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The EROS 300

Class A Monaural Power Amp


  • Description
  • Features
  • Specifications
  • Reviews
  • Downloads

Design Philosophy

Design Challenge

The challenge designing the Eros 300 was the conflict created when you want a lot of power, low noise and most importantly you want it to be musical. It took over 3 years to develop the Eros and 79 circuit designs later. I always look at the design from two points of view, as an audio engineer and an electrical perspective. Each design phase is based on a 50/50 balance between the technical spec and listening to each component change.  Ultimately, the final design must sound good because who listens to sign waves anyway.

Class A amplifier design

Class A has less cross-over distortion and therefore is more musical. The tubes are always on, so you have power when you need it. Class A is fast and responsive. There is no lag, providing your music more punch, transient response, better dynamic range and transparency.

Secret Source

The secret to a good power amplifier design is matching the circuits of the power supply to the amplifier. The Eros 300 is a choke based power supply with a custom toroid power transformer that gives you maximum power with no compression. It’s the most musical design because it retains all the dynamics from your sound source.

Lots of choices

How many tubes is enough? How much power is enough? I decided that 6 KT88’s and 150 Watts of Class A power was a good balance. More tubes and more power didn’t actually sound better, in fact, worse. Adding more tubes would have created more heat and less reliability.

What is EROS?

In Greek mythology, EROS is the God of Love and Desire.

  • Class A topology is the most musical of all amplifier designs because it has the lowest distortion.
  • Unlike solid state, this tube amp efficiently transfers power by matching the output transformer to the impedance of your speakers using 4 & 8 Ohm taps.
  • Two parallel options for your speaker cables; the binding posts or the SpeakOn connector.
  • The Eros 300 is easy to use. There are no adjustments or tweaking with Auto Bias technology on the output tubes. It’s safe, consistent and the settings are always perfect so you can relax and enjoy the music.
  • Transformer balanced XLR input offers true noise cancelling and rejection as well as ground isolation from the preamp.
  • Should you need to eliminate a hum? The Eros 300 has a XLR ground switch installed for your safety. This eliminates the dangerous temptation to lift the ground from the mains.
  • The combo of Push-Pull – Ultra Linear output circuit design gives you more power & lower distortion.
  • No Negative Feedback in the circuit design provides better dynamic range because it doesn’t compress the music.
  • Non Inverting Phase Polarity retains the directional information from the original source which gives your system punch.
  • A robust choke based power supply creates maximum immediate power with no compression.
  • Custom Toroid power transformer provides lower noise with plenty of available power.
  • The circuit design creates low noise levels with a wide frequency response. You will notice this in the really dark, quiet passages in your music.
  • Gold pin sockets are more reliable because gold doesn’t oxidize when it heat cycles.
  • A matched sextet (6) output tubes provides a synchronized balance for the lowest distortion and maximum output of power.
  • An extensive grounding scheme is designed into the power supply, wiring and all tube circuitry.
  • Elegantly designed, heavy duty 14 & 16 gauge zinc plated, fully grounded steel enclosure isolates external noise and vibration.
  • All fuses are easily accessible
  • It’s ready to enjoy with 50 hours of factory burn in on all circuits and vacuum tubes.
  • Each unit is hand built “Made in the USA”

Specifications per Amp


  • Pure Class A
  • 150 Watts per amp
  • 4 and 8 Ohm output taps
  • 8 and 5 Amp Slow blow fuses for outputs
  • Three 5 way binding post
  • One SpeakOn connector
  • Non inverting output phase polarity
  • Push-Pull and Ultra-Linear design


  • Transformer floating XLR input
  • Balanced inputs with pin 2 hot, impedance of 12K Ohms
  • Single Ended input, impedance of 100K Ohms
  • XLR ground lift switch on balanced input
  • 1.8 Volts RMS input to reach rated output

Detailed Specs

  • Monaural
  • Auto bias
  • No negative feedback
  • Frequency response 20Hz to 50Khz ~ 2db
  • Total Harmonic Distortion ~ 0.43% at 1W output into 8 Ohms
  • Gain of 26dB
  • Noise level > 5 mV RMS into 8 Ohms with input shorted
  • 1% metal film resisters throughout
  • Polypropylene capacitors throughout the audio path


  • Custom Toroidal power transformer with global mains
  • Choke based power supply design
  • 120 Volts AC mains
  • Optional factory installed 220V, 230V, 240V 50/60Hz
  • Power consumption at rated output 360 Watts
  • Power consumption at idol 348 Watts
  • On/Off switch conveniently located on front panel
  • 3 pin IEC power connector with fuse

Active Components

  • Matched sextet, set of six (6) KT88’s vacuum tubes
  • Two (2) Gold pin ECC82s (12AU7) vacuum tubes
  • High quality gold pin ceramic sockets

Other Features

  • Each unit is made by hand
  • 50 hour factory burn in on all circuits and vacuum tubes
  • Dimensions 17″ W X 20″ D X 10 ” H
  • Weight 64 lbs. (29.03 kg)
  • Shipping weight 80 lbs. (36.28 kg)
  • 14 and 16 gauge Zinc plated steel enclosure
  • Two year limited warranty
  • Six month warranty on vacuum tubes


The 2016 Rocky Mountain Audio Fest reports are in and Zesto Audio is very happy indeed.


Jim Hannon's "Best of Show"

"Best Sound" (for the money)

"It put many more expensive systems to shame."

Jim Hannon - The Absolute Sound, RMAF Show Report - October 2016

Read full report

"Most Coveted Product" and "Auspicious Debuts: Analog"

Jim Hannon - The Absolute Sound - RMAF 2016 Show Report

"Zesto Audio's new Téssera Phonostage made me think I was listening to master-tapes.

One of the best sounds of the show!"

Read more..



RMAF 2016: Would This Stairway Lead to Heaven?

"This was one of those happy, expectation-fulfilling rooms where recordings actually sounded like music."

Jason Victor Serinus - Stereophile - October 2016

Click to read full report


Rocky Mountain Audio Fest 2015 - Hot Product
Zesto Audio's Eros 300 Monoblocks

"..... impressively dynamic and rhythmically tight, clear, lucid and articulate,

with a natural warmth and rich tonal palette." 
Roy Gregory - The Audio Beat - October 2015

RMAF 2015: Zesto invokes Eros, Earns Kharma read more
Scot Hull - Part-Time Audiophile - October 2015


"Great sound overall, and creating systems that have collaborative synergy

under difficult show situations in hotel rooms isn't a given." 
By Sasha Matson-Stereophile- October 5, 2015
Read more