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help with op amp headphone amp hum and distortion
help with op amp headphone amp hum and distortion
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Old 17th June 2021, 06:38 PM   #121
Rick PA Stadel is offline Rick PA Stadel  United States
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Join Date: Apr 2017
Location: greater Kansas City
OK, great! Thanks Am relieved that they're a lot closer than I was worried they might be.

Wish I was quicker getting my part done -- still working.

Have you had a chance to check out LT Spice? It's a heck of a thing to be available free! But the learning curve can require some patience. Maybe try not to do the dumb thing I did and overlook the many examples that come with. Fiddling those is a lot better place to start than going straight into building expertise in schematic entry.

Last edited by Rick PA Stadel; 17th June 2021 at 06:40 PM.
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Old 17th June 2021, 08:39 PM   #122
gregas is offline gregas  Canada
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Location: Toronto
Hey Rick,
Glad to hear the results are close. I haven't had a chance to try the program.
I have a 17 month old so as you can imagine free time is sparse.
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Old 18th June 2021, 02:49 AM   #123
Rick PA Stadel is offline Rick PA Stadel  United States
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Join Date: Apr 2017
Location: greater Kansas City
Wow -- a 17-month-old! That is such an adorable age, but boy can I imagine limited free time -- pretty sure I didn't have 10 or 15 hobby minutes a month for the whole first 3 or 4 years! Nothing but baths, bottle making, diaper changes and laundry, laundry, laundry ..

Cheers

Last edited by Rick PA Stadel; 18th June 2021 at 02:55 AM.
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Old 21st June 2021, 02:52 AM   #124
gregas is offline gregas  Canada
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Yes. He's very adorable. Can't get enough of the little guy.

How's your calculating going? Will there be minor changes?
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Old 21st June 2021, 07:05 AM   #125
Rick PA Stadel is offline Rick PA Stadel  United States
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Join Date: Apr 2017
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Sorry, yes, I'm afraid so ..

It's looking more and more like my original objective to keep parts count and complexity to a minimum may have been unwise.



Will get after it in earnest again tomorrow.
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Old 24th June 2021, 08:02 AM   #126
Rick PA Stadel is offline Rick PA Stadel  United States
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Join Date: Apr 2017
Location: greater Kansas City
Hey gregas,

Guess I better go ahead and post these -- maybe some of the much-sharper-than-me folks on here will see the flaw in my thinking. I've been going around in circles for hours now, fixing one shortfall but causing another, can't remember what the original *supposed clever insight* was, and still haven't yet gotten to what was supposed to have been the *real point* of the exercise -- letting the simulator push the pencil!

What seemed like an obvious, simple exercise to simulate the AD817's decreasing input current with increasing temperature -- 3,75uA at 0 C to 2,75uA at 40 C -- and simplifying to linear over that range -- has not revealed itself to me, short of building a model. Of course that would be a worthwhile pursuit. But not something I'm capable of doing on a short timescale. Maybe someone on here knows how to implement a table of temp:current value-pairs for one of LT Spice's built-in current sources, but I haven't found a way to do that, either.

There ARE numerous, popular, nominally uncomplicated solutions to remedy this issue. But they'd require additional op-amps or other active circuitry -- nothing so simple as a diode and a few resistors.

Best practical suggestion would be to use it as is, adjusted for your typical ambient temperatures according to the season. Then be sure to have the headphones unplugged at power-up and power-down.

Or cave, at least temporarily, and fit the output coupling capacitor suggested by other members (which offers the best protection for the headphones, too).

Sorry for the limited joy,
Cheers
Attached Images
File Type: jpg AD817 temp compens attempt V3.jpg (208.9 KB, 62 views)
File Type: jpg AD817 temp compens attempt V3b.jpg (208.0 KB, 61 views)
File Type: jpg AD817 temp compens attempt V3c.jpg (216.0 KB, 64 views)
File Type: jpg AD817 temp compens attempt V3d.jpg (233.3 KB, 62 views)
Attached Files
File Type: zip AD817 temp compens 3uA input offset V3d.zip (14.7 KB, 0 views)

Last edited by Rick PA Stadel; 24th June 2021 at 08:06 AM.
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Old 25th June 2021, 02:03 AM   #127
Rick PA Stadel is offline Rick PA Stadel  United States
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Join Date: Apr 2017
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Oh, forgot to mention -- the zip is the complete collection of work-product that LT Spice generates for all 4 of those variations .. in case anybody's interested.
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Old 25th June 2021, 04:22 AM   #128
gregas is offline gregas  Canada
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Hey Rick

Thanks for all your help with this project.
I was hoping there was a simple fix to get our temp compensation circuit to behave.

Not sure if anyone else will chime in here.
It's been you and me for a while now.

So if I'm to adjust with the current setup for dc offset.
I let it warm up and adjust for lowest dc voltage on both channels.
What is considered ok? At what point would it not be ok? Possible damage to headphones?
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Old 27th June 2021, 06:08 AM   #129
Rick PA Stadel is offline Rick PA Stadel  United States
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Join Date: Apr 2017
Location: greater Kansas City
You're very welcome!

I don't have a good answer for 'how much is too much', especially for 'modern' headphones. Probably something in the 10 to 15 mV range should not cause trouble.

As for the procedure ..
- headphones unplugged
- cover on but unfastened, unit in normal operating orientation
- warm-up for 6 to 8 minutes
- attach the DMM black probe to headphone ground
- remove the cover, flip unit if necessary, make a quick adjustment -- less than 1 minute
- replace the cover, wait another 2 or 3 minutes, repeat for other channel

We'll want to aim for a setting that produces lowest maximum value, of the greater of the two absolute values. If one channel is +12mV, for example, and the other is -5mV, turn the trimpot CCW until their absolute values approximately match.

I'd say if it isn't possible to get both channels less than about |25|mV, a large electrolytic might be a wise temporary precaution -- especially if your headphones are expensive. If the caps end up retired sometime later, and your electronics hobby continues, you might be surprised how handy a couple of 1000uF/35V capacitors lying about can be.

Am still working on 'the problem', by the way -- but it'll probably be a couple/few weeks before there's any joy.

Regards

Last edited by Rick PA Stadel; 27th June 2021 at 06:33 AM.
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Old 27th June 2021, 05:23 PM   #130
gregas is offline gregas  Canada
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Hey Rick,
Thanks for the detailed explination.
I'll adjust and report back.
I did order bipolar caps for the output.
If i can't keep the offset low. I'll see about installing them.

Where does the dc voltage come from? Shouldn't the input caps
Remove dc?
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