Op Amp Circuits and boards

Some Op Amp Gain Circuits

One of those simple electronic things that is useful: get some gain circuits going using op amps, for whatever you need to apply a gain stage or buffer to.

Dual attenuator/dual buffer
Quad gain/buffer for old LFO circuit EFM LFO5A
Simple Quad Audio 210-Mixer
Small Op Amp Inverter PCB

GainBrain Repair Buffer Board

This started when I was trying to fix an old dynamics processor with 2 attenuator stages and 2 gain/buffer stages. A power cap had leaked and blown the crap up out of some of the adjacent input buffer circuitry. It was easier to just start over on the buffering vs. fixing a bunch of burned traces and desoldering parts....

Board layout looks like this:

Here is the schematic and the board. I created gerbers and sent this off to EASYEDA in China to have some boards made.

Worked first time! How about that? If you want to make some of these yourself gerber zip file is here. I tried to make the board flexible, so it's hopefully useful for more than just this repair--if you know anything about simple op amp circuits, you can AC couple betweens stages, stick a pot in negative feedback to have adjustable gain, make the 2nd stage a give you more boost by changing resistors in feedback loop, etc. This board tested working for me.

As audio electronics go, this is pretty easy stuff but for newbies here is a good youtube from afrotechmods about op amp gain etc., in fact IMO watch anything you can by afrotechmods, his tutorials are easy to follow, informative, and accurate.

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I needed an VCLFO for a studio a friend was building, so instead of just buying one (too easy right?) I dug out a really old LFO board I had lying around in my junkbox, from a now-defunct outfit called "Electronics for Musicians".

Wait, EFM? Who are they and where do I get this stuff? Not so fast partner....This outfit was apparently run by a single lone wolf tech dude named Tom Gamble.....Fitting name because if and when you got your boards from EFM was a gamble?

I remember grumbling about vanished paypal funds to EFM back in the day (2005?)

. Sad to say as far as I can tell Tom and EFM have vanished without a trace. can't even find him on social media? Tom and his outfit was like a girlfriend you complain about, But when she leaves for good you miss her? Tom had an eye for simplicity, low parts count, and in his day cranked out the designs.

so anyway here I am 12 years later and I still have a few unbuilt boards from EFM. LFO5A one of them. Time to put it to work.

And if you ever followed EFM at all, the EFM LFO5A board has all the EFM issues: egregious chip frying trace errors, BOM is incorrect, outputs are all over the map as far as P/P voltages and so on.

To correct all of the P/P output issues on the LFO5A board I downloaded this Dave Kendell mod schematic to correct, as part of the invaluable forum page for the LFO. (the original forum is gone, this is a copy)

This is built for the LFO5A as a daughterboard but overall the design is generic and modular enough that I figure it can used for reasonable audio or DC gain up/gain down or buffering solutions as they come up.

For this one here is the board:

Here is the schematic:

OK, for EAGLE users, board is here and schematic is here. I made gerbers and sent them off the China....we'll see if it works.

UPDATE!It works! EFMLFO5A works great with the addition of this gain board. If you want to get some fabbed for yourself, gerber file is here....I had another project where I needed to use a few of the gain stages, so I modified a few resistor values, left about half the board unpopulated, slapped the board on with some 90 angle clips, and we were good to go.

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Model 210 Mixer

I was hired to build a custom mixer for a small installation (this was quite a while ago) and there were a few requirements: the mixer had to be small and have both fixed and variable outs on 2 jacks. the metal work required that there only be 5 ports (1/4 inch?) for the following: 4 inputs, 1 fixed output, one variable output. what to do? Here is what I came up with. The thing I remember being a tad tricky (for me anyway) is that port 4 is variable as input and adjustable output, and a dual pot was used since I could only fit 4 pots in a very limited space. I don't recall doing a PCB for this, it was all built on perf. Hand drawn schematic (?) is here. I lost this documentation for a very long time, having recently dug it up. Oh yeh the mixer used a pair of Burr Brown OP627's which cost a whole lot but were very nice.

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Smallish OpAmp Inverter

1001 uses? Small (uses only through-hole, sorry!) and flexible PCB I hope. unlike non inverting op amp gain stages, Inverting can be configured to reduce gain as well as increase within the closed loop, also quickee AC or DC mixer front end: put more 100K resistors in parallel with R1 and connect one side each to the inverting input "2" on the op amp. Then, put a second one of these PCB's in series to re-invert the inverted signal after the mixer stage and set overall gain. (Why you might ask do I sometimes put 2 resistors in series in the closed loop of these sort of circuits? Usually because I need a certain gain value and don't have a single resistor on hand that works so i put 2 in series instead. That's the good thing about the new world of super cheap PCB fab--you can do this any way you want!)

Here's the schematic:

Here's the board:

Eagle files are here; schematic PDF is here; board PDF is here; gerbers here.

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