audioguru2
- Apr 6, 2004
- 12,026
- Joined
- Apr 6, 2004
- Messages
- 12,026
Hi Guys,
In another post we have discussed why this part of a project doesn't work at all, or produces severe distortion and noise.
This audio amplifier's input is near line level, and its output load is an earphone, 8 ohms or more, from a cell phone, home stereo or Walkman.
Why doesn't this 741 amplifier circuit work?
1) The 741's inputs are connected to its negative supply voltage, so they won't work properly:
Common Mode Voltage Limits
One of the practical op-amp limitations is that the inputs must stay within a certain voltage range (usually significanly less than the supply voltages) for proper operation. Op-amps are subject to drastic gain changes and bizarre behavior if these ranges are exceeded. For a 741 operating on +/-15 volts, the common mode input swing should be within +/-12 volts. Some other amps are designed to allow common mode voltages down to the negative supply voltage.
Maybe this 741 amplifier will work better if its negative supply pin 4 is connected to -9V, instead of to ground, then the grounded inputs will properly be within the common mode voltage limits.
2) No negative feedback. The 741 has a typical voltage gain of 200,000. Without an input signal, it will amplify its up to 6mV of input offset voltage so that its output will be resting near a supply voltage, instead of its output being biased near half-supply voltage. Therefore when an input signal is applied, its output will half-wave rectify it, causing severe distortion.
The high gain will also amplify the high internal noise level of the 741, so that its output will be full of noise.
3) 8 ohm load? That is like a dead short to a 741 that is spec'd for a minimum of a 2K load. A 741 is guaranteed to supply only 10mA of output current. Therefore its peak power to an 8 ohm earphone is only 800 microwatts, which is an extremely low sound level.
The faulty circuit that makes me sick to look at it is here:
View attachment 35946
In another post we have discussed why this part of a project doesn't work at all, or produces severe distortion and noise.
This audio amplifier's input is near line level, and its output load is an earphone, 8 ohms or more, from a cell phone, home stereo or Walkman.
Why doesn't this 741 amplifier circuit work?
1) The 741's inputs are connected to its negative supply voltage, so they won't work properly:
Common Mode Voltage Limits
One of the practical op-amp limitations is that the inputs must stay within a certain voltage range (usually significanly less than the supply voltages) for proper operation. Op-amps are subject to drastic gain changes and bizarre behavior if these ranges are exceeded. For a 741 operating on +/-15 volts, the common mode input swing should be within +/-12 volts. Some other amps are designed to allow common mode voltages down to the negative supply voltage.
Maybe this 741 amplifier will work better if its negative supply pin 4 is connected to -9V, instead of to ground, then the grounded inputs will properly be within the common mode voltage limits.
2) No negative feedback. The 741 has a typical voltage gain of 200,000. Without an input signal, it will amplify its up to 6mV of input offset voltage so that its output will be resting near a supply voltage, instead of its output being biased near half-supply voltage. Therefore when an input signal is applied, its output will half-wave rectify it, causing severe distortion.
The high gain will also amplify the high internal noise level of the 741, so that its output will be full of noise.
3) 8 ohm load? That is like a dead short to a 741 that is spec'd for a minimum of a 2K load. A 741 is guaranteed to supply only 10mA of output current. Therefore its peak power to an 8 ohm earphone is only 800 microwatts, which is an extremely low sound level.
The faulty circuit that makes me sick to look at it is here:
View attachment 35946
Last edited by a moderator:
