Audio OpAmp Gain values

20K!

John

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.

Please download

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

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

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

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:

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

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

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

John

--
Unfortunately, you\'re wrong in both cases.

A patent, or a copyright, is only a means for recourse, not a means
of prevention.