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Yamaha Stereo transformer

Discussion in 'General Electronics Discussion' started by Jeanguypataterubberboot, Jul 18, 2018.

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  1. Jeanguypataterubberboot


    Jul 9, 2013
    Hi all,
    I have a Yamaha RX-V640 stereo that I took apart and was wondering how to identify how many amps the transformer will handle. Is there any way to tell this? I can't see any specs on the transformer in the schematic. One side of it has a sticker with the following:

    X3919 A0 U7 75K027HDLP TAM M 0310

    Also what projects would you suggest I do with this transformer.

    Thanks very much


    Attached Files:

  2. Audioguru


    Sep 24, 2016
    The stereo probably has a label saying how much is its maximum AC current.
    If the current is not stated then its maximum RMS output power will be in its owner's manual.
    Then you guess that the amplifiers are class-AB and the heating is 45% of the maximum output power
    Then you use simple arithmetic to add the output power and the heating power and calculate the current with Current= Power/Voltage.

    BUT a stereo is not operated continuously at full blast, so de-rate the current from the transformer.
  3. kellys_eye


    Jun 25, 2010
    Power it up. Measure the open-circuit secondary voltage i.e no load. The add a load until the output drops by 10%. This will be your safe maximum load.

    Use 12V bulbs (in series/parallel connection for an easy-to-make load i.e. 12V 10W (or 25W or even 55W car side light/headlight bulbs).

    Transformers have all sorts of parameters you don't usually consider including a temperature rating which determines how hot you can get the core laminations (under constant load) before you cause damage and for some transformers this is quite hot indeed - hot enough to make it very uncomfortable to touch. Note that not all transformers can run that hot though....

    If you only intend using it for intermittent loading you can over-rate the output i.e. if you calculate, using the above method, that your transformer is safely rated at a constant 5 amps )with a 10% volt de-rating) you could probably use it intermittently to 50% over that i.e 7.5 amps (with attendant volt drop that has to be taken into account).
    Last edited: Jul 19, 2018


    May 20, 2017
    Bear in mind that the duty cycle for music can be quite low and volume manufacturers tend to rate their power supplies with this in mind.
  5. Jeanguypataterubberboot


    Jul 9, 2013
    Thanks guys.
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