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Douk ST-01 PRO Hybrid Valve Amplifier Big on the cute factor, this miniature amplifier glows warmly in the dark from its valve filaments, but has plenty of power thanks to a Class-D output stage. It costs $170, so how does its performance stack up? Review by Allan Linton-Smith W e have seen & tested myriad amplifiers, modules and kits. Many have totally fake or exaggerated claims about their performance, but this one is different. It is a well-engineered & well-presented package with decent performance. It combines modern technology like a Class-D amplifier, digital audio and Bluetooth with an old-fashioned triode preamplification stage. It only requires two speakers, cables and a signal source to be added. You can select between the RCA, optical (TOSLINK), coaxial (S/PDIF), USB and Bluetooth inputs with the press of a button. They include a remote control, instructions, valves, an optical lead and 24V, 4.5A plugpack power supply. A quick check on the internet uncovers many other products from them, which all appear to be well designed and presented. Features and design The heart of the Douk ST-01 amplifier is the Texas Instruments TPA3250 Stereo Class-D Amplifier IC, which is run in a bridge-tied load (BTL) stereo configuration. The small dimensions of this amplifier are a result of the absence of heavy, expensive power transformers and output transformers, instead using an external switch-mode power Fig.1: the signal from the various inputs is fed to the valve preamplifier via the tone controls. The pentode anode and screen grid are joined so it effectively becomes a triode, like the circuit shown here, creating ‘acceptable’ even harmonics. Pentodes have more of a tendency to create less-acceptable odd harmonics. 58 Silicon Chip Australia's electronics magazine supply and an efficient Class-D chip (see Fig.5). This chip can deliver 70W per channel into 8W loads with a 32V DC power supply (at TI’s specified 10% THD+N [total harmonic distortion plus noise]). However, the ST-01 delivers around 30W into 8W <at> 1% THD+N due to the limitation of the 24V 4.5A plugpack supplied. They state that you can upgrade it to 24V 6A if more power is required. By itself, the TPA3250 has very low distortion, quoted at 0.005% THD+N for 1W into 8W in the data sheet. However, this is a hybrid design with a pentode (wired as a triode) preamplifier to give a somewhat softer ‘valve sound’ – see Fig.1. This deliberately injects harmonics (but not much noise) into the system. As a result, we measured around 0.02-0.1% THD+N at 1W into 8W (see Fig.6), which is significantly better than most valve-only designs. The warm glow from the valves is complemented by a retro VU meter, which is a handy to monitor the signal levels to avoid objectionable clipping. Another interesting feature is the wide variety of valves that can be substituted for the ones supplied by Douk. There are 15 alternates listed! We tried a pair of 5654s in place of the 6K4s supplied and an audible difference siliconchip.com.au Manufacturer’s data (Douk Audio ST-01 PRO) » Audio inputs: Bluetooth, USB, coaxial [digital], optical [TOSLINK], RCA, U-disk [USB] » Audio output: Banana jacks / 3.5mm auxiliary socket » Maximum output power: 100W+100W (4Ω) [requires upgraded power supply] » Supported load impedance: 3-8Ω » Sampling rates supported: – USB input: 96kHz/24-bit – Bluetooth input: 48kHz/24-bit – Coax/optical inputs: 192kHz/24-bit » Supported USB formats: MP3, WAV, WMA, FLAC, APE » Maximum capacity of USB disk: 64GB » Treble/bass adjustment range: ±6dB » USB systems supported: Windows, macOS, Linux » Frequency response: 20Hz-20kHz (±1dB) » Signal-to-noise ratio (SNR): ≥98dB » Total harmonic distortion (THD): 0.07% » Working voltage: 18-30V DC » Dimensions & weight: 115 × 98 × 54mm, 634g Included in the package: ST-01 PRO Amplifier and user manual; 24V DC 4.5A mains power supply; remote control (without two AAA cells); USB cable; Bluetooth antenna was immediately obvious. Rather than leaving that as a subjective evaluation, we measured the actual spectra generated and examined the difference in the resultant harmonics, as shown in Fig.3. Most valve amplifier owners love to try different valves and/or different brands to evaluate the audible results. This is called “valve rolling”. Sometimes certain brands are considered superior and have superior prices, but our substituted 5654s only cost $7.63 each. By the way, if you look up the 6K4, you may find that it is a miniature triode, not a 7-pin pentode. Confusingly, there are two different “6K4”s! The Fig.3: this spectrum analysis at 1kHz & 1W shows the variation between different valves. The 6J1 is represented by the pink trace, and the 5654 by the blue trace. The total distortion is the same for both valves, but the harmonic differences are audible (eg, the blue trace is slightly cleaner below 5kHz). siliconchip.com.au triode is from Russia while the pentode is from China. The Chinese “6K4” is actually pretty close in performance to a 6J1 or 6J2 and comes in the same package, so you could substitute it with a 6J1 or 6J2. Performance The frequency response for the Fig.4: the amplifier’s response for various bass and treble settings: flat (cyan), maximum treble (green), minimum treble (blue), maximum bass (red) and minimum bass (magenta). Australia's electronics magazine June 2025  59 Fig.5: the TI TPA3250 IC contains four amplifiers that are bridged into two channels for stereo, similar to this example circuit from the device’s data sheet. The Class-D RF carrier is filtered out by sets of LC filters before the audio signal goes to the speakers. GPS-Synchronised Analog Clock with long battery life ➡ Convert an ordinary wall clock into a highlyaccurate time keeping device (within seconds). ➡ Nearly eight years of battery life with a pair of C cells! ➡ Automatically adjusts for daylight saving time. ➡ Track time with a VK2828U7G5LF GPS or D1 Mini WiFi module (select one as an option with the kit; D1 Mini requires programming). ➡ Learn how to build it from the article in the September 2022 issue of Silicon Chip (siliconchip. au/Article/15466). Check out the article in the November 2022 issue for how to use the D1 Mini WiFi module with the Driver (siliconchip.au/Article/15550). Complete kit available from $55 + postage (batteries & clock not included) siliconchip.com.au/Shop/20/6472 – Catalog SC6472 60 Silicon Chip Australia's electronics magazine siliconchip.com.au ST-01 was flat, within ±0.2dB from 20-20kHz when the controls were at the mid-position. The bass control added +12.5dB at 20Hz or cut it by -15dB. Bass boost can help make up for smaller speakers having less bass output. The treble control gave a boost and cut of +6dB and -6dB at 20kHz. That can be quite useful if your speakers have an overly muted or bright sound. Full bass or treble boost may not be effective at high power levels, as you could run into clipping. The frequency response plot shown in Fig.4 was made at 1W into 8W. The distortion (THD+N) shown in Fig.6 is 0.03-0.08% at 1kHz & 1W, which is acceptable for a hybrid amplifier. It is mainly due to the addition of harmonics (not noise) from the valve preamplifier, as was visible in the spectrum analysis graph (Fig.3). For these measurements, we used an Audio Precision AUX200/AES 17 ‘brick wall’ 20kHz low-pass filter system, which prevents the class-D carrier from interfering with the accuracy of the measurements. However, it also cuts out the second harmonic above 10kHz, third harmonic above 6.5kHz etc. Hence the dip above 6kHz in Fig.6. In this amplifier, we measured the carrier at 359kHz, and the filters attenuate that by more than 50dB. Clipping began at 20W with 0.1% THD+N into 8W, with the THD+N climbing to 1% at 30W. The clipping was noticeably ‘soft’ (see Fig.7). Around 33% more power can be delivered using a larger 24V/6A power supply than the one that comes with the amplifier. Conclusion The Douk ST-01 Pro amplifier is a very compact, lightweight and versatile device with quite decent power output and acceptable distortion levels. Its cute factor is bound to impress, especially for its warmth in small rooms. The ability to accept signals from various sources is very handy, and its price is quite reasonable for what you get. It is pretty unusual for a hybrid amplifier to accept 15 valve types. This is a plus for valve enthusiasts, and experimenting with it is quite SC fun too. Fig.6: the THD+N is 0.03-0.08% at 1kHz & 1W, which is acceptable for a hybrid amplifier and is mainly due to the addition of harmonics (not noise) from the valve pre-amplifier. The variation between the channels is due to differences in the valves, which are obviously not a matched pair. Fig.7: the THD+N vs power into 8W shows that soft clipping begins at around 20-30W, making it very usable in a smaller room, especially with large speakers, which are usually quite efficient. siliconchip.com.au Australia's electronics magazine The amplifier is tiny despite its reasonably high power output. The minuscule class-D IC has had its heatsink removed in this photo. June 2025  61