Hiraga 20 w DC threat

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Hi Guys, advice please from all Hiraga or Hiraga DIY users. I have found a factory Hiraga 20 watt to buy, original transistors and caps, nice condition, but am put off by stories i read about transistor failure putting DC through and frying my expensive speaker drive units. Is this a a genuine concern or a one-off for a poor unfortunate user. Thanks for your help, i have fancied this amp for 20 years, now so close, but i could not justify threatening my speakers. Are there any mods that can be done if this is a real concern?
 
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DC faults are a hazard of any direct coupled design, protection circuits not always providing the safety they should.

I can't comment on the reliability of this amp (I know nothing about it tbh) but can suggest that using two back to back large electrolytics (say 10,000uf each) of a voltage at least equal to the total rail voltage of the amp per channel to connect in series with the speaker will offer essentially total protection whatever happens.
 
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Not necessarily. Junction temperatures of silicon transistors are happy running at well over 100C. That translates to case/heatsink temperatures to hot to touch. A big reliability issue on hot running components is fatigue of the solder joints and that can cause just as destructive conditions as actual failure of a part. Its always worth checking and running an iron over such components for long term reliability.
 

fab

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Heat concern

Hiraga 20W and F5 amps are not worse than other class A amp for temperature management (big degeneration resistor value in the output stage in this case). I also use the 5-10 seconds touch to set the maximum bias for an heatsink.

However, by precaution I also install a 70 degree C thermal cut-off on each heatsink of all my class A amps to cover for possible failure and prevent heat burn. The normally closed Thermal cut-off is on the power supply line of the amp.

Fab
 
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