Doc modding Marantz imperial 7

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r3xj0hn570n
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Re: Doc modding Marantz imperial 7

Unread post by r3xj0hn570n »

karatestu wrote: Fri Oct 22, 2021 7:58 pm
If you think about it with a 6kHz xover, the tweeter is only 6dB down at 3kHz, 12dB down at 1.5kHz and 18dB down at 750Hz. A tweeter's resonance is usually around 1200Hz or higher and you really don't want it producing any meaningful output at resonance. A fourth order linkwitz Riley (24dB/octave) would be down 48dB by 1.5 kHz.
I've found odd order crossovers, well, 1st and 3rd, where there isn't any parallel component right next to the voice coil can interact with the driver resonance peak and product much horribleness. If you think about it, Your driver impedance has gone from ~5 Ohms to say 50, what has happened to that crossover order at resonance? 1st goes to 0th, 3rd goes to 2nd.

A first order crossover on a tweeter is much harder to do than you first think, at resonance the amp can supply pretty much no damping at all. It's not so bad when the resonance is out of the stop band, like in a low pass filter. Adding tweeter padding, series then parallel Rs, can help, then the series reactance can see these resistors at the resonance frequency.
karatestu wrote: Fri Oct 22, 2021 7:58 pm
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karatestu (Sat Oct 23, 2021 6:53 am)

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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Hey Rex. Thanks for your thoughts. Please excuse my drama queen hissing fit, this journey is littered with them. I feel better today after a sleep and I'm not gonna give up. Thing is I want to build something I will be happy with for the rest of my life. I believe in doing things right first time otherwise there is a lot of wasted time and energy.

Thanks for your thoughts on first order xovers. I did realise that even these single order filters are not as easy as they appear to be but I am a bit clueless as regards the details. When you are obsessed with removing components not adding them then adding resistors in the form of a simple series pad or a L pad seems just so wrong.

I am used to working with doped mid bass with no electrical low pass just the doped acoustical roll off. No idea what order that roll off is having not measured anything. I recently have become aware that this doping is strangling some dynamics and detail. It is a very safe and relaxing sound which I do like but as always with OCD types, you always worry about what you are missing out on. Hence my move to try and move away from doping.

Fist order passive line level filters in the input stage of the power amps (by just changing the cap values, no added components) seems to me to be the ultimate purist way of doing it. Nothing added and I don't have to worry about a lot of the speaker level filter issues like the xover being influenced by the drivers changing impedance and the inductive nature of the drivers voice coil. I am only dealing with resistors and they have very little inductance unless wirewound which nobody would use here.

The inductor I used on the doped 6.5" is a crappy unit harvested from a 1970's Marantz speaker. Not ideal and it sounded pretty poor when played without the full ranger. The DCR of the coil is maybe lowering the drivers output even more ? I can't believe I am having to think about padding down a 84dB full range to match the MB - if that is what the issue really is and not some full ranger resonance issue. Can it really be that insensitive ? Spec sheet says 88dB, winisd auto calculated it at 86.something which suggests the manufacturer is telling porkies. Add the dope and the fact it is pointing at the ceiling and maybe I shouldn't be surprised.

Emptied my head on here again :roll:
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Re: Doc modding Marantz imperial 7

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Looking for the easiest way to achieve the result I want with the fewest components possible. Maybe I should be using a single larger full range driver that can do bass to treble. I know they have their issues like intermodulation distortion from having to move so much for bass whilst at the same time trying to deliver the delicate treble.

How about using a wideband driver as the up firing mid bass married with a 2" full ranger firing forward in a diffusing sphere right above the MB ?

I have always been impressed with the Mark Audio full range drivers even though I have never heard one. I seem strangely drawn to them. They usually need to be put in a horn, MLTL or reflex design to get adequate bass output. I don't want to go there - I would have to use one of the many published designs by Dr Scott L as I wouldn't have a hope of designing my own. The one I have in mind can be used in a sealed enclosure although it's qts is marginal.

So I came across a Mark Audio driver which is a wideband driver, not a full range as it rolls off at 10kHz. It's a 7" cone with Fs of 40 something, qts 0.39, spl 88dB and vas about 40 Litres. It sims in winisd as maximally flat in 20 litres. Bingo ( possibly).

See here https://www.markaudio.com/online_sho ... 12pw/

At over £100 a pop then there is no room to get this wrong. I would probably try and use it without low pass firing up but if that doesn't work I would low pass it passively at line level. It need mors volume than one of my steel 30cm spheres can provide but maybe I can join two together which will look like a number eight.
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Re: Doc modding Marantz imperial 7

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To get a feel for full range drivers: if you go to the Audio Talk Owston meet, I will have my Fane 12-250-TC 12” aperiodic speakers on show. It’s nice to experiment, but the frustration can be terrible, if you are not getting anywhere. Sometimes it pays to hear what other people are doing and pick up ideas off them. Don’t know whether Scott L will be there, he usually turns up but never commits to coming, but there’s me and there’s Steve Shiels. :)
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karatestu (Sat Oct 23, 2021 9:20 am)
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

SteveTheShadow wrote: Sat Oct 23, 2021 8:22 am To get a feel for full range drivers: if you go to the Audio Talk Owston meet, I will have my Fane 12-250-TC 12” aperiodic speakers on show. It’s nice to experiment, but the frustration can be terrible, if you are not getting anywhere. Sometimes it pays to hear what other people are doing and pick up ideas off them. Don’t know whether Scott L will be there, he usually turns up but never commits to coming, but there’s me and there’s Steve Shiels. :)
Thanks for your advice Steve, much appreciated. I am thinking seriously of going to Owston but risk depressing myself even more with further proof of how inadequate my work has been.

I like the idea of your Fane 12" FR but I'm not prepared to accommodate boxes of that size in my little living room. It would have to be something smaller but then I am limiting myself to bass output and sensitivity. I don't think I know enough about the driver parameters to chose the best one.

I really like the 2" FR in small spheres above the up firing doped 6.5". Most would leave it at that and be done with it but oh no not me. I am my own worst enemy. Problem is I have had a taste of what a driver with less mass and no dope can do and I want it (the bits I liked about it). I don't want passive speaker level filters either for various reasons - I don't know what I am doing with them and the thought of using a coil fills me with horror.

So after thinking about it I have several options

1) Live with the doped 6.5" run fully open & 2" FR tweeter 1st order high passed passively at line level
2) Use an up firing wideband like the Alpair 12PW run fully open & 2" FR tweeter high passed passively 1st order at line level
3) Try and choose a down firing bass and up firing mid to marry with this 2" forward firing full range tweeter- all filtering done 1st order passive at line level.

One thing is for sure. There is no way I am going beyond 1st order even with the problems it throws up. The drivers overlap for many octaves so maybe better results come from using the same type of cone material for both. The Alpair 12PW was paired up with the Alpair 7P I think but that is no longer made and the resulting box size required means the sphere above the up firing driver would be too big.

At this rate I will be back at point & squirt having done a full circle of confusion. It's fooking annoying. :angry-steamingears:
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Interesting interview with robin Marshall who designed the legendary Epos ES14 (no low pass on mid bass, 1st order electrical on tweeter). He says that maybe small cones should be used for a tweeter rather than dome :think: And wide overlap between drivers is the best way rather than sharp 4th order cliff edges :think:

https://www.stereophile.com/interviews/ ... ex.html
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Further reading on Diyaudio.com confirms that the SBA65 2" FR diver I am using is a good choice for use as a tweeter. Many threads it gets mentioned as one of the best to use . As it has a sub-radiator (i.e a direct-coupled central cap which acts as a dome tweeter) this helps with dispersion of high frequencies. Also it's low sensitivity is mentioned but I can live with that as long as I can find a wide bander which when pointed at the ceiling marries well with it as regards volume at listening area etc. I listen relatively quietly in a small room.

I don't mind a dip in the mids. If a wide band is used firing up it is going to lose some of it's higher frequencies even though firing into a sphere. The bass won't care. It's a bit of a gamble whether a 88dB wide bander used in this way will blend with the 84dB forward firing 2" cone. The Aplair 12PW has a response that gently slopes down from 500Hz and it falls off after 9kHz.

Image

At 8kHz the SBA65 2" FR is starting to rise in output and is not that much behind the Aplair 12PW. If I had to I could cross the two at around 5kHz where the 12PW is at it's lowest dB. With the 12PW pointing up and 90 degrees off axis surely that will help marry the two up ?

Any thoughts Rex, Steve or anybody else ?

Image
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Re: Doc modding Marantz imperial 7

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A bit more experimenting done. Yesterday I added some little 3mm ply shapes to make the 2" FR seem to be round. This effectively makes the driver rebated as I have also added more decorators caulk to smooth out the transition. Haven't sanded it down yet but it's not bad.

That improved detail, imaging and a couple of petty annoyances. I then thought about the 2" tweeter's reflections and re-radiation off the top baffle of the 30cm cube that is underneath. I added some 10mm thick underlay type material out of an old car seat to the top baffle. That has further improved things. I don't know why I didn't do it sooner.

Oops, forgot the pic (just for Tomasz, I know he loves the look of my creations :grin: )

Image

It is clear that I will have to pay close attention to edges and reflections as it is a considerable cause of response errors etc than can be very annoying. Once I have the steel 30cm spheres in use that won't be as much of a problem as the top baffle slopes away. I may experiment with some reflection absorbing material on the front of the top half of the steel sphere.

Starting to feel a bit better about it all now. When I had passive xover at 1.5kHz then the little 2" FR will have been reflecting loads more stuff off the mid bass cube's top baffle as well as re-radiating from edges on the tweeter sphere. Paying attention to these things really does pay dividends.
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

'Any thoughts Rex, Steve or anybody else'

I should be used to being ignored by now :grin:

I keep thinking about a mini line array with these 2"FR's. Two of em will give me an extra 3dB if they each have their own amps. Can't wire them in parallel as that would be a 1.8 ohm load. In series you gain 3dB from an extra radiator but lose 3dB from the impedance doubling. Four of em could be wired in series parallel for the same 3.6 ohm load as a single one with one amp. But The number of radiators has been doubled twice so a 6dB gain. That would take the spl to 90dB. Anymore than that then the matching bass driver starts to become too big for my proposed box size limits.

You can't win at this game, the laws of physics are a real cant.
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

As I listen quietly nearly all the time then why not design these speakers with a smiley face frequency response ? Elevated volume at low and high frequency will make it a good listen at low volumes (Fletcher Munsen curve). As normsl speech is about 60dB J don't think I listen above about 65 dB :shock: The 2" FR have a rising response. All I need to do is engineer a bass hump :lol: or crossover at the middle 9f the baffle step and use a more sensitive woofer. Shouldn't be hard seeing as though the 2 incher is only 84dB :dance:

Being content with lstening quietly has so many advantages. Low sensitivity drivers, low amplifier power - no problem. Low intermodulation distortion :dance: Healthy ears :dance: :dance:
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