Need a easy to build signal amplifier

[Chisvasi Sebastian]

Hello, i need a easy to build signal amplifier (i'm a begginer), not a
complex one, my problem is that the input signal is below a millivolt
and i need to raise it to at least a copple of volts (tenths of volts
will be perfect) anyway the output signal is going to the mic jack of
my soundcard (because i intend to use it like an osciloscope).

Thanks ...

 

[John Popelish]

Hello, i need a easy to build signal amplifier (i'm a begginer), not a
complex one, my problem is that the input signal is below a millivolt
and i need to raise it to at least a copple of volts (tenths of volts
will be perfect) anyway the output signal is going to the mic jack of
my soundcard (because i intend to use it like an osciloscope).

Thanks ...

This is an ordinary job for opamps. These are universal signal
amplifiers that have two inputs (labeled + and -) and an output. The
opamp has a very high differential gain (it amplifies the difference
voltage between the two inputs) while using almost no current through
these inputs. When the + input is more positive than the - input, the
output heads positive.

You program an opamp for what ever gain you want with a few
resistors. It can produce a positive or negative output for a
positive input (called noninverting or positive gain versus inverting
or negative gain). The input voltages and output voltage are
constrained to stay within the supply voltages.

For a basic tutorial on these cheap and useful components and lots of
circuit ideas, see:

http://www.national.com/appinfo/amps/0,2175,967,00.html

 

[Allen Bong]

This site is cool! I really enjoy it. Thanks

Allen

 

[Robert Monsen]

I'd avoid the 741 op-amp. It's an ancient op-amp with quirks that can
be frustrating unless you know how its internals work. Radio Shack
sells a dual JFET input op-amp that's good enough for most uses. Some
fancy op-amps are specially made for audio use (low distortion open loop
gain).

Op-amps are generic term for a generic device that has a differential
input of nearly infinite gain. The circuits using them rarely depend on
a specific type. Use any basic circuit and any good op-amp.

Although the OP doesn't specify his needs very clearly, its clear that he is
working with audio. Are most run of the mill opamps fast enough to handle
high audio without distortion?

Thanks,
Bob Monsen

 

[Roger Johansson]

Hello, i need a easy to build signal amplifier (i'm a begginer), not a
complex one, my problem is that the input signal is below a millivolt

What is the signal source?

and i need to raise it to at least a copple of volts (tenths of volts
will be perfect) anyway the output signal is going to the mic jack of
my soundcard (because i intend to use it like an osciloscope).

The mic input is already made for very weak signals, below and above
1mV, so maybe you do not need any amplifier, just use the volume
control on the mic channel in the computer.

If you really need an amplifier you should know that amplification is
not the main problem with such weak signals, an op-amp can deliver a
lot of amplification, keeping unwanted noise out is the big problem.

You should look for microphone pre-amplifiers for good low noise
designs.

You need a low noise op-amp, like the 5534/5532, and you need to be
very careful with how you arrange the input cable, shielding of the
circuit, etc..

For such weak signals you need to shield the signal source, the cable,
the pre-amp, and the cable into the mic input.
Coaxial cable and metal boxes for the circuits, battery driven or use
the voltage output available in the mic jack.
There are 3 contacts on a mic connection on a sound card, ground,
input, and 5-9Volt output which is used to drive an electret mic.

That voltage output can you use to drive your pre-amp, if you do not
need so much current. Otherwise use a battery, and shield the battery
too.

If you want to use the sound card as an oscilloscope there are
problems with using the mic input, by the way. It often has an
automatic gain control and that will make the gain adjust
automatically, which means you cannot rely on the values you get.

If you want to use the sound card as an oscilloscope you should use
the line input instead, and then you really need a pre-amp for such
weak signals.

Your best option to begin with is to use the mic input without any
pre-amp, and see what results you get.
Check the soundcard docs to see if you can disable the automatic gain
control. If not you cannot trust the values you get on the strength of
the signal, but maybe it is not so important.

If you need to measure the signal strength, and the automatic gain
control cannot be disabled on the mic input you need to build a
sensitive mic pre-amp and use the line input instead.

 

[Bill Sloman]

Chrisvasi Sebastian wrote:

" Hello, i need a easy to build signal amplifier (i'm a begginer), not a
Complex one, my problem is that the input signal is below a millivolt
and i need to raise it to at least a copple of volts (tenths of volts
will be perfect) anyway the output signal is going to the mic jack of
my soundcard (because i intend to use it like an osciloscope)."

To which Kevin McMurtrie indirectly replied"

" I'd avoid the 741 op-amp. It's an ancient op-amp with quirks that can
be frustrating unless you know how its internals work. Radio Shack
sells a dual JFET input op-amp that's good enough for most uses. Some
fancy op-amps are specially made for audio use (low distortion open loop
gain)."

"Op-amps are generic term for a generic device that has a differential
input of nearly infinite gain. The circuits using them rarely depend on
a specific type. Use any basic circuit and any good op-amp."

Kevin -

I'd be interested in what quirks in particular you experienced with the
741. While it's true that the 741 has been around for awhile (I think it was
a follow up of the 709), it is still made because it is such a good,
reliable general purpose device. While I wouldn't use it for precision
instrumentation, it is admirably suited for the purpose intended in
Chrisvasi's original post. I think you would be amazed at how much military
and Fortune 500 equipment the 741 wound up being designed around. Would I
use it today for non critical applications? You betcha!

The LM324 quad and the LM358 dual op amps are cheaper, more compact,
just as crummy and work as single supply op amps. Using a 741 is pure
lazyness.

The guys who design for the military had an excuse - it takes a long
time to qualify an integrated circuit for high reliability military
applications, and their choice of op amps used to be limited.

Fortune 500 equipment represents a different problem, not unlike high
end audio, where the customer wants something idiosyncratic to
indicate their unique capacity to throw money down the drain, and that
market is too small to pay for good design - whence loads of 741's.

 

[Bill Sloman]

Bill -

Laziness is not exactly how I would characterize it. I think hermetic
sealing and reliability may have just had a little something to do with it

For military applications, and the occasional harsh industrial
environment, but even there, using the 741 is generally a cop-out.
These days there are always better amplifiers for any specific job.

 

[Zak]

Although the OP doesn't specify his needs very clearly, its clear that he is
working with audio. Are most run of the mill opamps fast enough to handle
high audio without distortion?

Well, the LM324 for example isn't.


Thomas