Hi,
I am having problems biasing my amp. What ever I try voltage drop across emitter resistors (R33 - R40) remain 0v if there is no signal at the input. This is what I am doing, pls correct me if i`m wrong:
1)Short signal input to ground
2)disconnect speaker from output
3)set the voltmeter to dc millivolts reading
4) Read voltage across one of the emmiter resistors (R33 - R40)
5) Vary R22 preset from maximum resistance to lower resistance
Voltage across emmiter resistors does not vary and remain 0.000v . How is this prossible?
Note: If a signal is given to the amp and a speaker is connected, it sounds good.
I am having problems biasing my amp. What ever I try voltage drop across emitter resistors (R33 - R40) remain 0v if there is no signal at the input. This is what I am doing, pls correct me if i`m wrong:
1)Short signal input to ground
2)disconnect speaker from output
3)set the voltmeter to dc millivolts reading
4) Read voltage across one of the emmiter resistors (R33 - R40)
5) Vary R22 preset from maximum resistance to lower resistance
Voltage across emmiter resistors does not vary and remain 0.000v . How is this prossible?
Note: If a signal is given to the amp and a speaker is connected, it sounds good.
Hi,
measure the voltage across C10 when VR22 is bottom, middle & top of it's adjustment range.
Disconnect and/or remove D3 & D4.
Repeat measurements across C10.
measure the voltage across C10 when VR22 is bottom, middle & top of it's adjustment range.
Disconnect and/or remove D3 & D4.
Repeat measurements across C10.
Have you tried measure the bias on R31 and R31.
I don't know this particular amp, but some amps run in Class B on the output devices, while the driver stage handles the low outputs 😉
I don't know this particular amp, but some amps run in Class B on the output devices, while the driver stage handles the low outputs 😉
Value of some device must be uncorrect. Control again all resistors value and connection of biasing transistor...
Measuring the bias across C10 is the quick way to really know, rather than haphazardly changing things in the blind. It should be above 2.5V for proper biasing (at least 4 Vbe junctions). If it is too low, increase R21 until you have a good adjustment range. remember that the voltage across R22, R23 total is always max .6V
What are the current values of R21, 22, 23?
Jan Didden
What are the current values of R21, 22, 23?
Jan Didden
voltage across R24 1.17v
voltage across C10 2.06v (varies when turning preset R22) (1v - 2.3v)
By the way something that I forgot to mention before is that I have included another pair of output transistors to the original circuit so it now has 5 npn and 5 pnp. Maybe that is the little alteration that is making the difference.
what do you think ?
voltage across C10 2.06v (varies when turning preset R22) (1v - 2.3v)
By the way something that I forgot to mention before is that I have included another pair of output transistors to the original circuit so it now has 5 npn and 5 pnp. Maybe that is the little alteration that is making the difference.
what do you think ?
Hi leander,
You need to increase the value of R23 to get into the proper adjustment range. Adding the extra outputs will not affect this.
-Chris
You need to increase the value of R23 to get into the proper adjustment range. Adding the extra outputs will not affect this.
-Chris
leander said:
Count the B-E junctions across C10: 2 x driver, 2 x output (this is independent of the # of parallel outputs. They are in parallel, so their Vbe is also in parallel, and parallel voltages stay the same. Making a baby allways takes 9 months, irrespective of the number of women assigned to the task😉 ).
So, total 4 junctions x .6V is at a minimum 2.4V. You have not enough biasing voltage. I think you must increase R21 (IIRC the diagram).
Jan Didden
Hi Leander,
increase R21 not R23.
Try about 50% larger.
Your voltages across C10 are varying as expected so you do not need to remove D3 & D4.
If you increase R23 the range of adjustment of the bias voltage will be reduced and may be too small to be useful.
increase R21 not R23.
Try about 50% larger.
Your voltages across C10 are varying as expected so you do not need to remove D3 & D4.
If you increase R23 the range of adjustment of the bias voltage will be reduced and may be too small to be useful.
Hi guys,
Got me on that one, I went the wrong way for sure! That should get you going leander.
Thanks Jan & Andrew.
-Chris
Got me on that one, I went the wrong way for sure! That should get you going leander.
Thanks Jan & Andrew.
-Chris
Hi,
changing to a smaller R23 will increase the total bias voltage and increase the range of adjustment.
Many bias circuits choose resistor values that use an extremely small current and this leads to modulation of the bias voltage due to base current demands nearing the resistor string current levels. So reducing R23 will alleviate this slightly but with out component values I cannot tell. I suggest you aim for resistor string currents about 10 times the Vbe base current and all the remaining VAS current passing through Vbe transistor. Check the gain of Vbe to determine the base current at this level of bias current.
changing to a smaller R23 will increase the total bias voltage and increase the range of adjustment.
Many bias circuits choose resistor values that use an extremely small current and this leads to modulation of the bias voltage due to base current demands nearing the resistor string current levels. So reducing R23 will alleviate this slightly but with out component values I cannot tell. I suggest you aim for resistor string currents about 10 times the Vbe base current and all the remaining VAS current passing through Vbe transistor. Check the gain of Vbe to determine the base current at this level of bias current.
Hi Leander,
Ampslab have chosen nice low values for the resistor string.
At max bias voltage the string = 390+150=540r. max string current is about 4*0.65/540 =4.8mA & min current=3.5mA.
if base current is = 0.35mA then max Vbe transistor current is about 40 * 0.35 =14mA. I think the red LED will give about 10mA to 12mA through VAS therefore Vbe current is about 12-3.5 =8.5mA < 14mA therefore OK.
Ampslab have chosen nice low values for the resistor string.
At max bias voltage the string = 390+150=540r. max string current is about 4*0.65/540 =4.8mA & min current=3.5mA.
if base current is = 0.35mA then max Vbe transistor current is about 40 * 0.35 =14mA. I think the red LED will give about 10mA to 12mA through VAS therefore Vbe current is about 12-3.5 =8.5mA < 14mA therefore OK.
I changed both R23 and R21
changed R23 from 150 to 50
changed R21 from 390 to 500
The amp is biasing now and is working fine😀
Thanks for your help ppl 😎
Am I supposed to hear any difference now that the amp is biased? , Because I did not heard any difference when playing music at low volume , the only difference is that the power transistors ar warmer at idle state.
How should I set the bias, when the amp is cold or when it is hot ?
reg
Leander
changed R23 from 150 to 50
changed R21 from 390 to 500
The amp is biasing now and is working fine😀
Thanks for your help ppl 😎
Am I supposed to hear any difference now that the amp is biased? , Because I did not heard any difference when playing music at low volume , the only difference is that the power transistors ar warmer at idle state.
How should I set the bias, when the amp is cold or when it is hot ?
reg
Leander
Hi,
set the bias cold and monitor it over a range of temperatures.
If it moves significantly when warm then the thermal compensation is not working too well. Consider resetting bias when warm.
Try to arrange for the minimum bias (at any temperature) to be equal to the min bias quoted by Ampslab.
Adequate (correct) bias should reduce crossover distortion. You would tend to notice this more at low volume or quiet passages between louder parts. It also tends to sound worse on the high frequency components in the spectrum. Hi hats, SSS on voice, strings etc.
set the bias cold and monitor it over a range of temperatures.
If it moves significantly when warm then the thermal compensation is not working too well. Consider resetting bias when warm.
Try to arrange for the minimum bias (at any temperature) to be equal to the min bias quoted by Ampslab.
Adequate (correct) bias should reduce crossover distortion. You would tend to notice this more at low volume or quiet passages between louder parts. It also tends to sound worse on the high frequency components in the spectrum. Hi hats, SSS on voice, strings etc.
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