Tube Tech SSA 2B Stereo summing amplifier - Manual

Lydkraft ApS
Mose Allé 20, DK2610 Rødovre
Denmark
1
TUBE-TECH SSA 2B
Stereo summing amplifier
Description:
The TUBE-TECH Stereo summing amplifier SSA 2B is aimed for use where extreme high
quality summing of several signals into two channels is the target.
It has 10 stereo inputs, which is summed into two outputs (LEFT and RIGHT).
Each channel of the first two stereo inputs can be switched to mono, giving a total of 4 mono
inputs.
The SSA 2B consists of a summing amplifier at the input, a 23 stepped attenuator with a
range of +/- 10 dB and an output stage.
Two large VU-meters show the levels on the XLR output sockets.
To avoid the distortion introduced by the VU-meters when connected directly to the output
XLR sockets, symmetrical configured OP- amps buffer the VU-meters.
The summing amplifier is a symmetrical two-stage amplifier, designed around two dual, low
impedance, and high transconductance triodes. The input transformer is part of the dual
feedback configured as a so-called zero field transformer.
The attenuator is a heavy gilded 2x23 position switch with steps of +/- 0.5, 1, 1.5, 2, 2.5, 3, 4,
5, 6, 8, 10 dB.
The output stage is designed around two dual triodes in push-pull and an output transformer
(with a static screen).
Both the summing amplifiers and the output amplifiers are symmetrical.
Summing-amplifier:
All inputs are balanced by two zero field transformers (one for each channel).
What is zero field transformer?
In the zero field transformer configurations, the transformer is part of the feedback circuit.
There are several benefits to this:
1. The voltage across the transformer is reduced substantially whereby a much
smaller transformer is needed for the same specification.
The reduction is of the same magnitude as the feedback applied.
2. The distortion at low frequencies is reduced substantially.
3. Frequency response in both the low and high range is extended.