How to set bias for this amplifier ?

Status
Not open for further replies.
shehantha

shehantha

Disabled Account
Joined 2014
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.
 

Attachments

  • c1.gif
    c1.gif
    57.7 KB · Views: 534
  • c2.gif
    c2.gif
    52 KB · Views: 517
  • c-audio-sr-series-amplifier-schematics.pdf
    c-audio-sr-series-amplifier-schematics.pdf
    127 KB · Views: 159
shehantha said:
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.
Click to expand...

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:
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.
 
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
 
vzaichenko said:
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
Click to expand...
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
 
vzaichenko said:
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
Click to expand...
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
 
shehantha said:
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
Click to expand...

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?
 
vzaichenko said:
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?
Click to expand...
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 ?
 
shehantha said:
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 ?
Click to expand...

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.
 
vzaichenko said:
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.
Click to expand...
I remember before I remove the FETs I checked the DC offset it showed about 150 or 200 mV not Zero. I cannot check exactly as i use analog meter. Is this ok. Also my protection circuit also works well and relay on
 
150-200 mV is a little bit higher than normal, but not critically high.
So go ahead with the measurements with the FETS - with no load first, and then with the load.
 
Many thanks for helping me. I fitted the fet s and rechecked the voltage with out load , result is same. then i connect the speaker and checked. Negative side 2sa1943 dirver emiter resistor ( R15) gave me now about 50mV ( half of previous value ) and positive side R17 gave me about 1.3 V . What should I do now Sir ?
 
shehantha said:
Many thanks for helping me. I fitted the fet s and rechecked the voltage with out load , result is same. then i connect the speaker and checked. Negative side 2sa1943 dirver emiter resistor ( R15) gave me now about 50mV ( half of previous value ) and positive side R17 gave me about 1.3 V . What should I do now Sir ?
Click to expand...

Actually, it looks very much like oscillation. 1.3V over 330 ohm gives us 3.9mA emitter current. Your powerful driver can't get hot with it.

Most likely, as soon as you connect the load, the amp starts oscillating at some MHz frequencies - you can't see it with the meter.

I'm afraid, without oscilloscope we can't really see the picture...
It will be possible to play with compensation caps, but you need to see the HF behavior on the screen.
 
many thanks for the help Sir. I will try to find a oscilloscope and try to find the issue. You are really helpfull for me and I learnt much from you today.
 
Status
Not open for further replies.
Back
Top