Doc modding Marantz imperial 7

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karatestu
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

20cm goosenecks arrived yesterday :dance: Playtime
DIY inspired by Richard "The Doc" Dunn RIP

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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

I was curious to go back to a 19mm dome tweeter since the xover frequency has been increased to 10KHz. First I stuck it on the front of the steel mid bass sphere (magnetism) as close to the mid bass as possible. This angled it upwards towards my ears. Even with a phase shift from the high pass cap the tweeter still dominates even when padded down (10R). I suspect it is too far forward.

Next I tried it centred directly above the up firing mid bass' voice coil. The driver was naked (no enclosure or faceplate) and mounted by magnetism to the top section of a steel cymbal stand - like people use to mount microphones when measuring. This was interesting - the nakedness of the driver (just 6cm diameter) gave a very open and airy sound that was very compelling. It is my firm opinion that the speaker just doesn't sound right when the tweeter is not above the cone area of the up firing mid bass.

I tried it with the high pass filter at speaker and line level and the results were the same. However, I still think small dome tweeters are not up to the job with a 1st order high pass even crossed over as high as 10kHz. They aren't even 18dB down by the tweeter's resonance frequency. Compare that to the tweeter crossed at 3KHz with a 4th order - it will be over 24dB down by the same frequency.

As much as I liked the open presentation that the naked dome tweeter gave when centred above the mid bass I could not live with the added distortion and tinny sound that the dome produced. This was underlined when I eventually put the 2" cone tweeter in it's sphere back in. There is something about small domes I don't like. It could by that they just sound small scale or the tinny sound that I suspect comes from the reflections off the pole piece behind the dome - yes even though there is a felt pad in there.

So 2" full range driver (well almost full range) brought back what i was missing with the dome. Much improved detail, ease and a complete lack of a "tinny" sound (don't know a better way to describe it). As the 2" cone will have more limited dispersion I suspect that may have a bearing on my preference as well.

I could mount the 2" cone just below the up firing mid bass BUT I can't live with the tweeter that far forward from the voice coil of the mid bass. It just sounds wrong to me even with the phase shift from the high pass capacitor. So, dome tweeters are definitely out as is having the tweeter forward from the up firing mid bass' cone.

I have had to compromise of course. The diffuser sphere above the mid bass is essential because the 2" needs an enclosure for the back wave (unless I ran it OB of course ) and the tweeter needs to be above the cone of the mid bass to sound right to me. Oh god. I just introduced another thing I haven't tried - open baffle cone tweeter :roll: Not sure how that would work. Lots of sharp edge to re radiate the mid bass wave. After all I have banged on about smooth spheres and all.

There will be no going back to speaker level high pass caps, padding resistors and dome tweeters. Got that ? Passive line level (1st order) is much better IMO. Sometimes you need to go back to things to realise how inferior they are.
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

More things to tell you about. :roll:

I removed the blutack I had around the junction of the mid bass' cone and dust cap. Reason being I decided I couldn't live with the bloody pale yellow of the cone. Decided to lightly spray the whole thing with black plastidip and then put some blacktack in there instead of the blutack - for aesthetic reasons only.

The blutack left a bit of an oily residue and a couple of little blobs on the cone. In my haste I doused a cotton wool bud with alcohol and got it off. Sprayed the cone and dust cap very lightly with black plastidip - just enough to cover the yellow of the kevlar. All looking good until later I went back for a look. Disaster, or so I thought at the time :( The glue holding the dust cap on had loosened and it was clearly not adhered anymore .

There is a happy ending to this but at the time my whole system was going in the skip. really. A fair bit of cursing later and I managed to calm down enough to think that maybe all wasn't lost. I could just glue the dust caps back on .....but wait, this might be an opportunity for a bit of experimentation. So I pulled the dust caps off completely which wasn't hard and set them carefully aside for the eventuality that I had to put them back on.

So i could now see the small area of cone below the glue line that was still yellow, the coice coil former and the cavity in the centre of that. The cavity is vented near the bottom of the voice coil former. Anyway, I haven't taken pics but what I was looking at was a bit like this

Image

The voice coil former sticks up a fair bit and the dust cap is glued on part way from the centre of the cone. I obviously didn't want crap being able to get into the voice coil gap but I thought I would have a listen with no dust cap on. It was pretty bad. Bass had diminished and there was a resonance from the open cavity even though it is vented to the rear.

Bass diminished probably due to the enclosure no longer being sealed and I had removed the added weight of the blutack which lowered Fs and increased Qts. the resonance from the exposed cavity was pretty nasty. That got me thinking about putting some open cell foam in there to see what happens. So I did. The resonance was gone but the bass was still a bit under nourished. So the cavity needs to be sealed off to stop any air leak.

That got me thinking. Well I need to add the blacktack at some point to damp the cone breakup and add weight (which brings the sensitivity down to match the 2" tweeter as well). The voice coil former sticks up a bit and there is a volume which could be filled with the blacktack and then a flat dust cap could be pushed in on top of that.

As I don't have any blacktack yet I put the blutack around and on top of the voice coil former. Next I cut a perfect circle of butyl rubber of the correct size to go over the top of all this and join up with cone. I carefully pushed down on the rubber circle with a length of solid brass bar of the same diameter to bed everything in, leave a flat surface and join the rubber and cone neatly all the way around.

Job done. Then I remembered I had left the foam in the cavity inside the voice coil former. Oh well, I can take it out if needs be. There is definitely a resonance from that cavity even when the back is vented in to the enclosure and it must make it's way through what ever dust cap is on. So I left it in. I realise the voice coil won't be able to dissipate heat as well from the area where I added the 10mm thick slice of foam but the cavity is still vented to the rear, my steel spheres are a good thermal conductor and I don't EVER thrash my speakers.

I really don't think power compression will be a problem at the levels I listen to or most hifi scenario's. There is obviously a resonance from the cavity and if the foam helps then it's a winner.

So now the cone is plastidipped, the dust cap is now flat and much thicker material and the blutack is completely hidden from view under the circle of butyl rubber. The cone and new dust cap are still damped by the blutack and the added weight is nearer the voice coil of the driver - it's actually touching it. I now have a much better looking driver but I hadn't listened to it yet :shifty: Making even small changes to a driver can have dramatic effects for good or bad.

I fired it up and wasn't disappointed. Not all breakup comes from the dustcap and inner parts of the cone so the very light coating of the cone with plastidip has imo been beneficial. Dust cap removal is very well documented on diyaudio. Some even replace with phase plugs but I'm not going there, seems to be too much of a shot in the dark. The voice coil cavity is a well known cause of resonance which is why venting or filling it with a solid phase plug is done (amongst other things). So I think my foam plug in the cavity and blutack around the outside of the bit of the voice coil former that sticks up will be staying even though there is a reduction in power handling. I reckon the trade off is worth it.

Subjectively I think there is a definite improvement in the FR of the driver. My enjoyment of the speaker is without a doubt increased. I have no before and after measurements to show so it is just my subjective opinion - of limited use I suppose but I only have myself to please after all.

I have nothing that stands out as a problem anymore. It's all good. Am I too fussy ? Probably but I need to get this right as these speakers are going to be with me for the rest of my little, unimportant life (hopefully) and have no measurements to help only my ears (and my family's ears). At least the drivers are much better to look at now as well :dance:

As you were. If you hadn't already lost the will to live then you surely will have now. One of my best friend's funeral today :( :( :( :(
DIY inspired by Richard "The Doc" Dunn RIP

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Re: Doc modding Marantz imperial 7

Unread post by antonio66 »

You lost me with your last couple of posts Stu, (doesn't take much doing), but bloody well done, you have certainly put in the effort:

Sorry about your friend, hope the day goes well for you:
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karatestu (Fri Mar 11, 2022 7:33 am)

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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

antonio66 wrote: Fri Mar 11, 2022 4:52 am You lost me with your last couple of posts Stu, (doesn't take much doing), but bloody well done, you have certainly put in the effort:

Sorry about your friend, hope the day goes well for you:
Thanks Dave. The important bits are 1)dome tweeters and 1st order filters suck 2) tweeters too far forward from the mid bass don't sound right.

We gave my friend a good send off. The church and pub after were absolutely rammed. I have never been to a better attended funeral. He was a beautiful man.
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antonio66 (Sat Mar 12, 2022 4:36 pm)
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

My head hurts.
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

I am no longer afraid of messing about with speaker drivers. I wanted to use these B&W drivers as they are very well made. Nice aluminum frame which gets out of the way of the back wave. The magnet is on the small side but again gets out of the way of the back wave better. The voice coil cavity is vented through the voice coil former - you can just see one of the holes in the pic below. The cheap Chinese drivers are a joke compared to these.

Image

There is no getting away from the fact that kevlar can have a nasty break up resonance that either requires a notch or crossed over low with a steep filter. In my P4 speakers which these drivers are removed from B&W crossed them over at 3.5KHz with a single series inductor. No wonder they annoyed me a little. I always blamed the metal dome tweeter but that was crossed with a 3rd order electrical high pass and I know better now - it was the mid bass' break up that was the problem.

When trying to use this driver wide open with no electrical filtering then something mechanical has to be done. The old way of pouring on contact adhesive was not an option for me anymore. It clearly kills a lot of a driver's life and get up and go. The resulting sound is very pleasant and soothing but too much is lost in the process.

I think I have found the best way of doing it now. A very light coating of plastidip. Remove the dust cap and put the doping around the voice coil former. Fil the cavity inside the voice coil former with open cell foam or something that won't turn to dust in time. Put a flat rubber covering over all this - everything is nearer the centre of the cone and nicely hidden by the new heavily damped dust cap.

It has worked very well with this driver, I am not saying that will be the case with all drivers. The FR is clearly better to my ears. There is damping of the break up and voice coil cavity resonance but we haven't thrown away the driver's life and get up and go in the process. I'm well chuffed with the result.
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savvypaul (Fri Mar 11, 2022 11:31 am)
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Re: Doc modding Marantz imperial 7

Unread post by SteveTheShadow »

Joe Ackroyd of Royd fame ran his bass/mid units sans dustcap. His pole pieces were not vented though, so he didn’t have the problem of the cavity resonance. They were decent speakers but lacking in bass, especially the Eden, which had the internal steel plates. Weighed a ton. Nearly killed me carrying the pair I bought, all the way from Sheffield Devonshire Green to the train station. Good job it was downhill and I was still in my twenties at the time. :lol:
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Latteman (Fri Mar 11, 2022 12:56 pm) • karatestu (Fri Mar 11, 2022 2:02 pm)
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Re: Doc modding Marantz imperial 7

Unread post by Latteman »

I had the minstrels- similar no dust cap design- they were sensitive to sunlight / heat which made the ‘dope’ run into the voice coil killing them if not maintained
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karatestu (Fri Mar 11, 2022 2:02 pm) • SteveTheShadow (Fri Mar 11, 2022 2:43 pm)
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Been playing around with the goosenecks. And decided to give my tweeters the dipole treatment ie no enclosure. Looks like this....

Image

Only done roughly by hot gluing the gooseneck at both ends and wiring not inside it. Just got the other one to do now then some listening. I left the butyl rubber that is stuck on the mounting frame. These are crossed over at 10 KHz and so baffle step loss is not high on my list of worries at the moment. it will make the roll off second order when combined with the electrical first order filter ?

Image
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Ithilstone (Fri Mar 11, 2022 5:01 pm) • r3xj0hn570n (Fri Mar 11, 2022 6:02 pm)
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