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Old 12th Oct 2019, 6:05 pm   #1
alanworland
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Default Repaired Quad 405-2 temperature

Just got my new to me Quad amp going after replacing caps, o/c resistors, op amps and triacs.
Sounds the business with 1mV of DC on each channel, had it running for a few hours and it is warm to the touch. It's only been lightly loaded and am I to understand it is in class A operation and at some point it moves into class B?
Would the temperature I have experienced be due to it operating in class A?
It's running warmer than my 303 does under similar conditions.

Alan
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Old 12th Oct 2019, 6:51 pm   #2
GrimJosef
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Default Re: Repaired Quad 405-2 temperature

The circuit of the power stage in the 405-2, and its predecessor the 405, is unusual. Quad called it 'current dumping'. I'm not sure that the traditional model of a single amplifier which moves from Class A (where neither of the push-pull devices cuts off) under small-signal conditions to Class B (where each of them does at some point in the cycle) at higher powers applies quite so cleanly here.

In Class A the amount of heat dissipated is (practically) the same whether the amp is playing music or not. So high power amps which stay in Class A all the way up to their maximum output will run hotter than people expect when they're running at low power, or with no music playing at all. I don't think this is the case with the 405-2 though.

If they run warm it will be down to the choice of quiescent operating point, the size of the heat sinks, the ventilation and whether anything else, e.g. power supply components, are also dissipating heat.

Cheers,

GJ
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Old 12th Oct 2019, 6:57 pm   #3
Mooly
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Default Re: Repaired Quad 405-2 temperature

The 405 runs its class A section at around 3.5 watt dissipation in the driver transistor and another 3 watts or so dissipated in the resistive loading for that stage. That is per channel and so yes, it will run warm.

As output increases it doesn't move into class B as such, its a bit more complex than that as the unbiased dumpers (main output transistors) supply the load current. The feedback signal is taken from a bridge with the class A stage attempting to correct the errors.
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Old 12th Oct 2019, 7:01 pm   #4
peterpiper
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Default Re: Repaired Quad 405-2 temperature

Hi Alan, the class A part does run quite warm (the 2 40872s) I had a lot of bother getting the DC offset down to 1mv, points to remember, check the 2 3.3 ks feeding the op amp zeners, make sure the zeners give the correct voltage, make sure the 10 ohm at the input is shorted. I found C2 the 100mf 3v tant was leaky reducing the dc feedback. I found it very instructive to disconnect base and emitter of both dumpers while trying to get the offset down, Its surprising how loud it goes without them!
Peter
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Old 12th Oct 2019, 9:19 pm   #5
alanworland
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Default Re: Repaired Quad 405-2 temperature

It sounds quite a complex operating circuit! Both of the 3.3k's had gone open circuit and after replacing other parts and getting to my latest offset I sort of called it a day as I thought that was quite good!
That sort of wattage would get it to the temperature being experienced, I shall put a probe on the heatsink next time.

Alan
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Old 13th Oct 2019, 4:29 pm   #6
alanworland
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Default Re: Repaired Quad 405-2 temperature

Purely out of interest the amp has been running for 6 hours now with reasonable volume at times (not 10's of watts) and highest temperature I have measured on the heatsink is 39 Deg C measured with an infrared 'gun' - just the job with winter coming!

Alan
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