Op-Amp for VGND Suggestions

[No-Talent-Wanker]

Hi, I am usually a lurker here because I can usually trouble shoot and solve my own issues due to the fact that the projects I work on are fairly simple, low parts count affairs that don't strain my simple, low parts count brain.

However, I have run into a situation where I could use some advice. Let me explain...

Right now, my breadboard power supply is basically a 12 volt or 9v wall wart plugged into this circuit. I also have a breadboard with a +5v supply that comes from a YwRobot 545043 breadboard power supply (It also has +3v but I have never used it). And then there's the 2x9v batteries hooked up for bi-polar stuff.

The wall warts irritatingly eat up space on my workbench power strip and if I add another breadboard (like I want to do), it will get worse. So, I looked around for bi-polar power supplies... Ouch!.

Something you need to know about me is that I am frugal - err, OK, I'm a tight wad that squeaks when I walk (no, really - I DO squeak when I walk). So, here is what I am building:



The "SC305" in that schemo refers to this 5 Volt, 2A,+12 Volt 0.8A,-12 Volt, 0.4 A Ault SC305 Power Supply. These are salvaged parts which are the same thing as these power supplies sold new on eBay but w/o the enclosure. Just to be clear, I have no association with MPJA other than being a long-time customer and they are in the same state.

This will give me the following voltages:

GND

+5v (regulated)

VGND  (-4.5v)
-9v (regulated)
+9v (regulated)

VGND (-6v)
-12v (un-regulated)
+12v (un-regulated)

The plan is to have all these voltages and ground come out to header pin buses - 4 pins per voltage.

Now, I know there is a limited amount of current available, but I don't plan on powering a large pedalboard or modular synth with it - this is just for a breadboard ps and I mostly stick to smallish projects.

Where I could use advice pertains to the "VFB Opamp-Based Virtual Ground Driver" circuit from this page (about half-way down) which is essentially what I have drawn but with filtering added.

I have 741's and TL071's on hand but since the supply already has a limited amount of current it can provide, I figured I would look into the LMH6642/AD817/"high output current" single op-amps mentioned in the tangentsoft link above.

I did find the LMH6642 and AD817 at Jameco but the AD817 in through hole are $4.15 each  and the LMH6642 was surface mount only (not as terrible at $1.75 ea. tho...). Since any circuit I commit to after breadboarding that uses one of these VGNDs needs one of these op-amps too - that can add up real fast.

Did I mention I was a tight wad?

I have tried searching for a "high output current" single op-amp and equivalent op-amps but cross referencing the results with what the merchants I normally shop at keep in inventory is a daunting task. Forgive me for being lazy, but spending countless hours sifting through hundreds upon hundreds of op-amp datasheets and then trying to find a candidate in a vender's catalog is not my idea of fun.

If it weren't for the fact that I am waiting on the 7909's to come all the way from China, I would have already popped in TL071's and called it a day. But since I'm sitting on my hands at the moment, and there are some very knowledgeable members here, I figured it couldn't hurt to ask...

Can someone suggest a "high output current", single-sided supply, single op-amp that will fill the bill better than the TL071 that is readily sourced and priced more reasonably than $4.00 a chip?

Thanx

[R.G.]

One very simple, low cost, and low parts count virtual ground driver is the LM386.

Connected across 9V with no input signal at all, it balances its output at half the power supply. It comes in an 8-pin DIP, and is cheap. It also can drive a few hundred milliamperes into an 8-ohm load, so it's capable of high currents for an 8-pin DIP, and only uses 4ma for itself.

[No-Talent-Wanker]

  Well, now I feel dumb (well, dumber than usual). I already have those but always thought of them as a mini amp. $0.25 a chip sure beats $4 a chip, that's for sure.

I searched TI for the phrase "high output current op-amp" and got over 400 results but didn't see the anything commonly used in audio circuits. Then again, I was a bit overwhelmed and didn't search to far or long...

What is the difference between a "high output current" op-amp and a "low power" op-amp any way? I am still a bit confused about that and why the author would suggest such an expensive chip - must have deep pockets.

Thanks R.G., I'll try the 386's. And, while I am at it, I would like to take the opportunity to thank you for the trouble shooting documents you have provided. They have helped me out on more occasions than I can count.

[R.G.]

The world is a place that makes for a lot of forehead slapping.    It was pure accident that I noticed that the LM386 would self balance to the middle of the power supply and thought "hey, that would be a great bias generator..."

Opamps are a specialized amplifier. They long ago got off into the stratosphere with accuracy and other specializations. You really have to work with them a lot and also ferret details out of the datasheets to figure out what kind and type would work better for a situation. A "low power" opamp is generally designed to have a very low idle current so they can be used in low power circuits. They are optimized inside for very low power usage, and the implications of that are usually that the output transistors won't do much current, and that the output stage is biased very close to class B to avoid standing currents. So the crossover distortion is bigger for audio use (which may not matter at all if it's not used for audio) and the lowest load it can drive may be 4K to 10K. "Low power" means "we got low power by compromising on other fancy stuff."

"High output current" opamp means that "this baby can start your car; well, we tried anyway". The optimization has been in the direction of making the output transistors BIG so it can shovel out lots of current. This may or may not have required compromising away other things, like DC accuracy, common mode range, idle power and so on.

In general, when a class of opamps is named for something, it means that other stuff is compromised. That includes cost. An opamp that does it all, high speed, high accuracy, low idle current, high output current, wide frequency response, high power, etc. generally means "high cost" too. 

"Low cost" is relative. I'm on the other side of that hump. I have limited time, so trading some money for time is good for me, at least in the difference between $0.25 and $4.00. On the other hand, I also do all my parts orders sorted by lowest cost, too. Buy what you need, but educate your self so you can know what the minimum you need is, and work up from there.

You're very welcome to any help I've provided. It's payback for the people who helped me.