Review Quiescent Audio T100SPA Power Amplifier

Apart from minor changes to amplifier designs, there have been very few radical changes in modern amplifiers. Yes, I know about Class D, but even the nCore is over 10 years old, with new variants being 2 years old and more recently, some have just been tweaked, which improves the sound, but it is not a huge change.

You could say it has become rather boring.

Part of the reason for this is the march of Corporate Owners who want to play safe to ensure a good return. This is more easily done if the product is not too radical and thereby runs the risk of disenfranchising new customers or previous owners who do not want to upgrade. Also, the increasing number of ‘new companies’ and nations entering the market means they will be tempted to take the tried-and-tested path.

But it is not totally stagnant. Some designers are thinking of new(ish) directions and ways of making amplifiers. One such example is the Quiescent T100SPA stereo power amplifier.

Superficially this amplifier looks to be following a tried-and-tested path. The amplifier has two amplifier boards, one for each channel. Each board uses 2 MOSFETs with bias operation for the 1st watt in Class A and the rest of the power in Class B. This pair of MOSFETs generates 130w/ch into 8 ohms and 200w/ch into 4 ohms. However, that is where the superficial analysis stops.

Quiescent have been working for many years on noise reduction using vibration isolators and specialised techniques for EMI and RFI. The company was previously known as Vertex and made a wide range of audio products. These products were based on work done with the MoD where the reduction of RFI and EMI is vital to avoid detection. The effectiveness of this approach in audio was measured and tested by a certified measuring house, Acuity. The key person involved with this work was Dr Gareth Humphries-Jones a Doctor of Applied Mathematics and a specialist in signal processing algorithms as applied to high-performance sonar and radar systems. He also developed a way to measure audio equipment in the frequency and time domain and then correlate it to sound quality. This was reported in an interesting paper: https://rightnote.co.uk/wp-content/uploads/2015/09/ka-paper.pdf. The methodology compared the before and after digital signals by analysing them for changes in the time and frequency domain. The sound quality of the audio systems was then compared using an experienced listening panel. Dr Gareth Humphries-Jones found a correlation between changes in the time and frequency measurements and the sound quality. These measurements were carried out using very sophisticated equipment and software. The intent was to use less complex equipment, but unfortunately, Dr Gareth Humphries-Jones died before that could be done. This work was done by testing Vertex’s audio equipment. I recently found that the further developed Quiescent equipment was even more effective in improving sound quality.

Quiescent’s key personnel Nigel Payne and John Cheadle have further explored what the impact would be if they applied their RFI/EMI reduction measures and vibration isolation to a fairly straightforward amplifier. By installing it inside the amplifier they reasoned it could mitigate the harm from RFI/EMI and vibration generated inside the box as well as that from external sources.

The power amplifier consists of two separated amplifier boards using high-powered MOSFETs mounted directly to the heatsinks using the appropriate vibration and RFI/EMI reduction techniques.

But that is not all. They also applied noise reduction around some of the individual components that generate audio-interfering noise. They used old and new patented techniques developed using complex vibration reduction materials, especially around the transformers and AC-to-DC conversion electronics.

This required a lot of development work applied to the components in the amplifiers and the box that housed them. Most of this is hidden from view inside the unit.



The amplifier cooling does not use fins as Quiescent believes that they can vibrate and add distortion. Amplifier temperature control relies on using venturi tubes on the amplifier’s sides with internally constructed spiral grooves and venting on the base plate and rear panel for convection cooling.



The outcome of this is a reasonably heavy stereo amplifier (27 kg) that has a thick, shaped metal front that looks quite stylish. The lid uses a specialised non-metallic material to reduce circulating eddy currents caused by the Faraday effect.


The amplifier has a balanced and an RCA input, selected via a rear switch. Unusually the amplifier has a fairly faint LED that can be turned off, as Quiescent believes that even the best LEDs add a small amount of noise. There are two ground connections one for the amplifier case and one for the amplifier board. Otherwise, the operation of the amplifier is straightforward. Speaker pole terminals are provided by WBT Signature nextgen which accepts either spades or banana plugs.

I listened to the stereo version of the amplifier, there is a monoblock variant. I tried out three very different preamplifiers a valve-based ARC Ref 6SE, a Luxman 900 solid state and a resistor passive from Hattor, using its balanced connections. The amplifier fed my Avalon PM1 speakers. The front end was a Jays CDT2 Mk III transport and a Mola Mola Tambaqui DAC with a GRIMM MU1 streamer and Roon software. I used Cut Loose silver ribbon cables and nearer the end of the review period, I tried the Quiescent cables.

The amplifier was well and truly run in but I gave it a good 24 hours playing to itself before I did a quick listen using my standard test list.

On goes 2 Cellos’ Game of Thrones Medley. The amplifier was astonishingly quiet and allowed a large 3D soundstage to develop with all the drama I had previously heard from this track. It also allowed the tension in the music to build up and get the blood flowing. This music leaves you in a slightly tense state as its aim is to build you up and leave you wanting to watch the programme that follows. In that, it succeeded. A great start.

Despite the amplifier only producing 130w/channel it sounds very powerful and fast. I believe it is because it reproduces the natural weight of voices and instruments and it is therefore not robbed of precious mid-range weight. That can happen with some fast amplifiers which start to verge on the boundaries of being thin and a bit lacking. When playing naturally recorded music from Chasing the Dragon, the music’s tone was fully saturated, which gives the music the right weight, neither thin nor heavy and slow. This amplifier is a ‘very fast’ amplifier. This was getting interesting.

I should mention ‘an audio trap’ here. There was something different with this amplifier’s sound and I needed more time and music to determine what was happening. I listened to a wider variety of music to find out what was happening.

It was actually a real positive.

In a lot of amplifiers I have reviewed and some that I have owned there seems to be a halo surrounding the voices, with an associated lack of total clarity with the voices and some instruments. After some time you get used to it and it no longer seems to be a problem. I originally thought it was an artefact of digital audio. But here was an amplifier that got rid of that halo. Quiescent say it is noise that intermodulates with the music. Whatever it is, it is not present with this amplifier.

I do not want to overstate this effect but that is what I heard. It is initially disconcerting until you realise that the Quiescent amplifier has got rid of that halo and its associated reduction in clarity. In that respect, this amplifier is unique amongst the amplifiers I have heard.

Back to the amplifier.

I tried playing well-recorded classical music to see how the amp coped with large hall recordings playing big powerful music. Beethoven and Chasing the Dragon’s vinyl obliged. And so did the amplifier. I played Beethoven’s 5th Symphony, played by the Locrian Ensemble of London recorded in St Martins in the Field and the system reproduced this recording with all the power and drama of the concert with the wonderful acoustic of St Martins captured by those fantastic valve microphones. The sound was not that far from the original as I was present at the original recording in St Martins. Whilst listening to this music it was easy to zoom in and focus on one/two instruments or just relax and enjoy the whole orchestra.

At this point, I stopped making this into an audio autopsy by analysing the music and just enjoyed listening to the music. And boy I did enjoy this music, for hours on end. From the big powerful drama of Pink Floyd and Roger Waters, especially the bass intro of One of These Days from the live Roger Waters recording, Us and Them. Or the delicate recording of Give Me Love from the Applewood Road Album. Or a gnarly Steve Wilson recording, which showed a real forte of this amplifier, playing powerful, dramatic music. There are too many amplifiers that can play music with a lovely balanced frequency response but just suck the life out of the music. Not here, I was getting a natural tonal balance and the life in the music.

The amplifier is an ‘honest product’ and therefore if you play badly recorded music, it sounds less than good, but it does not shout about it. Most of the time it was just a sheer joy listening to music.

I should say there were a few teething problems but they were soon sorted out by Quiescent.

As you may expect, this good an amplifier does not come cheap. It retails at £20,000. But if you listen to it and like the sound, I doubt you will get an amplifier with this sound quality anywhere near its price. So if you want to hear a well-made amplifier that competes with some of the best out there, this may be just the power amplifier for you.



https://quiescent.co.uk/