Reference Anniversary
True Balanced Line
Preamplifier ANL

The design is without compromise, the sound system with such a design allows for all the functionalities and settings that other users can realize only in a much larger number of individual units / components. In the design phase, the rationalization of prices was not a priority. The preamplifier and power supply unit are both in each dedicated enclosures. Chassis are made of high grade aluminum, copper and stainless steel sheets cover. Both components have additional aluminum heat sinks fins, that’s lowering internal temperature and prolong electronic parts lifetime. Both units are placed on the special and in-house developed anti vibration feet to minimize microphonics effect on vacuum tubes and capacitors.

  • Inputs

    3 x balanced XLR or differential RCA (selectable)
    2 x differential
    Tape Monitor Loop

  • Output

    2 x balanced
    2 x single ended (RCA)
    2 x single ended Monitor (RCA)

  • Gain

    Main gain selection per channel: 12 dB, 15 dB, 18 dB
    Additional gain selection per channel: -2 dB, 0 dB, +2 dB

  • Output Voltage (maximum)

    +29 dBU – 21,8 Vrms, 61 Vpp; 100 K Ohm load – 111 V peak, 78,7V rms

  • Frequency response

    20 – 200.000 Hz -0,1 dB @ 100K ohm load (no output transformer)
    10 – 120.000 Hz +/-0,5 dB @ 10K load (with output transformer)

  • Vaccum tubes

    4 x 6922 (E88CC) Triode – selection with gold pins; 2 x 6CA4 Rectifier Selected

  • Dimensions

    466 x 472 x 130 mm

  • Weight

    12.5 Kg Amplifying unit; 20 Kg Power supply unit

  • Finish

    Satin anodized aluminum, Chrome

Mono & Stereo
ReviewArticle 1Article 2Article 3Article 4


Amplification section concepts

After 10 years of design, projects and manufactured audio units was our intention and goal to build up a reference line starting with the Line preamplifier.

As Audio Alto mostly uses vacuum tube amplification the decision was to design and develop a Cost No Object line preamplifier based on vacuum tubes. Our idea is realized in a preamplifier that takes all the benefits vacuum tube triode amplification can offer but supported with surrounding circuits to minimize and overcome the triode shortcomings.

The audio paths consist just of strictly necessary elements. All used elements are high grade state of the art components. The circuitry is true balanced concept. Balanced concept in a way actually doubles the audio path length and reduces few decibels to the noise signal ratio, but we disregarded this drawback to take advantage of the much more important feature that balanced topology offers – eliminating ground loops, that are the mayor case of sound degradation of most systems and much better common mode noise rejection. One of the benefits is also lower 2nd harmonic distortion. All audio circuits are designed to consume constant power (current). With no change in power supply current consumption we also eliminated power supply voltage fluctuation related to signal and with this the possible unwanted influence between different circuits. With this approach and without using any global negative feedback the resulting circuit hum and noise is very low.

Inputs are balanced and single ended (RCA). All RCA inputs are differentially processed so the sources are not connected to common ground. Inputs which are not selected are all electrically disconnected from the preamplifier, so they have no influence on reproduction while not in use. The connectors that interface the outside world are WBT and Neutric connectors.

Inputs and outputs are also electrically isolated with a high grade output transformer. This arrangement also lowers output section impedance to less than 600 ohms. The transformer is made on amorphous core and wound with high grade Cardas wire.

Outputs are balanced and RCA and both as mentioned before are galvanic isolated. ANL provides also the mostly forgotten monitor input and output. This is the fifth input but also provides an output input return that can be used with different apparatus (also with sound and room correction systems).

All the switching functionality in the preamplifier is managed with hermetically closed signal relays with Au/Ag contacts. The relays are “latching” (even after the control voltage is interrupted, this relay maintains its set or reset condition until it receives the next inverting input). With this feature while the relay is in use it radiates no magnetic or any other annoying signal to the reproduced music. The relays are driven with impulses only when they need to change state and managed with special software logic.

The amplification circuits consist of two balanced connected triodes (per channel), that are matched and have golden pins, without using capacitors in the signal path (we are using only one oil capacitor in the parafeed configuration with the output transformer). The triodes are supported by FET current sources with cascade configuration for maximum current source output resistance. The resistance is higher than 30Mohm so the current sources are practically excluded from audio path. The gain level is selectable at rear of the enclosure. The main gain can be selected with a three position switch that selects main gain of +12dB, +15dB or +18dB. Each channel has also a so called balance switch with three positions that selects gain -2dB, 0dB and +2dB. This gain options offers the user gain optimization according to his/her system needs.

The power supply is regulated with very fast shunt regulators (with Zero-Temperature Coefficient reference Zener diodes). All shunt current regulators use FETs.

All the supporting components are high quality audio dedicated and selected components (Kiwame, Tackman resistors, golden pins sockets, polypropylene capacitors…). The printed circuit boards are made of special higher temperature stable material with gold plated copper connections. The boards are without protection solder layers lowering the dielectrically influence. Printed circuit boards are mechanically damped to lower vibration and stress for components and vacuum tubes.

The internal audio path connections are realized with silver wire in Teflon sleeve. As a volume control we use top grade 48 position golden contacts switch from Elma connected in shunt mode. In this configuration the balanced attenuator consists of just 4 precision and developed for audio purposes resistors (Tackman). The attenuator is truly balanced.

The whole enclosure, printed circuit boards and all components in the preamplifier follow mechanical symmetry design.

Amplification section concept summary:

  • vacuum tube preamplifier
  • no global negative feedback
  • input and output section realized as module
  • module base realized with 2 mm thick special treated copper sheet
  • minimalistic no compromise design (cost no object)
  • Electro Harmonix  6922 dual low noise matched gold triode amplification (balanced)
  • vacuum tube ceramic sockets with gold plated contacts
  • truly balanced design
  • dual mono completely separated channels
  • mechanical symmetry design
  • balanced and differential inputs (XLR and RCA)
  • balanced and single ended outputs (RCA)
  • WBT nextgen 0210 Cu RCA input and output socket connectors
  • Neutric XLR input and output connectors
  • signal passing wire is pure silver with Teflon sleeve to minimize impact on reproduced sound
  • not used sources completely detached from the preamplifier to minimize interference and noise impact
  • source selection with special latching signal relays with gold and silver contacts
  • all switchable functions active realized exclusively with latching relays
  • latching relays used to eliminate magnetic field when relay is active
  • no electrolytic capacitor in audio path (including in power supply circuits)
  • use of only high grade polypropylene capacitors
  • one or two high grade silver oil capacitors (Mundorf MCap Supreme Oil) in audio path (only one capacitor with output transformer installed)
  • Lundahl high grade output transformer for completely eliminate potential ground loops (transformer in as option)
  • Lundahl transformer wound with Cardas high purity post annealed audiophile grade copper wire
  • audio high grade low noise high tension resistors Kiwame
  • audio high grade resistors Tackman
  • Elma 48 positions shunt volume attenuator
  • volume attenuator with stepped microprocessor controlled motor for remote control (remote volume control is as option)
  • printed circuit boards use special board material that is thermal more stable
  • printed circuit boards are without solder mask layer to minimize dielectric constant influence on reproduction
  • printed circuit boards connections are copper gold-plated
  • printed circuit boards are mechanically damped to lower vibration and stress for components and vacuum tubes

Power supply

Mains power supply is separately filtered for each channel power supply. In the power path we use also two DC blockers to block potential DC voltage component that can be present in the mains power, so we eliminate the potential unwanted interaction with custom designed and wound toroidal transformers. As toroidal transformers have high inductance and low magnetic field dispersion mains DC voltage component can lead to buzzy toroidal transformers.

The left and right PSU are symmetrical modules using as base 2mm thick cooper sheets. Cooper is specially treated in a special way against corrosion while still maintaining its electrical parameters.

The whole power supply unit uses 4 custom wound toroidal transformers with electrostatic shield between primary and secondary windings. The shields eliminate the electrostatic – capacitive coupling between primary and secondary windings so noises from the mains power lead are eliminated. All transformers are also enclosed and potted to minimize vibrations in Ferro-Magnetic enclosures to lower external magnetic field and disturbances.

Power supply unit is connected to the preamplifier unit with two shielded multi core cables using professional stainless steel 12 pole connectors. Each cable delivers power to one channel. All the voltages and currents that are transferred trough the cables are constant, so the connections are free of any variable magnetic field and with it related radiations.

High voltage required for vacuum tubes operation are rectified with vacuum tubes diodes providing as low as possible current spikes and distortion. Smoothing the voltages is realized with high grade polypropylene capacitors (no electrolytic capacitors are used!). To lower any noise coming from power lead and lowering ripple from smoothing the DC voltage we use a specially designed capacitor with four contacts. The approach is known as Kelvin connection and gives amazing suppression of unwanted noise down to -120dB up to the more than 1 MHz bandwidth. High voltage is also smoothed with high grade Lundahl choke connected in symmetrical – balanced mode.

Heater power for vacuum tubes is done with constant current generators. With this approach we avoid high current spikes and stress for vacuum tubes when they are cold (turning on the system) and maximally prolonging their operational time.

Power supply unit delivers also required voltages and current for low voltage current generators and supply voltage for control unit and its functionality.

At the rear of the enclosure is mounted the hour meter that shows elapsed time. The hour meter can be reset opening the cover and is restarted from zero normally when we insert new vacuum tubes.

Power supply section summary:

  • power supply (PSU) in a separate enclosure
  • completely separated power supply for left and right channel
  • mechanical symmetry design
  • power supply section realized as module
  • module base realized with 2 mm thick special treated copper sheet
  • power line is separately filtered for each power supply unit with »PI« filter
  • toroidal transformers protected from power line DC voltage with special DC blocker circuits
  • use of four custom designed and wound 100VA toroidal transformers with high output power and low load for lower power supply distortion and unwanted harmonics generation
  • separated transformers for high voltages
  • separated transformers for vacuum tubes heater and low voltages
  • toroidal transformer enclosed in iron pots to minimize the magnetic impact
  • toroidal transformers with electrostatic shield between primary and secondary windings to eliminate the noise on power line
  • tube heaters energy is supplied with current sources to maximally extend vacuum tube life
  • high voltage for tubes is rectified by 6CA4 vacuum tube and smoothed with Lundahl symmetrical chokes
  • rectifier vacuum tube heater is time delayed to extend tube life
  • the high voltage power supply is designed in an innovative way to minimize the current spikes (drastically lowering PSU noise)
  • use of special polypropylene filtering capacitor with »Kelvin« connection to extend the low impedance filtering and noise reduction up to 1 MHz
  • all high and low voltages are regulated with shunt regulators
  • shunt regulators are without negative feedback, stable and very fast
  • digital counter of expired hours to display vacuum tubes usage time

Control unit

The control unit consists of a microcontroller that is enclosed in a special stainless steel enclosure. With this approach all the radiated noises are maximally reduced. The peripherals are connected and controlled by the microcontroller with static signals with no generated high frequency noise. All control logic signals are electrically isolated from the rest of analog circuits.

Remote control uses radio frequency receiver mounted on the main control board with a small antenna at the rear of the main enclosure. The control unit software is proprietary and developed in house.

Control unit section summary:

  • CPU control unit designed to drive latching signal relay and to minimize radiated noise
  • control circuitry enclosed in separated space to minimize radiated noise
  • CPU control unit is galvanic isolated from the rest of circuits for lower noise
  • remote control is activated with special radio frequency transmitter in the remote unit and receiver as part of the control unit
  • two +12V trigger outputs


Both enclosures are internally made from 2 mm anodized aluminum sheets; the separation walls between preamplifier sections and channels are made of polished stainless steel sheets. The exterior is made of CNC machined 5 mm anodized aluminum. Special parts are made of polished stainless steel or depending on options from gold plated brass. The inscriptions are laser engraved.

Both cases are suspended on specially in house designed low profile anti vibration feet to minimize reverse sound vibration effect on vacuum tubes and capacitors.

Enclosure section summary:

  • preamplifier and power supply unit in two enclosures
  • enclosures made of high grade aluminum, copper and stainless steel sheets
  • cover, front and rear plates from high grade high quality 5 mm thick CNC extruded anodized aluminum sheets
  • all enclosures design with addition aluminum heat sink fins to lower internal temperature to prolonged electronic parts life
  • enclosures include specially designed low profile anti vibration feet to minimize microphonics effect on vacuum tubes and capacitors