Doc modding Marantz imperial 7

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karatestu
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Today's experiment is regarding the subject of one of the quotes i pasted in this thread recently. I will put this in layman's terms if i can (i'm one too).

Not sure why but i became intrigued by aperiodic enclosures. To anybody who does not know they are basically a leaky sealed box (i.e not air tight) but usually involve a vent (not a port) with resistive material stuffed inside it. This resists air flow and stops the back wave from escaping the box (apparently). It is not like a port which outputs at certain frequencies and is tuned so can chuff and fart and generally sound bad (to some).

Seems like a halfway house between sealed and ported. Relieves the enclosure of pressure and lowers the magnitude of the bass driver's impedance at it's resonant frequency (Fs) which can be a good thing. It lowers the Q of the speaker which means a smaller enclosure volume can be used for the same results as before. The trade off is a Fs at a slightly higher frequency and a higher F3 (frequency at which output is 3 dB down)- bass output is slightly lower.

It used to be more popular with quite a few Dynaco models using this design. There are vents you can buy from scanspeak which are 11cm across and stuffed with resistive material.

I don't have any of those of course so thought of making my own. If it doesn't work out then i have lost nothing. Cutting a 11cm hole in the back of the big bass cube is no loss really if i have to seal it again. It;s going get thrown on the bonfire anyway when i finally build the finished article. And if i put my bass drivers between the floor joists i won't need bass bins anyway.

Not that this should stop the inquisitive from having a play. So i went to find the cordless drill and drill bits. A bit of chin scratching ensued whilst i pondered how big a hole to drill and where. Does the hole need to being the front isobaric chamber, the rear one or both ? Still not sure about that but decided to drill a 6mm hole in the back panel.

Sat down to listen not expecting much difference in bass quality as the hole is tiny and not stuffed with anything. But it indeed has done something. Even without any measurement gear i detect a more tuneful and even bass low down.

Thoughts move on to making a proper vent - a diy version of the scanspeak ones of course :grin:
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karatestu
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Boffle
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Who knows what a boffle is then. Without googling it ?
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Re: Doc modding Marantz imperial 7

Unread post by savvypaul »

I know what a bothy is...
I am in the hi-fi trade
Status: Manufacturer
Company Name: NVA Hi-Fi
https://nvahifi.co.uk/

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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Mentioned by LindsayT in a speaker thread this summer

Image
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Latest news from speaker bodge land.

Tried my isobaric 12 inchers in series. Overall i prefer parallel, just needed to remind myself. There is a loss of dB when going from parallel to series. Put it back to parallel.

Tried my MS902S cab with just the woofer in it. Of course i had to use the inductor on that as these metal continuous cones don't dope - i sprayed it with plastidip after i had removed the bison kit (quite a while ago). With these inward firing and in parallel with the two series wired 12" it was awful. Too much volume from the MS902 mid bass, it sounded tinny and the whole sound stage and imaging was completely goosed. The set up was very non-optimal so i won't discount side firing a 5" doped mid bass but it will have to be wired for the right spl and in the same enclosure as everything else.

Next experiment was a bit more involved. With inspiration from the boffle idea and other stuff i had read round and about i decided to remove the back panel from my big cube's. Found some old pillows and duvet's and shoved them in the back of the cab and tried not to leave any gaps around the edges.

Listening to it now with the 12 inchers wired in parallel. It's quite a difference to before. There is no lack of bass but it is not perfect however. The bass guitar sounds more natural and so do kick drums (which is not really low bass but it has had an effect on them). People go on about open baffle and how it sounds natural and without the boxy sound of sealed and reflex enclosures. I can hear that now although i do have an open backed box with a load of old pillows stuffed in the back :grin:

There is more detail to the bass quitar and more of it's natural tone but it does sound slightly woolly compared to sealed box. Maybe that is how it is supposed to sound ? or it is the pillows i have stuffed in the back (right up to the magnet as there is not a great deal of room with it being isobaric). On one or two CD's i have the bass has always set off a room mode (it's electronic music). It doesn't now and there is no shortage of low frequencies :think:

There is enough merit in this to investigate further although i don't have room in my living room at home to do open baffle justice. Maybe the boffle is a way to take this aspect further although i am in new territory for me here. :pray: Doc is probably shaking his head right now at the thought of me doing this but he did like open baffle (that's my excuse). Even after doing isobaric i have always thought that my big cube enclosure is too small for these 12" drivers to really do their thing. I am also going to try a clamshell isobaric arrangement and use the whole of the 85 Litre box for the rear driver (with and without the back panel on).
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

I know this is not strictly open baffle (i suppose it is one with sides and top) but i am really astonished by the amount of bass that there is. The back wave is said to cancel out the front to some degree as it is out of phase. My pillows and duvets wont be absorbing all the bass from the back as bass will travel right through it.

All i can say is that these 12" drivers must have a high qts (higher than any of the specs i have seen) probably due to the doping. I have seen qts figures published for these on an ebay seller's site which were much less than a thread i found on a forum where somebody had measured the qts as much higher. The bass goes deep, is detailed, textured, warm and does not seem to be as "in your face" - all at the same time.

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Re: Doc modding Marantz imperial 7

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Currently listening to Portishead Dummy and the room is shaking :shock: :shock: :shock:
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

I suppose isobaric open baffle or infinite baffle may be pointless . I will have to look in to that or try it with and without.
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Removed some of the stuffing and moved the speakers slightly further away from the wall.

I am hearing bass guitar detail i have never heard before on Herbie Hancock Secrets. Track no.4 (Spider) onwards is like :shock: :shock: :shock:
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