CV Pedal workarounds

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gb2
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CV Pedal workarounds

Post by gb2 » Tue Sep 15, 2020 5:39 am

I'm eyeing to get a Korg MS20 filter clone pedal for my bass rig (https://ftelettronica.reverbsites.com/l ... m/13879968), after contacting the company they sent me the manual of the original pedal ( which confirms my concern that i can't just use simple expression or volume pedals for the cutoff cv ins.

Interestingly the market for designated CV foot pedals is pretty scarce, especially in the 0-10v range.

The idea that mostly floats around is to hook up a 9v battery to the input of an expression pedal to get at least 9v. But I wasn't able to find a good building doc around, and also I'm not really into having batteries in any of my gear. Just one more thing to think about, and also not really environmental friendly.

Then yesterday I had another idea, but since I have no experience in this field, I wanted to ask around here if that's even possible.
There are a variety of 10v reference precision voltage ics around - would it be possible to build a little box with two of those, wall wart powered, with two output jacks, then go to a volume pedal, then to the filter pedal?
In my understanding a volume pedal is just an attenuator, but can they even handle 10v? The spec info on those pedals are incredibly non-descriptive...

Any input here, or other ideas?

tl; dr: I need two cv foot pedals with 0-10v output, any ideas?

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bloke_zero
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Re: CV Pedal workarounds

Post by bloke_zero » Tue Sep 15, 2020 6:23 am

I've looked at the EXACT same thing with a resonator - great pedals.

I just hooked up a 9V powersupply to a jack and plugged it in to the input of a volume pedal.

It's (according to my clueless way of thinking!) just a pot attached to a pedal and with no active parts functions well enough in the role you want (it was an ernie ball VP jR for reference).

I think a big part of wah pedals is the ability to properly 'tune' the sweet spots relative to foot position. In my memory it was really hard to get into the sweet spot between the volume pedal and the frostwave - it was pretty wild swings which made expression hard which might have been improved with practice. I can set it up again - the last time I did was about 10 years ago so memory is fuzzy.

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EATyourGUITAR
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Re: CV Pedal workarounds

Post by EATyourGUITAR » Tue Sep 15, 2020 7:03 am

what you want is an attenuator for your attenuator. run 10v to the attenuator. set the magnitude (amplitude). then after you have a more narrow range you can work the expression pedal over that narrow range. set the cutoff frequency on the frostwave pedal. or you can power your expression pedal from a 5v power supply. this is very simple either way. you are connecting a DC voltage source to a volume knob. simple. look up the schematic for a diy attenuator. voltage references are better than switch mode power supplies but you might want to buffer the voltage reference with an opamp. if not, your voltage reference must be stable at all loads that may be presented by the attenuator + the input impedance of the frostwave.

https://ohmslawcalculator.com/ohms-law-calculator

looks like 0.1mA load for a 100K volume pedal. you should be fine with any voltage reference. this assumes an inverting CV input on the frostwave with no bias resistor at the input. the current flowing into the input of an inverting opamp balances to zero. total load is 0.1mA = 0.0001A
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pricklyrobot
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Re: CV Pedal workarounds

Post by pricklyrobot » Tue Sep 15, 2020 8:52 am

Volume pedal will handle 10V fine. It is indeed just a pot attached to a treadle. You may find though, that the audio taper pot likely to be in a volume pedal does not give you a great range for controlling a CV.
I did the 9V battery wired to a volume pedal thing years ago to make a CV pedal for my Prophet 600. But I swapped the pot for a linear one to get a better range. I’m this case it involved drilling a small hole through the pot shaft to attach a cotter pin that tied it to the pedal mechanism. It was a little tricky but it worked.
The feasibility of swapping the pot in another pedal is gonna depend on its physical construction. So you don’t really know until you crack it open.

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Re: CV Pedal workarounds

Post by EATyourGUITAR » Tue Sep 15, 2020 11:04 am

yes it will handle 10v fine but you are sweeping over 10 octaves. that might be more than you need. having a knob on the side to set the max voltage might be very useful for performance where you want to sweep 5 octaves. with a second attenuator you can set the voltage reference anywhere you want from 10v to 0v. also, I would not swap the 100KA for 100KB. I would swap for 100KC but you can also wire the 100KA backwards to work like a 100KC. so you don't actually need to buy anything. this will put a log on your exponential for something closer to Hz/V (linear cv pitch input).
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Rex Coil 7
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Re: CV Pedal workarounds

Post by Rex Coil 7 » Tue Sep 15, 2020 11:28 am

The line of Ernie Ball volume pedals are excellent foundations for any type of foot controller tasks. I modified one of the VPJr. 25k volume pedals to deal with controlling voltage controlled synth modules. The stock VPJr. 25k pedal has an A25 pot, a B10k pot is a far better match for use with voltage controlled systems. I had to drill out the mounting hole for the pot so it would accept the Bourns B10k pot I elected to use .... FAR better quality than the stock unit, and it provides the correct control curve (linear).

Here's a link to a page in my project synth thread ... this should take you straight to the correct post within that thread. If not, once you get there scroll about halfway down the page that this link takes you to and you'll find the pedal modification entry. It's quite detailed and I go into how I did things, and why I did them.

LINK = viewtopic.php?f=24&t=78836&start=300

EDIT: I just did a copy/paste of that posting into this one. It's huge (25 pictures and video), but it may answer questions or even generate some questions ...

Rex Coil 7 wrote:
Sun Jan 20, 2019 11:50 am
MODIFIED ERNIE BALL VP Jr VOLUME PEDAL (for use with Dot Com Q181FJ):

Welcome back my friends to the show (that seems to never end!). The continuing saga of the Super Mini Modular project.

Well hello mens, five months and roughly 2,300 views have passed since my last update. Odd how there are 2,300 more views since my last entry, but no more comments ... Meh, everwhat.

Anyhow, I've come to accept that I go through these manic/depressive waves that seem to last anywhere between six months to over a year at each end of the pendulum's swing. I am clearly re-entering another manic phase (I call them "productive phases"). I just seemed to have popped into wakeness just a few days ago. ~CLICK!~ ... the depressive phase simply ended, just that fast. The joys of manic depression combined with combat related PTSD as well as a decade of pain meds to deal with a permanent nerve trunk injury in my neck. Oh the joy, indeed. (not even ... shit!).

Enough of that .... movin' on ....

Having been away from my workbench for just short of six months, I started with something small that would produce quick results and a promptly delivered sense of accomplishment to get the mental turbos spinning up before I jump into more heavy duty tasks. That task I selected was to modify one of my three Ernie Ball VP Jr. 25k mono volume pedals to be compatible with the Dot Com Q181FJ. The Q181FJ ("FJ" = "Foot + Jack") is a control module within the Q181 Controller Series. It is designed to accept expression pedals and foot switches. It can handle both at once.

WEB PAGE LINK = https://www.synthesizers.com/q181fj.html

It's visible on the right of this row of "Interfaces", next to the module with the red knob (it seems funny how every one of those modules has the word "Interface" in their names on the panels, HAHA!) ...

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When I first took delivery of the Q181FJ I tested it out with a stock Ernie Ball VP Jr 25k volume pedal and I didn't quite like the way everything was working together ... more like was not working together. The response curve just didn't work in a linear/controllable fashion. Not much response right off of the heel position, still not much halfway through the sweep of the treadle ... then suddenly a large jump in output from the Q181FJ during the last 20% of the treadle's sweep to the toe position. Yuk ass!

I took a few measurements of the volume pedal's resistance and discovered that the 25k pot in the pedal was most likely an A25K. Hmmm ... nope.

I know that the Q142 Pedal Interface module responds very well to a B10K pot, so I tested a B10K with the Q181FJ and that seemed to work far better. So the answer was clear ... remove the A25K pot from the Ernie Ball VP Jr. 25k volume control and replace it with a B10K pot. Yea, right ... easier said than done (I was to discover). If it were as easy as all that, then bing bang boom done. But noooooo .... it turned out that the pot had to have a very long pot shaft to accommodate the brass sintered/sandblasted pulley that mounts on the pot shaft. The pulley has a string wound around it so when the treadle is swept from heel to toe the string will turn the pot shaft and change the resistance of the pot (just the same as using a knob to turn a pot).

So I went on this quest to locate a high quality pot in the B10K configuration that had a long pot shaft that was not splined or split. I had to actually post a thread to recruit help locating one.

LINK = viewtopic.php?t=204247&highlight=

Long made short I found the magic bean needed to tackle this problem. Bourns makes the exact right pot I needed for this. At the time, these pots were new items, and Mouser was only stocking small amounts of a select few resistances. Mouser had the B10K, so I bought SIX of them. When they arrived it was clear why they cost $7.86 each .. plus shipping! Check the specs:

** Bourns PDF241-S425S-103B0
** 24mm outer diameter (same size and the large Dot Com pots).
** 15mm long smooth round 6mm slotted shaft (perfect!).
** 10mm long threaded barrel (another perfect!).
** B10K.
** No detents.
** Estimated life is 1 million cycles (dayum!).
** Carbon Element.

Yea ... $7.86 each plus shipping. But check those life cycles yo!! 1 Million!! Check the link to the other thread I posted above for more details on the pot itself.

When they arrived, I opened the bag and as per human nature I tested the feel of how the shaft spun. Holy shit, I've never felt a pot that turned so smoothly that didn't have any wiggle/wobble. None! And the finish on the shaft is centerless ground ... check the pic (Replacement Alpha A25K from Ernie Ball on the left, the new Bourns B10K on the right).

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The fit inside of the shaft collar is so good, due to the centerless ground finish of the shaft. Bourns is able to make a much better fit because the shaft is so precisely sized. Bourns also used a pot shaft made of far better material with a greater hardness than what your buck standard Alpha pot has. The set screw in the brass pulley barely left a signature in the Bourns shaft, but in the Alpha shaft the set screw dug in and left more than just a signature, I'd go as far as calling it "damage". The more I worked with the Bourns pot the less I objected to the $7.86 cost per unit.

It was clear (by just doing a visual inspection) that the Bourns threaded barrel was larger in diameter than the Alpha pot that Ernie Ball fits these volume pedals with. This meant that the mounting block that the pot fits into would require some machining (a fancy ass word for drilling out). That wasn't a huge surprise since I'd spec'd out the Bourns pot before buying them, so I already knew that the mounting block may require some work. But I wasn't totally sure about that until I had one in my hand. I got out the manual calipers and took a few measurements. I had some replacement Ernie Ball 25k pots on hand that I'd purchased for maintaining the volume pedals I had bought, so I took to measuring.

** A25k Alpha threaded barrel Outer Diameter = 0.270 inches.
** B10k Bourns threaded barrel Outer Diameter = 0.345 inches.


I set them aside and took to disassembling the pedal, preparing it for modification. As I took the pedal apart, and removed the pivot pin from the treadle, I could see that reassembly was going to be very tricky. VERY tricky. Especially refitting the stainless steel pivot pin into place since it used a washer stack on each side that had very little room between the pivot blocks of the treadle and the pivot blocks of the base. I reckoned I'd deal with that when it came time to put it all back together.

Image

After disassembling the pedal and removing the pot mounting block I measured the hole for the pot barrel:

** Ernie Ball VP Jr 25k pot mounting block barrel hole inner diameter = 0.274 inches ... roughly 0.004 inches larger than the stock A25k pot's barrel.

Since the Bourns B10k pot threaded barrel had an OD of 0.345", the mounting block would require a hole of roughly 0.350" to 0.355". So the task became locating a drill bit of that size in my massive collection of drills that I'd accumulated over the decades. Seriously, I have about one thousand drills! So I set about digging through all of them with caliper in hand to locate what I needed.

Now, this never happens to me .... ever .... but the 3rd drill I pulled from the toolbox .... beat up and ugly ... measured 0.358". Holy shit ... I was all set to spend an hour or so at this. Third one! And it was in serviceable condition! Not too dull, a little beat up (but still usable) .... more importantly it was straight, had good cutting edges, and it was a "boring drill" that has the correct type of cutting edges along it's length for increasing an existing hole size (rather than drilling a new hole). The perfect bit ... third one out of the damned box! It had to be 25+ years old, I bet I used it a dozen or so times before on long forgotten jobs. Banging around in a toolbox with a thousand other drills over the decades, and it still had an edge after all of that torture. As can clearly be seen in the picture, the cutting edges had some chips and nicks from being stored loosely banging around in a toolbox with other drills. But that edge was just barely good enough to get it done! You can see that at some point it had slipped in the chuck and buffed the part that fits in the chuck ... but again, that drill was in just good enough shape to pull off the job, with probably a few dozen more jobs left in it's life to go!! Here's a set of "before and after" shots of the pot block. It's also a good opportunity to really see that wonderful centerless ground finish on the pot shaft of the Bourns pot compared to what an Alpha pot's shaft looks like. MAJOR difference between the two, wouldn't you say? The smooth finish of the Bourns pot shaft lends to a nice tight seal between the shaft and the barrel, keeping Der Krap-n-shitz out of the pot and providing an immensely smoother spin. A test fit of the brass pulley also proved to be much more snug than the Alpha which keeps the pulley better centered and creating less off-center concentricity. In English, "it just works better".

Image

I mounted the block in the drill press, chucked up the beat-ass drill, and slathered some automatic transmission fluid made for GM transmissions on the drill and the part. "Type F" fluid for ford auto transmissions won't work, it must be auto trans fluid for GM transmissions because it has more lubricity than "Type F" and other properties that prevent aluminum from building up on the cutting edges of the bit ... which in turn prevents what is known as "gauling" the aluminum part. One of those things you learn after years of metalworking. I powered up the drill press and bored the hole in the block ... fortunately Ernie Ball uses high quality aluminum in their volume pedals, that block took to machining really well. After boring the hole, I used a bevelling taper to cut a nice finish bevel on both sides of the enlarged hole. Job done!

Cleaned up the part and went back inside to continue the job. I put that magic drill bit in the same box that I have the repair parts and the other five Bourns pots so that the next pedal I modify will have the right drill at the ready.

Image



Next up, I mounted the stock A25k pot and the new Bourns B10k in a small vice, desoldered the wires from the stock pot one at a time, and soldered each wire into place on the Bourns B10k pot, also one at a time. Doing each wire one at a time prevents soldering them into the incorrect place on the new pot.




Image



I mounted the Bourns pot into the block, and put the rest of the parts that went on the block in place as well. I mounted the assembly into place in the base of the pedal, and everything fit beautifully without any clearance issues.



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Next, the dreaded reassembly of the treadle to the pedal base. This took some doing. After about a half an hour of struggling with fitting those washer stacks into place while refitting the pivot pin, I stopped, took a few breaths, and rethought my approach. I knew that those pivot blocks had to be held into place by a pair of screws, very much like the way the pot mounting block was held into place. I sat and thought about how these pedals must be assembled at the factory ... how did it all go together during assembly? Then it hit me that those pivot blocks must have been installed before the "skateboard tape" on the top of the treadle, there was really no other way to do it! With those pivot blocks left loose, it would be simple to fit the washer stacks on the pivot pin .... THEN install the pivot block screws into the pivot blocks second. I had to peel back some of the skateboard tape from the treadle top to access the pivot block screws, but I only needed to remove one side (one pivot block) to install the pivot pin and the washer stacks. So I did exactly that, and it went together all hinky dinky perfect.

Now to put the string back into place.
About a year ago I saw a video by Ernie Ball about how to install the replacement string and spring into one of their volume pedals. So I hunted up the video and watched it a few times, replaying certain sections to get the method down pat. I gotta say, that video REALLY helped to get that done. It took me about five tries to get it done properly, but once I got it done it came out really great!!!

[video][/video]

You can tell this is the Bourns pot installation and the completed string installation since the new pot may be identified by the machined ~slot~ in the end of the pot shaft. And in the second shot below you can see (ever so slightly) the contact cement that I had to use to re-adhere the "skateboard tape" to the treadle after reassembling the pedal (which I've explained in greater detail below these pictures). So what you're looking at is the post reassembly "re-string" job I did ... not too shabby, yea?

Patience .... lots of patience, work slowly, don't let frustration get the best of you, and go in knowing you will have to take a few whacks at it, maybe even a half dozen shots at it! Just take your time, and work carefully.

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Next task, the skateboard tape needed to be reglued
after peeling back about four inches of it to gain access to the pivot screws. I used some Scotch 3M 77 spray on contact cement. I masked off portions of the treadle with "3M Blue" masking tape to prevent slop from getting to places I didn't want it to go. I sprayed a puddle of the 77 contact cement on a paper plate, then used a "Q-Tip" (aka "cotton bud" in the UK) to paint some contact cement to both the skateboard tape and the surface of the treadle.

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Allowed the cement to "tack up" for about five minutes before sticking the skateboard tape to the treadle, then once the contact cement was properly tacky, I pressed the skateboard tape down to the treadle. I placed a masked off block of wood on the treadle and clamped the wood down tightly on the skateboard tape and left it to dry for about two hours. Why "two hours"? ... well even though the contact cement was probably totally ~set~ inside of twenty or thirty minutes, when I had to peel back the skateboard tape to deal with the pivot block that effort stretched the skateboard tape out of shape a little bit. So by leaving it clamped for a couple of hours it helped to "reshape" the stretched skateboard tape and assure a good looking completion. Look at the pics, judge for your own damn se'f ....

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A wee bit of contact cement is visible along the edge of the skateboard tape, and there's a teensy smudge of contact cement visible on the surface of the treadle, near the heel (it's a sortof light spot about the size of a fingerprint) ..

:roll: :whistle:

All in all, the whole job came out just great. I now have a super tough, highly rebuildable expression pedal for use with my modular synth. If you look at the pedal carefully you can just barely see a teensy bit of contact cement on the one edge of the skateboard tape. I'm very happy with how good the entire job came out.

In the three pictures the first one is the "BEFORE MODS" picture. The next two shots are the "AFTER MODS". You can see the tiny bit of contact cement along the left edge of the skateboard tape in the two "after" pictures (along with the smudges of contact cement on the surface of the skateboard tape from sloppy workmanship) ... and in the "before" shot (first picture) there's no evidence of contact cement. Not too bad of a job if I say my own damned se'f ...

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The top picture of the group above is what it looked like BEFORE I did the whole job. You can barely tell which is which other than the tiny bit of contact cement showing itself in the bottom two pictures!

I just love how long and smooth the sweep is on these Ernie Ball units. The sweep is longer than the Moog EP units, and FAR longer than your average "Cry Baby Wah" pedal. The longer sweep makes them more controllable with your foot/ankle than the short sweep designs. The only other thing needed to make them compatible with Dot Com Q181FJ or Dot Com Q142 is that an "insert cable" must be used with the pedal. An insert cable has a Tip/Ring/Sleeve plug on one end, then the cable splits (like a "Y" cable) so there is a Tip/Sleeve plug on each of the two ends of the "Y". One of the TS plugs is connected to the Tip of the TRS plug, the other TS plug is connected to the Ring of the TRS plug. Using this type of cable with an Ernie Ball volume control pedal that has one input and one output "converts" the pedal into an expression pedal that has a TRS plug on the end that plugs into the controller module in the synthesizer. It works perfectly!

Only thing to watch for is if you plug all of this in and the pedal seems to be working all funky, most likely you'll need to swap the two TS plugs in the "input" and "output" of the volume pedal. I know it seems as though it shouldn't matter which of the two Tip/Sleeve plugs is inserted into the input/output jacks of the volume pedal .... BUT IT DOES!! So if you set one of these up with the insert cable and you're hearing fonky results, swap the positions of the two Tip/Sleeve plugs. I learned that through experience.

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These Ernie Ball volume pedals are SO MUCH MORE DURABLE than the plastic Moog EP2 or EP3 pedals, and these are fully rebuildable. They may also be modified for other uses as well. I reckon this expression pedal will outlive me.

Total cost of the pedal itself, and the Bourns B10k pot comes to roughly $75.00. I bought three of those same models of volume pedals one at a time on eBay as they popped up ("new open box" saved me roughly 50% on each pedal), one of which I modified as shown in this entry. I have six string/spring repair kits on hand, six Ernie Ball A25k pot replacement kits that include a new brass pulley, and five more Bourns B10k pots on hand as well. I'm all set when it comes to expression pedals or volume controls!!!

This "military grade" expression pedal is just one more component within the "super controllers" I've concocted for use with the synth. The pedal is an excellent match with the dual ribbon controller equipped QKB61 keyboard with joystick that I put together and showcased previously in this thread. It's all part of the entire instrument!

Yet to go:
** fitting the bus bars to the Euro "modulation cabinet".
** mounting the power supplies in the Euro cab and the 12U "utility cabinet".
** connecting the power cables from the bus bars to the modules.
** completing the QX675 Multi Circuit Dual VCF panel.
** wiring up all of the normalized back-panel connections for the VCOs and VCFs.
.... plus plenty more. At least six months of work to go yet when you include putting together the "Modular Computer" (see link to that thread in my signature). I'm going to do my absolute best to really take full advantage of this current "productive phase" I'm in and manage my energy to get as much done as I possibly can (maybe even completing this project as well as the others I've got going on). So plenty of fun coming!!!

Ok folks .... since I'm feeling so much better there will be more regular updates coming. What's next? Who fekkin' knows? One thing for sure it will be something to do with the ......

..... SUPER MINI MODULAR!

Later, Cats ~n~ Kittens ....

:cookiemonster:
5U NORMALIZING PROJECT (for your entertainment) viewtopic.php?t=78836&highlight=

SCREW IT ... PULL THE PIN

KSS
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Re: CV Pedal workarounds

Post by KSS » Wed Sep 16, 2020 6:58 am

Outstanding photo essay, Rex!

I've loved that line of pedals since they first came out. So good. <-- Both the pedals *and* your writeup.

gb2
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Re: CV Pedal workarounds

Post by gb2 » Sun Sep 20, 2020 7:42 am

Wow, thanks for the informative and extensive input everyone!!!
That really cleared some things up for me, especially about the linear/logarithmic response issue.

I don't feel experienced enough to change the pot of a volume pedal to a linear one, so I will go with an expression pedal that can attenuate and invert.

My plan now is to get a TRS to 2x TS splitter cable and just send 10V to the ring of the TRS - in my understanding that should do the trick, as the expression pedal then "thinks" it's getting 10V from the effect pedal.
A dirty solution would be to solder a power supply directly to the "right" TS, the clean one would be to build a little box with power supply in and jack out.

Happy to hear further comments on this, especially if my understanding is wrong!

In any case: Thanks so much!

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bloke_zero
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Re: CV Pedal workarounds

Post by bloke_zero » Mon Sep 21, 2020 4:12 am

I've done the power supply thing - it went well until memorably the power supply went on the fritz during a gig and started randomly modulating the filter at high speed - not optimal, but I guess everything is a potentioal point of failure!

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