The Yves Cochet ALS 1 Nocturne is a stereo vacuum tube power amplifier rated at 40 watts RMS per channel, designed in France for high-end two-channel listening. It is a rather rare unit on the used market and is highly appreciated by enthusiasts of tube amplification for its minimalist design, refined sonic character and ability to drive many home-audio loudspeakers without difficulty. Its construction follows a traditional approach, with a tube output stage, generous power supplies and a clean, well-organized internal layout designed to ensure dynamics, low noise and a believable reconstruction of the soundstage.
Like all quality tube amplifiers, the Yves Cochet ALS 1 Nocturne requires periodic maintenance and accurate instrument testing to continue performing at its best over time. Checking the condition of the tubes, the power-supply capacitors and the correct operating points is essential to preserve reliability and performance. On this page I document a complete inspection with power, distortion and frequency-response measurements, useful both for those considering the purchase of a used ALS 1 Nocturne and for anyone who simply wants to understand how this amplifier behaves under real-world conditions.
This amplifier was retrieved from a second-hand shop and brought to me for a full inspection and a deeper verification of its overall condition. Before installing it permanently in a Hi-Fi system it is wise to ensure that there are no aged components, bias drift or latent issues that could compromise long-term reliability. After an initial external check I inspected the wiring, connectors, switches and potentiometers, then moved on to the internal structure, examining the point-to-point wiring and the quality of the original solder joints.
The goal of this work was not just to “see if it turns on”, but to understand how the Yves Cochet ALS 1 Nocturne behaves after years of use, whether the tubes are still healthy, whether the power supply operates correctly and whether the output stage still delivers the performance intended by the original design. Only after these checks does it make sense to talk about listening impressions and proper loudspeaker matching.
I checked the condition of the tubes and the electrolytic capacitors, which both visually and instrumentally appeared in good shape, with no swelling, leakage or out-of-spec behaviour. I then performed several instrument tests to verify correct operation under real load. This is probably the second or third unit of this model that shows excellent instrumental performance on my test bench without requiring corrective interventions. The measured output power is about 40 watts RMS per channel on a resistive load, with a damping factor of 13.3, a value that indicates good loudspeaker control, especially in the low-frequency range.
A damping factor of this kind might suggest a somewhat generous use of global negative feedback, a common choice in many tube amplifiers designed to be compatible with a wide variety of loudspeakers. Since I did not have the schematic available and this was simply an inspection of a properly functioning unit, I deemed it unnecessary to go further with invasive measurements or circuit-modification tests. In any case, the measurements of power, distortion and frequency response show behaviour fully in line with what one would expect from a tube amplifier of this class. Below are some representative graphs.
0.13% distortion @ 1 watt
The spectrum at 1 watt shows total distortion around 0.13%, with a predominance of even harmonics typical of well-engineered tube power stages. This is a very low value that, in a real listening context, is essentially inaudible and contributes to a clean, smooth and fatigue-free sound character. The controlled presence of higher-order harmonics indicates that the output stage is operating in a linear region and that the power supply is adequately supporting current demand without significant drops.
The frequency-response graph highlights a transformer resonance around 95khz which has not been compensated, unlike other designs that use a Zobel network or small correction capacitors. This makes it difficult to determine the actual useful bandwidth of the amplifier because the resonance peak affects the reading at very high frequencies. Nevertheless, within the frequency range that truly matters for music reproduction, the response appears smooth and free of major irregularities.
It is worth noting that a resonance so high in frequency is usually not a problem for listening, but rather an indicator of how the output transformer was designed and constructed in terms of parasitic capacitances and leakage inductances. For a complete assessment, one would also test the amplifier on more complex loads that better simulate real loudspeakers, but even on a purely resistive load the ALS 1 Nocturne behaves in a consistently orderly manner.
The 10khz triangular wave also confirms good performance, without particular distortions or strong phase rotation as often observed in less refined designs. This suggests that the Yves Cochet ALS 1 Nocturne handles fast transients correctly, without smearing or rounding them off. The triangular wave is extremely sensitive to bandwidth limitations and output-stage sluggishness, so the preservation of edge sharpness indicates that the amplifier responds quickly and cleanly.
The absence of evident rounding or steep tilting in the 10khz triangular wave also suggests that the parasitic capacitance seen by the output stage is under control and that the driver stage has enough current capability and slew rate to feed the output tubes without strain. In practice, these measurements confirm behaviour consistent with a well-built tube power amplifier, suitable for serious Hi-Fi use and not merely aesthetical appeal.
Below are square waves at 100hz, 1khz and 10khz, used to evaluate the amplifier’s behaviour with impulse-type signals and to analyse stability, low-frequency performance and transformer-resonance control. The 100hz test shows how the transformer behaves near the lower frequency range, revealing possible saturation or waveform deformation. At 1khz the amplifier should show its most linear behaviour, while at 10khz overshoot, ringing or slow damping can reveal compensation issues or excessive negative feedback.
