Bias Circuit Operation - explanation

[Watthour]

You are most likely correct. It kills me that we have a dozen different terms for the same thing.

Right. To me, "bias" is the root cause of someone not liking an amplifier just because it says "Onkyo" on the faceplate.

 

[Yused Towork]

I remember when "bias" was like "pressure"....
it all had to do with tires..

 

[hopeless]

Right. To me, "bias" is the root cause of someone not liking an amplifier just because it says "Onkyo" on the faceplate.

I just never warmed up to their sound. It was better than most, but just very "polite". You would expect the M504 to really slam, but it just doesn't. The relatively diminutive Citation 24 next to it at the store I worked at was a far better amplifier. Just spanked it. Very often, the big sexy meters and overall size won.

 

[rcs16]

Need to mention thermal bias stability as part of the Vbe multipler ( bias generator) design. Lots of designs do not do it properly where all devices after the bias gen are not part of the thermal feedback mechanism, thus some thermal instability results. Bob Cordell explains it well in his power amp design book.

 

[Watthour]

That was an argument for hybrid output modules/circuits when in their infancy, and it applied to a lot more than audio amplifiers. I cut my teeth on EDM power supplies and in both analog and digital control of those. The guy I worked with who taught me the most was Eiichi Takarada (look him up), Takarada-san had some very clever and innovative designs garnered from many manufacturers and of his own design. That translated to a lot of other applications.

One concept was to mount all power and drive analog components on a common heat sink, be it air or liquid cooled.

 

[hifihoosier]

Just found this two year old post today. Awesome! Thank you @Hyperion

 

[markkb]

Just found this two year old post today

Yay! Thanks for replying, I missed this 2 years ago.

Thanks John, this will help me wrap my brain around a unit that I have on the shelf with a biasing problem.

 

[tutor_turtle]

Here are some tips which you might find useful when you have a bias problem.


Below is a schematic snip of a driver board for an AU-517.

View attachment 2799286

TR06 (centre left) is the bias control transistor, just below it is VR03 the bias trimmer, the transistor and the trimmer are vital to the operation of the bias circuit.

How this bit of circuitry works to control the bias and make it adjustable.

TR05 and TR07 are turned on enough to provide current flow through R23 (from the positive rail) and R25 (from the negative rail). This current also flows through R20, R21 and VR03, these last two present a voltage to the base & emitter of TR06 turning it hard on, corresponding to minimum bias.

With the bias trimmer VR03 wound all the way to the bottom of the track to present all of its track resistance to the circuit TR06 will be turned fully on. With TR06 turned fully on the bases of TR08 and TR11 will be effectively almost shorted together – along the red lines – and so will have insufficient voltage to switch on so, little or no bias current should flow in the output transistors. The voltage present at ‘V’ will be 1 to 2 Volts or thereabouts with the bias trimmer set to minimum bias.

Adjusting Bias
If we then try to adjust bias we will alter the slider position on the track of VR03 in the direction of the purple arrow (upwards). As we do this TR06 will gradually turn off. As TR06 turns off, the voltage at ‘V’ is allowed to increase. As this voltage increases, the voltage across the base and emitter of the driver transistors increases, (green lines) thus gradually turning them on. When this happens the Output transistors also turn on. And with the output transistors turning on, bias current can be measured (as a voltage drop) across the output transistor emitter resistors - This is normal operation. Note that representative voltage drops can be seen across R34 & R35 and also R36 & R37 indicating current flow in the respective pairs of driver transistors.

If you can’t adjust bias:-

i.e. no bias, - OR bias stuck at wrong reading.

– things to check.

First check your power supply voltages (always).

Check that TR05 & TR07 are turning on. voltage B-E=0.6V and their schematic voltages are correct*.
*Use minigrabbers attached with the power OFF then power on to measure the voltage.

Check TR06 – adjust the trimmer and see if the voltage at ‘V’ alters** – if not suspect the trimmer or TR06 faulty. Voltage at ‘V’ will be around 1-4Volts, dependant on bias setting, and amp design.
**Use minigrabbers attached with the power OFF then power on to measure the voltage.

If good up to this point check R29 & R30 (out of circuit - with Ω Meter). Also check R38 & R43 – same procedure. If any of them are high value or ‘open’ they will either starve the drivers of current (R29 & R30), or the Output transistors (R38 & R43). .

If still no-go check the driver transistors, by removing them from circuit and using either multimeter on ‘diode test’ range, or a transistor tester.

Check the Output Transistor emitter resistors, lift one end of each and test.

If still no-go check the Output transistors (remove them) and test as above.

These are the basics, I have tried to explain the operation of the bias circuitry in as simple terms as possible – I hope this helps.

Many thanks, once again you've taken the mystery out of the puzzle.
Fluke makes some nice needle pointed leads that are insulated down to the last millimeter.
A little pricy, but they let you probe live circuits with confidence.
Not the exact ones I have, but very similar. (the Fluke product has less exposed metal)
As seen on Amazon.