Left: Tannoy MG15 crossover schematics. Right: Up-Grade filter layout.
Click images to view large.

The following file describes how the Tannoy MG15 crossover can be up-graded to modern standard, in this case even very high standard using silver/gold caps and foil coils wound on waxed paper. More options are available to suit any budget.
Before you start be aware you have to reuse the tweeter attenuation transformer. To my knowledge this transformer is not available anywhere.
As the crossover was the same for MG12 as for MG15, this kit is suitable for the MG12 as well.
I assume readers here are well aquainted with the Tannoy Monitor Golds and will skip any intro. For further information you may study this page: http://www.troelsgravesen.dk/tannoyMG15.htm - and here: http://www.hilberink.nl/speaker.htm

Should you be one of the many MG15 owners, thinking the MG15 is just "it" - the one and only speaker - the upgrade kit here may be an option to further enhance performance. The crossover shown below was made for a client and is close to the best money can buy. Foil coils wound on waxed paper and capacitors made from silver/gold metalized polypropylene with pure silver leads.

Option #3: Wax foil coil for bass and Superior Z-cap for bass section and tweeter notch filter. For the tweeter series capacitors and tweeter roll-off function silver/gold caps are used. Not cheap, but if it has to be the best available.
Option #2: Wax foil coil for bass and Superior Z-cap for bass section and tweeter notch filter. For the tweeter Silver-Z caps are used.
Option #1: 1.8 mH wire coil for bass and Standard Z-Caps for bass section and tweeter notch filter. Superior Z-caps for tweeter series cap and tweeter roll-off section.
Go to Kit Presentations here.
I don't think an upgrade from standard polyprop caps will offer significant improvement over the most common polyester caps found in the original MG15 crossover. It may however be an advantage to change the bass section's parallel 15 uF cap to a Standard Z-cap, in particular if the present cap is an electrolytic cap. These may vary a great deal after some 40 years of age.

The tiny 1.2 mH trafo* used by Tannoy for the 15" driver is hardly optimal and although the present crossover features film caps throughout, we can do better these days. The two coils in the tweeter section are re-used as a replacement for the tweeter attenuation coil cannot be found and there's hardly a rationale for a new one being better.
The tiny coil for tweeter notch filter is also unlikely to cause any sonic degradation although it can be replaced by a 1.0 mH coil from e.g. 0.5 mm wire and adding 10 ohms series resistor.
* The 1.2 mH coil turned out not to be 1.2 mH, rather 3.2 mH! Read more below.

The thin wires and rotary switches are likely to further degrade the sound of the compression driver and below you will find a hard-wired replacement that will - I admit - not offer the convenience of the rotary switches. If tweeter level and tweeter roll-off has to be adjusted regularly, I suggest making a switch-board on the rear of the speakers where you by jumpers can configure the crossover to your liking.

The client for this up-grade also wanted a plug'n play solution, so the original speaker plugs were cleaned at wired to the crossover. Maybe not 100% optimal, but I guess it would be very difficult to hear any difference between this solution and a hard-wired connection to the voice coils

The Original MG15 Crossover
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In this late - and very well kept - version of the MG15 crossover, the electrolytic cap is replaced by a new capacitor, most likely made the same way as film caps, only here from etched foil based on physical dimension. Also for the tweeter section new - and better - caps have been used.
I can only admire the nice wiring job done here. All in best of order with twisted wires and secured leads. It takes time to make a crossover like this.

The Monitor Gold MG15 Problem
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Go to page with full description of the MG15 problem

With the availability of original Tannoy schematics, everything looked straight forward. The crossover module received had the label LSU/HF/15/8 and LSU/HF/12/8, meaning suitable for 8 ohms version of either 12 or 15 inch drivers. So far, so good.

Dealing with vintage British gear, be prepared to surprises. After finishing the crossover it was obviously tested on dummy loads as had been done on the original crossover.
The new crossover did in no way perform like the old crossover. Checking the new layout over and over again, nothing appeared to have been wired wrong. Hmm....

The only thing left to do is starting measuring component values in the original crossover. Quite tedious as most components need un-soldering.

The long and the short of it is this: The series coil wasn't 1.2 mH, rather 3.2 mH! And this despite the original crossover carrying the label LSU/HF/15/8 and LSU/HF/12/8 for an 8 ohms driver.

Two things come to mind here: Does my client have a 16 ohm drivers? Or - and this is more tricky - have the Tannoy folks tried to solve a potential 2 kHz bump. The latter requires some explanation. 

I had this comment from Edward/CA on the caps. Having measured all the electrolytic caps to be spot on with regard to capacity, the 16 uF turned out to be actually 30 uF! It's unlikely a 16 uF cap would achieve double value due to age and either the label is wrong or the  cap manufacturer really had a bad day at the assembly line. As stated above, be prepared for surprises!

The New MG15 Crossover
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Preparing solver tag bar and central ground copper rail. Further I always secure the solder tag bar by a strip of Superfix as can be seen on photos.

Placement of solder tag strips.

Left: Tweeter roll-off section and notch filter. Right: In front tweeter series caps and attenuator coil.

The finished crossover.

To the left the new cross-board providing the same tweeter adjustments as original rotary switches (right).
Check crossover board layout at top to see how to connect the three wires.

Left: For those interested, here are the tweeter attenuator coil inductance measurements.
Right: Connecting wires to MG15 socket. Remove 5 mm teflon insulation. Cut off a few wires (5-6) of the multistrand wire to slide
into the the socket pins. Teflon wire takes a lot of heat and so does the pin. Heat thoroughly until a solid joint is made.

Left: Clean pins with some grade 400 sand paper. Right: Plug'n Play!

How the tweeter roll-off function works

To the left the roll-off adjustment levels. Click image below to view the different scenarios.

Click image to view large.

The Crossover Upgrade Kits
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DOWNLOAD KIT SALE PRESENTATIONS  

All kit and component prices may be subject to change and are always to be confirmed by Jantzen Audio Denmark.

Contact Jantzen Audio for full quotation incl shipping. Remember to include address.
Mail: contact@jantzen-audio.com

All technical questions to me at troels.gravesen@hotmail.com

The finished crossovers
After the crossover seen below was finished, the discrepancy between Tannoy schematics and actual Tannoy crossover became apparent.
Thus, final crossover will have a second 15 uF cab + an additional 6.8 ohms resistor.

Silver-Z caps have been replaced by Alumen-z and Silver-Z for level 1, thus a single 1.5 uF Silver-Z will replace the two Silver/Gold caps
and one 6.8 uF Alumen-Z will replace the two 3.3 uF Silver/gold.

Finished!

Plug'n Play MG15 crossover.

Left: Typical SPL 1 meter, 2.8 volt to the bass driver. Overall system sensitivity appears to be in the 91-92 dB range
Right: This to illustrate the MG15 crossover problem. Based on measurements of seven MG15 drivers, the crossover as
illustrated in the original Tannoy data sheet will make a response like seen in blue (1.2 mH and 16 uF to ground).

Left: Final system response from Up-Grade Filter. 1/6 octave smoothing to make things look nice.
Right: Individual and summed response from drivers driven from Up-Grade filter.

Left: Blue = system response from filter with inverted tweeter polarity. Red = inverted polarity (as should be).
Right: Tweeter impedance profile from two drivers. Due to the large resistors in series with the tweeter, the variation here is insignificant.

Left: Impedance of client's two bass drivers. Doesn't get much better than this.
Right: Final system impedance with various (100 L) box tunings. Red = Aperiodic tuning. Green = vented, Fb = 43 Hz.
Purple: Fb = 37 Hz. I suggest Fb in the 30-33 Hz range for this driver. Suggested box volume 150-200 liter.

Left: Tweeter roll-off function. Right: Tweeter level function. Se crossover layout for wiring.

Left: I don't think we've ever seen dispersion from an MG15 driver. Here at 0, 10, 20, 30 and 40 deg. Not bad at all.
And distortion? Right: Distortion at ~90 dB/1 meter level. Distortion rise = 15 dB.

Left: Distortion at 1 meter/96 dB. Right: Distortion at 1 meter/102 dB. Distortion rise = 15 dB.
Overall these distortion measurements display very low levels at significant sound levels.

Left: Final fine-tuning of tweeter level - to my taste. Red = tweeter level "yellow". Tweeter roll-off = "grey". Blue: Level "orange", same roll-off.
Right: My clients two drivers driven from new crossover. Quite a matched pair!

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