Voltage across 0R22 tells you the bias current.
The resistor itself would heat up in ~5 minutes.
But the MOSFETs on heat sink take long.
Half and hour or more.
You should first trim at room temperature to 1.2~1.5A (0.33V).
Then wait half an hour till heatsink is hot.
Then readjust both for bias and for DC offset (relative to Gnd).
Patrick
The resistor itself would heat up in ~5 minutes.
But the MOSFETs on heat sink take long.
Half and hour or more.
You should first trim at room temperature to 1.2~1.5A (0.33V).
Then wait half an hour till heatsink is hot.
Then readjust both for bias and for DC offset (relative to Gnd).
Patrick
I am setting the left channel first. For the initial settings, I dialed the trimpot for about 2.3v for the all Mosfet's Vgs. What I get is this :
Transformer before Reg. for - sideis: 20.65V and + side is 20.13V.
After Reg. for the - side is at 16.29v and + side is at 16.57v.
As for the 0R22, it kept going up for 3 hours! It starts at about 0.368V and climb all the way to 0.383v for 3 hours.Then it kinda stabilised there for half an hour or so and then it came down again after i took the rest of measurements (and then restarts it's climb again).
First power up :
R39 = 0.368v (went up to 0.372v after an hour)
R37 =0.367v ( to 0.366v one hour)
R7 = 0.363v
R6 = 0.370v
After 3 hours :
R39 = 0.383v
R37 =0.380v
R7 = 0.380v
R6 = 0.381v
gate and source measured at :
Q15 = 2.216v
Q19 = 2.273v
Q1 = 2.145v
Q 8 = 2.165v
Offset for (-) side is 58mV and for (+) side IS 34mV.
Heatsink at the center where the fets are is at 78c. and at the sides isat 65c. I don't know how accurate is my measuring device.
Is there something wrong?
I will bias the right channel tomorrow to see if this is happening to both channels.
Transformer before Reg. for - sideis: 20.65V and + side is 20.13V.
After Reg. for the - side is at 16.29v and + side is at 16.57v.
As for the 0R22, it kept going up for 3 hours! It starts at about 0.368V and climb all the way to 0.383v for 3 hours.Then it kinda stabilised there for half an hour or so and then it came down again after i took the rest of measurements (and then restarts it's climb again).
First power up :
R39 = 0.368v (went up to 0.372v after an hour)
R37 =0.367v ( to 0.366v one hour)
R7 = 0.363v
R6 = 0.370v
After 3 hours :
R39 = 0.383v
R37 =0.380v
R7 = 0.380v
R6 = 0.381v
gate and source measured at :
Q15 = 2.216v
Q19 = 2.273v
Q1 = 2.145v
Q 8 = 2.165v
Offset for (-) side is 58mV and for (+) side IS 34mV.
Heatsink at the center where the fets are is at 78c. and at the sides isat 65c. I don't know how accurate is my measuring device.
Is there something wrong?
I will bias the right channel tomorrow to see if this is happening to both channels.
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I believe Patrick recommended 1.5mm wire. I myself use 16g. Try measuring the temp on the 18g.
Today, it starts at 0.330v at room temp 31c. Yesterday it was warmer, probably at 33c or 34c. At which point do I start to trim the dc offset? I trim it by adjusting the trimpot, correct? But this will affect all others measurement. Yes, using the kerafol with thermal grease. No, I did not measure the sinewave.
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I use 1.3mm wire. Including the sleeve will be close to 1.5mm so it's a bit thick and hard to twisted around.
> At which point do I start to trim the dc offset?
You trim it when you first power on, and then every 10 minutes until temperature is constant.
> I trim it by adjusting the trimpot, correct?
Yes.
> But this will affect all others measurement.
No significant change if you only trim 50mV or so.
> Yes, using the kerafol with thermal grease.
The whole point about Kerafol is that it does NOT require thermal grease.
Adding grease will make things worse.
Please read my article at Linear Audio.
> No, I did not measure the sinewave.
I suggest you do before building the 2nd channel.
Patrick
You trim it when you first power on, and then every 10 minutes until temperature is constant.
> I trim it by adjusting the trimpot, correct?
Yes.
> But this will affect all others measurement.
No significant change if you only trim 50mV or so.
> Yes, using the kerafol with thermal grease.
The whole point about Kerafol is that it does NOT require thermal grease.
Adding grease will make things worse.
Please read my article at Linear Audio.
> No, I did not measure the sinewave.
I suggest you do before building the 2nd channel.
Patrick
Sorry, typo. I am using the kerafol without the thermal grease. I do not have capability to measure the wave.
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If you intend to use your amp at 35°C, I suggest either you reduce the bias to 1.2A,
or you add two slow running low-noise fans blowing towards the heatsinks.
NF-A14 ULN
Patrick
or you add two slow running low-noise fans blowing towards the heatsinks.
NF-A14 ULN
Patrick