Help, how to adjust trimer(potentiometer) on transistor amplifier

[RealForce]

Ok I really need help. I have this amp that I build and it uses 2x IRF640 and 2x IRF9640 transistors. It has 0.33Ohm/5W output resistors and it has 2.2k trimmer(potentiometer) for adjusting the working state of the transistors. I really don't know how to adjust this trimmer (potentiometer). Can you please tell me how do you adjust this?

[mickeymoose]

And the schematic is?

[jaycee]

Connect a voltmeter over one of the 0.33 ohm resistors. Through I = V/R you can determine the idle current flowing through the output stage.

How much idle current to set depends on the design of the amp. I belive around 70-100mA is typical for these MOSFETs but it really does depend on the design. I would start at 50mA.

[RealForce]

Originally Posted by jaycee

Connect a voltmeter over one of the 0.33 ohm resistors. Through I = V/R you can determine the idle current flowing through the output stage.

How much idle current to set depends on the design of the amp. I belive around 70-100mA is typical for these MOSFETs but it really does depend on the design. I would start at 50mA.

Thank you so much my friend. IT WORKED like a charm.

[RealForce]

Can you tell me what is the difference with this trimmer.
I set it to 33mV as said in schematic but works the same if I set it to 0mV except it doesn't get that hot when set to 0. So can someone tell me what exactly does this trimmer? I don't feel any power loose execept the output transistors are getting warm or cold.

[RealForce]

Ok so I set the trimmer to around 33mV. But why all 0.33 resistors show different mV. Some of them are 33mV some 15mV some 20mV. Is that normal?

[nigelwright7557]

Originally Posted by RealForce

Can you tell me what is the difference with this trimmer.
I set it to 33mV as said in schematic but works the same if I set it to 0mV except it doesn't get that hot when set to 0. So can someone tell me what exactly does this trimmer? I don't feel any power loose execept the output transistors are getting warm or cold.

The trimmer adjusts the standing current through the output transistors to get rid of crossover distortion.
Having too much standing current is wasted as heat.

The 33mA sounds about right for this type of amp.
Any less and you will get crossover distortion.

[RealForce]

Originally Posted by nigelwright7557

The trimmer adjusts the standing current through the output transistors to get rid of crossover distortion.
Having too much standing current is wasted as heat.

The 33mA sounds about right for this type of amp.
Any less and you will get crossover distortion.

But the problem is that not all resistors are 33mV. Some of them are 15 some 20 and some are 8. The amplifier sounds great but why all resistors have different voltage?

[jaycee]

I'll clarify the procedure a bit better. Biasing is not really set and forget.

Start with the amp cold. Turn on, adjust to 33mV. Then let the amp heat up. You will probably notice that this voltage will increase as the amp warms up - adjust for 33mV as it warms up. After about 30 minutes it should be stable and not changing.

I'm not sure exactly why you're getting different measurements. Seeing the schematic would help. It may be better to measure across both 0.33 ohm resistors for one pair of MOSFETs, and use that value (halving the resulting reading of course)

The bias current is needed to keep the MOSFETs partially on, otherwise you get a kind of distortion called "crossover distortion". How much bias current is needed depends largely on the type of MOSFETs and the design of the amp. Setting zero bias current will work, but you will get distortion in the output.

[richie00boy]

You are getting different voltages across the resistors because the voltage/current sensitivity of MOSFETs varies quite a lot between devices, also the 0.33 ohm resistors will all be slightly different values.

Just set to an average of about 50mA in each pair.