SPL TwinTube, User Manual

Page: 19 / 52

19
Analog Elemental Series
Premium Mic Pre | Technology
Triple Stage Preamp
The preamplifier circuitry of the Premium Mic Pre is composed of three stages: a trans-
former, a discrete differential amplifier stage and an instrument preamplifier all contribute (in
different shares) to the main amplification. The three stage setup firstly ensures a load distri-
bution that minimizes the risk of overloads. Secondly, each stage can be optimally configured
through select components and sophisticated circuits – a decisive advantage over single IC
circuitries where the main amplification is achieved by just one stage.
In addition, the potentiometer (as the control element of the differential amplifier stage) only
has to cover a range of approximately 68 dB while the maximum amplification is at around
80 dB. In practice this means that even very high amplification values are still outside of the
extreme control range positions – which are critical for any potentiometer.
Stage 1 – The Input Transformer
Inherent part of the Premium Mic Pre design are the input and output transformers of the High
Performance Series by Lundahl. These transformers are lavishly handcrafted. They replace
common input and output balancing stages. These classic components offer a very high
sound quality and common mode rejection but they are also very reliable and provide connec-
tions and signals of superior quality.
Lundahl transformers have a very high reputation in terms of manufacturing quality and the
resulting longevity as we have seen proven impressively throughout many years of experi-
ence.
In any microphone preamplifier the input transformer is of special relevance as it is an inte-
gral part of the preamp circuitry: it contributes to the main amplification through a passive
and permanent boost of, in this case, 6 dB. The advantage of passive amplification over
active amplification is that it does not add any noise. A second advantage of integrating this
passive amplification is a signal processing that is, as a matter of principle, lower in overdrive
and noise throughout all following stages simply because the rest of the electronic circuit is
charged with 6 dB less at any setting of the amplification.
In addition, transformers ensure a galvanic isolation, preventing any disturbing or damaging
voltages from being carried in any of the two directions. Electromagnetic, high frequency or
digital interference has no more influence. Problems with humming e. g. in a live environment
that have been caused by differences in the potentials between the stage and the FOH do not
occur. Even a voltage that has accidentally or through technical failure been connected to a
ground line cannot be transmitted. So transformers can exclude even mishaps or problems in
an installation reaching the categories “improbable” till “unbelievable” ...
Further, the phantom powering of microphones does not require any condensers in the
preamp socket which has further sonic advantages.
From our personal listening impression we can recommend transformers in any case. The
advantages in operational safety can not be overestimated especially in critical or complex
installations for studio, live or broadcast applications.
Stage 2 – The Discrete Differential Amplifier
From the input transformer, the signal is routed to a discrete differential amplifier based upon
a quad parallel transistor circuitry. This parallel circuit of eight single transistors reduces
noise remarkably through distribution of load.
The discrete differential amplifier is the central amplifying stage; here the amount of the
amplification is controlled by a current-carrying source. Control systems that are triggered
by current rather than voltage have the main advantage that possible negative effects of the
potentiometer will not affect the audio signal.
The discrete differential amplifier offers a maximum amplification of up to 68 dB. A servo
drive circuit actively eliminates DC offsets of the differential amplifier. Servo drive circuitry
minimizes DC offsets more effectively than conventional solutions that utilize capacitors by
setting the DC offset to almost 0 mV. In addition, the active servo drive solution has sonic
advantages over passive capacitors as it tends to produce less distortions.