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OP275 to drive low impedance network?
OP275 to drive low impedance network?
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Old 29th October 2017, 04:30 PM   #1
Fast Eddie D is offline Fast Eddie D  United States
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Default OP275 to drive low impedance network?

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.
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Old 30th October 2017, 02:21 AM   #2
PRR is offline PRR  United States
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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?
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Old 30th October 2017, 01:57 PM   #3
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by PRR View Post

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.
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Old 31st October 2017, 04:39 PM   #4
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by PRR View Post

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.
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Old 1st November 2017, 05:27 AM   #5
ammel68 is offline ammel68  United States
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OP275 to drive low impedance network?
Quote:
Originally Posted by Fast Eddie D View Post
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.
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Old 1st November 2017, 09:39 AM   #6
rmaudio is offline rmaudio  United Kingdom
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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 ?

Last edited by rmaudio; 1st November 2017 at 09:41 AM.
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Old 1st November 2017, 09:47 AM   #7
Mooly is offline Mooly  United Kingdom
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OP275 to drive low impedance network?
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 ?
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Old 1st November 2017, 06:26 PM   #8
Fast Eddie D is offline Fast Eddie D  United States
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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.

Quote:
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.

Quote:
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.

Quote:
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.

Quote:
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.

Quote:
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.

Last edited by Fast Eddie D; 1st November 2017 at 06:38 PM.
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Old 1st November 2017, 08:09 PM   #9
Mooly is offline Mooly  United Kingdom
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OP275 to drive low impedance network?
Tough luck on the job

The wandering offset sounds odd, and yes, definitely could be a sign of instability. I take it the 5532 has no such issue. Lol, you need a scope
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Old 1st November 2017, 08:23 PM   #10
rayma is offline rayma  United States
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Quote:
Originally Posted by Fast Eddie D View Post
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.

Last edited by rayma; 1st November 2017 at 08:25 PM.
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