OP275 to drive low impedance network?

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I'm designing a continuously adjustable "loudness" (Fletcher-Munson) control. It's just a buffer driving a passive network, and a buffer on the output. The network is just a scaled down version of the old Yamaha loudness circuit - a few resistors and a couple of capacitors.

The impedance of the network drops to around 2.5 kOhms in the midrange (approximately 1 kHz - 4 kHz). If I use an OPA2134 for either buffer, it will oscillate. If I use a 5532, it is rock solid. This is the first time I have encountered a circuit where the 5532 and 2134 were not functionally interchangeable for a prototype circuit, except for offset voltage.

I was looking at other op amps and thought about the OP275. OP275GPZ Analog Devices Inc. | Integrated Circuits (ICs) | DigiKey I've heard of it but never used one. Looks like a logical choice to drive a low impedance network. It seems like there's some pros and cons highlighted in the internet chatter, so I thought I'd get some straight dope here.

I'm building it all on a very small board. The whole board will hang off the back of the panel mount potentiometer, with just one connector. I built a volume/balance control with onboard voltage amplifier (2134 works without a hitch here) the same way and it's very compact and works excellent (uses 100K Blue Velvet pot :)). I like building these modular circuits because they can be switched around in a hurry and are super easy to replicate (once I get it right ;)).

So thanks for sharing your comments and experiences.
 

PRR

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Would be nice to see the actual circuit.

If you can draw an all-caps path through the load, most opamps will get unhappy; '5532 marginally less than some others (but still may be on the edge of screaming, so not keeping its mind on your audio).

Put 100 Ohms in series between the opamp (and its NFB) and the load being driven. Does that settle it?
 
Put 100 Ohms in series between the opamp (and its NFB) and the load being driven. Does that settle it?

I tried that. It doesn't work.

5532 has always been the better choice when driving cables. I've used both chips in active filter circuits without a problem. I was surprised when the 2134 fell apart driving a passive RC network.

I will get a couple of these chips and see what's up.
 
If you can draw an all-caps path through the load, most opamps will get unhappy; '5532 marginally less than some others (but still may be on the edge of screaming, so not keeping its mind on your audio).

There is not an all-caps path through the load. The passband load impedance is defined by two series resistors, total resistance 2.5K. I thought this was good enough to keep me out of trouble.

Commercial applications all use a similar basic design as mine; a buffer driving a passive network driving another buffer. But commercial designs use a separate board for the potentiometer and network, which has no opamps on the board. All buffers are on main board. This is how my prototype was configured. I tweaked it a lot before committing to building it. And now it's throwing me a curve.

This is the second time I've revisited this concept. First time I gave up and thought about it for a couple months before trying again. It can't be that hard. I've spent two weeks spinning my wheels and at some point must move on again.

Commercial designs use 4556 and devices I don't know about. I never used 4556 for anything. Part of my trouble is that I didn't do this stuff for over 30 years and now I'm back. I first designed line level circuitry using the CA3140, which was super expensive, cranky, and too easy to damage with ESD. Nevertheless I did earn the praise of the guys that taught me (who thought only tubes could be used in audio). They thought I was a real maverick but shut their pieholes when they auditioned my preamp with their custom built tube equipment. I came back to this vocation when I saw how awesome the modern parts were - parts we dreamed about in school were now available for peanuts. So I have a lot of catching up to do.

By the way, thanks for commenting.
 
I never used 4556 for anything. So I have a lot of catching up to do.

Yeah, it sounds like you do have a lot of catching up to do.

The JRC 4556, though not the greatest sounding option, has been used for years as a reliable, bipolar dual op-amp with 70 plus mA of output current per channel.

There are better sounding single op-amps available with as much or more output current, but most are CFAs.
 
As PRR says - it would be good to see a schematic. It does sound odd that it doesn't work but I don't think we'll be able to make much progress without seeing the circuit.
Agree that the opamps are usually interchangeable as long as power requirements are met and 5532 dc offsets acceptable.
One thought - what is the bypass cap scheme / values ?
 
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I'd be curious to see a diagram as well.

2.5k isn't a low impedance load in the grand scheme of things tbh, even a TLO72 would be more than happy driving that. This sounds more like a problem related to layout or even the OpAmps having a suspect provenance......

Are we talking rail to rail oscillation here, or just a hint of something furry on top of the signal ? What is the frequency of oscillation ?
 
I will draw up schematics of where I was and where I think I'm going. I just got my first "smartphone" (I'm a dinosaur) and will see if I can snap some pics and post. This will surely require some fumbling.

I'm so blind that I have a very hard time with phones and cameras. I used to be a good photographer but I just don't see well enough any more. In fact I just got turned down for a job because I couldn't pass the physical - my eyesight is too poor. They wanted to hire me too. And this breaks my heart - if I can't work I'm in trouble. My mind still works but the rest of me not so much.

2.5k isn't a low impedance load in the grand scheme of things tbh, even a TLO72 would be more than happy driving that.

I thought about that during the design process. I thought I was on safe ground.

This sounds more like a problem related to layout or even the OpAmps having a suspect provenance......

Same thing I always do with 100% success so far. That doesn't mean it doesn't need improving.

Are we talking rail to rail oscillation here, or just a hint of something furry on top of the signal ? What is the frequency of oscillation ?


Don't know the frequency because I don't have a working scope right now. I do have an oscillation detector circuit that uses a germanium diode rectifier and an op amp buffer. Plus the DC offset drifts about 400 mV when measured on a VOM, a sign of oscillation. And it's just a little "fuzz" not rail to rail.

Yeah, it sounds like you do have a lot of catching up to do.

The JRC 4556, though not the greatest sounding option, has been used for years as a reliable, bipolar dual op-amp with 70 plus mA of output current per channel.

I know about the 4556 and why it's used. I work on equipment and a lot of consumer grade equipment is stuffed with them.

This is why I'm asking about the 275. I would use the 4556 but want something better.

There are better sounding single op-amps available with as much or more output current, but most are CFAs.

I know. I've spent a lot of quality time browsing datasheets looking for something I can use. I avoid smd but did successfully design and build a circuit with a couple of smd parts - only because I couldn't find satisfactory legacy (through hole) parts.
 
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I'm so blind that I have a very hard time with phones and cameras. I used to be a good photographer
but I just don't see well enough any more. In fact I just got turned down for a job because
I couldn't pass the physical - my eyesight is too poor. They wanted to hire me too. And this
breaks my heart - if I can't work I'm in trouble.

Have you contacted your local low vision support organizations? There are many in your area.
You also can use a large screen WiFi tablet as a phone via Google.
 
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Have you contacted your local low vision support organizations? There are many in your area.

I never heard of such a thing. I'll look into it.

Maybe they can help me get a job. People want to hire me. They know I can still work and have a lot to offer. But corporate eggheads don't want the liability of employing an over 50 person with less than perfect health. Or so I'm told anyway.

You also can use a large screen WiFi tablet as a phone via Google.

Thanks.
 
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Sucky. One of my best developers has 10% vision. He has a guide dog and 2 very large monitors on the desk and does a great job. And I get a dog to pet in the office. His web pages are always compliant to visibility requirements as well, which saves a lot of rework.

Fingers crossed someone does pick you up soon.
 
Thanks to all.

It's lawyers and insurance companies that call the shots in the boardroom now. The last "real" job I had was in 2009. It's been hustle hustle since then.

This legal environment has skewed employment opportunities for many deserving candidates. This isn't the first time something like this has happened to me.

I hope it's OK to say that my country is in a very dark place now. (No pun intended.)
 

PRR

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I am well past 50. I was not seeing too well. Because of another problem, I didn't attend to it. When I was better, I went "for new glasses" and the eye-guy sent me right over to the cataract guy. I am now SO MUCH better; better than new (always was some near-sighted). I hope that is your problem, and that you can get funded to fix it. I do know many other less solvable eye problems (my friend left nose-prints on his monitor; another friend flirts with macular degeneration).

As for that circuit: fat crayon, big sheet paper. Get the local kid to snap a picture and post it somewhere, tell you the "URL" or any breadcrumb trail so as we can find it.
 
I already took some pictures with my new cellphone. Most did not come out very well. I fumbled around but was unable to upload any. It is very frustrating and embarrassing. I do not like asking anyone for any help either. I am fighting to remain as independent as possible.

I do need to go to the doctor, but I haven't even had insurance for 4 years now. Don't tell me about Obamacare - it would cost about 80% of my income. It's a sad pathetic joke. What I need is a job with insurance. And it's hard to get a job when your health is obviously deteriorated - not only am I Mr. Magoo, but I have two shot knees (obvious when I walk), a hernia the size of a baseball (I already had four fixed when I was a first class citizen with real insurance), a bad ankle, a bad shoulder, and I found out I'm pre diabetic (which would explain my weight going up and down so much). So I'm in that hard place. I need a job to get insurance, and I'm too much of a wreck to get a job. I'm not exaggerating when I say that people LAUGH IN MY FACE when I go for an interview. It's quite humiliating.
 
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I do need to go to the doctor, but I haven't even had insurance for 4 years now.

Sorry. I do think that PRR could be right. Cataracts can be completely "cured" by replacing
the natural lens with a plastic lens. You should be checked out by an ophthalmologist soon.
Maybe your Skokie low vision group will have some suggestions. Medicaid might be an option for you.
 
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