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Telstar 29th November 2012 12:59 AM

Extended and Efficient midrange: what do we have here?
 
I'm collecting candidates for a 3-way I want to build in the foreseeable future.
The design goal is (real) 93db efficiency (update: had to lower a bit to have more options).
-The tweeter is chosen and is my beloved scan-speak 7100-03 crossed pretty high (from 3k to 5k -I will experiment)
-Bass will be some sort of long folded TL tuned at 25hz to 200hz (absolutely NOT higher than 250hz). There is plenty of suitable drivers so this is not an issue. My current favourite is the 10" scan-speak Discovery.

-The mid is where life is and the doubts too :) I decided that will be in closed box, well isolated from the LF.
The crossover between midrange and tweeter should be first order acoustic and the least amount of passive equalization should be used, where possible mechanical filtering.
Minimum impedance should stay well above 6 Ohm, unless the sensivity is so high that I can connect the drivers in series (as it is the case with the PHL).

Update: After some more research I decided to settle with this configuration:
A pair of midrange in MTM, ideal size between 4" and 6,5"
The single 8" option has been discarded.

Drivers marked with * are on paper the most suitable.
Drivers marked with - are decent alternatives.
Cost is limited to about 250€/speaker (since 4 are required).

The very scarce list of candidates so far includes:
6-6.5":
*Acoustic Elegance TD6H-8 (excellent driver 200-4k, a bit inefficient)
-Acoustic Elegance TD6M-8 (slightly less lowend than the H version, same 90dB)
-PHL (1120, successfully used 250-3k; other models???)
-Supravox 165GMF (excellent linearity usable above 5k, possible distortion around 2k)
-Audax 170Z0 (efficient, needs some EQ **NB: discontinued)
P.Audio SN6-100N (usable between 150 and 3,5k) - carbon fiber cone and neo magnet
P.Audio SN6-200F (150-3k) -more traditional paper cone and ferrite magnet
B&C 6MD38-8 (decent to 3.5k, lacking midbass)
18sound 8NMB420 (decent to 3.5k, lacking midbass)


smaller than 6":
*Scan-Speak 12MU/8731T00 (veery nice little driver capable of 200hz-over 5k, bit inefficient ofc)
-ATC 3" dome mid (good 350-5k, lacks low-end)

8":
*Jantzen JA8008 (ok 200 to 4k, smooth, no breakup)
Beyma 8M70v2 (usable up to 5k, good low-end, cheap)
PHL (2440/50, 2460, lack published data, which I'm searching)

LineSource 29th November 2012 07:06 AM

Telstar, I think you have a love square here :-)

What do you love more:
~94db/watt efficiency,
OR your Scan-Speak 7100 playing at 3+KHz,
OR good polars and controlled directivity,
OR 1st order Xovers ? ? ?

The current trend in speaker design is to avoid lobing in the polar response, and to have a smooth controlled directivity shape function from bass to treble.

To avoid lobing, the center-to-center distance of ALL speakers operating over the crossover frequency should ideally be less than (wavelength/4) , and at the worst case less than (wavelength/2) . For an M-T-M this wavelength/(4-->2) spacing applies to the M-T and M-M distances. Hence, great M-T-M typically use a low frequency steep slope Xover(~1.4KHz with 3rd or 5th order acoustic slope) and 3"-4"M mounted as close as possible. A small tweeter like the SB29RDNC fits MTM rules, as it can be crossed low and has 0.5mm Xmax.

To use a high'ish efficiency large'ish diameter(7"-8") midrange will also demand a steep slope low Xover frequency small tweeter like the SB28RDNC, or a waveguide to control directivity.

I think you must break your "love square" to have a great speaker. It may even end up as a lower efficiency 4-way to showcase the ScanSpeak 7100.

Telstar 29th November 2012 09:22 AM

Do I have to follow the trend? ;)
I can give up on the MTM config for the high ways, but not on the other parameters.

wintermute 29th November 2012 11:09 AM

Why do you need an 8" or 6" if you are crossing 200 -250 Hz to the bass? 5" drivers should be fine down to 200Hz, and will certainly allow a higher crossover frequency before beaming.

Actually you are going to need some very efficient midbass drivers to get your 95db once you take baffle step into consideration, which I see the beyma and supravox are! However the supravox has an re of 5.5 ohms, and you would need two of them in paralell (taking baffle step into consideration) to get your target of 95db) which would bring you down to less than 3 ohms minimum impedance.... My MTM's have lows of 2.7 ohms at around 200Hz -300 Hz.... and they are nominally 8 ohm drivers with re around 5.6 ohms.

Tony.

Telstar 29th November 2012 12:11 PM

Quote:

Originally Posted by wintermute (Post 3262295)
Why do you need an 8" or 6" if you are crossing 200 -250 Hz to the bass? 5" drivers should be fine down to 200Hz, and will certainly allow a higher crossover frequency before beaming.

Because I don't like the sound of small transducers in the midrange, and especially in the midbass. Besides, I didn't find any good 5" driver, none that would go low to 200hz in acoustic suspension or (even worse for LF efficiency) open baffle.

Quote:

Actually you are going to need some very efficient midbass drivers to get your 95db once you take baffle step into consideration, which I see the beyma and supravox are! However the supravox has an re of 5.5 ohms, and you would need two of them in paralell (taking baffle step into consideration) to get your target of 95db) which would bring you down to less than 3 ohms minimum impedance.... My MTM's have lows of 2.7 ohms at around 200Hz -300 Hz.... and they are nominally 8 ohm drivers with re around 5.6 ohms.
165 GMF SUPRAVOX
Yes, you are right, I didnt consider baffle step yet.

If I choose the Supravox I would connect them in series. In the case of the Beyma 8M70v2 I may go for parallel (3.15 min Re), but I need to run some simulations first. An 8-incher is easier to beam and also more problematic for the polar response. Compromises.
But The 8M70 is VERY tempting due to the low price, and I have had good results with beyma in the past, so I could just try a pair, and if i dont like it move on with a different transducer, like the smaller supravox.

If I get 94dB instead of 95 is still fine, cause that's the efficiency of the tweeter above 2khz, but I cannot trust published specs, which tend to be too optimistic regarding efficiency. AFAIK series connection doesnt add the classic 3db, but would still help the low registers. A hourglass shape for the baffle top part is probably ideal.

And there is also the tempting option of the double tweeter (also in series, which bring Re to a comfy 7,4 ohm) like for instance in Dynaudio confidence speakers
Dynaudio Confidence C4
Dynaudio DDC
but I haven't listened to such configuration. The alternative to keep the Re high is to add a rear-firing second tweeter, which can work if I choose a high XO point. Anyway, I like these speakers to be an easy load and efficient since there will be only 20W class-a to power them.*

*Note: the bass section (25-250hz) will be separately amped and should be a long TL like PMC.

ORNJ 29th November 2012 02:39 PM

if you like the array design then you can also look at copying one of these designs.

http://www.pbnaudio.com/

they use mostly all Scan Speak drivers from what I can tell. However, I believe that the majority of them are rated more around 92 db.

Telstar 29th November 2012 03:19 PM

They use seas drivers which are VERY inefficient. I guess their speakers are 85-89db max.
I like the KAS-2 design btw, it employs the sloped baffle around the MTM section that I have in mind.

ORNJ 29th November 2012 03:41 PM

Well stereophile had the EPS rated at 92db. I dont know about the others. and you can see the Illuminator drivers in some of them.

Good luck though, I was trying to go for a similar efficiency in a speaker with a small TMW monitor but have lowered my expectations.

I am going use the ScanSpeak 4" 10F from approx 500hz to 3.5khz and the ScanSpeak 8" 22W to fill in down below and then probably the Seas 22TAF for the tweeter which will be about 87-88 db sensitive. I just couldnt get very high sensitivity with the size requirements I am working with. The only other option would be to go with a 8" Coaxial and another 8" woofer to cross to the coaxial before the midrange driver starts to color the highs too much.

zmyrna 29th November 2012 03:42 PM

If you want OB, then:
8" Beyma (haven't seen many people using this)
8" Supravox
Visaton B200
My choice would be vintage alnico 8" Isophon (paper cone & surround)
You can't get down to 200-250 with a low qts mid (without EQ that is)

Telstar 29th November 2012 03:53 PM

I have tried 8" fr in OB: lacking at the top and at the bottom. I had to raise the crossover point to 300hz with a very steep slope (I guess equal to 48db). In OB at the very least 2x 8" are required, but I think I'll go with closed box this time and pretty slim baffle =high WAF).
A pair of the above cited supravox 6" MTM seems the best option at the moment. Their price is ok too (double of the beyma but still affordable).


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