Biasing and safe operating area region

[Shelah]

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?

 

[AndrewT]

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.

 

[Vrystaat]

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.

Regards
Jan
We love brandy here but Whiskey is also great !

 

[AndrewT]

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.

 

[mark31385]

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?

 

[AndrewT]

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.

 

[mark31385]

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

 

[AndrewT]

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.

 

[mark31385]

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.

 

[AndrewT]

All equipment that consumes any finite level of current will get warmer than their surroundings. Plain science.

 

[mark31385]

All equipment that consumes any finite level of current will get warmer than their surroundings. Plain science.

I understand this lol, but the amp is as warm to touch as when it is under full load and outputting quite high. It could probably do with re-calibrating due to the age.