Tannoy Monitor Gold 12" build.

[awkwardbydesign]

I recently bought a pair of 12" MGs in Chatsworth cabinets, as a project.  Looking around for ideas I came across Paul Coupe's (Reffc) design for 12" HPDs.  https://www.hilberink.nl/tannoy2/paulc/paultannoy315hpdproject.pdf  So I am shamelessly stealing it!  My drivers are not  exactly the same, but close enough that his cabinets are a starting point.  Mine will be slightly smaller, with one port rather than two, and tuned to either 23Hz, or 26Hz for a bit more thump.  My room isn't very big (5.25 x 4.4 x 2.3 metres), and where I have to sit against the back wall there is a room node at around 30Hz.  So tuning for level(ish) down to 30Hz would probably exacerbate that.
I will be using the original crossovers with better caps and resistors for now (Clarity Cap ESAs and Mills) plus a reliable 4 pin chassis plug.
I have started asking questions over on pfm, and getting some help with computer simulations (I am useless with computers), so hopefully I will be able to start drawing up some plans soon.
Now the GL75 is more or less finished, I need something else to play with, and building speakers is what I have enjoyed for the last 50 years.  Let's hope I get this one right!  

[ian]

Paul is a member here so he may well see your plans.  Speaker building is fun!

[Reffc]

Indeed.

That design with the curved cabs is much misunderstood (too much speculation) and facts are short on the ground when they're discussed as a result.  To help you along, be careful not to go too low on volumes.  I have built and tested many Tannoy speakers over the years, and those were the starting point for a low colouration design optimal for the HPD 12's.  Gross volume (from my notes) was 156 litres BUT net volume is a fair bit less....people forget about this as it's the volume chosen for tuning purposes, taking into account T&S parameters.

Your best bet is first to test your own drivers for free air measurements using DATS3.  Factory specs for HPDs was 25Hz Fs but most pairs, especially if not reconed are between 18 and 22 Hz (I've tested dozens).  I recently re-coned a pair for my own factory Cheviots and they tested bang on 25Hz.

Knowing this, the VAS, Qtc and a few other parameters, you can use something like WinISD, or do the calcs longhand and determine your F3 tuning point (ie 3dB below program, free space).  With Tannoys, you get a gently sloping curve which comes back to a slight peak (as per Alex Gardners  advice) at tuning when used in larger cabs and it's the peak you tune for if using in anything over about 125 to 130 litres.  My Fidelio Cabinets (which I think are the ones you refer to) were tuned on a NET cabinet volume of around 135 litres and exhibited (tested) around a very slight peak at roughly 34Hz, as that is what I wanted for my listening space.
 
Use DATS, not a program, to set the correct tuning point for the cabinet volume.  At tuning, your first and second impedance peaks should be roughly equal.  Mine have the first peak about 3 Ohms raised, which is insignificant really.  Careful with the ports as I selected two 4 inch ports to control turbulence which ideally you ought to keep under 7m/s port velocity for the resonating column (WinISD models this for you and tells you whether you will have turbulence or not).

For the best designs using DCs however, I have gone since to distributed vents as they have a slight attenuation of impedance and always sound better to my ears, although a port properly tuned should not be heard.  If people hear ports, then the speakers are badly designed.

To test once you think you have it right (and here I'd caution before going to the trouble of building the final Fidelio shape as that is troublesome to achieve and expensive to make) I would first determine the volume for your drivers and the tuning point them make a rectangular mock up simply to test tuning .

At resonance, (use a signal generator or test tones) you should have NO observable driver movement.  If you tune too high, you will have both a bass peak, which may sound impressive but can interact badly with a room, and more importantly, you will see excessive cone movement as frequency drops.  This happens a lot in DIY speakers I've seen and often the result is that the pole piece is smacked by the voicecoil former which then rounds off and starts buzzing or seizes.

Similarly, avoid EBS tuning.  It was fashionable amongst some in DIY circles but this runs your driver through resonance in an uncontrolled manner and can also cause unwanted excursions along with sapping a lot of amp power.

There is one optimum volume enclosure for most 12 inch HPDs and this may lay anywhere between 110 and 135 litres depending on the individual drivers tested.  Obviously, match the volume and tuning for each cab.

The reason for falling Fs is both the increase in older suspension compliance and can sometimes be exacerbated with loss of magnetic flux strength due to magnets losing strength.  AlNiCo can lose it's strength from excessive power delivery, high temperatures and shock.  Most pairs by now will have a little loss but it is rarely significant so not worth worrying about unless tested (DATS again for BL force factor) but suspension compliance increase usually is significant which is why I usually recommend reconing or replacing the suspension as well as foams when the time comes.

PM me or email me via my website for any assistance needed....just be aware that the plans are here and available for sale if you want to benefit from them.

P.

[awkwardbydesign]

Thanks Paul.  I will email you when I have got my head around it all a bit more.
I use REW for acoustic measurements, but at the moment I measure impedance with a big old valve sig gen and homemade millivolt meter and then plot it on graph paper.  It seems to work. (I think I will even be able to manage impedance measurements on REW eventually!)
Both my MG12s resonate at 20Hz, but with different impedances, so my WinISD plots are based on that.

My plan is to build the cabs (probably now at 135 litres effective), and tune the port(s) in room.  It's not really feasible to build a plain box to test and then the curved pair.  Not got the energy any more. Getting too old; I bought a pair of QUAD 2805s hoping not to have to build any more speakers, but that didn't work out.  Anyway, I need to build stuff, the GL75 is done, I already have two DHT preamps, so it's speakers again.
But one question; why two ports? My sim makes two ports way too long!
The distributed port idea does appeal to me more, but I doubt I could tune them well enough with my limited skill set.  I'm fine with construction, but lousy with anything computery!

[Reffc]

Sounds like you're well on with testing and planning...but if I may suggest an alteration...mg12's are better suited to 110 litres net. It's unusual unless the rubber surround versions to see fs as low as that with MGs because what tends to happen is the pleated surround doping hardens with age and raises fs, at least on the pairs I've had in.  They're short throw so the surrounds have quite an effect on fs. I would expect fs of more like 50 to 60hz.  The last pair I had in of 15 inch MGs tested at 40hz instead of factory 25hz.

Just get in touch if you want some input

P.

[awkwardbydesign]

Mine ARE the rubber surround version! So the 20Hz figure is correct, approx 50Hz in the Chatsworth cabs.

[Reffc]

Mine ARE the rubber surround version! So the 20Hz figure is correct, approx 50Hz in the Chatsworth cabs.

20 Hz sounds about right then.  They ought to perform similarly to HPDs.  My Rutlands were designed for these and they're 110 litres, and my RFC Cheviots are about 106 litres.  Both sound great.  Anywhere between 110 and 135 should work fine.  Best power efficiency is somewhere in between those figures.

[awkwardbydesign]

I've been modelling 135 litres in WinISD, 26-32Hz.  30Hz looks promising, and that larger volume gives me options to reduce it when I actually test them.  I have bricks! 
I think I will make the lower front section removable, so I could try different ports, or even slots, if I learn enough to make that worthwhile.  Nearly time for pencil and paper and a calculator; safer than computers for me.

[Reffc]

The beauty of DATS systems is that no calcs are needed.  You make the port area roughly 30% of the moving cone piston area, then do a rough calc or use a modelling suite for lower tuning than you want (ports are easy to cut down but harder to extend!) then reduce port length until you have equal peaks.  Tune either side of this to taste if you wish as long as the peaks are within say 3 to 5 Ohms of each other.  Job's done.

[awkwardbydesign]

I've been looking at the DATS and can't see where to buy one in the UK.  And buying from the EU at the moment is fraught with uncertainty!

[awkwardbydesign]

One of the 4 pin connectors on the chassis was faulty (broken sockets) so I am replacing them with locking connectors.  I had a couple of 5 pin ones, so they will do.  Fixed to a piece of copper clad board; easier than cutting aly.  While I am at it the lead is replaced with some  cable with seperately insulated strands and ptfe insulation, which I also have.
Caps and resistors too.  A mixture of Clarity Cap ESA (new) and Russian PIO, ERO MKT, Philips polypropylenes (which I have).  Russian wirewound and Mills resistors because I like the sound of them.  It all just fits into the original crossover box. Eventually I may make complete external crossovers, but not until the cabinets are made.

[Reffc]

Probably a sensible refurb.  I would personally only have replaced the caps. Resistors resist so unless they were of a type that were high PPM (thermal drift) they are benign wrt signal so unlikely to have any significant effect on sound, at these voltages/currents anyway (noise simply isn't a consideration in these crossover circuits where resistors are concerned but power rating and PPM are). Tannoy only used wirewound ceramics and most that I have tested have been well within spec except some in the LF shunt range which tended to be under rated so would suffer some thermal drift....or burn out as was commonly the case with the 2.2R of the HPD 10 and 15 inch HPD crossovers.  The mid to HF frequency ones would be fine and not worth changing....but always wise whilst rummaging in there to re-flow the solder joints.

Is that green cap the original Electrolytic?  Replace if so.  ESR will be miles out by now even if capacitance tests good.

[awkwardbydesign]

I have replaced white coffin wirewounds and Jantzen MOX resistors in other builds with Mills, and heard an improvement, so they are my go-to choice these days, particularly in HF circuits.
If you mean that green capacitor hiding under the Russian K40Y-9 PIO cap, that is an ERO MKT, which replaced the 'lytic.
But I have just discovered that the second 4 pin chassis socket has the red wires on the right, while the first socket (as shown in my picture above) has them on the left!  I suspect one tweeter is wired out of phase.  Or rather in phase.  I can't compare them until tomorrow, but I probably won't be able to sleep now! 

[Reffc]

Be aware that MOX resistors are nearly always high PPM so not the best choice for crossovers imho where high power dissipation is called for.  They'd be ok in the notch filter position but asking them to dissipate lots of power would see them drift more than an equivalent wirewound which remains the better choice.

It's not unusual to see drivers which may have previously been messed with and are out of phase.  The tweeter ought to be positive phase (ie the diaphragm moves forward when you apply a very small DC voltage from a triple A battery across the terminals (always safest to do this with a cap in series to protect the windings of the HF) and the woofer ought to be wired 180 degrees out of phase.  The battery test should show the woofer moving inwards when you short the battery across the terminals.  If both are wired in the same phase you'll get a dip in response at crossover.

[awkwardbydesign]

I just did the battery test on the woofers.  They both move the same way, that is with the battery positive to the left tag the cone moved forwards.  One of the tweeters has the red wire to that side, the other to the other side.  So if I connect the battery with the positive to the same side as the red tweeter wire, the bass cone should move backwards.  Which would be what you said, I think.  So the commoned connection should be on the negative tweeter wire side.
I haven't tried the battery test on the tweeter, as that would mean taking the back tweeter cover off, and besides, it makes me a bit nervous!
If that is right, I can put them back together (yet again!) and listen to them.  I have a little Ming Da EL84 amp driving them, with a Concordant Excelsior Clone preamp, and the GL75/AT1503/SPU-E as a front end.  A fun loft system.
Eventually I would like to build an SET power amp (GM70? 845?) to match so it would all be home built.  Next lockdown, maybe.