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Old 8th March 2013, 01:19 PM   #21
agdr is offline agdr  United States
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Quote:
Originally Posted by weslito View Post
How on earth did you figure that one out?
Yeah, that one is obvious, isn't it? Sorry! I'll shutup.
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Old 8th March 2013, 07:13 PM   #22
weslito is offline weslito  United Kingdom
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Quote:
Originally Posted by agdr View Post
Yeah, that one is obvious, isn't it? Sorry! I'll shutup.
The 40.2 K resistors are in parallel with the input bias current, not in series. Have I missed something?
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Old 10th March 2013, 08:27 PM   #23
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Quote:
Originally Posted by agdr View Post
I'm guessing one problem may have been his 40.2K return resistors. With 4 op amps rather than two that input bias current would double and the voltage drop across the 40.2k would double. The DC offset of the O2 may have gone up to 6mV.
Thought about this some more, as I was sure I was just being dumb.
I was rather foolishly thinking about it from the perspective of the current being sourced from the pot/coupling cap.
Got it now. Double the bias current flowing out through the 40.2K = double the DC voltage at non-inverting input = worse offset. Does that sound about right?

Edit: cocked up my maths.
4 * 180nA typ. input bias current over 40.2K = 28.9mV. I see what you mean now, that's rather large.

Last edited by weslito; 10th March 2013 at 08:41 PM. Reason: Extra dumbness.
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Old 10th March 2013, 10:18 PM   #24
agdr is offline agdr  United States
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weslito - sorry, I mis-read your post. Easy to do with text. Most of my posts are geared towards helping people just learning electronics and tend to be on the fundamental side. The forum here has many true audio pros posting and some real audio legends. Occasionally someone misses the intended audience and I get swatted.

The input bias current is what is flowing into (or out of) the input terminals to the op amp. The same amount may or may not flow through both the inverting and non-inverting pins depending on op amp design, but for sake of discussion I'll assume both are the same here. In the case of the NJM4556A chips in the O2 the data sheet shows a typical value of 50nA (0.05uA), with a max about 10 times that. Given the coupling capacitor in the O2 design, all of that current has to flow through the 40.2K ground return resistor since it has nowhere else to go (or come from).

So with two NJM4556A inputs tied together that would be a total of (2)(50nA) = 100nA = 0.1uA into (or out of) the two chip non-inverting inputs and through the one 40.2k resistor, for a voltage drop of 40200 ohms * 0.0000001 amps = 4mV. Multiplied by the 1x (current buffer) closed loop voltage gain of those stages that is still 4mV offset reflected to the op amp output.

The O2's I've built typically have 3mV of DC offset, so the result is in the ballpark. In addition there is the inherent DC input offset of the NJM4556A, which is listed at 0.5mV typical and 6mV maximum, times the closed loop voltage gain of 1 (buffer).

In one of the mods I posted in the O2 mod thread I cut the feedback PC traces (output to non-inverting input) on two of the NJM4556As and soldered in 40.2K resistors in the feedback loop to cancel out that input bias offset. That works since that same input bias current flows out of (or in to) both op amp inputs (assumption from above, which may or may not be valid). Adding a resistor in the feedback path of the same amount produces the same voltage drop, but now will get a polarity change at the op amp's differential input and cancel in the sum with the non-inverting input of the same DC voltage amount.

When I actually tried and measured it the result was the output offset voltage of that channel of the O2 amp dropped by about half, from 3mV to 1.5mV. I took the difference to be the input offset voltage of the NJM4556A or a difference in inverting and on-inverting bias currents. But as I write this I'm thinking I may have made a mistake on that one. Since the two halves of the NJM4556A are in parallel, and double the input current is flowing through that external 40.2K resistor to ground, the nulling resistors in the feeback loop of each op amp half may actually need to be 80.4K (82K standard value) to null properly when summed back together. Hmmm.... I'll have to ponder that one.

Last edited by agdr; 10th March 2013 at 10:26 PM.
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Old 10th March 2013, 10:46 PM   #25
weslito is offline weslito  United Kingdom
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Yeah, that makes sense. Presumably the offset nulling effects of the feedback resistor would come at the price of rather high Johnson noise. 40.2K seems quite a large value to use there. 3mV offset isn't very harmful, is it worth the trade off?

Another question, if you don't mind me picking your brains. The 'typical' and 'max' input bias current often vary by quite a large margin, as you point out. Are the bias currents of non-inverting and inverting within the same device usually fairly well matched? If not, your feedback resistor would have to be carefully (and painfully) matched.
Also what about the two sides of a dual opamp (like the NJM4556)?

Last edited by weslito; 10th March 2013 at 10:50 PM.
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Old 10th March 2013, 10:50 PM   #26
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Actually, thinking about it, the 40.2K to ground on the non-inverting input. Would that also contribute it's own Johnson noise?
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Old 11th March 2013, 12:09 AM   #27
agdr is offline agdr  United States
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weslito - good questions! I'm still stuggling with the Johnson noise stuff myself with this ODA project so I'm not going to have any useful answers, unfortunately. In general though you are probably right, the resistor in the feedback loop (especially one that large) will probably be a trade-off of adding some Johnson noise in return for reduced output DC offset.

One of the slick things RocketScientist/NwAvGuy did with the O2 design was arrange things so that the pot and the large(r) 40.2K resistor were at the input of the stage with just 1x closed loop voltage gain. By putting more resistance at the input of a voltage gain stage, as with my ODA design here, any noise gets multiplied by the gain of the stage, up to 6x in this case.

But then again it may turn out that Johnson noise isn't the predominate source of noise in a particular design. I've been kind of amazed so far that I'm not getting more noise with the pot up front. In fact I just placed an order with Mouser for a 50K pot and intend to swap it out with the 5K and see what I can hear.

The data sheet for the OPA627 has an interesting graph on pp. 4 of source resistance vs. voltage noise.

http://www.ti.com/lit/ds/symlink/opa627.pdf (opens pdf)

The inflection point in op amp noise vs. source resistor noise comes at about 10k.
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Old 11th March 2013, 04:52 PM   #28
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Very interesting, never noticed that graph before. It goes on to say "above a 2kΩ source resistance, the op amp contributes little additional noise. Below 1kΩ, op amp noise dominates over the resistor noise".

This can be seen on the graph also, as 2kOhms is the point at which resistor only noise is 4.5nV/√Hz; equal to that of the amplifier.

So as you say, 10K (it's actually ~8K if you zoom in on it) is the point at which the op amp contributes no considerable noise of it's own.

Up at 40K it's about 26nV/√Hz. Is it possible to convert this into say, dBV, for a given frequency? I've no idea how you'd go about that.
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Old 11th March 2013, 06:31 PM   #29
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See Noise in Audio Amplifiers.
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Old 12th March 2013, 02:11 AM   #30
agdr is offline agdr  United States
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xnor - good link! Rod Elliot has has great stuff on that site. His power supply page here in section 5

Small Power Supplies

leaves little doubt that the power supply I have laid out for this project is utterly ridiculous in terms of noise removal overkill.

RocketScientist said he measured the O2 noise floor with his dScope and said the LM7x12s didn't make any difference. I know that opc has written the same about the LM3x7s and his Wire amp with the AP measurements. So me adding the LT regulators here is essentially just $15 in the wastebasket. But I've had a pair of them here for awhile and have wanted to try them out, and this is DIY audio after all (!), so in they go! But back OT, that is a helpful noise info page.

weslito - I haven't worked that part of the math out yet. I'm going to just swap in the 50K pot first and see if I hear background noise show up. If not I'll probably futz with the numbers a bit to figure out why. At 50K I should be well up on that "resistor over amp noise" curve. But even there it seems max noise will be at the middle of pot range. Lower in the range, which is where I usually run the pot due to "hot" sources, and both the signal and noise from the other half of the pot should get attenuated by the voltage divider. Or at least that is how it appears, I may have that mucked up.

Being able to use a higher resistance pot like that, if excessive noise doesn't show up, would make fuller use of the FET input on the OPA627. I've added a 500R series resistor too between the op amp and the pot wiper to help lower the DC change as the pot it turned from one end to the other vs. the parallel resistance combo of the feedback network out the inverting input.

Last edited by agdr; 12th March 2013 at 02:19 AM.
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