Aleph 3, option to get less current temporarely

[rmgvs]

I plan to build an Aleph 3. There has been a lot of discussion about the temperatures of heatsinks/reliability and total dissipation versus sound quality. Suppose that I do not need 30 Watts at all times. I have the same voltage rails (25 Volts) but wish that I can turn the amp to an idle current that corresponds with a decent output (soundwise) of 10-15 Watts. I guess an idle current of around 700 mA (instead of 1 A) per transistor should do the job? What resistor(s) value(s) should I change to do the job? Maybe some small switch can be built in the amp so that you can change the amp at will to a current of 1 A (normal 30 Watt operation) and 0.7 A. I guess the life expectancy of the amp will increase considerably?

[wuffwaff]

Hi,

you can change R114 for different bias points. Just put a pot in here and set the two or more bias points you want. Measure the resistor values and put in a switch to change between values.
You must measure both channels cause the resistor values tend to be different in each channel because of transistor tollerances.

As long as dissipation per fet is below 30-35watts and cooling is sufficient the amp will life longer as is needed anyway so no use changing this from 20 to 40 or 60 years

William

[GRollins]

If you build a stock Aleph 3, the device dissipation is around 25W/device. With decent heatsinking, your output devices should last quite a long time. I've not heard of any consistent reliability problems with the Aleph 3--most problems arise because people do not use sufficient heatsink. When in doubt, buy more. MOSFETs will not die from being too cool, but they will surely perish from excess heat.

Grey

[rmgvs]

Thanks for reactions. So it is R113 in the circuit diagram of Aleph 3 that has to be changed. What would be the problem of using a pot permantly anyway? I guess in this place it will not degrade sound quality?

I understand that there is no problem when ample cooling is used. But as it often goes, I have a enclosure in mind that is nice and seems apt, with a temperature coefficient of 0,31 per side/channel. But I had not counted with the correction factor (around 1.33 I guess), so for 55 degrees I can only put away 70 Watts of heat.

Besides that, what is wrong with using some less idle current when you are not using it most of the time? Anybody has experimened with 700 or 800 mA per mosfet and want to tell about the sound differences with 1 Amp?

I do not understand the circuit in its single-ended class A nature. The amp can put 60 Watts into 4 Ohms. Normally a single-ended amp works only in class-A and cannot put more power into low impedances. Of course it has something to do with the special Pass-arrangement of the upper mosfets.

What would be the output capabilities into 8 and 4 Ohms using 700 mA per mosfet (1.4 Amps total) using +/- 25 V of DC?

[wuffwaff]

Hi,

at 50% ac-current gain peak current would be 2.8A. So max power into 8Ohms is 30watts (voltage limited) and 16 watts into 4 Ohms.

You can use a pot all of the time

Even a single ended class A amp has a maximum output power defined by supply voltage and bias. This can be at 16 Ohms but also at 2 Ohms depending how you set them.

Lowering the bias will change the sound character. I would suggest to listen for yourself but in my case upping the bias gave a more relaxed sound with better ambient retrieval capabilities.

William