Biasing amp

[leander]

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.

[leander]

This is the amp

[leander]

here

[AndrewT]

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.

[Jan Dupont]

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

[Upupa Epops]

Value of some device must be uncorrect. Control again all resistors value and connection of biasing transistor...

[janneman]

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

[leander]

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 ?

[anatech]

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

[janneman]

Originally posted by leander
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 ?

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