# Audio OpAmp Gain values

Discussion in 'Electronic Basics' started by [email protected], Oct 26, 2005.

1. ### Guest

Hi all,

I hope no one will mind if I jump in and ask a bit of a beginner's
question.

The circuit below is from the first stage of an audio distribution switch.
Input will be 1V P-P over the audio range of 20-20K, but I am not well
schooled, and do not know how to determine the values of the two
resistors, the feedback, and the divider.

Given the input values at 20K, how do I calculate the values for the other
resistor pair in order to create a unity gain stage? I just want to put in
the 1V P-P and get 1V P-P out.

I know its rather a lame question for folks here who are wiser than I, but
I am just getting started, the boss gave me the box and said the first
stage gain is wrong, so I would like to fix it up quick, and learn
something in the process.

Thanks a lot for your tolerance and patience

Mark

___
|---|___|--|
| |
20K | +12V |
___ | |
-|___|-+ | |\| |
|--+----|-\ |
| >--+---
20K +-----|+/
-|___|---+ |/|
| MC33078
| -12V
.-.
| |
| |
'-'
|
|
===
GND
(created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)

20K!

John

3. ### ChrisGuest

Hi, Mark. Welcome, the water's fine. This is where electronics newbie
questions belong (unless it's somebody in an undergrad or tech college
course who doesn't want to take the time to do their homework, in which
case they'd better bring 20 zloty). Oh, and by the way, more knowledge
doesn't necessarily mean more wisdom ;-)

I took the liberty of putting labels on your resistors and input and
output voltages above.

Your circuit is usually called a "difference amplifier", or diff amp.
For the case where R1 = R3 and R2 = R4 (which is usually the case),
then

V(o) = R2 / R1 * (V2 - V1)

If you plug in your resistor value of 20K, you can see it would only
work with a gain of 1 (R2 / R1 = 1) when your mystery resistors are
also 20K, as Mr. Larkin said.

http://www.national.com/an/AN/AN-31.pdf

The National Semiconductor appnote AN-31, "Op Amp Circuit Collection",
is a great emergency cheat sheet to keep on your workbench. Your diff
amp is on the first page, along with both the equation where R1 = R3
and the more complicated equation for when they don't. It covers a lot
of the basic op amp circuits, providing one typical schematic, and one
equation (if necessary) for each circuit. This may be of great help to
someone like yourself who doesn't do circuit design for a living, but
will occasionally have to replace component values to tweak something
or just wants to figure out what an op amp configuration is doing.

Good luck in your education
Chris

4. ### Guest

My great thanks to John and Chris for the answer to the question, and the
help on how to figure it out myself in the future.

Sure appreciate you both taking the time to bail me out and head me in the
right direction. I was guessing 20K, but not knowing what kind of
amp/buffer I was seeing, I just wasn't happy with just a guess.

Much obliged for your assistance.

Best Regards,

Mark

5. ### Paul BlitzGuest

If its of any help to you, I've designed a load of simple circiuts for use
at Hospital Radios. One of these is a 1:4 audio distribution amplifier. The
whole article is at:
http://home.btconnect.com/woodgreen/tech_tips/techtip/art10.htm. If you need
stereo, you build 2 of them. If you need fewer or more outputs, delete or
add extra output buffers!

You'll find both circuits and description there.

If you go up a level, to
http://home.btconnect.com/woodgreen/tech_tips/techtip/art_ind.htm, you'll
see the list of other bits I've designed.

(I have no problems with individuals using any of my circuits, but they may
NOT be used for commercial purposes without contacting me!)

Hope this helps!

Paul Blitz

6. ### John LarkinGuest

Incidentally, you can't prevent (at least in the US) anyone from using
a circuit you designed, unless it's patented. All a copyright does is
prevent them from using the literal image that you publish.

And a 2N7000 is *not* a Fetlington (which was a fet-bipolar hybrid,
now obsolete.)

John

8. ### John LarkinGuest

Unfortunately, you're in a bad mood, again.

John