ADA8000 +/-18V regulator swap: lower heat dissipation?

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nevercroak

Member
Joined
Jul 29, 2016
Messages
11
Hi,
as the regulators in these (ADA8000) are running very hot, does anybody see a problem with replacing the LM78-/LM7915 with LM78-/LM7918 regulators (effectively making op amp and preamp supply rails +/-18V) to get the heat dissipation in the regulators down?
From looking at the gyraf schematics and a quick check on the PCB I can't see any reason it should not work, other than the clip level metering being wrong. For now my head can't really figure if the total power dissipation of the unit will go down, or the difference in heat dissipation at the regulators is now just dissipated in the op amps.

I appreciate any comments on this.

-Arne
 
You will shift the dissipation to the ICs. Possibly increased even more than the nominal 20% increase if the ICs draw more current at +/-18V vs +/-15V.

+/- 18V may be marginal high voltage for some ICs.

Running the ICs hotter will reduce MTBF (mean time between failure) hurting reliability.

JR
 
Thank you!
Maybe I did not put this right, I know I am shifting the dissipation to the OPs, question was if the total dissipation will differ - what you luckily addressed as well.

It is an old unit, with TL074s which are OK with +/-18V (yes, max rating so not the best idea of all). The datasheet shows a decrease in current draw (no signal and no load) when increasing temperature of -0.2mA/+25°C. There is no information about supply voltage vs. current draw, so it possibly is constant, independent of the supply voltage (looking at the schematic in the datasheet I doubt so...). All in all, they might not get that much hotter considering the current draw vs. temperature relation, as they dissipate more power they get hotter and draw less current.
The input amps are pretty warm (not exceptionally hot, can touch them continuously) so it seems like it would be worth a test if the run to hot.
When using +/-18V rails no working input protection for the TL074s is present, possibly not the best idea as well.
So,
is there anything apart from the fact, that the OPs run hotter and one loses input protection, that should keep one from trying +/-18V rails in this unit?

-Arne
 
Bigger heatsinks?

Series resistance before the regs to waste-off some voltage? (At the risk of loss of regulation when the power company is having a bad day.)
 
nevercroak said:
is there anything apart from the fact, that the OPs run hotter and one loses input protection, that should keep one from trying +/-18V rails in this unit?

You have to make sure that your worst-case (low line) regulator input voltage doesn't go below the specified drop-out. For an 18 V regulator, you want a 21 V input (and similar for the negative rail).

That said, I think the problem with these units is that the transformer output is too high, so maybe you'll get away with it.

Since ± 18 V is right at the op-amp process limit, you need to verify that the regulator outputs are really 18.0 V and not higher.

And, as noted, you're simply moving the power dissipation from the regulators to the op-amps.

I think I'd replace the power transformer with something that has a lower secondary voltage.
 
Read this entire thread:

http://groupdiy.com/index.php?topic=20591.0

I went with the step down PT for 120v for the 2 I have:

http://www.mouser.com/Search/ProductDetail.aspx?qs=atHKiefXm7SRpqo30qeT4g%3d%3d

I have not run mine for long but they seem to work fine. I have not tested any voltages though.



 
nevercroak said:
Thank you!
Maybe I did not put this right, I know I am shifting the dissipation to the OPs, question was if the total dissipation will differ - what you luckily addressed as well.

It is an old unit, with TL074s which are OK with +/-18V (yes, max rating so not the best idea of all).
In my judgement a bad idea to ever run parts at maximum rating.
The datasheet shows a decrease in current draw (no signal and no load) when increasing temperature of -0.2mA/+25°C. There is no information about supply voltage vs. current draw, so it
figure 12 in TI data sheet

Current looks relatively flat with voltage but i find it instructive that they do not even continue plot past 15V rail. 
possibly is constant, independent of the supply voltage (looking at the schematic in the datasheet I doubt so...). All in all, they might not get that much hotter considering the current draw vs. temperature relation, as they dissipate more power they get hotter and draw less current.
The input amps are pretty warm (not exceptionally hot, can touch them continuously) so it seems like it would be worth a test if the run to hot.
When using +/-18V rails no working input protection for the TL074s is present, possibly not the best idea as well.
So,
is there anything apart from the fact, that the OPs run hotter and one loses input protection, that should keep one from trying +/-18V rails in this unit?

-Arne
If you have "too much" voltage there are tricks that can scrub off some un-regulated voltage.

#1 use half wave rectification- this will double the PS ripple voltage  but drop the dissipation in the regulator some (make sure extra ripple doesn't cause regulator to drop out).   

#2 add a small resistance in series with rectifier diodes (not in series with regulators). A resistor in series with the regulator just trades dissipation in the regulator for dissipation in the resistor. A resistor in series with rectifier will reduce ability to charge the caps reducing average unregulated voltage.

#3 use smaller reservoir caps in PS. This will increase ripple voltage but drop average voltage and dissipation in regulators (again make sure ripple voltage is tolerated by regulator low voltagte drop-out). .

JR
 
I would not change the voltage from 15 volts to 18 volts as this may cause the MTBF to drop or cause other problems. I would relocate the voltage regulators to a new location where they can be BOLTED to the chassis. John's suggestions could be an alternate solution, however the ripple may be present with low line voltage.
Duke :)
 
I did not meant to make another general: make my ADA8000 not fry itself. I could just not find any discussion on using +/-18V rails, as these would move some heat away from the regulators, and the caps which are in direct proximity, as these both are the parts that fail. I am quite aware of the other methods discussed to get the unit run cooler: Resistors, relocating regulators to a heatsink/chassis, fans, other transformers (and this being the optimum solution). Simply swapping regulators would in my opinion seem like the easiest way to keep the psu from dying, as they cost next to nothing and you have some lying around anyways, no? Yes your op amps will die sooner, but do they die within a relevant time frame?


JohnRoberts said:
In my judgement a bad idea to ever run parts at maximum rating.
Andy Peters said:
Since ± 18 V is right at the op-amp process limit, you need to verify that the regulator outputs are really 18.0 V and not higher.
Sure, could be brought out of the danger zone with a series diode. TL07x are working in quite a few places at 17v+ reliably. Does not make it the best choice, but seems to work fine.

Andy Peters said:
You have to make sure that your worst-case (low line) regulator input voltage doesn't go below the specified drop-out. For an 18 V regulator, you want a 21 V input (and similar for the negative rail).
That's quite obvious, but measurements should be taken. Anyways, it does not seem to be a problem here.

JohnRoberts said:
figure 12 in TI data sheet
Thanks, was only looking at the ST data sheet, as these are STs, but seems to be quite linear.

JohnRoberts said:
#1 use half wave rectification- this will double the PS ripple voltage  but drop the dissipation in the regulator some (make sure extra ripple doesn't cause regulator to drop out).   

#3 use smaller reservoir caps in PS. This will increase ripple voltage but drop average voltage and dissipation in regulators (again make sure ripple voltage is tolerated by regulator low voltagte drop-out). .
This is quite an interesting idea and I took some measurements on mine and decided to give half wave rectification a try for the 5V rail.
So: Desolder both middle diodes of the 5V rail rectifier (D9, D10) and add a 220µF capacitor in the now free holes for "safety". Scope shows minimum of 8,5V + ripple at 231V mains voltage. Seems pretty good, as it leaves enough voltage for -10% drop in mains voltage, the additional 220uF capacitor could be even smaller. Current consumption on the 5V rails should be quite constant as well. And of course: they run a bit cooler, but not something to really rave about for IC1 and IC2, but IC7 dropped its temperature significantly.


Andy Peters said:
And, as noted, you're simply moving the power dissipation from the regulators to the op-amps.
Yes, as noted above it will likely shorten the op amps lifespan, but how long is it and how much does it shorten it? Subjectively they are not near dying from over temperature.


Ok, the biggest problem I see here is frying your input OPs with your transistor preamp when running it with +/-18V as they only tolerate +/-15V on the inputs, regardless of the supply voltage being higher. This goes for the TL074 at least. So not a good idea.
I don't see any problems with the output stages regulators, even the OPs run cooler than these in the input stage. But: The regulators themselves are not even that hot... so possibly a waste of time to swap them - will think about it.

-Arne
 
nevercroak said:
Yes your op amps will die sooner, but do they die within a relevant time frame?
The +/-15V rating on TL07x is the power voltage for "Recommended Operating Conditions" (it's the input voltage where most of the other characteristics are rated) just like many other opamps that run on that voltage.
Reading the whole data sheet, including footnotes, can be very instructive:

http://www.ti.com/lit/ds/symlink/tl071.pdf

From page 5:

(1) Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated under Recommended Operating Conditions is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.

Reading a little further on page 5 shows a table called Recommended Operating Conditions in which the minimum and maximum power voltages are +/- 5V to +/- 15V. No doubt any manufacturer will tell you you should only operate their devices within these and other ratings given.
 
benb said:
The +/-15V rating on TL07x is the power voltage for "Recommended Operating Conditions" (it's the input voltage where most of the other characteristics are rated) just like many other opamps that run on that voltage.
Reading the whole data sheet, including footnotes, can be very instructive:

http://www.ti.com/lit/ds/symlink/tl071.pdf

From page 5:

(1) Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated under Recommended Operating Conditions is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.

Reading a little further on page 5 shows a table called Recommended Operating Conditions in which the minimum and maximum power voltages are +/- 5V to +/- 15V. No doubt any manufacturer will tell you you should only operate their devices within these and other ratings given.

As we are talking about ST components here. why not just referr to the ST datasheet? http://www.st.com/content/ccc/resource/technical/document/datasheet/16/b5/cf/d3/34/29/4b/09/CD00000489.pdf/files/CD00000489.pdf/jcr:content/translations/en.CD00000489.pdf p. 4 Table 2, operating conditions: 6-36V, equaling +/-3 - +/-18V. Not only reading datasheets is a good source of information, but looking into existing applications as well. Just from my head: Old Soundcraft desks built around TL072s (even TI) using +/-17V and eela using TL07x for broadcast duty with +/-18V and series diode on each module. It does not really seem to be a problem using +/-18V regs with a series diode.

When saying they will die sooner, I was talking about the increased heat in the OPs.

As stated befor, it will not be a good idea for the input stage, as you could exceed the OPs input voltage when powering the transistor based pre amp with +/-18V and for the output stage, the regulator is cool enough. So not that interesting anymore.
Though it would be easy to rewire the PSU to have one +/-15V rail for the preamp and a +/-18V rail for the OPs, but I think I am to lazy right now. Would be a matter of swapping regulators with series diode on output pin, bending up two pins on the other regulators, removing two pins from the front board connector carrying the power supply rails and running four wires, so not even cutting traces. Maybe I give it a try later.


-Arne
 
nevercroak said:
As we are talking about ST components here. why not just referr to the ST datasheet? http://www.st.com/content/ccc/resource/technical/document/datasheet/16/b5/cf/d3/34/29/4b/09/CD00000489.pdf/files/CD00000489.pdf/jcr:content/translations/en.CD00000489.pdf p. 4 Table 2, operating conditions: 6-36V, equaling +/-3 - +/-18V. Not only reading datasheets is a good source of information, but looking into existing applications as well. Just from my head: Old Soundcraft desks built around TL072s (even TI) using +/-17V and eela using TL07x for broadcast duty with +/-18V and series diode on each module. It does not really seem to be a problem using +/-18V regs with a series diode.

When saying they will die sooner, I was talking about the increased heat in the OPs.

Here is the deal with "Absolute Maximum Ratings Over Operating Free-Air Temperature Range" (the wording used in the TI TL07x data sheet). There is a note attached to that statement:

"Stresses beyond those listed under 'absolute maximum ratings' may cause permanent damage to the device. These are stress ratings only, and functional operation at these or any other conditions beyond those indicated under 'recommended operating conditions' is not implied. Exposure to absolute-maximum-rated conditions for extended periods of time may affect device reliability."

In other words: they don't test beyond the absolute maximum ratings. They don't guarantee anything other than what is in the data sheet under "recommended operating conditions," and operating in excess of the absolute maximum ratings (which include not only supply voltage, but also input voltage and power dissipation) is "undefined." The parts may work forever at ±18.5 V rails. They may blow up in your face. A black hole might open up in Bayonne, NJ. The traffic lights might turn blue. None of those results are precluded by "undefined."

And further: one batch of parts from a specific lot might work forever at ±19 V. Parts from an order the following week might come from another lot, which fail at ±18.05 V. You won't see a production design running a TL072 at ±19 V, and when you see them with ±18 V rails, there is a diode or resistor in series with the op-amp supply pins to drop the voltage to something safe.

Finally. Note the absolute maximum ratings -- they do include thermal ratings. It is possible to overheat the op-amps without exceeding the supply rails. If the thermal design is marginal, which seems to be the case for the ADA8000, then you can have problems. And increasing the supply voltage would seem to exacerbate that.
 
Andy Peters said:
Here is the deal with "Absolute Maximum Ratings Over Operating Free-Air Temperature Range"
[...]
they don't test beyond the absolute maximum ratings. They don't guarantee anything other than what is in the data sheet under "recommended operating conditions,"
Andy Peters said:
one batch of parts from a specific lot might work forever at ±19 V. Parts from an order the following week might come from another lot, which fail at ±18.05 V
Yes, I know.

Andy Peters said:
You won't see a production design running a TL072 at ±19 V, and when you see them with ±18 V rails, there is a diode or resistor in series with the op-amp supply pins to drop the voltage to something safe.
Yes and for reasons. Mainly cause of  the operating conditions the manufacturer guarantees operation as expected in. Further I was never talking about +/-19V and addressed the whole series diode thing before.

Andy Peters said:
Finally. Note the absolute maximum ratings -- they do include thermal ratings. It is possible to overheat the op-amps without exceeding the supply rails. If the thermal design is marginal, which seems to be the case for the ADA8000, then you can have problems. And increasing the supply voltage would seem to exacerbate that.
I'm perfectly aware of this, the heat dissipated by the op amp depends on idle power consumption, load driven, output voltage and supply voltage and you can easily exceed the thermal ratings without upping the supply voltage to the maximum.

I am aware of the risks in increasing the dissipation in the OPs and using them near maximum ratings. If swapping the regulators I would be taking the risk of frying the OPs just from power disspipation, yes, and I would be willing to take that risk if I try. And yes I see that likely the total heat generated by the unit will rise, but getting it distributed will allow the formerly hot parts to transfer their heat to the air better as heat transfer is relying on a gradient. The temperature inside the unit is not distributed evenly (stock, and never will be) so you have to transfer a lot of energy an a relatively small surface, distributing it will aid the heat transfer from inside the unit to the outside as well.
My question was more about the circuit itself and the undocumented parts of it, is there something relying on the 15V rails that would fry or lead to unwanted behavior that I did not see?

But this does not seem to go anywhere else than: Do you understand datasheets, electronics and heat transfer in general, or: Why don't you only do proven things.
 
nevercroak said:
Andy Peters said:
Here is the deal with "Absolute Maximum Ratings Over Operating Free-Air Temperature Range"
[...]
they don't test beyond the absolute maximum ratings. They don't guarantee anything other than what is in the data sheet under "recommended operating conditions,"
Andy Peters said:
one batch of parts from a specific lot might work forever at ±19 V. Parts from an order the following week might come from another lot, which fail at ±18.05 V
Yes, I know.

Andy Peters said:
You won't see a production design running a TL072 at ±19 V, and when you see them with ±18 V rails, there is a diode or resistor in series with the op-amp supply pins to drop the voltage to something safe.
Yes and for reasons. Mainly cause of  the operating conditions the manufacturer guarantees operation as expected in. Further I was never talking about +/-19V and addressed the whole series diode thing before.

Andy Peters said:
Finally. Note the absolute maximum ratings -- they do include thermal ratings. It is possible to overheat the op-amps without exceeding the supply rails. If the thermal design is marginal, which seems to be the case for the ADA8000, then you can have problems. And increasing the supply voltage would seem to exacerbate that.
I'm perfectly aware of this, the heat dissipated by the op amp depends on idle power consumption, load driven, output voltage and supply voltage and you can easily exceed the thermal ratings without upping the supply voltage to the maximum.

I am aware of the risks in increasing the dissipation in the OPs and using them near maximum ratings. If swapping the regulators I would be taking the risk of frying the OPs just from power disspipation, yes, and I would be willing to take that risk if I try. And yes I see that likely the total heat generated by the unit will rise, but getting it distributed will allow the formerly hot parts to transfer their heat to the air better as heat transfer is relying on a gradient. The temperature inside the unit is not distributed evenly (stock, and never will be) so you have to transfer a lot of energy an a relatively small surface, distributing it will aid the heat transfer from inside the unit to the outside as well.
My question was more about the circuit itself and the undocumented parts of it, is there something relying on the 15V rails that would fry or lead to unwanted behavior that I did not see?

But this does not seem to go anywhere else than: Do you understand datasheets, electronics and heat transfer in general, or: Why don't you only do proven things.
I am not familiar with specifics of the ADA8000 and frankly don't want to know.

Several here have shared advice from decades of design experience.

Another rule of thumb about thermal management, is for every increase of 10'C in device operating temp, MTBF drops in half. Of course this may still be a very long time in human terms.  Modern ICs rarely drop dead from old age, usually it's some external stress.

I have already said this but I always try to engineer in some operating headroom or margin of safety. Coincidentally the manufacturer's selling the parts also have engineers on staff and they presumably try to provide some safety margin too. Please draw the intended lesson from this anecdote.
=======
In the design of audio power amps with unregulated power supplies the PS reservoir capacitors can be a significant expense.  Specifying capacitors that are voltage rated high enough to survive mains over-voltage events, can bump the capacitors into the next higher voltage category and more expensive parts for hopefully rare high-line mains voltage events. I knew a design engineer who shall remain nameless (no not me), who ASSumed that the capacitor company engineers were under-rating the voltage breakdown for their capacitors, so operated them in his design using a few percent of the capacitor maker's unwritten headroom. This was not a good plan and the reason I am sharing this anecdote is because his amp design crashed and burned because of his cut corners.

Happiness in product design is never having to explain design related flaws like that to customers and dealers, and why they spell ASSume with an ass in it. 

Over the decades I used about a bazillion TL07x and 553x op amps. I never once felt compelled to run them at +/- 18V .

JR
 
JohnRoberts said:
Another rule of thumb about thermal management, is for every increase of 10'C in device operating temp, MTBF drops in half
Tanks for that, did not know this rule of thumb.

JohnRoberts said:
Of course this may still be a very long time in human terms.  Modern ICs rarely drop dead from old age, usually it's some external stress.
That was what I was referring to in one of my first posts, the lifetime is, when not running the devices out of spec, in most cases very long in human terms. Parts will fail, as there are defects in the silicone and failure in manufacturing but which part will fail (even within specified operating conditions) you can't tell till it fails.


JohnRoberts said:
I have already said this but I always try to engineer in some operating headroom or margin of safety. Coincidentally the manufacturer's selling the parts also have engineers on staff and they presumably try to provide some safety margin too. Please draw the intended lesson from this anecdote.
JohnRoberts said:
Happiness in product design is never having to explain design related flaws like that to customers and dealers, and why they spell ASSume with an ass in it. 
True words. I am an engineer myself, not electrical though. I got a degree in mechanical engineering and safety margins are in most cases quite a lot higher there, so I am aware of the concept and would love people paying more attention to this in their designs, electrical, mechanical and what not, as this may  separate good products from average or bad ones. And this makes customers/contractors come back to you or not. Follow up below...


JohnRoberts said:
Over the decades I used about a bazillion TL07x and 553x op amps. I never once felt compelled to run them at +/- 18V .
20% more heat dissipation asking for trouble versus ~1.5dB increased headroom and that little lower noisefloor, compared to running it on +/-15V. Possibly lower costs for filter capacitors, and transformers - yeah, who is the winner now? ;)

If I had designed the unit in question I would have gone for a proper transformer in the first place and a different layout of the board.
But there comes purchasing department and tells you they got a good deal on these millions of transformers that don't fit your spec. Then you think about that you are only Behringer and people will upgrade anyways or not use it regularly and you hope that the live guys will smash it for other reasons beforehand - hey it will last these two years we have to provide warranty.... bad if it's that one product you manufacture that gets some recognition.

So, my intent was not to discuss if it would be a good idea to run these with increased voltage, just for the idea of running them with increased voltage, but look if it could do something to this unit to make it less prone to failure. I guess nobody can tell before somebody tries it (that may be me). The idea was to think about a quick and anything other than optimum fix for the main reasons these units go down, and a fix that would cost you ~0.5€ (or parts you likely already have) and 10 minutes. If somebody wanted to make a living repairing these units, they better do it right ;)

@JohnRoberts: Thanks for the comments, I appreciate them! And especially for bringing the half wave rectification back to my mind, as this really took off some heat from the unit.

-Arne
 
nevercroak said:
If I had designed the unit in question I would have gone for a proper transformer in the first place and a different layout of the board.
But there comes purchasing department and tells you they got a good deal on these millions of transformers that don't fit your spec. Then you think about that you are only Behringer and people will upgrade anyways or not use it regularly and you hope that the live guys will smash it for other reasons beforehand - hey it will last these two years we have to provide warranty.... bad if it's that one product you manufacture that gets some recognition.
OK you made me look, this is apparently a Behringer product. There is a strong possibility that was an existing  transformer already in their system used in other SKUs. It is always cheaper to use common components than to bring new parts in for every new SKU.

At one point, I had tens of SKUs (or more)  using the same transformer. 


JR
 
nevercroak said:
JohnRoberts said:
Another rule of thumb about thermal management, is for every increase of 10'C in device operating temp, MTBF drops in half
Tanks for that, did not know this rule of thumb.

For aluminum electrolytic capacitors, it's not a rule of thumb, it's actually a law.

-a
 
There is another thread about this where people discuss at great length various ways of improving the situation, but no, you have to be different!  Why dont you go back and read that thread, and DO WHAT THEY SUGGEST!!!
In particular the buck transformer method gets you a big improvement with minimum modification.
PUTTING 18 volts ON YOUR TLxxx IS NOT A GOOD IDEA!!!
Clear enough for you?
 
Andy Peters said:
nevercroak said:
JohnRoberts said:
Another rule of thumb about thermal management, is for every increase of 10'C in device operating temp, MTBF drops in half
Tanks for that, did not know this rule of thumb.

For aluminum electrolytic capacitors, it's not a rule of thumb, it's actually a law.

-a
Thanks I may have recalled a discussion about capacitor life and capacitors are a weak link inside most hot chassis.

I have found the Arrhenius equation applied to semiconductor reliability too (not just chemical reactions like inside electrolytic caps) but the math is generally over my head.

Which is why I gravitate to simple rules of thumb. 

JR 
 
A few simple ideas that may help:

1) You can reduce the voltage drop across the existing 15V regulators by adding one or more power diodes in series with the regulator inputs. This moves some of the dissipation out of the regulator and into the diodes, just as a series resistor would. However, unlike a series resistor, the voltage drop across a diode is a lot less dependent on the load current of the regulator.

To get this to work right, you need to measure the voltage drop across the regulator (input to output), determine what the regulator dropout voltage is, and add only enough drop to remove some heat, without starving the regulator for input voltage. Also keep in mind that most regulators work more poorly when you reduce their input voltage headroom. So, while you're moving dissipation out of the regulator, you're possibly forcing the regulator to perform more poorly, and also making the unit as a whole more susceptible to problems with brownout input AC voltages.

2) While +-18V is really risky, almost all op amps used for audio can handle +-17V well. There is no such thing as an LM7817 or LM7917, but you can use an adjustable LM317 and an  LM337 each with a pair of adjustment resistors if you're careful. Teflon sleeving and some careful mechanical work can make this less of a nightmare, but this is still slightly challenging to shoehorn an adjustable regulator circuit into the footprint of a 7815 or 7915. This is more or less the same as reducing the input voltage with diodes, but a little more tricky to work mechanically, and it also forces you to make sure that +-17V is actually OK for the circuit as a whole, so maybe this is not such a great plan.

3) The best answer may be to simply improve the heat dissipation of the existing regulators. Thermal epoxy is available, and while it's not ideal, an extra heatsink could be thermally epoxied onto the epoxy side of a TO-220 regulator chip, and it could help.  I have no idea what the mechanical design is like, but there maybe other ways to improve the heat dissipation of the regulators, and this would be electrically the simplest solution - you're not changing the circuit at all, just fixing the obviously substandard thermal design and packaging.

Best of luck!
 
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