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Old 22nd June 2014, 03:57 AM   #1
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Default How to set bias for this amplifier ?

Hi, recently I build a C Audio SR707 amplifier using the schematic diagram. Outputs are Fet's and drivers are C5200 and A1943. ( originals are different ) My question is positive side driver is getting hot even thers is no signal . with in few seconds of power on the + side driver gets hot .

For testing purpose I did not mount the drivers and FET's on a heat sink. Because of that I could able to noticed that positive side driver 2SC5200's behave.

is this normal ? because I have seen some amplifiers one side get hot with out signal.

if not caold you please tell me how to set the bias for this amp. I tried with trim pot but no change on the heat of the driver transistor. I have attached the scamatic diagram. for FET's I used 2sk1530 and 2SJ201. Could anyone tell me using analog multimeter how to set the bias for this amp ?

thanking you.
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Old 22nd June 2014, 05:39 AM   #2
bimo is offline bimo  Indonesia
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Quote:
Originally Posted by shehantha View Post
Hi, recently I build a C Audio SR707 amplifier using the schematic diagram. Outputs are Fet's and drivers are C5200 and A1943. ( originals are different ) My question is positive side driver is getting hot even thers is no signal . with in few seconds of power on the + side driver gets hot .

For testing purpose I did not mount the drivers and FET's on a heat sink. Because of that I could able to noticed that positive side driver 2SC5200's behave.

is this normal ? because I have seen some amplifiers one side get hot with out signal.

if not caold you please tell me how to set the bias for this amp. I tried with trim pot but no change on the heat of the driver transistor. I have attached the scamatic diagram. for FET's I used 2sk1530 and 2SJ201. Could anyone tell me using analog multimeter how to set the bias for this amp ?

thanking you.
Bias voltage are 2x VBE of transistor driver + 2x VGS(OFF) Mosfet (0.8..2.8V). I think you must lower R21 value. With this schematic, max. bias voltage is 2.35x VBE.

Last edited by bimo; 22nd June 2014 at 05:40 AM. Reason: typo
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Old 22nd June 2014, 06:06 AM   #3
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many thanks for the quick reply. I will lower the R21 but could you please tell me from which points i should get the readings of multimeter. I saw many says take from the output across emiter resisters. But this diagram dosent have emiter resistors.
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Old 22nd June 2014, 06:26 AM   #4
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Why start another thread when you already have a thread running with this question.
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Old 22nd June 2014, 08:09 AM   #5
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Yes, the second thread with exactly the same question looks strange to me also.

Anyway, I would approach it the following way:

1) Remove the output MOSFETs. Don't connect any load. With no output transistors and no load the circuit will balance itself anyway and we will see if everything is fine or not relatively safely;

2) Set VR1 pot to the maximum R position.

3) Connect the V-meter (DC) to the output (R17 / R15 junction);

4) Now switch the power on. You would see something close to zero (less than 100 mV) at the output. If you see higher DC - something is wrong. Solve the problem until you see less than 100mV;

5) Assuming you've got "0" DC at the output, connect the V-meter between the emitters of Q2 and Q3. As you carefully rotate VR1 pot, voltage has to change from almost zero to a number of volts (up to 5-8 or so). Do it slowly and watch out for the drivers' temperature. If nothing happens - something is wrong with your circuit.

6) If everything is fine, set the VR1 pot back to the maximum R value, connect the MOSFETs and connect the A-meter serially with the MOSFETs, let say, the N ones (top shoulder). I would put all the necessary parts on the heatsink at this point (including TR6 sensor) in order the thermal feedback to work properly.

7) Carefully rotating the pot adjust the quiescent current. Give it some time to settle, adjust again (several times) - see if it is stable during up to half an hour.

If you have got an oscilloscope - keep it connected to the output all the time in order to watch for possible oscillations and other artifacts.

Cheers,
Valery
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Old 22nd June 2014, 09:13 AM   #6
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Quote:
Originally Posted by vzaichenko View Post
Yes, the second thread with exactly the same question looks strange to me also.

Anyway, I would approach it the following way:

1) Remove the output MOSFETs. Don't connect any load. With no output transistors and no load the circuit will balance itself anyway and we will see if everything is fine or not relatively safely;

2) Set VR1 pot to the maximum R position.

3) Connect the V-meter (DC) to the output (R17 / R15 junction);

4) Now switch the power on. You would see something close to zero (less than 100 mV) at the output. If you see higher DC - something is wrong. Solve the problem until you see less than 100mV;

5) Assuming you've got "0" DC at the output, connect the V-meter between the emitters of Q2 and Q3. As you carefully rotate VR1 pot, voltage has to change from almost zero to a number of volts (up to 5-8 or so). Do it slowly and watch out for the drivers' temperature. If nothing happens - something is wrong with your circuit.

6) If everything is fine, set the VR1 pot back to the maximum R value, connect the MOSFETs and connect the A-meter serially with the MOSFETs, let say, the N ones (top shoulder). I would put all the necessary parts on the heatsink at this point (including TR6 sensor) in order the thermal feedback to work properly.

7) Carefully rotating the pot adjust the quiescent current. Give it some time to settle, adjust again (several times) - see if it is stable during up to half an hour.

If you have got an oscilloscope - keep it connected to the output all the time in order to watch for possible oscillations and other artifacts.

Cheers,
Valery
Many thanks for the very detailed explanation. Before I received your reply I tried removing speaker wires and observed the heat of the driver transistors. i noticed both drivers are cold. So final conclution is positive driver get hot when the speaker is connected.

Anyhow i am now going to remove the FET s and try to follw the steps you mention. But one clarificatin I need sir. Could you please tell me how to connect the FET with A meter in series.

Thanks waiting for your valuable reply
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Old 22nd June 2014, 10:02 AM   #7
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Quote:
Originally Posted by vzaichenko View Post
Yes, the second thread with exactly the same question looks strange to me also.

Anyway, I would approach it the following way:

1) Remove the output MOSFETs. Don't connect any load. With no output transistors and no load the circuit will balance itself anyway and we will see if everything is fine or not relatively safely;

2) Set VR1 pot to the maximum R position.

3) Connect the V-meter (DC) to the output (R17 / R15 junction);

4) Now switch the power on. You would see something close to zero (less than 100 mV) at the output. If you see higher DC - something is wrong. Solve the problem until you see less than 100mV;

5) Assuming you've got "0" DC at the output, connect the V-meter between the emitters of Q2 and Q3. As you carefully rotate VR1 pot, voltage has to change from almost zero to a number of volts (up to 5-8 or so). Do it slowly and watch out for the drivers' temperature. If nothing happens - something is wrong with your circuit.

6) If everything is fine, set the VR1 pot back to the maximum R value, connect the MOSFETs and connect the A-meter serially with the MOSFETs, let say, the N ones (top shoulder). I would put all the necessary parts on the heatsink at this point (including TR6 sensor) in order the thermal feedback to work properly.

7) Carefully rotating the pot adjust the quiescent current. Give it some time to settle, adjust again (several times) - see if it is stable during up to half an hour.

If you have got an oscilloscope - keep it connected to the output all the time in order to watch for possible oscillations and other artifacts.

Cheers,
Valery
dear Valery,
I removed the FET and set the VR1 to maxiumum level. Then I checked the DC voltage accrossed the Gate Resistor which is 1K as you mention. But I could observed 0 to 0.4 voltage rise when turn the VR1. not up to 5-8 volts as you mention. Why is this ?.

However my amp is working only thing is the positive driver get hot when conect to a speaker. So what should i do. I did not go to fit FET s again as i did not get the expecting voltages ( 5-8 V) as you mention. Please help me
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Old 22nd June 2014, 10:13 AM   #8
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Quote:
Originally Posted by shehantha View Post
dear Valery,
I removed the FET and set the VR1 to maxiumum level. Then I checked the DC voltage accrossed the Gate Resistor which is 1K as you mention. But I could observed 0 to 0.4 voltage rise when turn the VR1. not up to 5-8 volts as you mention. Why is this ?.

However my amp is working only thing is the positive driver get hot when conect to a speaker. So what should i do. I did not go to fit FET s again as i did not get the expecting voltages ( 5-8 V) as you mention. Please help me
Can you please check the voltage across every R15 and R17 resistor separately. The one connected to the hot transistor will have a higher voltage across it. What are the values?
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Old 22nd June 2014, 10:39 AM   #9
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Quote:
Originally Posted by vzaichenko View Post
Can you please check the voltage across every R15 and R17 resistor separately. The one connected to the hot transistor will have a higher voltage across it. What are the values?
Thanks, I checked the values and got around 100 mV (I am using analog meter ) for both R15 and R17. so both are same . However with fitted FETs positive driver do not get hot if theres no speaker connection. Shall I connect FET and speaker wire and then check the voltage accross R15 and R17 Sir ?
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Old 22nd June 2014, 10:47 AM   #10
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Quote:
Originally Posted by shehantha View Post
Thanks, I checked the values and got around 100 mV (I am using analog meter ) for both R15 and R17. so both are same . However with fitted FETs positive driver do not get hot if theres no speaker connection. Shall I connect FET and speaker wire and then check the voltage accross R15 and R17 Sir ?
Another question - are you sure you have got zero DC voltage at the output?

If yes - ok, pls try to connect the fets and see what the voltages across R15 and R17 show. Then (if still zero DC at the output) connect the load and measure again.
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