Hercules II Stereo



The same effortless, extraordinary sound.

The horizontal format of the Hercules II amplifier made it practical to create a stereo version – an amplifier in the same chassis as the Hercules II Mono, with the same effortless, extraordinary sound, but with two 500-watt channels instead of a single 1,000-watt channel.

The Hercules II Stereo uses exactly the same circuit as the Hercules II Mono and the original Hercules. The same circuit that inspired a leading reviewer to describe our original Hercules amplifier and Altair preamp as “the most transparent, detailed, and cleanest electronics I’ve heard.” The Hercules II Stereo cannot quite match the dynamics of the Hercules II Mono, but the fundamental sound quality is the same.

A Balanced Bridged circuit topology.

The revolutionary Balanced Bridged circuit topology used in all Constellation Audio amplifiers gives the Hercules II Stereo exceptionally low noise and outstanding common-mode rejection ratio. But those are just technical terms. What’s more important is that with an essentially perfect balance between the positive and negative halves of an audio signal, the Hercules II Stereo achieves a level of musicality previously heard only in our other amplifiers. The key element in our topology is the use of only N-type output transistors, instead of a mix of N- and P-type transistors as found in other fully-complementary solid-state amplifiers. Because every element of the musical signal passes through exactly the same types of circuit components, all the musicality of the original single-ended module is preserved.

500 watts per channel into 8 ohms

Our 500-watt/8-ohm per-channel power rating for the Hercules II Stereo is actually pretty conservative; it’s really more like 550 watts. And 750 watts into 4 ohms, and a full 1 kW into 2 ohms. Thanks to its increased power supply capacitance and ample cooling capacity, the Hercules II Stereo drives low-impedance speakers with ease.

Modular circuit concept

Instead of achieving the Hercules II Stereo’s in the traditional way – just bulking it up with more output devices – we build it from numerous 125-watt single-ended modules. It thus has the musicality of smaller amplifier, and the power and control of a large amplifier.

Balanced drive design for superior musicality

In the Hercules II Stereo, as in all Constellation Audio amplifiers, both the positive and negative halves of the audio signal see exactly the same type of output devices: N-type transistors. In other amps, the transistors are half N-type and half P-type, resulting in an inherent signal imbalance.

Inherently stable circuit design

Our circuit design combines high bandwidth with inherent high stability. Unlike many high-powered amplifiers, the Hercules II Stereo does not need a Zobel network at the output terminals to ensure stability at high frequencies.

Dual-mono design with one 3,000-watt transformer per channel

Each of the Hercules II Stereo’s channels has its own custom-wound, 3,000-watt toroidal transformer. Both transformers contain multiple isolated sets of windings to supply independent power feeds to various circuits within the amplifier. The isolation of the windings assures consistent voltage and current supply to all of the amplifier’s circuits, regardless of the demand placed on those circuits. Because each channel has its own transformer and supply capacitors, it is unaffected by the operation of the other channel.

High-efficiency passive cooling

With more than 1,000 watts total power into 8 ohms, the Hercules II Stereo demands a constant, free-flowing supply of cool air. To achieve this without the use of fans, we incorporate two huge heat sinks and hundreds of large ventilation ports to minimize airflow impedance and maximum cooling.

Perfect balancing through Line Stage Gain Module

The Hercules II Stereo incorporates two Line Stage Gain Modules, one for each channel. This module uses servo circuits and hand-selected FETs to assure that all incoming signals are perfectly balanced by the time they reach the amplifier stages. This is the same exact circuit found in our Altair II and Virgo line stages, and in all of our source components.

Constellation Direct balanced interface

If you are using a Constellation Audio preamplifiers, you can use the Hercules II Stereo’s Constellation Direct input. This input bypasses the amplifier’s Line Gain Stage Module, which simplifies the audio chain for the best possible detail and musicality.

4 XLR (2 Constellation Direct), 2 RCA
metal binding posts
Power Output:
550 watts, 8 ohm (1 kHz @ 1% THD+N)
750 watts, 4 ohm (1 kHz @ 1% THD+N)
1 kW, 2 ohm (1 kHz @ 1% THD+N)
Output noise:
<500 ?V, -100 dB @ 250 watts
32 dB
Damping factor (8Ω load):
<0.05%, 1 kHz @ rated power
Frequency response:
10 Hz to 100 kHz, +1/-0.5 dB
Input impedance (direct):
100K Ω
Input impedance (balanced):
10K Ω
Input impedance (single-ended):
10K Ω
Output impedance:
0.05 ohm
220 lbs / 100 kg
Dimensions (W x H x D):
19 x 13 x 32 in (whd)
48.26 x 33 x 81.3 cm (whd)

The Absolute Sound

The Absolute
Sound Editors
Choice Awards

Constellation Audio won multiple awards this year in The Absolute Sounds Editors’ Choice Awards 2021.

The Absolute Sound

Editors' Choice
Award 2019

Constellation Audio received a number of Editors’ Choice Awards for 2019 from The Absolute Sound.

The Absolute Beat

Jimmy Awards

” Constellation Audio’s Dream Team of designers consists of stars in the audio firmament. Among others, John Curl, Bascom King, Peter Madnick, and the late James Bongiorno have contributed to various aspects of Constellation product design. The legendary Irv Gross handles sales and marketing. “

The Absolute Sound

Product of the
Year Award

Reference Series Altair II Preamplifier and Hercules II Amplifier have been awarded the Absolute Sound 2015 Product of the Year award.

The Absolute Sound

The Absolute
Sound Editors
Choice Awards

Constellation Audio won multiple awards this year in The Absolute Sounds Editors’ Choice Awards 2022.

The Absolute Sound

Stereo Sound
Grand Prix 2014

Stereo Sound Grand Prix 2014 Winners:
Hercules II Power Amplifier and Altair II Line Stage

Altair II;
“A system that delivers such an incredible sound that you become lost within the music.”
“One of the best preamplifiers existing today.

Hercules II
“A stunning performance level that outstrips the original.”
“A product that will leave you yearning for it all the more.

The Absolute Sound

The Absolute
Sound - Editor's
Choice Awards

Editors’ Choice 2012

Hercules and Altair have been awarded the Absolute Sound 2012 Editors’ Choice award .

The Absolute Sound

Absolute Sound
2011 - Product
of the Year

Hercules and Altair have been awarded the Absolute Sound 2011 Product of the Year award.

The Absolute Sound

TAS Editors
Choice Awards

The Absolute Sound granted a number of our components with their Editors Choice Award 2020, including:

Altair II. Argo. Centaur II, Hercules II/Hercules II Stereo, Inspiration 1.0, Inspiration Mono 1.0, Inspiration Preamp 1.0, Inspiration Stereo 1.0, Perseus, Andromeda, Pictor, Taurus Stereo, and Taurus Mono Amplifier.

The Absolute Sound

The Absolute
Sound - Editor's
Choice 2016

Inspiration STEREO 1.0, Inspiration MONO 1.0, Inspiration PREAMP 1.0, Centaur Stereo, Argo, Perseus, Virgo III, Altair II, Hercules II Mono have been awarded the Absolute Sound 2015 Editor’s Choice award.