Thermidt is a stereo tube amplifier positioned at the crossover between high-fidelity listening and studio monitoring, built by IGS Audio for users who want tube character without sacrificing bass control and dynamic accuracy. The circuit uses Negative Cathode Feedback (NCF) instead of a conventional global negative feedback loop: distortion is managed locally at the cathode stage rather than being corrected after the fact by wrapping the entire signal path into a global feedback loop. The power stage runs four 7189/6N14P-EV tubes in Class AB push-pull with fixed bias; the driver stage employs two 6BL8 triode-pentode valves. Output power is 2×21 W into 4 or 8 ohm loads. At 29 kg, the weight is a direct result of the UTM Industry-designed output transformers – wound in multiple sections with insulated layers over a Z11 Grain-Oriented laminated steel core, chosen specifically for its low core losses and superior low-frequency transfer characteristics.
NCF topology and output transformers – what actually determines the sound
Conventional tube amplifiers using global NFB reduce distortion at the cost of dynamic envelope and the tonal character listeners associate with tube amplification – the feedback loop smooths the response but simultaneously removes some of the transient openness of the circuit. NCF applies correction locally at the cathode, at the point where distortion originates, without routing the signal globally backwards through the full amplification path. The result is THD of no more than 0.15% at 20 W alongside the dynamic behaviour and sonic openness of a low-feedback design. The UTM output transformers with Z11 Grain-Oriented core reinforce this directly: grain-oriented silicon steel reduces hysteresis losses in the core and improves low-frequency magnetic coupling, which translates into tighter, more defined bass at full output power rather than the soft, slow bottom end common in budget transformers.
Connectivity and studio integration
Thermidt accepts analog sources via RCA Cinch inputs at 775 mV RMS sensitivity, and includes a built-in DLHiFi Bluetooth 5.1 LDAC module operating at 24-bit/192 kHz with a 100-metre range – high-resolution wireless streaming without external converters or cables. A dedicated headphone output handles headphones from 16 ohms upward. Three operating modes – Flat, Bass, and Loudness – allow the frequency response to be adapted to room characteristics or monitoring preference. The 8 mm CNC-machined top plate with chimney-effect ventilation slots provides sustained tube cooling during extended sessions. Frequency response of 10 Hz – 80 kHz ±1 dB and IMD of no more than 0.85% at 15 W place Thermidt well within reference-grade amplifier territory.
KEY FEATURES
- NCF (Negative Cathode Feedback) topology – local distortion control without global NFB loop
- Class AB push-pull, fixed bias, 2×21 W into 4 or 8 ohms
- Power stage: 4× 7189/6N14P-EV tubes
- Driver stage: 2× 6BL8 triode-pentode
- UTM Industry output transformers, Z11 Grain-Oriented core, multi-section winding
- Built-in Bluetooth DLHiFi 5.1 LDAC, 24-bit/192 kHz, 100 m range
- Dedicated headphone output: from 16 ohms
- Operating modes: Flat, Bass, Loudness
- 8 mm CNC-machined top plate with chimney-effect ventilation
- Weight: 29 kg
TECHNICAL SPECIFICATIONS
- Type: stereo tube amplifier, Class AB push-pull (fixed bias)
- Output power: 2×21 W (4 Ω or 8 Ω)
- Power stage tubes: 4× 7189/6N14P-EV
- Driver stage tubes: 2× 6BL8 (triode-pentode)
- THD: max. 0.15% @ 20 W
- THD: max. 0.15% @ 15 W, 100 Hz
- IMD: max. 0.85% @ 15 W, 50 Hz
- SNR: above 52 dB
- Frequency response: 10 Hz – 80 kHz ±1 dB
- Input sensitivity: 775 mV RMS
- Inputs: RCA Cinch, Bluetooth 5.1 LDAC (24-bit/192 kHz)
- Headphone output: yes, from 16 Ω
- Power supply: 230 V / 110 V AC, 50/60 Hz, 150 VA
- Weight: 29 kg
