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Old 15th April 2012, 08:27 AM   #1
Shelah is offline Shelah  Zambia
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Default Biasing and safe operating area region

To come up with proper working of output transistors, we use the bias to come up with an accurate voltage across the collector and emmiter of the bias transistor. This also helps output transistors remain in the SOAR. My question is does this voltage remain constant in all amplifiers and if not how can one come up with the actual bias voltage value?
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Old 15th April 2012, 02:05 PM   #2
AndrewT is offline AndrewT  Scotland
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I think you have the two, bias and SOAR, topics confused.

Bias is an attempt to minimise crossover distortion, whereas SOAR is a limitation imposed by the device on the maximum current that be be passed to the load.
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Old 15th April 2012, 10:14 PM   #3
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Default bias of transistors

Hallo AndrewT
I have read a lot of your posts, and it seems that you have a lot of experience. I have got a NAD 214 power amp, the spec say's that it's bias should be set at 20 mv. A friend mentioned that it should be set at 25mv to give it a warmer sound for classical music. Do you think it is well with limit or do you think it can be even a little bit higher. I live in a country where it gets to 34 dec C in summer inside the house. Will it be beneficiary to set it a bit higher or should one leave it as is.

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Jan
We love brandy here but Whiskey is also great !
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Old 16th April 2012, 09:43 AM   #4
AndrewT is offline AndrewT  Scotland
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If NAD say 20mV then leave it at 20mV.

If you have valid technical evidence for changing from Nad's 20mV, then consider experimenting with that other value.

Do not guess that some other bias voltage could be better than Nad's.

BTW,
whisky in Scotland is without an e. It is whiskeys from other countries that generally add the e.
Scotch must be made in Scotland and will always come without the e.
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Old 16th April 2012, 10:30 AM   #5
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Hi, Just to jump in on this thread as it saved me asking the question. I'm a newbie so forgive me. But to set the Biasing, do you connect the meter up to the speaker output with no load going through the amp, then adjust the bias pots for 20mV or am I off the mark?
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Old 16th April 2012, 10:52 AM   #6
AndrewT is offline AndrewT  Scotland
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No.
You must identify the test points to allow measurement of the bias voltage.

Many amplifiers do not have hook on test points and you must find the exposed terminals of the components that will give the equivalent of the test points. This leads to accidents and damaged gear.
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Old 16th April 2012, 12:04 PM   #7
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Ahhh thankyou for clearing that up for me Andrew

I might be at a loss then or treading in to the unknown. As I can't locate a service manual for my amp anywhere. Where would be a good place to start looking for a test point. My amp is a Hitachi HA 250 inverted darlington amplifier. Or would I be best starting a new thread and uploading pics.

Thanks in advance
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Old 16th April 2012, 12:06 PM   #8
AndrewT is offline AndrewT  Scotland
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We need pics. As close as you can manage and a detailed overall view.

Your amp almost certainly does not have hook on test points.
That takes you into the risk taking arena. Tread very carefully with a good eye and a pair of very steady probes.
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Old 16th April 2012, 12:13 PM   #9
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No worries, I will take some pictures tonight of the circuit board and post them on my thread already started in this section.

It is a late 70s amp and the sound quality is good, so don't want to mess it up. I'm confident enough with probes lol use them in my line of work. It's funny really I specialized in audio & visual repairs at college but didn't realize how hard it was to find an opening in that line of work. So work as a domestic engineer now. Anyway divulging abit.

I will upload the pics, as I have noticed that with no load going through it, the amp still gets warm to touch, so was thinking it may just need a general service and the bias resetting.
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Old 16th April 2012, 12:15 PM   #10
AndrewT is offline AndrewT  Scotland
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All equipment that consumes any finite level of current will get warmer than their surroundings. Plain science.
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