Forrest Mims Audio amplifier circuit does not work..

[chriswilliams]

I have armed on breadboard the audio Amplifier circuit that this in
page 122 of the book Getting Started in Electronics of Forrest M. Mims
but for some reason it does not work.

Already double checked components and connections but for some reason
it does not work.
Someone that already have armed and could orient me on the reason why
it does not work.

Do you believe that this circuit does not work if is armed on
breadboard

Thanks in advance for any comment.

 

[Michael Black]

I have armed on breadboard the audio Amplifier circuit that this in
page 122 of the book Getting Started in Electronics of Forrest M. Mims
but for some reason it does not work.

Already double checked components and connections but for some reason
it does not work.
Someone that already have armed and could orient me on the reason why
it does not work.

Do you believe that this circuit does not work if is armed on
breadboard

Thanks in advance for any comment.

It looks fine.

Have you looked up the datasheets for the op-amp and the 386 amplifier IC,
to make sure the pinouts he gives are correct?

Remove the first stage, and apply some audio through a coupling capacitor
to the input of the 386. For that matter, just lift the pot and touch
the input pin of the 386, and you should hear some hum from the AC line
surrounding us all.

Do you have the electrolytics placed the right way, so the polarity is
correct?

Do you have the opamp, the 741, running off two 9 volt batteries, or
did you think that meant to run it from a 9v battery with the - terminal
connected to ground? That preamp won't work as is unless it has a
dual-voltage supply, ie two 9volt batteries, one supplying the +9volts
to the +v terminal of the op-amp (and the negative side of the battery going
to ground), the other battery supplying the -9volts to the -v terminal of
the opamp (and the plus side of that battery going to ground). If you
don't have that, the preamp is either not going to work, or barely work.

When tracking down something like this, it's always best to simplify,
figuring out one stage at a time, then adding the other stage and
seeing that they work together.

Michael

 

[phaeton]

Yes, what Michael said.

FWIW, i think the general consensus about Forrest Mimms books is that
all the circuits are verified to work over and over again. (Anyone can
correct me on this)

I have troubles like you mention too. It's always because I've screwed
something up, or (in very very rare cases) I have a dead component.

I have built that circuit you have there, and IIRC it took me a couple
of tries. Sometimes when I can't see the problem I pull everything out
of the breadboard and start over. It's a shotgun approach that doesn't
teach you anything, but 50% of the time the circuit works.

good luck

-phaeton

 

[[email protected]]

Yes, what Michael said.

FWIW, i think the general consensus about Forrest Mimms books is that
all the circuits are verified to work over and over again. (Anyone can
correct me on this)

I have troubles like you mention too. It's always because I've screwed
something up, or (in very very rare cases) I have a dead component.

I have built that circuit you have there, and IIRC it took me a couple
of tries. Sometimes when I can't see the problem I pull everything out
of the breadboard and start over. It's a shotgun approach that doesn't
teach you anything, but 50% of the time the circuit works.

good luck

-phaeton

 

[[email protected]]

Chris,

Sorry about the problem. I'm away from the workbench for a while and
will check the circuit when I have a chance.

All circuits in "Getting Started in Electronics" were built and tested
at least 3 times. So you might try checking the ICs since your wiring
is apparently OK.

Best regards,

Forrest

Forrest M. Mims III
www.forrestmims.org

 

[Eric R Snow]

Chris,

Sorry about the problem. I'm away from the workbench for a while and
will check the circuit when I have a chance.

All circuits in "Getting Started in Electronics" were built and tested
at least 3 times. So you might try checking the ICs since your wiring
is apparently OK.

Best regards,

Forrest

Forrest M. Mims III
www.forrestmims.org

Oh Man! I can't believe Mr Mims himself responded! Greetings Forrest,
I have built many circuits from your books and have learned much of
what I know about electronics from books I bought at Radio Shack.
Thanks a bunch for writing 'em. I still even use "Getting Started in
Electronics" from time to time when I've forgotten something simple
and am trying to fix something or build something. The 555 timer book
is another good one. I didn't know you had a web site. I'll have to
check it out.
Cheers,
Eric R Snow

 

[[email protected]]

Eric,

Thanks for the kind words.

I stop by sci.electronics from time to time and will continue doing so.

I'm totally busy doing science these days and writing a new book about
the 50th anniversary of Hawaii's Mauna Loa Observatory. So I don't have
time to help design projects.

Please visit the nonprofit THE CITIZEN SCIENTIST when you have a chance
(www.sas.org/tcs). I serve as editor and am looking for some good
electronics projects that have science themes or applications.

Best regards,

Forrest

Forrest M. Mims III
www.forrestmims.org

 

[Matt J. McCullar]

I have armed on breadboard the audio Amplifier circuit that this in
page 122 of the book Getting Started in Electronics of Forrest M. Mims
but for some reason it does not work.

Already double checked components and connections but for some reason
it does not work.
Someone that already have armed and could orient me on the reason why
it does not work.

Let's see here... (pulls the book off the shelf)

Ah. This is an audio amplifier using two chips: a 741 op-amp as a pre-amp,
and a 386 as the power amp.

I remember this circuit; not only does it work as drawn, it works VERY well!
I recall using it many years ago with a hand-held magnetic microphone and it
was LOUD. Better than being good, it was LOUD!

I've got a lot of experience using solderless breadboards (great
invention!). If you can take a digital photograph of your circuit and can
post it on a web site so the rest of us can look at it, perhaps we can point
out the trouble.

I can testify to this: the 741 op-amp is a dual-polarity device. In this
case, that means that it requires both +9 volts AND -9 volts. Negative 9
volts does NOT mean ground. That's why this circuit requires two 9-volt
batteries.

The 386 is a very versatile audio amp and it can be configured in a great
many ways to change how much gain (volume) you'll get out of it. Bychance,
are you using some capacitors with values quite different from the listed
values? That can make a difference.

What is your input audio source? Are you using a microphone, a CD player
output, or what?

Matt J. McCullar, KJ5BA
Arlington, TX

 

[phaeton]

Forrest M. Mims III
www.forrestmims.org

Forrest,

If I may take a moment outside of someone's thread, and if you are *the*
Forrest Mims III (you never know on the Internet these days).



Thankyou. Thankyou for everything you've done and written. I wouldn't be
here if it weren't for you and your writing style. You're a man among men.


-phaeton

 

[Eric R Snow]

Eric,

Thanks for the kind words.

I stop by sci.electronics from time to time and will continue doing so.

I'm totally busy doing science these days and writing a new book about
the 50th anniversary of Hawaii's Mauna Loa Observatory. So I don't have
time to help design projects.

Please visit the nonprofit THE CITIZEN SCIENTIST when you have a chance
(www.sas.org/tcs). I serve as editor and am looking for some good
electronics projects that have science themes or applications.

Best regards,

Forrest

Forrest M. Mims III
www.forrestmims.org

I will. Thanks for the reply.
Eric R Snow

 

[chriswilliams]

Thanks to all for your answers. I guess that the problem is with the
dual voltage supply and, probably, the integrated circuit pins. I
will check. Thanks to Mr. F. Mims and excuses by using his name.