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Old 5th May 2007, 12:32 PM   #1
AndrewT is offline AndrewT  Scotland
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Question 3 stage LIN topology - NFB tappings?

Hi,
Doug Self makes a big play on where to tap off the upper leg of the NFB from the output. But I cannot recall anyone suggesting where best to tap off both the upper leg of the NFB and the lower leg of the NFB.

When the Amplifier is drawn in the same format as a balanced input to unbalanced output, it becomes obvious that the output leg could tap off the speaker terminal (but before the stabilising Thiele network) or as Dr Cherry suggests between the Zobel RC, but where should the lower leg go to?

All the way back to the source? then NFB compensates for the interconnecting cable?
or locate the lower leg resistor at the source component (and the source resistor controls the amp gain, =input 3),
or tapping into the RCA ground at the input to the amplfier casing (= input 1),
or as conventionally done, just to the PCB signal ground point? (=input 2)

Which is likely to give best results and which may be adversely affected by long wires?
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Old 25th September 2008, 10:29 AM   #2
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Or maybe this?:
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Old 25th September 2008, 11:05 AM   #3
AndrewT is offline AndrewT  Scotland
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where does the input return current go to?
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Old 25th September 2008, 01:18 PM   #4
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Well, I didn't figure that part out.

The point of my (above) post is that any resistor sturdy enough to last more than a month on the zobel is also the same sort that would put infinite gain on HF noise if used for the hi leg of the NFB. So, I thought that also using one of those (exact same model) for the lo leg of the NFB might cancel that potential problem.

Sorry that the response took a bit. I model via soldering iron, because "applied" is the only thing that makes sense to me.

And, here's a guess on the input area. Once again that sturdy power resistor was a problem for me as it couldn't provide a sufficient protection against HF noise if used for the input ground, so here's (photo below) the homely bus ground system that results from chasing this little issue about.

This (pictured below) is obviously wrong. The return line from the load is pack full of AC (audio that we just amplified).
If that 8R load involves a speaker with a crossover involved (an even worse noise on the speaker return line), I'd like to show load/speaker return line connecting to an offboard power supply (which is, hopefully, the only location of caps large enough to mash the unwelcome signal into DC, so we don't mistakenly amplify it)--but I didn't know how to show that.

Is there not some way to employ a diode to protect the amplifier against the return signal from the speaker? I didn't know how to do that either.
But, if our power supply is either regulated or switcher, then it probably won't have the 6000uF (minimum) resource to neutralize the speaker's very powerful return signals. Without that resource (or "something" to do that same job), the bass response is awful. I wouldn't put large capacitance at the amplifier board because that's yet another way to make a noise. Howabout a diode to neutralize the load/speaker return signal? Help?
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Old 25th September 2008, 02:55 PM   #5
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Andrew, don't forget that the impedance to gnd at the + node will be different from that at the - node. This will cause the input lines to be unbalanced form an impedance point of view.
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Old 25th September 2008, 04:38 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi T,
it is not a balanced input. It is only drawn to look like that for the sake of clarity so formumites can see the circuits are identical (except Cherry a/b)
What I am asking is which if any are better for an NFB topology and whether any of them are bad.
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Old 25th September 2008, 04:45 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi Daniel,
the gobblede gook has lost me again.
But back to your last posted diagram.
Show the input ground attached to the left side of C2+R2.
Move the signal ground connection from the far end of R6+C1 to between R3 & R6.
You now have a working circuit that looks like input 2 but with the power ground connected to signal ground with a 12r resistor just like Leach and many others (except you show the Cherry/b NFB point).
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Old 25th September 2008, 04:51 PM   #8
Mooly is offline Mooly  United Kingdom
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Hi Andrew,
Going back to post#1 I followed Dougs work at the time but there were aspects I couldn't "reconcile" and I think your asking the same question.
I found it helped to just think of it as a DC problem, but the real issue was "what happens when the input two grounds of a stereo pair" are connected together, wherever that may be.
Does working it as DC problem help if you put in nominal resistance values for all the wiring. I found it did
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Old 25th September 2008, 05:08 PM   #9
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Quote:
Originally posted by AndrewT
Hi T,
it is not a balanced input. It is only drawn to look like that for the sake of clarity so formumites can see the circuits are identical (except Cherry a/b)
What I am asking is which if any are better for an NFB topology and whether any of them are bad.
I may be missing the point (it wouldn't be my first time!). Ignoring the Cherry switch for the moment...I sort of look at your circuits like the attached. So the output is going to be approximately proportional to (Va - Vb). You seem to be asking where the best place is to attach Vb to signal ground, and where to locate R3.

With NFB you have to worry about at least two things: loop stability and noise. Provided the op-amp is unity gain stable, then the circuit stands a good chance of being stable regardless of where Vb is grounded, noise notwithstanding. Noise is important and will affect both terminals of the op-amp. To minimize it usually requires the track areas of those pins to be minimized. So R1, R2 & C2 should be as close as possible to the + pin and R3, R4 & C3 as close as possible to the - pin. So I would be inclined to rule out your "input 3" option.

I think "input 1" is attempting to sense gnd at the source end of a cable so as to reduce cable effects. I think that will only work if any unwanted noise/voltage drop on the signal part of the cable is equal to that on the gnd part. This is why I pointed out that the impedances are different.

Any help?
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Old 25th September 2008, 07:50 PM   #10
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Default Noise at great force.

Any effort we do to send a lovely signal to the load (speaker) then comes back as a fiercely strong loud noise via the load (speaker) ground line!
Gobbledegook + amperage, is indeed, a description of the problem.
So, please don't ground the NFB at that point.

Input ground versus NFB ground:
At high output usage (or transients), the load return line (speaker ground) will lift the Input ground, with terrible results if Input ground goes same or higher than NFB ground (unbalanced non-inverting amplifier, as shown).
Not traces, nor cables, nor rectifier, nor transformer is sturdy enough to prevent this.
A 1/2w or larger carbon resistor (groundlift) added to the NFB ground leg is daffy-looking but works in practice (retards capacitance / low capacitance). A Caddock price is unnecessary when a half penny, half watt, will retard capacitance just as well. That's an exploitation of component variances, just like the Cherry switch.

Ditch the noise?
Instead of thinking like the year is 1920 and sending the unwanted AC signal (from speaker ground line!!) into the power supply board neutral, where it could be rectified (flattened) by big capacitors (if using a linear supply so equipped), lets please figure out how to do that with a diode?
If my imagination is working correctly, that's two series pairs of diodes required to ground Either the speaker OR NFB to V+ and V-, with a creative virtural ground--And thus the point is keeping load return (speaker ground) noise permanently away from the NFB ground.
Otherwise. . .

However, if you really want to restrict this to linear supply, then this might do (photo below). What do you think?
At this point, I'm a little lost. See photo. That's as far as I got.
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