Koos Boshoven: The introduction last year of the new brand PrimaLuna
with its first model ProLogue One can be considered now as very successful. Rave
reviews start to show up worldwide in magazines and several user-reports have
already been received from PrimaLuna owners. The great value for so little money
and its high quality sonics are constantly praised by both buyers and reviewers.
This all promises, without doubt, a very bright future for the PrimaLuna brand.
Also because the amps have proven to be very reliable, which unfortunately isn’t
always common case with high-end equipment in general and tube gear
specifically.
Last year we sold a customer -on his special request- an expensive Italian
wellknown ****** tube-amp. This amp suffered quite a lot from bias-deviation
problems, driving the tubes in the amp, the customer himself and us almost to
complete madness. Learning from this, our boss Herman van den Dungen decided he
wanted to have developed an automatic bias module for his new PrimaLuna range.
He wants to guarantee his customers in and outside The Netherlands maximum
performance, maximum pleasure and a minimum of technical issues. What to do if
we exchange the EL-34 tubes? What happens if a tube is "gone"? The
answers are simple: re-bias the circuit and check it every 3 – 6 months.
Replace the tube, and if the the resistor and/or the fuse is gone too, replace
these as well. But what if you are living in Texas and your closest dealer is
500 miles away? What if you don’t know how to adjust the bias, or how to
replace a resistor? Herman’s answer: "We have to come up with a system so
that the customer doesn’t have to worry about any adjustments, a system that
in case of a problem just blows a fuse and nothing more than that". His
"master of design" of the AH! and PrimaLuna brands, Marcel
Croese (the former Goldmund man whose knowledge of High-End equipment is widely
known and appreciated), came up with some good ideas and finished his job
succesfully.
I start with some information about the use of bias and for that I quote here
the "white paper"-explanation from Marcel Croese:
Everything has a Bias
All audio-amplifying devices
require some sort of "bias", be they solid state devices - bipolar
transistors, FETs- or vacuum tubes. In fact "bias" is nothing more
than a voltage that is required to "open up" the device, enabling it
to pass a certain constant flow of current.
Various "levels" of standing current are common, and they represent
either class A operation, class AB operation or class B operation, in descending
order of actual current, and increasing order of non-linearity.
Generally all audio circuitry is biased in class A for the absolute minimum of
distortion, with the exception of power stages. For reasons of efficiency here
mostly a form of AB is chosen, where the trade-off in linearity and heat
dissipation is well thought out by the designer of the audio-amplifier, and
distortion is kept to the minimum by clever design and subtle fine-tuning of the
electronics.
Bias is mostly achieved by some passive circuitry that is tied to an active part
in the signal path. Generally it only works in the DC-domain, passing all
delicate audio-information without affecting it.
For Tube-output stages the "fixed bias" system is most commonly used,
because of its superiority in efficiency and least intrusion in the signal path.
The most well-known other option is called "self-bias" or
"cathode-bias", and this requires a big capacitor to bypass a big
resistor, with the potential of affecting audio quality a lot more, and
definitely burning a lot of wasted power in the already quite inefficient tube
output stage.
The "fixed bias" system however suffers from some serious drawbacks,
most of them only apparent to us when a tube blows or when the amplifier is
driven hard; where tube failure mostly is the result of incapability of the bias
circuitry to track the tubes' changing characteristics over time, or holding a
firm grip on the tubes' standing current when it is pushing a lot of power for a
long time and runs real hot. In both cases the tube may suffer from
"thermal runaway" and blow itself up.
Also a couple of parts in the "fixed bias" circuitry do -against
common belief- exhibit nonlinear behavior under standard conditions, adding
distortions and unwanted signals to the audio signal, polluting your music.
In an effort to get rid of these major drawbacks and push tube audio quality
into not yet achieved territory we decided to develop a new bias solution:
The PrimaLuna Adaptive AutoBias
The most important requirement was that it was
not allowed to compromise audio quality in any way, and at the same time had to
improve amplifier stability and reliability substantially. And it had to be
maintenance free: once adjusted no further adjustments ever in the lifetime of
the amplifier. Not even when changing tubes.
The conditions under which the fine labour of output tube
biasing has to be executed are harsh: tubes run hot and voltages run high and
vary wildly, situations that common electronic devices find unpleasant to
function in properly. So the AutoBias had to be absolutely stable under severe
conditions, which was no trivial task.
Common "fixed bias" systems have no possibility for compensating
high-temperature related issues like the earlier mentioned thermal runaway. The
bias-pots (any pots!) are highly nonlinear when cold, this gets even worse when
at temperatures of 65ºC(150F) and the noise that they generate is put on top of
your precious audio signal, where it gets amplified by the power tubes too!
The same counts for the highly nonlinear devices called electrolytics, that are
always there in a fixed bias system, trying to short any unwanted signals to
ground, but miserably failing.
So the simple conclusion was reached that the new AutoBias system had to be
inherently linear and present no reactive load to the tube-circuitry whatsoever,
as well as being totally stable under all possible temperature and voltage
conditions.
This we achieved by employing high quality parts throughout the circuitry that
have precisely defined temperature coefficients, and have those work together
and cancel differences out totally. Now we have total control over the
temperature tracking behaviour of the bias system. Without needing to resort to
any form of active or passive feedback of any part of the audio signal, the
various active parts in the system work closely together in total transparency
to guarantee unconditional DC and AC stability over all voltage and temperature
regions.
The second obnoxious thing that a fixed bias system does is that it is not
compensating the tube's tendency to use the audio signal as an extra bias
voltage, especially at high levels and low frequencies. The fixed bias, being
deaf, dumb and blind, keeps on injecting the fixed voltage, upon which the audio
signal rides. This results in moments of "impoverishing" the tubes,
"pinching them off" slightly, and produces instants of rapidly
elevating crossover-distortion, compression, bringing in certain harshness to
the sound.
The PrimaLuna ProLogue Adaptive AutoBias system avoids this by
"reading" the audio signal and inaudibly adjusting the bias voltage
ever so slightly, resulting in far superior tube behaviour and dramatically
dropping distortion levels by more than half.
What you hear is Clear, Warm, Detailed, Dynamic and Harmonic TUBE sound at its
best!
(so far the technical information by Marcel Croese)
The standard bias board (14 x 3.7 cm) from the PrimaLuna ProLogue One is
(in-house) replaced by the slightly larger new autobias board (19 x 4.1 cm) and
head to head compared against the standard PrimaLuna ProLogue One.
What was very obvious, is that the Adaptive Autobias-version took a
substantially shorter period to settle in. Within a few days the ProLogue One
Adaptive Autobias-version came on song and a few differences were immediately
very easily to detract:
Bass-control, (already a strong point of the standard ProLogue One) improved
substantially. It became more cohearent and rhythmically tuneful.
The system as a whole is more quiet too, although this is not the right way to
say. Already with the standard ProLogue One no noises or any hum is audible,
neither from the amp nor through the loudspeaker. But with the Adaptive
AudioBias installed it seems that the background of what you hear is more
silent, more dark. Micro-information comes out from this very black background
and details are a tad better defined than in the already very fine standard
version. Soundstage has widened up, dynamics improved and decays are more
natural.
The Adaptive Autobiased ProLogue One was also judged with the upcoming AH!
Black TjoeLipS loudspeakers. Both (PrimaLuna and AH! Black TjoeLipS)
are made in China (sorry, I think in "heaven") and after our
modifications added overhere, these products performare no less than parts of an
expensive high-end system. In all three dimensions a very reliable picture fills
the room, no matter what kind of music is playing or which playback-level is
chosen.
The system seems like a Husky dog, even under heavy conditions. Hungry for the
job even with the most demanding music. From Richard Bona's cd "Munia/The
Tale" the track "Couscous" plays, showing way better grip on the
bass-lines and in the drum-kit. There's also a fair improvement to hear on the
percussion (better dimensioned in a bit wider stage), but most progress is
evident in the midrange. Both male and female singers are showing more 'body'.
Thomas Quasthoff energetic voice in "Schuberts Winterreise" is very
demanding and therefore asking very much from the amp’s power supply. But with
the Adaptive AutoBias board installed his singing is utterly stable and totally
free of congestion. I heard the same control in one of my other favorites:
"Mahler's Das Knaben Wunderhorn". Here a full orchestra, with Matthias
Goerne, is singing "Des Anthonius Von Padua Fisch predigt". One can
see through the stage as if it was an X-ray picture. Also very convincing is
Jamie Cullum singing "I get a kick out of you". Pace, rhythm and
timing are absolutely first class suggesting a very expensive high-end system.
Modestly priced AH! power-, signal- and speakercables have been
used during this evaluation.
The standard PrimaLuna ProLogue One is a very fine amplifier, but modified with
the Adaptive AutoBias board it becomes a GREAT amplifier and a clear winner over
the standard version. The PrimaLuna Adaptive AutoBias Board is (apart from the
better sound) also a very elegant solution for a number of long excisting
problems. This amp, fittted with this new intelligent AutoBias board, gently
slips into the F.O.R.M. (Free-Of Regular-Maintenance) division. And … not to
forget to mention that due to this new board the distortion-specs of the
amplifier improved around 50%!!!
The standard ProLogue One is priced 950.= Euro, while the optional Adaptive
AutoBias board cost you a very modest 100.= Euro extra!!!!