output valves clean requires a driver valve of excellent linearity.
One could use an indirectly heated valve such as the 6BX7, very linear,
or the slightly less linear 6BL7, but low impedance, low mu
directly heated valves are definitely the best choice.
Since this amp has to be built
using valves which will be available for some time into the future,
so that replacements can be made throughout its life, it was necessary
to use modern versions of either the 2A3 or 300B. I originally experimented
with the 2A3 as I wanted a measure of its clarity and immediacy, but
these valves have a very nasty habit of making toilet related noises
even in the output stages of amps, and using one as a driver was impossible.
I even tried some NOS samples but many were only marginally better,
only the best and therefore rare and expensive samples were quiet.
So the 300B was chosen, and
it brought its characteristic warmth and musicality to the amp as well
as a greater impact to the bass. The 300B is operated with 300V across
it and an anode current of 45 mA so it will last for ages, no more current
or voltage was necessary for driving the 845s to full output. The 300B's
output is in fact so large that the 845s will be freaking (and so will
your wife and the neighbours) before it runs into trouble, which makes
its jobs and the job of the input stage easier.
Various input configurations
were tried, all using the E182CC/7044 valve for its powerful sound.
The original and best sounding configuration gave the amp so much gains
as to be impractical. Long speaker leads acted like antennae and transmitted
the amp's output into the input leads causing instability. Super high
quality cables and careful system set-up would eliminate the problem
but as this is a commercial amp, it has to be dealer-proof, so a simple,
single-stage RC coupled affair was settled upon. The 7044 was always
run at a high current to really bring out its flavour.
The Ankoru is interfaced
to the preamp via a coupling transformer to allow balanced operation
and to properly ground the grid of the red hot 7044. A switching system
permits regular unbalanced input as well. The Ankoru is intended
for use with the Audio Note M3 which has output transformers and balanced
Ideally transformer coupling
between the input stage and 300B would have been used but even super
quality transformers impart a signature upon the sound (ultra mega quality
ones don't however) so a special copper foil capacitor with paper/oil
dielectric was used to couple from 7044 to 300B. This capacitor, like
all caps, has a sonic character but it was used to avoid a build-up
of one type of timbre caused by the cascaded transformer coupled stages.
The power supplies are fairly
standard, and of course valve not silicon, remember microprocessor parts
in the power supply equals computerized sound. If you want your record
collection to sound like a bunch of cheap CDs, then use silicon rectifiers
for the audio PSUs like all the other junk in the shops. In fact, I
would use valves for the filament supplies if I could - Tungar rectifiers
such as the Ediswan 68506 would work for those who dare [Cool! -
ed.] or AC straight from the mains trannie, but then punters would
whinge about hum. I could have built gargantuan supplies which would
have caused the primordial fires of a nuclear power station to die but
this amp had to fit into an (almost) domestically acceptable chassis.
So a sensible but effective
approach was taken; capacitor rather than choke input filters were used
to get maximum voltage efficiency and chokes were used to get ripple
down. The capacitors in the PSUs are directly in the signal path so
they need to be of excellent quality and here the Cerafine types come
into their own. They have a smooth and refined sound. Energy storage
was not taken to extremes but the main HT for the 845s holds 50j of
energy (the caps on the input side of the filter are isolated from the
audio circuit by the choke and therefore don't count).
It is necessary to have a rigid
supply. Smaller caps generally sound a bit sweeter in the mid and treble,
but if you want a decent bass quality, the last thing you want happening
is the PSU flapping about all over the joint. You don't put a lawnmower
carburetor on a Ferrari engine.
Going for oil drum sized caps
doesn't work either (Question: Can you think of a trannie amp with super
solid bass and complete and utter crap everything else?). Super sized
capacitor supplies can pump out heap big LF current transients but they
take heap big time to recover as well, and the impedance of the giant
electrolytics just skyrockets as the frequency rises.
Regular capacitor power supplies
integrate the demands placed on them so a bigger supply reacts a smaller
amount but everything takes longer. So the PSU for the 845s is suitably
scaled for an excellent all-round performance, solid bass through to
sweet and delicate treble. Things are made a bit easier because the
energy storage of a capacitance is proportional to the square of the
voltage on it and at 1200 V it doesn't take a big capacitance to store
a lot of grunt.
To minimize the effects of the
845s on the preceding stages, the 300B and 7044 have their own supply
from a separate mains transformer. Both are run from the same rail so
that the 7044 has a really juicy supply to suck from, and remember the
300B is running into an unloaded transformer so there is minimal supply
draw variation due to constant current operation.
The 845 supply is rectified
with two 5R4s in a voltage doubler configuration to ease the peak inverse
voltage requirement, the output impedance and peak current go up but
it is still satisfactorily within the valve's limits. The driver stage
HT is via a 5Y3 rectifier and the bias supply uses a 6X5. The main HT
is delayed by the bias supply, the driver stage and bias power is applied
when the amp is switched on. The 6X5 is indirectly heated rectifier
and so takes a little while to come up.
When the bias voltage reaches
a safe valve the big 845 power transformer is switched in by a relay.
If the bias fails for any reason, the relay will drop out, cutting the
power to the 845s.
All in all I am pleased with
the end result, the Ankoru when partnered with a good preamp
such as the M3 and a good turntable, gives a superb musical performance.
It can resolve the smallest nuances and subtle timbres of classical
music and deliver the visceral impact of techno, even with relatively
In short, this was the design brief: A single ended amp which would give that SE charm and musicality but which would also send the big solid state boys back to their silicon shrines to have a serious rethink.
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