Tube Tech SSA 2A User Manual

TUBE-TECH SSA 2A

Stereo summing amplifier

Description:

The TUBE-TECH Stereo summing amplifier SSA 2A is aimed for use where extreme high
quality summing of several signals into two channels is the target.

It has 4 mono input (hard wired to CH1 and CH2) and 8 stereo inputs, which is summed
together into two outputs.

The SSA 2A 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 for 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.

2. Frequency response in both the low and high range is extended.

3. Phase response in both the low and high range, is also reduced

(050320)

Lydkraft ApS

Mose Allé 20, DK2610 Rødovre

Denmark

1

To avoid a reduction of the crosstalk between the channels, it is important that all four mono
inputs are terminated, either with a shorting plug or by a signal source.

2

Mose Allé 20, DK2610 Rødovre

Lydkraft ApS

Denmark