The Tubecore 500 is a vari-mu tube compressor in API 500 format, hand-built by IGS Audio as one of the very few genuine tube compressors available in this standard. Compression is achieved via remote cut-off of tube biasing – the classic vari-mu topology used in legendary units such as the Fairchild 660/670 and the Manley Variable Mu. There are no op-amps in the signal path: a Carnhill input transformer and Sowter output transformer bracket a fully symmetric, balanced tube circuit. JJ tubes are powered at 250 V DC – a voltage many times higher than the 500-series power rail, requiring an internal voltage step-up stage within the module. Every unit is hand-assembled with matched components – 1% metal film resistors and 2.5% Wima capacitors – and individually calibrated before shipment. Shipping weight: 2 kg.
Vari-mu – how tube biasing compresses and why it sounds different from VCA or FET
In a vari-mu compressor the tube is both the amplification element and the gain-control element: as the signal level rises, it changes the tube's operating point via the grid voltage, which directly changes the gain. There is no controlling transistor, no VCA – the tube does both jobs simultaneously. The result is a compression curve with a logarithmic threshold character and an exceptionally musical response at low and moderate gain reduction levels. Attack times of 10–300 ms and release times of 0.1–4 s are intentionally slower than FET or VCA designs – the compressor does not catch transients but shapes the dynamics of the material over a longer time window, which means the programme retains its life and energy even under deep gain reduction. Maximum gain reduction is 30 dB.
Stereo linking and mastering applications
Two Tubecore 500 modules can be linked as a stereo pair: the unit with the deeper compression setting or slower attack/release becomes the master and controls the gain reduction of the other. Additional jumpers on the rear of the PCB allow independent adjustment of the input and output levels of each module – particularly useful when working with very deep compression where gain staging between modules affects the tonal character. The needle VU meter with an analog scale monitors the amount of gain reduction. Frequency response: ±0.2 dB across 20 Hz – 20 kHz. Maximum gain: 35–40 dB. Maximum levels: +22 dBu input and output.
KEY FEATURES
- Vari-mu tube compressor, API 500 format, mono
- Compression via remote cut-off tube biasing (vari-mu topology)
- No op-amps in the signal path – pure tube circuit
- Fully symmetric signal path from input to output
- Input transformer: Carnhill
- Output transformer: Sowter
- JJ tubes powered at 250 V DC
- Hand-assembled, matched components (1% resistors, 2.5% Wima capacitors)
- Individual calibration per unit
- Stereo link: two modules linkable as a master/slave stereo pair
- PCB jumpers for independent I/O level adjustment under deep compression
- Needle VU meter with analog scale
TECHNICAL SPECIFICATIONS
- Type: vari-mu tube compressor, API 500, mono
- Gain control element: remote cut-off tube biasing (vari-mu)
- Tubes: JJ (powered at 250 V DC)
- Input transformer: Carnhill
- Output transformer: Sowter
- Attack time: 10–300 ms
- Release time: 0.1–4 s
- Maximum gain: 35–40 dB
- Maximum gain reduction: 30 dB
- Maximum input level: +22 dBu
- Maximum output level: +22 dBu
- Frequency response: ±0.2 dB, 20 Hz – 20 kHz
- Stereo link: yes (master/slave via PCB)
- Components: 1% metal film resistors, 2.5% Wima capacitors
- Format: API 500, single slot
- Shipping weight: 2 kg
