Tannoy Monitor Gold MG 15 problem
Copyright 2010 Troels Gravesen

CONCLUSION TO STUDY               

As described on former page, the Tannoy LSU/HF/15/8 and LSU/HF/12/8 crossover schematics clearly states 1.2 mH coil to the bass driver.


Original Tannoy Schematics for MG15 or MG12.

Let's model my client's driver:


The driver as-is, no crossover. It's seen worse, in particular for a 15" driver, but smooth is hardly the right word.

The problem is this:


Above MG15 impedance plot (free air). This 8 ohms MG15 has a steadily rising impedance towards higher frequences,
and it starts at ~150 Hz, reaching some 19 ohms already at 1000 Hz.
Quite unusual - and it has implications as the crossover
doesn't face a linear impedance.


Above the MG15 impedance plot compared to e.g. SEAS 26WFX002. Quite a difference.
For subwoofers, this doesn't matter, but when we have to make a point of crossover above 1000 Hz it does have implications.


Linearising MG15 impedance takes an RC circuit of 47 uF + 8R2. Tannoy didn't do this, so let's go back to the -

- original crossover and see what happens:


This looks anything but ideal. A huge 10 dB bump at 1500 Hz.

- and the impedance:


Looks bad. 3 ohms at 1500 Hz.

What if we used an impedance correction circuit?


This looks much better!

Let's try my client's crossover:


This really doesn't solve the problem. The peak has moved down to 1 kHz and the result is
recessed midrange and still a bump some 5 dB above average level.

And here's the impedance:


- where we have a strange dip in impedance at 1 kHz. This is just not a proper designed crossover.

What would happen if we tweaked the "1.2 mH" crossover, by adding a resistor to the 15 uF capacitor?


Still don't like the 1 kHz peak - and the dip in the midrange.


If we were to maintain the 1.2 mH coil, this topology seems more likely. Is this an LR4 @ 1150 Hz or an LR2 at 900 Hz?
I wouldn't put too much emphasis on what kind of filter topology we're using with these drivers.
Main target is to get the driver down some 6 dB around 1100-1200 Hz.

Measurement - my client's drivers:

Conclusion to this study:

First of all: Does the Tannoy crossover (the one on the drawing) make a 10 dB peak at 1.5 kHz as predicted from old 2005 files? Yes it does! (I'd be really surprised if they didn't). Above SPL of bass unit without crossover (red) and driven from 1.2 mH with 16 uF to ground. If you have an 8 ohms MG15 and a crossover that says 1.2 mH with 15-16 uF to ground, you most likely have this peak.

Now, listening to this really isn't nice to put it mildly, so I wonder if most MG15 crossovers really have 1.2 mH? From what I've seen on the web, most parallel caps are 15 uF, so my guess is that we really do not find 1.2 mH in most crossovers, rather 3.0-3.2 mH.
Would some of you MG15 owners please unsolder at least one lead of your bass coil, measure inductance and report back?

Comments from Mark:
Hello Troels, I was looking at the Tannoy monitor gold information from http://www.troelsgravesen.dk/MG15-problem.htm and your request for confirmation of the low pass inductor value. I have attached a document with several independent measurements referenced in message #25577 by members of the yahoo tannoy group. Another set of measurements is also available at http://fn8142b.tripod.com/crossover.htm
All of the values for the wc3120 low pass inductor are around 3mH as you have measured and nothing like the 1.2mH handwritten value at
http://www.hilberink.nl/codehans/tannoy37.htm
It is unfortunate that this misinformation hasn't been corrected.
Sincerely, Mark Howell
Download reported data here. Thanks to Mark for reporting :-)

Looking at the frequency response from the driver alone, this driver appears to need no crossover at all. It's flat. We only need to make it roll-off somewhere around 1.2 kHz. The driver does well up to 2.2 kHz, but we need a series coil to generate a low-pass filter. So why not stay with the 1.2 mH coil? Below can be seen what happens from A) no filter, B) 1.2 mH alone and C) 1.2 mH and 6R8+30 uF to ground. This is indeed very close to a first order filter with some impedance flattening from the RC circuit. All in all it makes something close to LR4 on the acoustic side - or LR2 depending on target point of crossover.

The bass' low-pass section can be made in numerous ways and there may be differences between drivers that may require minor adjustments.
Based on modelling seven different drivers, the solution presented here will with a great deal of certainty provide an overall balanced presentation.

Did I forget to tell how much I've enjoyed hearing the MG15s again? Even from these non-optimised 100 L test cabs:

MG15s can do things very few speakers can do. Music all of a sudden becomes not only an intellectual/emotional experience, but physical. What the MG15s do better than most, is reproducing the energy in upper bass/lower mid. The absence of points of crossover in this region is a key parameter - not to forget the very size of the membrane. If anything should prove that size matters, this driver does. In fact, it does it so overwhelmingly that it may take some time to adjust to the sound. We're simply not used to feel the physical impact of music in our homes. We may very well know what it's like in live concert, but getting this within our four walls calls for mental adjustment - and not least finding the things that rattle around the house.
Treble from the compression driver driven through the silver caps is never heard better, it's clear, crisp and transient like conventional domes can never match.

This speaker does Siri's Killer Note without problems. I was quite surprised to hear this. I didn't have this CD back in 2005 and considering the lack of proper phase integration between drivers, this came a bit of a surprise.
In fact, this speaker seriously questions the relevance of "proper phase integration". The question is how good is has to be to be audible? Reversing tweeter polarity here does sound bad, no doubt about it, and the impact on overall response can be seen from the measurements shown in the
MG 15 Up-Grade file. The crossover here is basically low order on the electrical side, producing a LR4 on the acoustic side helped by the inherent roll-off of the drivers. One thing is phase shift, another thing physical size and low inherent distortion. Use big drivers that don't have to move much, and life starts getting better and if I had a bigger house the MG15 would be one of my stock speakers.

By the way:I couldn't help thinking of the Chario Sonnet speaker when doing this crossover. Same point of crossover, basically same filter topology. Does Tannoy's chosen point of crossover in reality help mating the two very different drivers, the huge 15" pulp cone and the weightless compression driver dome? Seems like the ear is less sensitive to less than perfect phase integration in this area.

End of MG15 story. I think....


MG15 from three different scenarios.


Modelled response of C.