IGS AUDIO DTL – dual microphone tube preamplifier
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 that attenuates audio signal. Working of the limiter is indicated by a LED
labeled “Limit” on the front panel.
Audio path of the preamp is a two stage tube amplifier with individual drive of both steps. The
saturation at the first step depends on the input signal level (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 set close to 300V
DC.
For each of the channels Output and Limiter threshold can be regulated smoothly. The featured
switches are: GAIN, +48V, Mic/HiZ, Phase, Pad -20dB. Hi-Z inputs are located on the front
panel. The meters show output signal level.
Usage:
The DTL is a typical tube 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 with reamping
tracks from DAWs. The DTL adds a nice tube saturation to the sound. The high impendence
inputs (HiZ) allow connecting a electric 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
mix buss compressor. It can improve the overall dynamics of the mix and give it a brighter
sound. This is a very versatile device for audio processing.
Block diagram
