Review: Keces S125 Power Amplifier

Firstly I would like to thank Keces and Arthur from their USA distributor for arranging this sample for me to review.

*disclaimer: This sample was provided for the purpose of writing a review, no incentive was given to write a favourable review. All opinions expressed are my own subjective findings.

Tech Specs:
  • Power Output - 125W x2 @ 8ohm / 225W x2 @ 4ohm
  • BTL - 360W x1 @ 8ohm
  • Power bandwidth - 5Hz – 100kHz
  • Current capacity - 30 amperes peak per channel
  • Total harmonic distortion (THD+N) - <0.004% at 1kHz A-Weighting
  • S/N R - 113dB 20Hz-20kHz A-Weighting
  • Input sensitivity - 1V
  • Input Impedance - 38k ohm.
  • AC Input Voltage - 110V-120V, 60Hz / 220V-240V, 50Hz (Switchable)
  • Power Consumption - 500 Watts
  • Casing - 4mm Aluminum
  • Dimension (WxDxH) - 300x220x133mm
  • Weight - 12KG
  • MSRP - $999.95

Packaging, Build Quality and Accessories:
The S125 comes in a big brown card box, one that is heavy duty and protects the heavy unit well during shipping. Inside the amp is held tightly in place with cardboard and is very well packaged. The box is nothing fancy, with no real information on the outside, but it is what’s inside that counts anyway.

The S125 is built like the rest of the Keces products, and that is solidly. The outer chassis is machined aluminium with heavy duty feet, they may not be the fanciest looking units, but they work well to reduce electronic interference and keep the components inside safe. The binding posts are chunky, and accept regular banana plugs; the inputs are all tight and of very high quality, the power switch is simple but works very well. Keces don’t seem to worry about making the unit look fancy, but they focus on the internal circuitry, I actually like the more utilitarian look anyway. The amp is also quite small for the power it outputs, the top has cooling vents and never gets too hot (it does get hot due to the class A/AB operation).

Accessory wise all you get is a power cable, but you don’t really need anything else. Of course if you are getting this you already have a suitable pre-amp, I am using the Keces S3 which stacks very nicely on top of the S125.

  • 1 kVA encapsulated toroid power transformer with independent secondary windings for each channel.
  • Class A/AB operation.
  • Balanced inputs with discrete circuits and XLR connectors.
  • 40,000 μF power supply filter capacitance.  
  • DC Servo circuits.
  • Overload/Over Temperature Protection.
  • Gold-plated RCA input jacks and Heavy-duty 24k gold-plated speaker binding posts.
  • The 4mm thickness of the aluminum chassis to effective to blocks EMI/RFI interference
  • The full-metal body realizes both vibration control and elegant style in an A4-size body that enables placement on a desktop to be a PC HIFI system or to be a small stereo system.
  • Keces are known for their power supplies, and the main toroidal transformer is a key element of the S125 delivery very clean power to the amp circuits.
  • It can be used as a 2 channel power amp, or in bridged mode as mono-blocks if you have 2. The S125 has plenty of power for most standard stereo setups.

Let me first say that this unit is whisper quiet in use, with no hiss even with sensitive speakers, the unit also has relays so that there is no power on/off thud.
The S125 follows in the footsteps of the S3 headphone DAC/Amp in that is has a very effortless but dynamic sound. This seems to be Keces house sound, and one that I personally appreciate a lot, the S125 is very neutral, but without being clinical. It is very clean sounding with very good detail retrieval, the midrange is very open and nuanced, but never loses out on dynamics and musicality.
Most people link musicality to being coloured, with the S125 you get musicality without the heavy colouring of some other amps. 

The S125 is not a lush sounding amp, it is fast and clean, expertly delivering very clean sound to your speakers without highlighting any part of the spectrum. Bass hits hard and digs deep but also with precision, the midrange is wonderfully open and the highs are effortless without a hint of harshness. The S125 has excellent separation and air, the soundstage is wide and effortless without sounding disjointed.

One of the great things about the S125 is that you can use the rest of your system to tune the sound, and the amp plays very little in colouring the overall system. The S3 + S125 system is superb if you want a very clean sounding rig that errs on the side of analytical but just steers clear retaining some musicality. You could use a valve pre-amp to get a little more warmth, or pair it with some fuller speakers if you want that sound. The S125 like the S3 tries hard to get out of the way and allow you to enjoy the music and not the equipment.

On a side note, you can use the S125 with inefficient headphones if they are used in balanced mode, I made a speaker tap to 4-pin balanced XLR cable and used the S125 with the HifiMan HE-6, HE-500 and also the MrSpeakers Ether Flow. With the Ether Flow there is some background noise but this is due to the higher sensitivity of the headphones, with the HE-500 there is only the slightest background noise and is not an issue when music is playing, and they sound very dynamic and punchy when used with the S125. The HE-6 had no background noise, and when using them with the S125 it was the best I have ever heard them, the bass is bottomless and so powerful without being overbearing, the rest of the sound was incredibly well separated and detailed, one of the best headphone rigs I have ever heard.

Conclusion: With the S125 you get a powerful power amp that can handle most speakers you throw at it. What I like best is the way the S125 doesn’t try to grab your attention with a romantic signature, it gets out the way of the music and delivers a powerful and dynamic sound that is also perfectly balanced from lows to highs. One word I now associate with Keces equipment is effortless; there is never any hint of strain or compressed dynamics from their units. The music flows with ease and I find myself just enjoying the music more, and analysing the equipment less.

Sound Perfection Rating: 10/10 (does what it sets out to do with ease and excellent sound quality)