Setting Bias

[kptseng]

I read from TNT that setting the bias will improve the image especially for aged amps.

Does it mean that if I just need to adjust the trmmer and measure the speaker output +/- and ensure I have the same reading? Should be mV I guess.

Volume should be down?

Do I need to hock the amp up with a CD player?

Many thanks.

[richie00boy]

No source should be connected at all.

That is DC offset what you will measure the way you say. Bias you must open the amp and measure across some resistors.

I do not recommend you adjust the bias as it will make the amp run hotter, maybe too hot. In my opinion, you should only change the bias if you intend to increase it a lot, so that the class-a region becomes significant, but this means additional cooling and maybe output devices are needed. Otherwise you just move away from the optimised point set by the manufacturer who probably set it on a distortion meter.

[Jan Dupont]

kptseng;
What I recall from the TNT site is, that they recommed to check the bias on older amps, as this can change over age..... Not that they tell you to set the bias higher or others.

Also they suggest that you replace the old 3/4 turn bias pot with a 10-24 turn trimmer with the same value.
I'm not sure, but I think they called it "Put a tiger in your amp".

Checking the bias on old amp's is a good idea

[kptseng]

Hi Jan and Richie,

Thanks for your input.

I have no intention to increase the bias, but to ensure the left and right channel has the same reading, as that is suppose to improve the imaging.

I hope I am correct.

I just did a quick test in measuring the amp output.
It start to read about 500+ mV and start to decrease by 4mV every second. Is that normal? If it is normal then at what point do I take the measurement?

[Jan Dupont]

The Bias is normally measured across one of the output device Emitter resistors (one resistor on positive and one on negative side).
What amp is it ?

[kptseng]

Hi Jan,
It's an Onix OA21s. The problem is I don't knwo which resistor and they are not marked at all.

[Nordic]

For a single output pair, can the Vbe of one of the output transitors be used as a guide?

Sadly I do not have the benefit of your education...
I can see that using only the Vbe one could exceed the amp's rateings, not knowing before it is too late.

But, I have found there seems to be a sweetspot in the Vbe on the output pair.. but once again this may be down to my experience not being wide enough to form statistical conclusions on this topic.

Not mentionion numbers to prevent distracting those that are even more misguided than me..

[Jan Dupont]

kptseng;
I did some repairs on such amp some time ago.
As I recall, the output transistors are attach to a L-shape aluminum angle that is attached to the chassis (so the chassis acts as a heatsink......)
Measure between Ground and the legs of the output transistors.
One leg will be + or - DC (35V as i recall), another leg will measure app. 0.7V and the last leg is the Emitter which should measure somewhere in between 12-17 mV..............

[jrockhead]

kptseng said;
I read from TNT that setting the bias will improve the image especially for aged amps.

I tried it on one of my Marantz 2270's and it sounded like crud. Highs and mids were hard and irritating, don't do it. Class AB amps sound best with a little bit of bias as they were DESIGNED!!!

Check the service manual for the correct bias and you will be in Audio Heaven(TM).

Marcus

[vynuhl.addict]

Hi Kptseng,



Upon startup it is very normal to watch the bias wander a little as the dc operating points settle down during warm up. For measuring the bias you can do it two ways, both require opening the amp though. First with a dmm on the 200mv setting and measure the voltage drop across the emitter resistor in one of the output devices and divide the voltage drop for eg 14mv/0.22R a common value which would be 63.636ma, you can measure across a pair NPN/PNP but the first is easier as you can clamp one probe to the speaker jack positive, less chance for probe slippage . I must also say, I strongly would recommend against the TNT article on raising the bias above manufaturer specs and Xover distortion from overbias can be just as bad and in many cases worse than underbias. The idle bias is often set at spec for a number of reasons, for some this has been found to give the best xover performance, for others, namely mid fi gear this bias is as so due to insufficient heatsinking and cost cutting to deal with the extra heat associated. But bringing it back up if it has drifted, to spec will improve imaging if it has drifted, dc offset seems to be quite high though, 500mv?, i would have that adjusted if there is a provision for it.. And as said by others, there must be no load connected, the only purpose for plugging in a CD player is to short the Inputs, no music need be played...



Colin