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The TTSH reverb driver and return amplifier are compatible with any Accutronics (or clone) model whose
second and thi
rd digit in the model number are “AB”. The Synthcube kit and BOM specify a 1AB2A1B.
This is a two spring, medium decay unit in a short pan. The fifth digit “A” specifies a grounding
configuration where both input and output have a common ground with the pan. This ground
configuration is required for operation in the TTSH.
You may substitute any reverb unit so long as the second and third digit in the model are AB (input
impedance ~10 ohms, and output impedance ~2K ohms). Observe, however:
If the fifth
digit (grounding) of the model number is not “A”, the builder will need to modify the ground
configuration of the pan such that the input and output sleeve connectors are common with the pan itself.
This is usually just a matter of installing a wire jumper between the sleeve connectors on the jacks.
Noise Reduction
The signal level in the return from the reverb unit is extremely weak, and very close to the noise floor.
Noise is induced in the sleeve of the return cable, and because the connection is not balanced, it can get
added to the reverb signal. This is specifically a problem when the 0V rail is referenced to an external
brick. Most bricks have a floating ground, and do not provide a shunt path for signals induced in the
shields of audio connectors. The shield needing this shunt path the most is the one in the cable for the
reverb return. The gain of the return amplifier is very high and so the noise will be brought up to audio
level. There are two ways to improve this (in order of effectiveness): 1. Connect the 0V plane of the
synthesizer to a true Earth ground; 2. Improve the rejection of noise in the return cable.
Earth Ground
Although not discussed in this guide, some builders substitute a linear bipolar power supply for the
external brick and DC-DC converters. In this arrangement, there is a strong earth ground brought to the
0V reference in the TTSH. This approach yields inherently quieter reverb, but at the cost of introducing
50/60hz line noise to the reverb signal. When the TTSH is built using the DC-DC converters and an external
brick power supply, the reverb can be particularly noisy. The solution is to bond 0V on the synth to true
earth. This may be as simple as placing a patch cable from any jack on the synth to another system whose
0V plane is referenced to earth. Another approach, recommend here, is to add a grounding lug
connection to the case.
It is left to the builder to determine the best way to bond the TTSH to Earth in the studio, so no particular
solution is advocated here. In a typical studio, there are many places and options to tie the TTSH 0V plane
to Earth.
In the author’s studio, a banana receptacle (green, of course!) has been added to the case, and this is
wired inside the case to the 0V pin on the barrel connector accepting the input power from the brick. On
standard USA appliance outlets, the center screw on the wall plate functions as an Earth connection. A
ground wire was constructed with a banana plug on one end, and a C type spade connector on the other.
This cable runs between the screw on the wall plate to the banana plug on the synth.
Regardless of how the synth is bonded to Earth, if it is powered using the external brick + DC-DC
converters, there will be a dramatic reduction of reverb noise when the Earth connection is established.
Improved Noise Rejection in the Wiring