The Double Triode Limiter (DTL) is a two-channel (stereo) tube microphone preamplifier with a built-in opto limiter on each channel, manufactured by IGS Audio. The signal path is fully tube-based: a two-stage triode amplifier with independent saturation control on both stages, a Carnhill input transformer (300 Ω) and an Edcor output transformer (600 Ω). Anode voltage is set to approximately 300 V DC. The limiter is based on an opto topology using an IGS Audio custom-designed photocell: an electroluminescent subcomponent illuminates a photo resistor that directly reduces the gain of the signal path – more light means less amplification. Maximum gain: 60 dB. Maximum gain reduction via limiter: 18 dB. Maximum output level: +22 dBu. Hi-Z inputs: 1 MΩ, on the front panel.
Two saturation stages – how the DTL shapes the sound
The two-stage triode amplifier architecture allows deliberate shaping of tube saturation at each level of the signal path independently. First-stage saturation depends on the input signal level and the Gain setting – higher input gain means deeper harmonic saturation on the first triode before the signal reaches the second stage. The –20 dB input pad enables working with line-level sources (synthesizers, drum machines, DAW reamping) without exceeding the operating point of the first stage. Second-stage saturation is controlled by the Output regulator – independent adjustment of the output level after the first stage changes the operating point of the second triode without affecting the first stage's saturation. The Hi-Z input (1 MΩ, TS jack on the front panel) routes guitar or bass signal directly to the first triode grid, providing direct electrical contact with the tube and its characteristic colour without an active buffer.
Opto limiter and studio applications
The IGS Audio custom photocell works on a principle analogous to the LA-2A: the electroluminescent element's light output is proportional to the signal level, and the photo resistor in the signal path reduces gain accordingly. Attack and release times are natural and program-dependent – opto elements do not allow precise time constant adjustment but respond musically without the harshness characteristic of VCA or FET limiting. The threshold range covers everything from gentle dynamic cohesion to hard peak limiting. An LED labelled "Limit" on the front panel indicates when the limiter is active. Phantom power +48V, phase reverse switch, and Mic/Hi-Z switching complete the controls. VU meters display output level. The unit is stereo with two XLR inputs (rear) and two Hi-Z jack inputs (front panel).
KEY FEATURES
- Two-channel (stereo) tube Mic/Hi-Z preamplifier with opto limiter per channel
- Two-stage tube signal path with independent saturation control on both stages
- Opto limiter based on IGS Audio custom-designed photocell
- Input transformer: Carnhill (300 Ω, balanced)
- Output transformer: Edcor (600 Ω, balanced)
- Anode voltage: approx. 300 V DC
- Hi-Z inputs: 1 MΩ, TS jack on the front panel
- Maximum gain: 60 dB
- Maximum gain reduction (limiter): 18 dB
- Pad –20 dB, phantom power +48V, phase reverse switch
- VU meters (output) + "Limit" LED indicator
TECHNICAL SPECIFICATIONS
- Type: two-channel tube microphone preamplifier with opto limiter
- Signal path: fully tube-based, two-stage triode amplifier
- Limiter: opto (IGS Custom photocell)
- Input transformer: Carnhill (300 Ω)
- Output transformer: Edcor (600 Ω)
- Anode voltage: approx. 300 V DC
- Input impedance (Mic/Line): 300 Ω (Carnhill)
- Input impedance (Hi-Z): 1 MΩ
- Output impedance: 600 Ω (Edcor)
- Maximum gain: 60 dB
- Maximum gain reduction: 18 dB
- Operating level: +4 dBu
- Maximum output level: +22 dBu
- Pad: –20 dB
- Phantom power: +48V
- Inputs: 2× XLR (Mic/Line, rear) + 2× TS jack Hi-Z (front panel)
- Outputs: 2× XLR (balanced)
- Metering: VU (output) + Limit LED
