Tip & How-To about Audio Players & Recorders

Speakercraft Vital 1250 Class D amplifier

The Vital 1250 is a class D amplifier designed to deliver 170watts per channel (x12) at 4 ohms impedence.

There is however a number of these which are starting to come through with amplifier modules that fail.

There are 6 stereo class D modules using the TDA8970 design.

In breif the issues are down to the transformer secondary windings delivering slightly over spec voltage to the modules. Over time with the audio sense active this initial pulse from the transformer at power up sees off the amplifier chip and in many cases damages the PCB links between the double sided print making it impossible to remedy the exsisting module, so replacement is the only option.

The transformer on this unit is unique to this model and give 6 pairs of outputs for the 6 modules supplying 2x 29V AC which when converted to DC is around 41Vdc to each module, this can surge up on power on for a split second.

The TDA8970 is designed to take an absolute maximum of 39V and there is little or no protection on the output stage to stop a DC output occuring on the speaker outputs.

My therory is that this will be only the case for UK owners on a 240v mains supply as the tollerance may be acceptable for USA 110V users.

I have repaired these units by replacing all 6 modules with better units that have additional output protection between the TDA8970 and the speaker outputs. also adding resistors to the supply rails to help bring them in line with the correct tolerance and give longer life.

I am an engineer at MusicView Services and we speacialise in high end audio products and profesional music sound equipment.

I hope this helps someone with this amp, these problem will be specific to this model and should not be considered to relate to anything other than the Vital 1250.

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