IGS Audio Thermidt 4520 – 845 Class A Tube Monoblock Amplifier
The IGS Audio Thermidt 4520 is a 20-watt pure Class A monoblock amplifier built around the directly heated 845 power triode. Each audio channel has its own chassis, power supply, output stage and custom UTM Industry transformers.
A stereo system uses two independent monoblocks. This separates the channels electrically and mechanically instead of placing both amplification stages inside one shared enclosure.
Designed in Poland, the Thermidt 4520 combines a classic transmitting triode with a dedicated high-voltage circuit, transformer-balanced XLR connectivity and a precision stepped level control.
845 triode amplification in pure Class A
The 845 output tube operates in pure Class A and delivers a manufacturer-stated 20 watts per monoblock. A 6BM8 triode-pentode provides the drive for the output stage, while a 6CA4 rectifier tube serves in the power-supply section.
IGS Audio specifies an internal anode supply of 1,040 V DC. This is a specialist high-voltage valve design, not equipment intended for user adjustment inside the chassis. Installation, ventilation and servicing must follow the manufacturer’s instructions.
The 4520 is aimed at listeners who want the architecture of a directly heated triode amplifier with more available output than many lower-powered valve designs.
Independent resources for each channel
In a stereo configuration, each Thermidt 4520 operates from its own power supply and transformer set. Neither channel shares the other’s chassis or output stage.
This gives each amplifier a self-contained electrical and mechanical environment. It also provides flexibility when arranging the system, provided both units have suitable ventilation and safe placement.
The monoblock layout does not remove the need for careful speaker matching or sensible cable routing. It provides a dedicated amplifier for each loudspeaker.
Custom UTM Industry transformers
UTM Industry develops and manufactures the output transformers and other magnetic components specifically for the Thermidt 4520.
In an 845 amplifier, the output transformer must accommodate the circuit’s operating voltage while providing the required impedance transformation and audio bandwidth. IGS Audio places particular emphasis on core quality, winding geometry and insulation design.
These transformers are part of the amplifier’s electrical design rather than an interchangeable cosmetic feature.
Selected components and stepped level adjustment
The signal path uses Mundorf Silver Oil capacitors and Vishay Dale resistors specified with 0.1% tolerance.
Level adjustment is handled by a Swiss-made, 24-step ELMA attenuator. Its defined positions support repeatable manual settings and corresponding adjustments when using two monoblocks.
This is not a promise of automatic stereo tracking or remote volume control. If you want a single master volume control for the complete system, plan an appropriate upstream preamplifier or volume-controlled source.
RCA or transformer-balanced XLR input
Each monoblock provides a single signal input available through RCA or transformer-balanced XLR connections. These are connection options for one audio channel, not a multi-source input selector.
RCA accommodates suitable unbalanced sources. The transformer-balanced XLR connection provides an alternative for compatible balanced preamplifiers and source equipment.
Confirm the connection method and level compatibility before installation. Do not assume that the RCA and XLR inputs should be driven simultaneously.
A system built around music, not wattage alone
IGS Audio describes the 4520’s listening goals in terms of scale, spatial depth, natural instrumental texture and dynamic expression. The design is intended for a broad range of recordings, from voices and acoustic instruments to orchestral music and rock.
These are the manufacturer’s design intentions, not guaranteed results with every loudspeaker. Speaker sensitivity, impedance behaviour, listening distance and desired playback level determine whether 20 watts per channel is appropriate.
Choose the amplifier and loudspeakers as a system rather than relying on nominal impedance or power figures alone.
Rigid duralumin construction
The Thermidt 4520 uses a duralumin chassis manufactured in Silesia, Poland, with production partner MJK Zabrze.
Its mechanical construction supports the substantial transformers and high-voltage power supply. A CNC-machined top plate and ventilation openings provide a functional enclosure for the heat-producing Class A output stage.
Allow the clearances specified by IGS Audio. The exposed tubes and surrounding surfaces require safe positioning away from accidental contact and heat-sensitive objects.
Is the Thermidt 4520 right for your system?
The Thermidt 4520 suits a hi-fi system built around 845 triode amplification, dedicated channel architecture and straightforward analogue connectivity.
Choose it for its pure Class A output stage, independent power resources, custom transformers and stepped level control—not for unconfirmed streaming functions or an extensive source-selection system.
For stereo listening, two monoblocks are required. Contact Falkenstein ProAudio to confirm the supplied quantity, loudspeaker compatibility, mains configuration and current delivery information.
Discover the complete portfolio: IGS Audio Studio Equipment.