Audio amplifier circuit does not work as expected

[CHINGUETAS]

Someone that already have armed and made work the audio amplifier
circuit of the book Getting Started in Electronics by Forrest Mims , p.
122.

Already I have made the following revisions:

1) Dual voltage supply checked
2) IC pins 741,386 checked (wrong 386)
3) Value components checked
4) Wiring checked

After this now I hear a strong ¡hummm¡ but not sounds from the
microphone.

Is this what I should hear or what I should hear?
Something missing?
Thanks in advance for any comment

 

[petrus bitbyter]

Someone that already have armed and made work the audio amplifier
circuit of the book Getting Started in Electronics by Forrest Mims , p.
122.

Already I have made the following revisions:

1) Dual voltage supply checked
2) IC pins 741,386 checked (wrong 386)
3) Value components checked
4) Wiring checked

After this now I hear a strong ¡hummm¡ but not sounds from the
microphone.

Is this what I should hear or what I should hear?
Something missing?
Thanks in advance for any comment

Mm... Don't have the book just like many others so you'd better provide a
copy of the design for serious conversation. Meanwhile, the hum may be
50/100Hz or 60/120Hz from the power supply when not carfully smoothed. Other
possibilities are lang power lines, especially when not decoupled correctly,
long (unshielded) signal lines, bad solder joints, incorrect grounding and
may be some other things I forgot. If you touch the input with a finger the
hum should increase, otherwise the amplifier does not work at all. May
indicate an open in the signal path.

petrus bitbyter

 

[Michael Black]

Hmm, lets see, i don't think i have ESP..
i just can't seem to see how it would or
would not work!.
maybe if you would supply us a schematic of what your
using it could help greatly!

Well he did say what book it was from. And this is his second
thread on it, it would have been better if he'd just kept
to the same thread, and Forest Mims himself made a visit to the
newsgroup to say there were no errors in the schematic in the book.

Michael

 

[Jamie]

Someone that already have armed and made work the audio amplifier
circuit of the book Getting Started in Electronics by Forrest Mims , p.
122.

Already I have made the following revisions:

1) Dual voltage supply checked
2) IC pins 741,386 checked (wrong 386)
3) Value components checked
4) Wiring checked

After this now I hear a strong ¡hummm¡ but not sounds from the
microphone.

Is this what I should hear or what I should hear?
Something missing?
Thanks in advance for any comment

Hmm, lets see, i don't think i have ESP..
i just can't seem to see how it would or
would not work!.
maybe if you would supply us a schematic of what your
using it could help greatly!

 

[David Harmon]

On Mon, 17 Jul 2006 22:31:43 +0200 in sci.electronics.basics,

Mm... Don't have the book just like many others so you'd better provide a
copy of the design for serious conversation.

Schematic at http://i2.tinypic.com/20a4c2d.gif

 

[petrus bitbyter]

On Mon, 17 Jul 2006 22:31:43 +0200 in sci.electronics.basics,


Schematic at http://i2.tinypic.com/20a4c2d.gif

Well, no big problems in the design.
Are the +9V and the -9V equal? When using 9V batteries, the +9V will have a
heavy load compared with the -9V. A difference of some tenths of a volt may
rule out the 386. (The offset of the 741 may do the same when amplification
is set too high.) You can check by measuring the output of the 741. The DC
voltage should be around 0V. +/- 0.1V may be acceptable, +/- 0.5V is fatal.
Is the microphone a dynamic type? Electret micophones may work well but the
ones I know, require to be powered.
Once more, you should hear the mains hum when touching in input of the
amplifier (both sides of the input capacitor.)
You should hear some hum also when touching the input (pin 3) of the 386. If
not, disconnect the volume potmeter from the output of the 741. If still no
hum, your 386 fails. (Blown 386, wrong or defective components, wrong
wiring, bad solder joints etc.)

petrus bitbyter

 

[me]

From what I can see, the problem is operator error...

 

[ehsjr]

Someone that already have armed and made work the audio amplifier
circuit of the book Getting Started in Electronics by Forrest Mims , p.
122.

Already I have made the following revisions:

1) Dual voltage supply checked
2) IC pins 741,386 checked (wrong 386)
3) Value components checked
4) Wiring checked

After this now I hear a strong ¡hummm¡ but not sounds from the
microphone.

Is this what I should hear or what I should hear?
Something missing?
Thanks in advance for any comment

Make sure you take ground from the junction of the two
batteries. And make sure it is a dynamic mike - an electret
requires power and the circuit does not provide it.

Ed

 

[Skeptic]

I see two problems with the design. The first is that the only DC
connection I see between the +9 V, -9 V and ground is through the 386 amp.
The ground should be connected to the junction of the two nine volt
batteries.

The second problem I see is that output of the 741 is referenced to 0 volts
because the noninverting input is tied to ground. This means that the
output will go above and below 0 volts. Since that output is DC coupled to
the 386 amp, the input will be dropping below the negative supply voltage (0
VDC) and will cause severe distortion.


jgreimer