IGS Audio: DTL Double Triode Limiter

2 channels Tube preamp with opto-limiter. 

DTL Double Triode Limiter is an expanded version of the Double Triode microphone preamp.

The Double Triode Limiter (DTL) is an expanded version of the Double Triode microphone preamp. The design was upgraded with a opto-limiter.

The limiter is based on classic solutions used in the 60’s and 70’s. The component in charge of compression is a IGS Audio custom designed photocell. It consists of a electroluminescent panel and a photo resistor. This panel illuminates the photo resistor which reduces the audio path amplification. Limitation of the amplification is directly proportional to the light intensity in the photocell. The limiter threshold is indicated by a diode labeled “Limit” on the front panel.

Audio path of the preamp is a 2-steped tube amplifier with individual drive of both steps. The saturation of the first step is dependent on the input signal (with possibility to use Pad -20dB). The saturation of the second step is dependent on the Output regulator. Inputs and outputs are balanced with Sowter and Edcor (USA) transformers. The anode voltage is close to 300V DC.

For each of the channels Gain, Output and Limiter threshold can be regulated smoothly. The featured switches are: +48V, Mic/HiZ, Phase, Pad -20dB. Hiz inputs are located on the front panel. The meters show output signal level.

Usage: The DTL is a typical microphone preamp. It is adapted to work with any type of microphone. A wide range of amplification makes it possible to record drums as well as the quietest ribbon microphone. The Pad -20dB makes it possible to work with line signals as well. In this case the DTL does a great job of recording tracks from external devices such as synthesizers or drums machines (like Akai MPC). Great results are achieved through reamping tracks from DAWs. The DTL adds a nice tube saturation to the sound. The high impendence inputs (HiZ) enable connecting a guitar or bass guitar directly into the grid of the first step of amplification (Typical HiZ solution).

Additional device capabilities are activated by the Limiter section. This can be used to limit peak levels of the input signal. The wide range of Limiter settings goes from slight compression to hard limiting. This is especially useful in vocal recording where the wide range of signal dynamics is controlled. The limiter controls the louder parts and prevents from the digital clipping at the inputs of you ADC. Another trick could be using the DTL as a drum compressor or sum compressor. It can increase the overall dynamics of the mix and give it a brighter sound. This is a very versatile device for audio processing.


Inputs: 2 Mic/Line XLR and 2 HiZ Jacks on the front 
Outputs: 2 balanced XLR 
Tube Only audio path 
Operating Level +4dB 
Maximum Output +29 dBV
Meters reads Output VU
Max. Limiter Reduction 18dB 
Gain Boost 60dB 
THD < 0.1% @ 1Khz 
S/N Ratio: 104dB
Power consumption: 80 watts 
Soft Start Power Tube Timer 
Shipping weight: 8 kilos



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