Balanced Line Driver and Receiver Schematics

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MisterCMRR

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Thanks a lot for this lengthy post. I am familiar with your work and published articles on AES.org where I am a member myself. I was just confused with what you meant by "laboratory signal sources".
I’ve always wondered why this InGenius topology wasn’t used in the mic preamp ICs that THAT produces, and where a large CMRR would be far more necessary because the cables can be long and the signal very small.
You're very welcome! By "laboratory" source I mean one that uses source resistors of extreme (better than 0.1%) precision. In fact, the IEC test (based on a BBC test) for CMRR guaranteed absolute precision using a clever method. One of the two source resistors was trimmable. A DPDT switch was put between generator and DUT (device under test) and the switch swapped the inputs. The trimmable source resistor was tweaked until the CMRR reading was identical - which guaranteed a match as good as the resolution of the test instruments! Clever, but that's not even close to what real-world sources are like. Most gear uses "build-out" resistors of 1% to 5% tolerance (and often electrolytic coupling caps of far looser tolerances), so imbalances of 10 Ω are not unsual (and used in my test that the IEC adopted in 2000). It does little good to laser-trim the diff-amp resistors to ±0.005% when a ±5% resistor will be put in series with it - as it is with real-world sources. So my point is that the CMRR numbers quoted for input stage ICs are essentially LIES, because that number is achievable only in a lab setup with PERFECT source resistance matching. With a typical device connected, CMRR may be some 20 to 30 dB less. Real input transformers, and the InGenius IC, reduce this dependence on the source by a factor of roughly 1,000! I started digging into all this back in 1994 because Jensen customers would report that adding an input transformer at an equipment input always made noise disappear. At the time, I couldn't explain why, so I started researching and experimenting to find out ... and it all boils down to common-mode input impedances. I've been trying to "spread the gospel" since ...

I tried to interest THAT in doing a mic-pre using the technology but, rightfully, they raised two issues. First, to do it properly would require DC-coupled inputs for the preamp - and its common-mode voltage range would need to include +48 V. The IC would still need a negative rail to support bi-polar output signal swings. This would require a 70 V manufacturing process for the IC ... not trivial! Second, the amount of common-mode noise in mic signals is generally very low - largely because no one in their right mind would make a second ground connection at the microphone itself. Significant common-mode voltages almost always result from a "ground" connection at each end of a signal cable. With a mic, one end invariably "floats" - so the only current flow (and resulting voltage drop) in the shield is very small. Only when something unusual happens (like a mic on the floor rolling into contact with, say, the flange of some stage lighting gear) are there common-mode noise voltages to reject. But, in theory, the pair of 6.81 kΩ resistors for phantom power can be "bootstrapped away," greatly reducing their negative effect on common-mode input impedance. But, for the reason just cited, it's an academic exercise. In practice, RF suppression and avoidance of a "pin 1" problem at mic input jacks is a far bigger issue than CMRR.
 

Matt Syson

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Hi
There is of course an discrepancy between 'lab' results and real world usage.
[(Quote from above)Real input transformers, and the InGenius IC, reduce this dependence on the source by a factor of roughly 1,000! I started digging into all this back in 1994 because Jensen customers would report that adding an input transformer at an equipment input always made noise disappear.]
Not disputing this BUT WHICH noises disappeared and was it ONLY the addition of a transformer? Although providing galvanic isolation it also may represent a complex LP filter setup. As it is a component with defined size, it also implies that other things were 'moved' to fit the transformer.
In the 1950s/60's the predominant 'noise' in most environments would be magnetic hum field (and thermal of the gear itself). Now there is a multitude of 'environmental noises from powerline (possibly even modulated powerline as electric companies read your meters remotely) and of course switchmode supplies and mobile phones so filtering and proper 'RF immunisation regimes are almost arguably more important than 'common mode rejection. With the internet spreading articles that are 'economical with the truth' leasing those who aren't prepared or able to think through the complete scenario that some things are either vital, or others are 'automatically' discredited. A possible 'requirement' for a mic input stage to be able to survive mains (240Volts AC) applied common mode was proposed for some gear in the 1980's, for which many transformers would actually 'pass' that requirement although it was rather 'niche' (outside broadcast where the 'far end' may be supplied by a generator for example. Maintaining CMMR from 'near DC' to several MHz is no mean feat and requires very careful board and component layout.
Matt S
 

JohnRoberts

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DC coupled mic preamps have been a holy grail pursuit of cap haters for decades. There are sundry approaches with potential but IMO the result does not justify the complexity. I have been scribbling schematics for several decades but Wayne Kirkwood melted the most solder making working proofs of concept for DC coupled mic preamps.

For the mic preamps used to provide make up gain following passive mixers since phantom power is not needed, the large input phantom voltage blocking caps could be removed, but then DC offsets need to be dealt with.

The ugly truth for cap haters is that many microphones have capacitors inside negating the effort.

===
I appreciate that you are talking about CMRR not capacitors. I recall in Paul Buff's Transamp application notes that he used a separate CM trim for HF since HF and LF CM was dominated by different mechanisms.

JR

PS; Another idea I liked was flying the mic preamp up to follow the phantom input voltage and then include an on board A/D. The digital output could be optically coupled to deal with DC, grounds, whatever.
 

FIX

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What are the aspects of performance that make them a better choice than let's say a 4562 in this application?

Not every engineer that works in a studio is the owner. Shorting one leg of a balanced connection is just a high-probability fact of life.
Listening. I don't ever pick an op-amp based on specs. I listen to them. The 2134 is my favorite choice for gain (other than my own op-amp like the Tonelux stuff or the API 2510) I cringe when I see specs like .0000000003% distortion. I use the dual THAT amps because they are low gain and sound really really good. When an op-amp has so much gain that the distortion is almost zero, I'm suspect. My 2510 and TX-240 op-amps were both low gain, (75 dB or less) and because of that, and having one gain stage, the phase margin and stability are excellent. The TX-240 can run without a compensation cap it's so stable. it's open loop pole is 10K where most of these op-amps are 250-1K. It also has one dominate pole instead of 2 like some amps. I didn't design it to be unity gain stable, though both are, because doing that is the worst thing you can do with an opamp. If I want unity gain, I'll put a 10K in the feedback loop with a 20p cap, with nothing to ground. That way, the amp isn't at full gain with 100% fed back to make it unity. You would be surprised at how nice any op-amp sounds that way.

As far as the studio owners, etc, I don't care and neither do they, they have cables that lift one side, and if the engineer can't hear that the gain is down 6dB or there is distortion, then you might as well let him go and continue working. LOL. If a device is unbalanced in, like the 550, 550A or 550B, then you will need a transformer. I go for tone before specs. Listening to op-amps may seem like a form of birth control, but you can year the differences. I do like the tone of a TL072 for direct out as well...
 

FIX

Paul Wolff
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DC coupled mic preamps have been a holy grail pursuit of cap haters for decades. There are sundry approaches with potential but IMO the result does not justify the complexity. I have been scribbling schematics for several decades but Wayne Kirkwood melted the most solder making working proofs of concept for DC coupled mic preamps.

For the mic preamps used to provide make up gain following passive mixers since phantom power is not needed, the large input phantom voltage blocking caps could be removed, but then DC offsets need to be dealt with.

The ugly truth for cap haters is that many microphones have capacitors inside negating the effort.

===
I appreciate that you are talking about CMRR not capacitors. I recall in Paul Buff's Transamp application notes that he used a separate CM trim for HF since HF and LF CM was dominated by different mechanisms.

JR

PS; Another idea I liked was flying the mic preamp up to follow the phantom input voltage and then include an on board A/D. The digital output could be optically coupled to deal with DC, grounds, whatever.
I've never been clear as to what DC has to do with music, and one of my products, the Sunset Sound TUTTI mic pre has 2 transformers, 2 990s, and several caps inline, and it is probably the most famous mic pre in the world after of course the original, like EMI and Telefunken ect. It's recorded every artist from the Doors to Led Zeppelin and everything in between and I've never head anyone say that When the Levy Breaks would have been so much better if the mic pres were DC coupled.

When I owned API, we had a distribution amp that we designed for ABC, called the 318, it had a 1M and 10p cap trimmer on it to get the CMMR to -105 at 100KHz, the highest int he industry at the time. Between ABC and NBC, we probably had 10K of them in their facilities and trucks. They would test the racks by pushing a walkie talkie into the cables and clicking the WT on and off talk. If they saw anything on a scope, they would reject the whole system. They did that because internal security would walk the building and report in, which is the last thing you want over the air. Also in "those days" many TV and radio stations had their antenna's on the roof.
 

JohnRoberts

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Music clearly does not contain DC, that would be pretty boring music, and bad for gear like loudspeakers. There used to be an old DC coupled power amp (Crown DC300) and it was notorious for killing loudspeakers.
=======
The near religious dislike of capacitors is not only irrational but all but impossible to avoid.

I used to be amused by audiophools claiming they could hear switch contacts in their audio path..... Perhaps they could hear a faulty switch. They don't want to know how many switch contacts their audio signal routinely encounters getting from here to there.

That said I try not to argue with people on the internet about what they say they hear.

JR
 

MisterCMRR

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Hi
There is of course an discrepancy between 'lab' results and real world usage.
[(Quote from above)Real input transformers, and the InGenius IC, reduce this dependence on the source by a factor of roughly 1,000! I started digging into all this back in 1994 because Jensen customers would report that adding an input transformer at an equipment input always made noise disappear.]
Not disputing this BUT WHICH noises disappeared and was it ONLY the addition of a transformer? Although providing galvanic isolation it also may represent a complex LP filter setup. As it is a component with defined size, it also implies that other things were 'moved' to fit the transformer.
In the 1950s/60's the predominant 'noise' in most environments would be magnetic hum field (and thermal of the gear itself). Now there is a multitude of 'environmental noises from powerline (possibly even modulated powerline as electric companies read your meters remotely) and of course switchmode supplies and mobile phones so filtering and proper 'RF immunisation regimes are almost arguably more important than 'common mode rejection. With the internet spreading articles that are 'economical with the truth' leasing those who aren't prepared or able to think through the complete scenario that some things are either vital, or others are 'automatically' discredited. A possible 'requirement' for a mic input stage to be able to survive mains (240Volts AC) applied common mode was proposed for some gear in the 1980's, for which many transformers would actually 'pass' that requirement although it was rather 'niche' (outside broadcast where the 'far end' may be supplied by a generator for example. Maintaining CMMR from 'near DC' to several MHz is no mean feat and requires very careful board and component layout.
Matt S
The customer-reported "disappearance" of noise was due to the addition of an external "transformer-in-a-box" (Jensen's ISO-MAX model PI-2XX in most cases) - and nothing else. Sorry I used a term as vague as "noise" in this context. What I refer to is not "white" or thermal noise (that's generally a gain structure issue). Nor is it "out of band" ultra-sonic and RF interference (those are filtering issues). What I refer to is the generally dominant source of noise in signal interfaces of any sort - "ground" voltage differences between the signal source and destination equipment or "GVD" as I refer to it in my seminars. Although a properly Faraday-shielded input transformer will maintain significant common-mode rejection up to 100 kHz or so, common-mode filtering is generally the answer to those EMI issues. The input transformer, if properly designed, can provide near-ideal differential (signal) low-pass filtering. Jensen input transformers in particular (when used with the recommended load resistance and/or RC damping network) are intentionally tailored to behave as 2nd-order Bessel low-pass filters to produce near-perfect time-domain ("phase") response as indicated by lack of overshoot and/or ringing on square waves or flat deviation from linear-phase or DLP - a true measure of phase distortion (which phase shift is not). The InGenius IC has a unique feature that minimizes the CM input impedance degradation caused by CM bypass capacitors at its inputs. The same bootstrapping feedback that increases LF CM input impedances can be used to effectively change the value of the bypass capacitors with frequency. For example, a pair of 1 nF capacitors effectively become 100 pF capacitors at audio frequencies but become their full value at RF frequencies due to the frequency-selective bootstrapping - see Fig 11 on the data sheet at http://www.thatcorp.com/datashts/THAT_1200-Series_Datasheet.pdf.
 

Tubetec

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I thought it might be a good place to ask a question here ,

Theres a profusion of cheap dacs on ebay with ESS ES9018,28,38 series chips , typical op amp unbalanced out on phonos.
Spec is quite good at -127 to -130 db SNR for the chip itself but the supporting 5532 op amps dont stand any chance of meeting those specs . Many boards socket the op amps so there is at least a possibility of swapping in a more modern SMD component on a sub board for instance . Forget about all that for now .

Instead of left /right signals on their respective output channel could I instead use the two channels of the dac in mono, each as half of a balanced signal path , If I make sure that both channels are fed from the same source but one with inverted phase , can I now just get my + signal from the left output and - from the right and feed a balanced line in the usual manner ?
 

MisterCMRR

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I've never been clear as to what DC has to do with music, and one of my products, the Sunset Sound TUTTI mic pre has 2 transformers, 2 990s, and several caps inline, and it is probably the most famous mic pre in the world after of course the original, like EMI and Telefunken ect. It's recorded every artist from the Doors to Led Zeppelin and everything in between and I've never head anyone say that When the Levy Breaks would have been so much better if the mic pres were DC coupled.

When I owned API, we had a distribution amp that we designed for ABC, called the 318, it had a 1M and 10p cap trimmer on it to get the CMMR to -105 at 100KHz, the highest int he industry at the time. Between ABC and NBC, we probably had 10K of them in their facilities and trucks. They would test the racks by pushing a walkie talkie into the cables and clicking the WT on and off talk. If they saw anything on a scope, they would reject the whole system. They did that because internal security would walk the building and report in, which is the last thing you want over the air. Also in "those days" many TV and radio stations had their antenna's on the roof.
While DC from a microphone would be of interest only to a meteorologist, extended low-frequency response is important to music. A typical signal chain for recording and reproduction may contain dozens of high-pass filters, mostly in the form of coupling capacitors - but also those present in the microphone and loudspeaker. Just as in low-pass filters, there is a phase shift in a high-pass filter that's related to its "order." Since most coupling capacitors form single-pole filters, their phase lead approaches 90° below their -3 dB "cutoff" frequency. So the phase response of the signal chain becomes that of a very high order high-pass filter. And this phase shift affects signal frequencies at least a decade higher than the cutoff frequency of each stage. Unfortunately, there is no equivalent of a Bessel high-pass filter to bring linear phase response to all this. The only way to undo most of this true phase distortion (or deviation from linear phase) is to move the -3 dB "corner" or "cutoff" frequency down ... way down! Marshall Leach of Georgia Tech wrote a paper about this back in the 1980s. Therefore, sizing coupling capacitors for -3 dB at 0.5 Hz is not unreasonable! It's also why most Jensen transformers have low-frequency response down to well under 1 Hz. Of course, this phase distortion is cumulative - the longer the signal chain, the worse it becomes. Because kick-drums get much of their character from frequencies affected by this time domain distortion, long signal chains often reproduce kick-drums that sound nothing like the real-thing. It's also why I've always preferred the sound of a woofer in a sealed box ("acoustic suspension") to one in a vented box. The former is a 2nd-order high-pass filter while the latter is a 4th-order. The higher cutoff slope directly translates to increased time-domain distortion. This time-domain distortion at low frequencies is, for me at least, reason enough to use DC coupling when feasible and "over-sized" coupling capacitors (and Jensen or other "over-designed" transformers) in signal paths. And, obviously, the shortest possible signal paths will generally sound better in this regard.

I don't want to start on a rant about negative feedback itself because the subject gets really complex really quickly. But I think that extreme open-loop gains (with corresponding extreme feedback factors) to drive THD numbers down is generally a bad idea. Better IMHO to have an amplifier with high open loop linearity and keep the feedback factor reasonable - as Deane Jensen did when he designed the 990, which also uses inductors in the input stage emitters to stabilize HF response without paying the usual penalty in slew-rate or equivalent input noise. The 990 design was also in keeping with the "spectral contamination" paper that Deane wrote with Gary Sokolich just before Deane died. Less spectral contamination happens in signal chains with higher linearity, lower feedback factors, and perhaps most important, bandwidth limiting. IMHO, it's very misguided to think that bandwidths over 50 kHz have any benefit. Well, I've already gotten deeper into this than I intended!
 

moamps

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Real input transformers, and the InGenius IC, reduce this dependence on the source by a factor of roughly 1,000! I started digging into all this back in 1994 because Jensen customers would report that adding an input transformer at an equipment input always made noise disappear. At the time, I couldn't explain why, so I started researching and experimenting to find out ... and it all boils down to common-mode input impedances. I've been trying to "spread the gospel" since ...

Do you have any information on how many of these InGenious opamps are used in modern consoles and devices?
I started my career working on consoles full of transformers, both microphone, line and output. Over time, things have changed a lot, but I don’t think for the better. And these days when I need to do something like live gig or OB, the standard equipment includes a variety of iso boxes that I made myself and adapted to my needs. The ground lift is a switch I adore. :). (for officers on duty, it is not a ground lift that separates the safety ground from the metal box)

I tried to interest THAT in doing a mic-pre using the technology but, rightfully, they raised two issues. First, to do it properly would require DC-coupled inputs for the preamp - and its common-mode voltage range would need to include +48 V. The IC would still need a negative rail to support bi-polar output signal swings. This would require a 70 V manufacturing process for the IC ... not trivial!

Sounds pretty complicated.

Second, the amount of common-mode noise in mic signals is generally very low - largely because no one in their right mind would make a second ground connection at the microphone itself. Significant common-mode voltages almost always result from a "ground" connection at each end of a signal cable.

I had one situation where I needed all the CMRR I could get from a mic preamp. It was the installation of ambient microphones in a theater where the installation of fire alarms was poorly designed and radiated huge repetitive bursts demodulated in audio band. The microphones were AKG electrets transformerless, impedance suboptimally balanced and the mic preamps was also transformerless, standard LTP + opamp design. Also, the length of the cable between the microphone and the preamp was more than 30m. The sound volume at the position of these microphones was very low on average, which means that the preamp has to work with the highest gain, more than 70dB, which led to the interference being quite audible. Additionally, this signal was processed dynamically in order to obtain a higher average volume for the needs of the inspicient (don't know is it correct word) and the background loudspeakers in the corridors, which made the situation even worse. In the end, the solution of the problem was to install small Sennheiser microphone transformers at the preamps inputs.

I went to OT little bit, I'll stop now.
 

Bo Deadly

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Instead of left /right signals on their respective output channel could I instead use the two channels of the dac in mono, each as half of a balanced signal path , If I make sure that both channels are fed from the same source but one with inverted phase , can I now just get my + signal from the left output and - from the right and feed a balanced line in the usual manner ?
I'm sure you could but what would be the purpose of this? You want a balanced output? DACs are probably already differential out followed by an instrumentation amplifier to debalance to make an unbalanced out for a typical RCA.
 

Bo Deadly

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I don't want to start on a rant about negative feedback itself because the subject gets really complex really quickly. But I think that extreme open-loop gains (with corresponding extreme feedback factors) to drive THD numbers down is generally a bad idea. Better IMHO to have an amplifier with high open loop linearity and keep the feedback factor reasonable
Yes we've been through that rant. Strangely a surprising number of the higher level folks here have expressed that high levels of feedback are bad in one way or another. Now you and FIX too?

For the record: Hogwash I say! There has been no reasonable explanation for high levels of feedback being bad other than proclamations about how it "sounds". Null testing is hard to argue with.
 

JohnRoberts

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Like capacitors negative feedback is under appreciated.

Without NF the modern telephone system would have never happened. Imagine hundreds (thousands) of open loop gain stages in series. without it we'd have difficulty getting a passable signal 6' across a console.

Of course as with everything execution matters.

JR

PS: I too am suspicious of modern uber-op amps that specify non-linearity down more than -140dB. That can't even be measured without gaming the test fixture with elevated noise gain, not exactly real world. That said better is always good, perhaps just not very necessary.
 

Tubetec

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Well these off the shelf dacs boards are 10-15 dollars , anything with XLR's is much more costly .
Using I2s format several dacs could be fed their respective signals then the outputs combined for better performance .
 

Bo Deadly

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Well these off the shelf dacs boards are 10-15 dollars , anything with XLR's is much more costly .
Using I2s format several dacs could be fed their respective signals then the outputs combined for better performance .
You don't even need to do that. Just parallel the outputs into the same output buffer. You would have to hack the board though. Not sure if paralleling after the buffer would give you the same THD / SNR improvement. Look at the datasheet for those chips. They are differential out. So unless that PCB is doing something cheesy, there should be amps on the two outputs that you can tap onto to get balanced outs. Add build-out resistors though.
 

abbey road d enfer

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It does little good to laser-trim the diff-amp resistors to ±0.005% when a ±5% resistor will be put in series with it - as it is with real-world sources. So my point is that the CMRR numbers quoted for input stage ICs are essentially LIES, because that number is achievable only in a lab setup with PERFECT source resistance matching. With a typical device connected, CMRR may be some 20 to 30 dB less.
It is importani to keep in mind that the actual global CMRR depends as much on the source than on the receiver.
The Wheatstone analysis shows that CMRR can be adjusted as close as perfection as can be by tweaking either the source or the receiver.
It is notable that CMRR is almost exclusively spec'd for inputs. This individual published CMRR value is almost unrelated with the actual performance, since the output stages of most equipment are not perfectly balanced. Even those using xfmrs have often a significant HF unbalance due to distributed capacitance.
 

MisterCMRR

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Spot On! That's the biggest misconception of all ... that an input stage is solely responsible for CMRR. In truth, CMRR applies only in a complete interface: line driver, cable, and line receiver. Usually "specsmanship" marketing omits that little fact - implying that the receiver will always reject noise just as it does in their (unspecified) test conditions.
 

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Since this thread has a lot of folks smarter than me answering questions, here is one regarding THAT balanced out drivers. I have a circuit with a low impedance single ended output, perfect for using the 1646, however there is no output level control. Is it a horrible idea to use, for example, a balanced 600 ohm attenuator after the 1646? If that is a bad idea, how else might one incorporate a trim control (could even be 0-6dB) without adding another opamp?
 

JohnRoberts

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Of course CMRR can be degraded by either end, or both (and even the middle... like wire).

Design engineers typically have control over only one end at a time, so focus on what they can touch.

Marketers merchandise what they have to sell.

At peavey I had the luxury/pain of dealing with both ends and even some middle. :unsure:

JR
 

abbey road d enfer

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Since this thread has a lot of folks smarter than me answering questions, here is one regarding THAT balanced out drivers. I have a circuit with a low impedance single ended output, perfect for using the 1646, however there is no output level control. Is it a horrible idea to use, for example, a balanced 600 ohm attenuator after the 1646? If that is a bad idea, how else might one incorporate a trim control (could even be 0-6dB) without adding another opamp?
You've more or less answered your own question. Though I don't particularly like the idea of dissipating energy in an attenuator, it's a valid (but costly) solution.
Have you considered adding a gain control on the existing output stage? It's not always possible or easy, but it's worth investigating.
 

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