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Mission 775 To plug or not to plug?

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Mission 775 To plug or not to plug?

Postby ef37a » Sat Dec 01, 2018 6:18 pm

I have just inherited a pair of these speakers along with an Arcam Alpha 6+ amplifier and a Rega Planar turntable. (all old kit I know and Hugh will be apalled!) .

These are twin bass units some 43" high (taken off the bases and the daft spikes) and of some 2cu ft voume. Pretty big mothers. They are reflex loaded with a flared front port.

What I would like to know is if it would be in order to block up the ports? I do not need high levels and if conversion to IB would give me a better bass quality with perhaps more extension I would be happy. Of course I could just try it and see but I wondered if I might be doing something dreadful to the response? The speakers were of course originally designed for port loading.

My intention is to run them for a few months then if I can get silly money for them, knock them out and put the cash in the Result 6* fund.

*My present monitor lust but that could change because it will be some time until the Plastic recovers from the seasonal bashing and Jan thru April is heavy for car stuff.

Dave.
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Re: Mission 775 To plug or not to plug?

Postby Hugh Robjohns » Sat Dec 01, 2018 6:42 pm

ef37a wrote:(all old kit I know and Hugh will be apalled!)

There's now't wrong with quality vintage kit as long as it has been well-maintained -- and I have a soft-spot for early Rega turntables anyway!

What I would like to know is if it would be in order to block up the ports? I do not need high levels and if conversion to IB would give me a better bass quality with perhaps more extension I would be happy. Of course I could just try it and see but I wondered if I might be doing something dreadful to the response? The speakers were of course originally designed for port loading.

Phil Ward is your man for this query, and I'm sure he'll be along soon, but my two-penneth is that you won't harm them in the experiment. Of course, the bass response will change dramatically, and whether that works for you or not will be a subjective decision. Of course, you don't have to block the ports completely (as in air-tight), and it leaky blockage might actually be a better option, using an open-cell foam bung.

By reducing the port's effectiveness you will reduce the rate of the LF roll-off, making it shallower (which equates to greater LF extension), but you'll also raise the start of that roll off and so reduce the overall amount of bass. It may also reduce any corner peaks in the response, which might translate as less boomy or just lacking in oomph! :-)

H
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Re: Mission 775 To plug or not to plug?

Postby ef37a » Sat Dec 01, 2018 8:19 pm

Thanks Hugh. The Rega has an Ortofon MM cartridge, can't see the model until I get a magnifier and lot of light on it. Sounds ok with Hard Days Night, first disc I had handy.

The CD player is another Arcam and the speakers sound terrific playing the Saint-Saens Organ Symphony. Mind you the room is very small, 12X 12.5X8.5 ft and bass is a bit OTT. Only trouble is the left channel cuts out but I can live with that, lots of ways to play CDs but only have this integrated amplifier.

The Denon cassette playing Supertramp sounds good though. Bass G plays tunes and the kick is clearly defined (Right, Right, Your Bloody well Right!) . #

I was not worried that I would damage the speakers. They are rated to 200W and the amp is only 50+50. No, I was concerned I might screw a carefully designed LF response. In this room and with MY mid>hf loss I could definitely do with less bass!

Phil's input would be very welcomed but Monday will do!

Dave.
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