Ok guys

My question is this, nothing major, more of a curiosity. When playing an album does it make a difference if you leave the dust cover up or down? Probably stupid  question but hey there are lots of opinions out there
I'd base it on how it sounds - whichever sounds better.  There's no hard/fast rule that dictates if it should up or down when playing.
Ideally we'd want the turntable in a different room. :)

Dust cover off during play,! No other way.
ok that was my question, thanks
Up or down, either way, as long as its attached to the table its channeling acoustic energy straight into the turntable. 
Ok thanks that was my stupid question like I said more of a curiosity 
I do not use dust covers at all
Not using a dust cover is a sure way to destroy your records but it is great for the record cleaning industry. I never clean my records. Don't have to.
As for sound that depends on the turntable and the dust cover. This may be the reason there are so may opinions. Most turntables do not come with a dust cover.  Those always sound better without if you catch my drift. A suspended turntable like the SOTA always sounds better with the dust cover down. The sub chassis with the arm and the platter are floating inside so the dust cover is not connected to it. The dust cover attenuates the noise in the room 20 dB or so, like having hearing protectors on.
Unsuspended tables with a fixed plinth might have a problem with the dust cover attached directly to it. Close your eyes and have a friend or spouse raise and close the dust cover to see if you can identify when the dust cover is down. If not, you are in business. Same if it happens to sound better. 
Playing a record generates thousands of volts of static. The record becomes negatively charged but dust is positively charged. So the charged record pulls dust into the groove like a magnet. If you use a dust cover and a conductive sweep arm this does not happen at all. Records do not get noisier and you never have to clean them. Preserving your record collection is paramount more so than even sound. 
Making a dust cover for odd turntables like a VPI Avenger or an SME is not hard at all. You just hinge the dust cover to a plate on which the turntable sits. If you can not do it yourself these guys would be glad to. 
https://www.displaycasej.com/custom-audio-covers  A turntable covered in dust and grime is a travesty. Precision instruments should not be treated this way. To me a turntable like the Clearaudio Statement is worthless because you can't put a dust cover on it. But I can put a dust cover on an Air Force 1:)
Dust cover off and placed in storage: 4
Dust cover on at all times: 1 (Mijo)

I added my vote for no dust cover to those of Chak, Slaw, and Miller Carbon, for a total of 4.

To be clear, leaving a dust cover attached to the plinth in “up” position is also a no-no. Lets keep a tally.
I still use the SOTA Sapphire turntable I purchased in 1983, which btw still works as well as when it was new. I can pound its plinth with my fist with light to moderate force while a record is playing, with no audible consequences. Given that, it’s hard for me to imagine how vibrations induced in its dustcover and conveyed to that plinth could have any audible consequences. So in the case of that particular table, and presumably its later incarnations as well, I agree with Mijostyn.

-- Al
Surprised, Al.  Have you tried without?
mijostyn makes a good argument for using a well-designed dust cover. The key words being well-designed. I've thought about it before and never bothered because of the difficulty of the well-designed part. Its not that I don't know how to do it. Its that I know it will be hard to do! But I'm getting to the point where .... and ditto static. What's that gizmo you use again, Mike?
Have you tried without?

No, I haven’t. Even if taking the dustcover off would somehow result in minor sonic improvements, which I can’t imagine with that particular table, I’d give a higher priority to keeping dust off of the platter (which is made of a material that is not necessarily the easiest to keep clean). And I’d rather not have to take the dustcover off and put it back on before and after listening sessions.

Best regards,
-- Al

Put me in the no dust cover camp, that's +5 and counting. Enjoy the music

A dust cover is a no-brainer to be desired. Nobody wants dust on their records, turntable, tone arm. Nobody wants the acoustic energy in the room vibrating the sensitive cartridge/arm system. The trick is to do it in a way that doesn’t create more problems than it solves.

i guess dust covers are like politics, religion, olives, and Kenny G. All taboo subjects in polite company.

It’s interesting that the pro dust cover guys, so far, are both owners of SOTA turntables. I owned a Star Sapphire Series III with vacuum  for most of the 1990s, and never used the dust cover. Al is correct that the felt platter mat does/did accumulate dust.
Mc,  https://www.sleevecityusa.com/Antistatic-Record-Cleaning-Arm-p/tac-01.htm. If you get one let me know there are a few tricks to setting it up right. 
Guys, as I said before the SOTA sounds better with its dust cover on and down as does the SME. It is like taking the blur out of a photograph. I have done the fore mentioned experiment and I can routine tell when the dust cover is down blind folded. If your turntable sounds worse with the dust cover down then have a platform dust cover made or get a new turntable. Not using a dust cover is not an option. Record safety is paramount. Using a conductive sweep arm will help but still incidental dust will fall on the record not to mention get into your tonearm and bearing.  
Lewm, I also own an SME and it benefits also as does the Linn LP 12
All suspended turntables. I suspect the Air Force 1, 2 and 3 will also benefit. My opinion is based on suspended turntables. They are the only type of table I have ever own except for my very first one (TD 124). I would never own anything else even though my equipment cabinet rests on concrete. The SOTA is a brilliant turntable for the money. It easily runs with the SME, a table that costs 4 times as much! The only down side is that it does not take a 12 inch arm and some arms like the Tri Planar will not fit. I also must warn that Putting a 4 Point 9 on the newest version of the Cosmos is apparently difficult. An A goner ordered one with a 4 Point based on my opinion and SOTA is having a hard time getting it to fit. I think they will shoe horn it in but just barely. SME's and Origin Live arms fit perfectly. Unfortunately, SME arms are going to get hard to find because the company will only sell them with their tables, a stupid decision IMHO. SOTA is thinking about making it's own arm. Lets cheer them on!
I’ve got carbon fiber just sitting in my shop. So I’m thinking instead of a contraption that runs across every record inevitably picking up crud it then smears all over my pristine White Hot Stampers what I do is stick a little tuft of conductive carbon fiber on a stubby little arm that holds it to the edge of the record. Then it doesn’t have to go anywhere, doesn’t have to be moved every record, just sits there and each record contacts it on the edge.

Might not be quite as good as grounding charges closer to the stylus but then again I don’t have to worry about cleaning it after playing any of my less than pristine records- which is most of them!

A proper cover for my non-suspended table would be a project but probably worth the effort. I mean the way things are now my system is so freaking awesome instead of being content its actually making me want to make it even more and more perfect! This is sick! Its a sickness, I am telling you! I’ve got food and electricity and tunes and practically looking forward to being quarantined just so I can have the time to build my super static destroyer dust cover!
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If my friends are having a few drinks its on.If not its off
Miller is wavering but if he sticks with his original position I think the poll is now running 7-2 against using a dust cover when playing LPs. (I have no beef with covering a turntable when it’s not in use.)

I must be lucky; I have no special problem with dust in either of my two audio systems. My Kenwood L07D has a clever dust cover for the platter that is used only when the turntable is out of service. It is the size and shape of an LP, made out of Lucite, fits over the spindle, and covers the entire platter surface. When I want to play a record, I remove it. It’s a factory original part that came with the turntable when new.
I'm a SOTA owner and I'm with mijostyn.  The machinery sounds better with the dust cover down.  And I'm a happy camper that the preferred dust cover position keeps the LP cleaner.  It's only when I'm spinning 7 inch 45's that I have the cover up.  My 45's are often in terrible shape, and the lead-out grooves are usually so noisy I can't bear the din a second more than necessary.
Mc, silly you clean the brush off between plays. Once in a blue moon I'll clean the brush with alcohol. It won't smear anything and it will keep any incidental dust away from the stylus. The sweep arm is only 20 bucks. Spend some money and experiment a bit. Who knows. You might actually like it. 1++ edcyn! 

Neither up nor down. Off when playing records, on when not playing records.
It does appear that only the users of suspended tables like the SOTA are fully in the camp of dust cover on and down.

I guess I have to say yes and no... Lol.

The lift off cover on my Garrard 401 is only on while not being used to play records so definitely off on that one.

My Technics SL-10 being a linear tracker has to have the cover down and closed to even play so .....

Back in the day, I discovered that my LP12 sounded a lot better with no dust cover. I was in the listening chair when a friend lifted the lid while music was playing. With it on, the music and soundstage sounded literally closed in. So it was a ritual to take it off and slot it back in when finished. Now I never even consider using a cover when playing.
So, suspension per se does not determine the benefit of a dust cover, based on noromance’s testimony. I’m not keeping track any more, but I think there are 3 who use a cover while playing LPs. The rest of us are on the other side of the fence. If I had to put it in a nutshell, I would say that the use of a dust cover sounded closed in to me, just as no romance says. By now I guess we could collectively advise the OP to simply try it for himself and make his own judgment. By the way this has everything to do with sound quality and nothing to do with dust protection.
I take it off while playing. Simply because the hinges broke a couple of decades ago so it is either on/covered or completely removed. I do not put it back on while playing not to accidentally make some damage while positioning. It comes close to the weight, etc.
Technics SL-1200G and SL-1600MK2 both dust cover down
The manual of the VPI HW-19 MKIII recommends that "the dust cover be down when playing records. Under most circumstances this will result in cleaner sound since the cover acts as a 'shield' against acoustic feedback." And of course keep the record and stylus clean. 
Put. me in the cover on camp. SQ better.
What brand ,model turntable and cartridge ? How good it is will have a lot to do with its stability and vibration control.  I don’t know of many quality turntables with a built in lid.
This discussion illustrates beautifully the problem with technical attributes. The subject at hand is dust covers. But it could just as easily be amplifier watts (is it better to have more or less?), speaker cables (is it better to have silver or copper?), or any of a hundred other technical items. 

Everybody wants a simple answer when the truth is there are no simple answers. Its better off, probably, unless its a table designed to be better with it on and down, in which case it should probably be on and down. Probably. The only thing we know for sure is which way it sounds better determines which way it goes- and not the other way around. So once again might as well just cut out the middle man. Go and listen. You will see.
It's good to have a dust cover to keep your table clean when not in use.  When using, do some blinded tests and see which you prefer.  If you can't tell the difference, then play records without cover so you don't have to bother with it when changing sides.   
...The stylus may "resonate" inside the dust cover, creating unwanted reflections. In addition, if the cover picks up vibrations, it creates an internal rumble that aggravates what has already been said. We must try
By no means do I have a "fancy" turntable, its a Pioneer PL-550.

But.... I've tried listening with the cover down, cover raised, and cover completely off.

The best sound achieved for me was with the cover completely off. My turntable is about 10 Feet to the left of my other components / listening area, and with the cover down or in the raised position, I get some stray vibrations / rumbling. (it is worse with the cover raised).
@kewm  Kenny G?  Kenny G??

Good grief, man!
stani? When you put on hearing protectors does your head rumble and your tongue vibrate?
Lewm, tell me your system is in a clean room. That would be a $250,000 upgrade. Records that have dust ground into them do not sound so hot.
A dust cover has to be hinged to be useful. Again the best way to do this for unsuspended tables is to have what I call a platform dust cover. You have a lexan plate on which your turntable sits. The dust cover is then hinged to the platform. If there is limited space behind the turntable hinge the dust cover at the top with the back fixed to the plate. This version requires a prop to keep it open. If you have plenty of room behind the table the you can hinge it at the bottom and the dust cover will counter balance itself so friction hinges will do the job. You can use any thickness of lexan you want. I would bet that 1/4" lexan would drop sound 30 dB under the dust cover. That would be like putting your turntable in another room. These guys will do anything you want,  https://www.displaycasej.com/custom-audio-covers

I was playing with my dust cover up and down last night when it dawned on me. There can be a significant delay in the sound getting to the turntable depending on where it is in the room. If the delay is long enough this causes an echo which can give you the sense you are in a larger room which might appeal to some people. A euphoric distortion. I would rather listen to the acoustics on the record. 
Well for once VPI and I are on the same page. Go figure.
I would not have a TT without a dust cover when not in use. It's a great place to place a tray of house plants on, while keeping everything clean, especially the refined moving parts of the tonearm.

Open or off when playing. If you believe in microphonics, open, let em go.

Hinged dust covers up while playing? Keep a bubble level on the deck. Does it change when the dust cover is on-up? surprisingly a few I had did not effect level. 

If effects level, you can remove the hinges and just put it on whenever.

Seriously, we bought a very nice unit for audio equipment and I told Donna she could have the top glass shelf , table height, for plants. Well, eventually, to get some stuff on that shelf, and keep my word, I painted 2 cafeteria trays, big felt strips on the bottom, and they actually look better higher up. I trust her watering skills. Just move the tray off the TT to the top of the Reel to Reel Dust cover, play on.
It would be nice if for once you could just accept the fact that your opinions are going to differ from other members.
Nobody is arguing that what you state in your system is what you hear and great for you.
However maybe you could just accept that it is in fact just your opinion and not try to foist that off to everybody else here as being gospel.
It works for you and that's wonderful, apparently it does not work for a lot of other members myself included and that should also be wonderful, not an area of contention. 
Richopp, I did not endorse Kenny G. I included him in a list of things about which people disagree vehemently. If you must know, I regard his music as a particularly brutal form of torture.

Mijo, you have an inordinate dust phobia. I am glad your dust cover ameliorates your anxiety. I however will never ever even consider using a dust cover whilst playing an LP. I clean my LPs with the Audioquest carbon fiber grounded brush just before each play and I clean my stylus with Magic Eraser before each side. LPs that don’t have groove damage from previous owners play silently in both of my systems on any of my 5 turntables. I do also clean any used LPs I buy on an VPI HW17. If your sota sounds better with cover down, that’s a good reason to use it. It’s fine with me, even if you like Kenny G.
If the table has a hinged dustcover, it should be down. Look, it's your table...do what you want! I am sure that the reason that manufacturers included a hinged dustcover was so that it could be closed while in use.  
+1 to @audioman58 for asking what turntable the OP is using.

If the OP’s system description is up to date he is using a ProJect Debut Carbon DC, which is a non-suspended belt drive table weighing about 12 pounds and costing ca. $400 including an Ortofon 2M Red cartridge. Certainly a very different animal than the SOTA tables which are the only ones to which my earlier comments pertain.

-- Al

For my part, I apologize if my last post was a bit rough on Mijo. Everyone should always do what sounds best to that individual. I guess I made my dust cover decision years ago and am now too old to change.
If the table has a hinged dustcover, it should be down. Look, it's your table...do what you want! I am sure that the reason that manufacturers included a hinged dustcover was so that it could be closed while in use.
Or so that it could be opened to actually place an album on the platter and then play it open or closed and see which they prefer.
Very few absolute black and white ( open and shut) cases where audio is concerned.

But the only true test is to try both ways in your own system as it could give an entirely different result in another persons system depending on a whole slew of factors outside the table and dustcover.
Obviously, YMMV....🤔
Lewm and unberwaltz, you can be as hard on me as you wish. The sun rises in the east and sets in the west. Playing records creates static electricity. Static electricity attracts dust, dust damages records, dusty records get noisier and noisier. I hate noise. I hate worn out records.
Using a Zerostat or conductive brush before play discharges the static electricity from the last play. As soon as you put the stylus down new static electricity is generated attracting dust not just on the surface but into the groove. None of this is my opinion. I promise I will put IMHO anytime it is just that. Going farther as you put your staticy record away more dust is attracted and you drag all that dust into the record sleeve. You might as well throw your records into a sand box, oh IMHO. 
I hardly ever have to clean my stylus. If you have to clean your stylus before every play your records are filthy. 
I have no idea who Kenny G is. I generally do not listen to either Rap or Pop music. My wife has dragged me into country but just barely. I like Alice in Chains, Wayne Shorter and Stravinsky.
It’s easy to try it both ways and find out for oneself.

My personal experience is it can help isolate lesser tables like many entry level tables better which is a good thing sonically and to help keep dust off things while playing.

With better isolated, heavier tables on a solid foundation I notice no real difference in sound but the dust still stays off better when the cover is on so that is a plus.

Can’t say I’ve ever noticed any down side to cover on while playing.  I've both sold and heard many turntables over the years.