Engineering Bulletin

Eliminating Popping Noise from TTI-2

PCB Revision 3

This information applies only to TTI-2 revision 3 boards. The board revision is printed on the component side of the board.

Some customers have noticed a popping noise just before or just after message delivery. The noise is caused by a DC voltage shift when the ISD2560 IC goes into or recovers from low power state. This is part of the design of the IC.

There are capacitors in the TTI-2 that typically mask the disturbance. The severity of the DC shift varies among the ICs and it is noticeable on some units.

The reason for placing the IC in a low power mode is to limit the amount of noise generated by the TTI-2. When the ISD2560 is enabled but not actively generating audio it may generate noise at a very low level. This noise is not typically audible but some stations process in such a way that this noise is amplified to an audible level.

Other audio systems eliminate low level noise but do not tolerate the audible popping noise. The solution is to modify the TTI-2 board so that the ISD2560 IC is forced into its powered state full time. This eliminates the DC shift and the associated noise.

Modification

This modification is easy for someone who is technically knowledgeable. Contact someone who is suited to the task if you are not proficient at soldering. While this modification does not void the factory warranty, damage caused by a poor modification attempt will not be corrected under the factory warranty.

The modification involves cutting one trace on the bottom side of the TTI-2 board assembly and bridging another pair of pads with a short strand of wire. The pads and the trace are on the bottom (solder) side of the board under the ISD2560 at location U5 shown below.

TTI-2 PCB location of ISD2560

The pads under the socket will look like those in the image below when the board is turned over. Cut the small diagonal trace between the two pads as indicated. A small X-acto knife works well for this task.

TTI-2 modification at ISD2560

Use a short piece of wire and create a bridge between the two pads on the dotted line in the image above. Fine gauge wire-wrap wire is typically used for board mods. If none is available, a single strand from a heavier gauge multi-strand wire will work. No significant amount of current passes through this wire. It is okay to use bare wire but be careful that it does not short against any other pads on the board.

If it becomes necessary to undo the modification, simply remove the jumper from the two pads on the same horizontal line and reconnect the pads on the diagonal line.