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Old 25th September 2007, 04:11 PM   #1
Mike C is offline Mike C  United Kingdom
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Default Mike's Proposed MTM

I was considering a DIY project about 2 years ago but have been too busy until now; I posted about it at that time.
My ideas have firmed up and I'm posting them here for a sanity check.

Background:
Room size 18ft x 14ft x 7.5ft.
Amp is a 300B SET of about 7 watts (subjectively far more powerful than you'd expect). I really love this SET sound.
I don't want headbanging volume; equivalent to 3/4 of the way back in a concert hall is the most I want.
Musical tastes are classical, folk, jazz, some rock and popular; and with a special liking for good female vocals.
Speaker size is limited (for WAF reasons) to about 1000mm high, 210mm wide, about 265mm deep. Regard this as a 'hard' constraint.

Current speakers are IPL S3TL trans lines with Seas drivers; 6.5 inch bass and 1 inch tweeter (H400 or similar). I like the sound but I'm sure I can get more ......

General Plan:
The size constraints limit the bass driver to 6.5 inch. I want good sensitivity and reasonable bass; for this there is simply no substitute for driver area, which suggests an MTM design. I have checked my amp driving weird loads (speakers in parallel etc) and it seems OK; I can use a 4 ohm tap if necessary.
MTM for SET is often used to get sensitivity - note Eros, Living Voice Avatar, North Creek Catamount and others.
It is difficult to make a TL work well with twin drivers, and the pipe area would mean a box well outside my limits; so I give up on this and will look to a normal vented box, tuned for my kind of music.

Kits or Established Designs:
If the North Creek Catamount were still available, I'd be tempted. MTM with Scanspeak 5.25inch slit cone drivers, said to be ideal for SETs.
Others, such as Eros, I'm somehow not totally convinced; remember I'm looking for a beautiful midrange that does wonders for female vocals (as well as as much of the rest as I can get).
Solen's SSUR seems to have promise, but evaluations of the design here on this board (and a builder's comments) leave me underwhelmed.
The nearest good design I have found so far is Zaph's large MTM using Vifa XG18 and Seas 27TDFC.

My Proposal:
MTM using Vifa XT18WH09-08 and Seas 27TDFC. 1000mm x 210mm x 265mm, with curved sides giving some extra width. Allowing for bracing this should give 40 litres. Ported to 38Hz.
This is of course a unique design, but very much inspired by the floor standing version of Zaph's large MTM, and to some extent by Newfinish's project using similar drivers, and by Pjay's work with the same bass unit.

The volume of 40 litres is a lot less than that used by Zaph, and about the same as Vas for *one* driver. It may be an issue. However, modelling it suggests it may be OK. The bass roll off doesn't look bad. F3 is about 50Hz; at 40Hz it gives -6.2dB which is 2.2 lower than Zaph's large ported design. Now I reckon to place the speakers marginally closer to the rear wall than ideal, and I would expect this to compensate for the 2 to 3 dB loss given by less than ideal box size. I'm not a bass freak, (not into rock), but do want a reasonably tuneful bass. This box and tuning give a group delay of 10mS at 40Hz which I believe to be reasonable. Naturally I'm prepared to play with port size to get ythe sound I want, and if push comes to shove could convert the box to an IB of Zaph's volume though I think I'd lose some bass.
I've mainly considered bass here; but I understand and expect that these drivers can give an excellent midrange. If I get that, and compromise the bass slightly, that will be acceptable to me.

I propose to borrow Zaph's Xover point and alignment; 2nd order L-R at 1700 Hz. I can see why this fits these drivers very well so why change this. I expect to start with Zaph's exact design, but with two small changes. First, a low DCR choke on the bass, for a marginal increase in sensitivity and to partially compensate for the highish Zout of my amp. And a lower value series R on the tweeter, to compensate for the lower DCR choke and also the slight extra sensitivity of the XT18 over the XG18; about 2dB in total.
Some modelling of the Xover may be useful especially if I can factor in the Zout of my amp .....

I'm prepared to buy some test equipment to help with final tuning, and I'm OK at this kind of thing (built my own amp, I have a signal generator and a couple of scopes so you get the idea).

Anyway ... does this look like a rubbish project or does it have some promise?
Cost and VFM are not big issues.

Any advice welcomed ... but do please note the hard constraint on size, and the fact I am totally a SET fan! :-)
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Old 25th September 2007, 10:30 PM   #2
Jay_WJ is offline Jay_WJ  United States
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Looks like a feasible project. By experimenting with port length you'll find tuning that suits you even with a less than ideal cab volume.

You'll be okay with using Zaph's XO for his XG18/TDFC design. But I strongly suggest experimenting with the tweeter's padding and its lower rolloff. You can adjust these by changing the R2 and C4 values in Zaph's schematic. Although the XT18 looks a bit more sensitive than the XG18 on paper, you never know the right amount of padding that will suit your taste unless you actually try different values. Use two resistors in parallel for higher power handling and finer possible values. For example, you can try two 3 ohm resistors for 1.5 ohms, 3 ohm and 4 ohm resistors for 1.71 ohms, two 4 ohm resistors for 2 ohms, 4 ohm and 5 ohm resistors for 2.22 ohms, and so on. This is not a waste of money. Don't rule out values greater than 2 ohms for the reason I said above.

And according to my experience, adjusting the tweeter's lower rolloff is very important in the voicing process for 2-way speakers. Buy 10 uF cap and a few 1 uF caps. Then you can try 11 uF, 12 uF, 13 uF, and 14 uF. By increasing the value, you will have higher upper midrange SPL. I personally like a bit strong upper midrange for classical music. But you may have a different personal taste.

This fine tuning won't much affect designed phase alignment between drivers. So don't worry about it. If you have any doubt, try xo simulation with different component values I suggested above. But you can trust me. I've done this before. ^_^

Take a look at my web page: http://www.geocities.com/woove99/Spk...esigningXO.htm

Scroll down to the section "Voicing your speakers." You'll find some more information about how to fine-tune your speakers to your ear.

Good luck!
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Old 26th September 2007, 10:12 AM   #3
Mike C is offline Mike C  United Kingdom
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Default Thanks!

Hi Jay,
Thanks for the encouraging and most helpful reply - info of this quality is why I love this board!

I've seen your site and in fact printed off your notes on tweaking the Xover, as it's logical and understandable; it gave me some confidence that I might, with some effort and a bit of luck, be able to do some of this. I'm an engineer so I'm used to the cycle of design/model/build/test/measure/correlate to the model/redesign.

A bit more background. Few ready made speakers fit my exacting needs; one is the Living Voice Avatar. So why not buy one? Well, I'm sure it's possible to do better; the bass drivers are decent enough but perhaps a bit run of the mill ... probably not of the standard of the Vifa XG or XT. I wonder if the Xover freq/tweeter choice has been done as effectively as Zaph did in his MTM. And of course I can build to a high standard, with laminated curved sides and a lot of bracing. So I think this XT18/27TDFC MTM has the potential to improve on the Avatar. Whether I have the ability to do so is another question! And I'm sure I know the answer (no ... or not yet).

I have to aim high, as the IPL isn't bad. But modern drivers seem to be somewhat better (the 27TDFC improves on the H400 for a start). However, I see no point in using 'average' drivers as I doubt if they will give much improvement over what I have now.
Hence the choice of drivers.

And above all, it's *fun* !!!!
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Old 26th September 2007, 01:26 PM   #4
Jay_WJ is offline Jay_WJ  United States
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Hi Mike,

The choice of 27TDFC is excellent. No alternatives to 27TDFC under 100 US dollars except the Peerless HDS 810921. But I don't think the more expensive Peerless is necessarily better than the TDFC.

The XT18 is also good. But as you may have noticed, the XT18's distortion performance in bass and lower midrange is only average according to Zaph's test. Also, Mark K's multi-tone test show that it---XG18 in this case, but I believe they share the same motor---is not a top performer even in the midrange.

Have you considered the new Seas ER18RNX? Its bass and lower midrange performance is among the best. Upper midrange distortions are a bit higher than the class leading SS Revelator or the Usher 8945P. But this won't be noticed much if you use a low crossover point (e.g., 1.5 kHz) with the TDFC. It is also suitable for a small vented design.

Just my 2 cents.
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Old 28th September 2007, 05:00 PM   #5
Mike C is offline Mike C  United Kingdom
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Default Significant Rethink

Thanks for the suggestion of ER18RNX; it is now a strong contender for this reason.

I have factored in the amp Zout into the simulations. Some good technical info suggests 3.8 ohms for a 300B SET Zout - in my case less as I have a higher ratio OPT. So 3.0 ohms will be close for Zout and cable R (I have long cables) and I've used this.

This has a profound effect on Qts !
The XT18WH goes from 0.37 to 0.47
The XT18WO goes from 0.37 to 0.54

Modelling the XT18WH in a box of 42 litres and various ports gives what I think are somewhat poor results. Tuned to 30Hz gives a peak of 1dB at about 80Hz; F3 about 52Hz, F6 about 42Hz. That's about the best I can get .... I don't think I'd like any more of a peak than that.
The XT18WO gives worse results.

The ER18, with Qts corrected for Zout, gives what seems to me to be a better curve. Box is 40 litres, port 30Hz. There is a peak but it's less, about 0.2dB, and broader - roughly 100 to 150Hz.
F3 about 48Hz, F6 about 38Hz, and F9 about 30Hz.
The nature of the response curve for this design with ER18 seems a lot better to me than the XT designs (but what do I know!).

I have a long way to go to get a design out of this, but hey I think I'm learning.
I know the Zout has an effect on the Xover design, so I can factor this into some simulations ans see where I get to.

I'm finding it very interesting, 'seeing' the effect of Zout on the speaker design!
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Old 28th September 2007, 06:35 PM   #6
Jay_WJ is offline Jay_WJ  United States
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If you look at Zaph's t/s parameter measurements, you'll find that the XT18/XG18's measured Qts is even higher than the ones published by the manufacturer. In contrast, the ER18's measured Qts is close to the one measured by the Seas.

I think the XT18/XG18 are more appropriate for a sealed design for two reasons: average bass/midbass performance and highish Qts (plus you have a high output Z of your amp).

ER18 will be an excellent choice for your project. It has high efficiency, top-performing bass / lower midrange, and low Qts like other Seas 6.5"s. Required cabinet volume can be even as small as 30 liters. 35 to 40 liters will be better in your case since you can get deeper bass and don't need high power handling.

If you're interested, I have a preliminary crossover design for ER18RNX/27TDFC 2-way at my website. This is for a TM vented 2-way. But I also have an MTM version of it (not on my website yet)---it turns out to be simpler than the TM version! Believe it or not, I'm pretty sure that this preliminary design will require only fine-tuning process to sound good. The reason is that I used Zaph's same measurements to design and voice my other 2-way projects. John uses exactly the same measurement settings in each of his driver test groups. So it is possible to obtain high consistency among different xo simulations if we use his data for the same kind of designs. I bet by tweaking the capacitor and padding resistor values of the tweeter network, you can make them sound and measure right to your ears and eyes. But I know that you want to do it yourself in every step. Anyway, this will be used at least as a reference or a starting point for you. If you want, I can send the xo design and its predicted responses (or my Speaker Workshop file itself). Let me know.
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Old 28th September 2007, 09:29 PM   #7
Zaph is offline Zaph  United States
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Hi Jay,

If you're curious, here's how your crossover models using actual in-box measurements. (the design axis was the tweeter @ 1m) In this box, it was a 9" baffle, and the tweeter offset 1" which is probably pretty close. Note that this particular 27TDFC has a slightly different top end than the one I posted a year or two ago in the mishmash, but the sensitivity, impedance and low end response were the same. Your model looks pretty close. Differences are probably related to an increased resolution using near field summing techniques for a more accurate curve, some cavity effect from the woofer's cone on the tweeter's response, and potentially the couple of degrees difference in the off axis response of the woofer on the tweeter axis. There might be some mild level differences in there also. But overall, what you got with tracing and the FRD tools looks pretty close to reality.

Your crossover was almost exactly the same as one of my iterations. I've got several versions done myself and I'm trying to decide which one I like the best. I might end up crossing a bit higher than 1.5kHz as the tradeoff between some other design elements seems to be worth it.

My plot also reflects my bass tuning preference. (this is a .7 cu ft box) This "lean and low" tuning sounds pretty good in a medium sized room.

In regards to the XT and XG woofers, yes Qts has traditionally been a bit higher than spec making for a sealed box preference. I also recommend a single Scanspeak aperiodic vent with these in a smaller sealed box. Also, a bucking magnet really helps these speakers. I think I mentioned the Qts improvements on the PE board somewhere. Performance of the XT and XG woofers is much more level dependent than some other woofers. At lower levels the midrange performance is excellent, but it goes away quickly at higher levels. Generally, people who like it loud are better off with a Seas driver. I don't listen loud myself, and as such the XG18 was still one of the best midranges I've heard. (and tested)
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Old 28th September 2007, 09:47 PM   #8
Zaph is offline Zaph  United States
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I've got HD plots of the XT18 from Hobby Hifi and Klang&Ton at different and multiple levels that nicely illustrate the level dependency of the distortion. Not sure if I could post them here without violating some sort of copyright. (mods what do you think?)

At 80dB/1m, there is almost no better midrange than what comes out of a XT or XG woofer. At 90 dB that lead disappears. My 6.5" HD plots have the level set at 92 dB @1/2m, which falls between the HH and K&T plots I've seen.
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Old 28th September 2007, 11:23 PM   #9
Jay_WJ is offline Jay_WJ  United States
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Thanks for the post, Zaph.

Yes, your simulation of my xo using your in-box measurements is close to my simulation result that used my modeled in-box responses. Indeed they're pretty darn close to the extent that even I am surprised!


Click the image to open in full size.
Click the image to open in full size.


In fact, I wanted to use a primary inductor value of 2.8 mH and a shunted cap value of 19 uF in the woofer network to more finely match my voicing preference I had found in my previous building experience. If I did, the measured response would be even flatter than the above! But I simply used the values of available stock components.

BTW, I really appreciate your contribution to the DIY speaker building community. Without your driver test data, my project would not have been possible. I look forward to seeing your ER18RNX 2-way design at your website.
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Old 28th September 2007, 11:32 PM   #10
Jay_WJ is offline Jay_WJ  United States
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Quote:
Originally posted by Zaph
At 80dB/1m, there is almost no better midrange than what comes out of a XT or XG woofer. At 90 dB that lead disappears. My 6.5" HD plots have the level set at 92 dB @1/2m, which falls between the HH and K&T plots I've seen.
Thanks for the good information. Level dependency, probably, is a key to understanding the behavior of the XT/XG18.
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