Quasar Crossovers

Christmas rather got in the way but the inductors and capacitors for the Quasar crossovers finally arrived. A rather reassuringly heavy package turned up from Hi Fi Collective with two Mundorf 6.8mH air cored inductors and two Mundorf 150uF Evo capacitors.

The series crossover is an integral part of the Quasar design, crossing over the drivers around 150Hz. This removes the bass frequencies from the wide band driver (which can only be a good thing) and limits the bass helper to the bass notes. The series crossover does some nice things compared to a parallel crossover too.

So this is James' suggested design for my Fostex 208E sigmas

And as I intend to use the 208s eventually, this is the design I used to buy the bits. Fortunately the values are near enough for the Alpair 12Ps I'm currently using too. So without too much thought I soldered the inductor and capacitor together, along with a resistor to drop the efficiency of the Supravox bass helper a touch and clipleaded to the drivers.

I wasn't expecting much of an improvement, but was rather surprised by how much things cleaned up. Before, I was running the Supravox full frequency, so rolling off about 5kHz, and using a 6uF PIO capacitor to roll the 12P off at about 4kHz. And it sounded great. There was a bit of a muddle which I put down to the band between 4kHz and 5kHz where both drivers were operating, but the clarity of the 12Ps shone through, and the underpinning of the 285s was firm and taught.

With the proper crossover in place the muddle disappeared which helped to clean the sound up quite significantly. Not only did the muddle go, but the overall detail improved as the 12P was running lower and seems a touch more articulate than the 285 above the bass helper frequencies.

But then James reminded me he'd designed a parallel crossover for the 12P as his experience with the original Alpair 12 wasn't good with a series crossover. Mark Fenlon does suggest that the 12s need as clean a supply as possible. So I changed the components around for the parallel crossover below and reconnected the clipleads.

I wasn't sure what to expect, and didn't think it would make any difference in all honesty. Well, it's early days but I think there is a change. At first I thought there was a little less punch and dynamics, and possibly a little more sibilance. The more I listen though the more detail I can hear coming from the 12Ps. I need to listen over a longer period but I think I can hear greater subtlety and tone with the parallel crossover. Just shows how small differences can have an effect.

Coleman Regulators - GM70

 

Heating of directly heated filaments is a much discussed subject. Unlike indirectly heated valves directly heated filaments are a whole lot more picky about how they're warmed up as they are directly in the signal path. And they have a tendency to hum (and the more power they consume the more they want to hum).

Many swear by AC believing it's more natural; there's less circuitry in the signal path, and it's cheaper on hardware too. Others believe that DC is the only way to go - not only should it remove any hum but it's cleaner too removing the harmonic fringes of the 50Hz AC (or 60Hz depending on where you live).

But even in the DC camp there are different ways to go. You could try passive DC with lots of high current chokes and big capacitors, and LCL is said to sound the best and means there isn't a whacking great electrolytic cap strapped across the filament. It's not so easy to set the voltage though.

Or you could try a current source (leave the voltage sources for your indirectly heated valves). These can be fairly simple, or fairly complicated. Fortunately there are some options available if you want to have a go but don't know how to design a good one. If you want a ready made option then the Tentlabs modules will do the job admirably. DIY Hi Fi Supply used to have their own modules too, but they look to have stopped making them. Anyway, just wire 'em up, set the voltage by twiddling a screwdriver and away you go.

Or if you like to build stuff (and save a few quid) then another option is Rod Coleman's regulators. Rod has been developing his modules for the best part of a decade now I'd guess and has been supplying them for maybe three years. Send him some money and you'll get a compact PCB and all the components to populate it, plus instructions to build and test them. You just need to supply a raw DC power supply, and advice is given in the instructions. Rod is contactable by PM on the DIYAudio forum.

I've built three pairs of Rod's modules before for my 26-10Y-300BXLS monster and each one improved the sound over the passive DC I was using before. So when I finally started the GM70 amp I'd been meaning to build for about three years it was an obvious "fit and forget" choice to build a pair of Rod's modules again. And here's a series of photos of the build sequence.

First I like to lay out all the bits, check them off, and mark them up with which component they are

So start with the bare board

And then the smallest components first - the resistors. And then the first capacitor.

Then the variable resistor used to set the voltage

Then the first two transistors, to the same height as the variable resistor

Next the second capacitor

Then the third of the transistors, again to the height of the variable resistor and adjacent capacitor

 

Next the thermister which stabilises the current temperature (in position R11) (note that only the higher current GM70-type modules use these)

Then I add the sense resistors as I prefer to mount the two 3 legged chips when I assemble the heatsink. I find this the best way for me to get a good job, but it is a bit fiddly soldering the chips as access is a bit tight. Note that the resistors need to be stood off the board by at least 14mm as they get very hot.

So here's the module mounted on the heatsink - a piece of aluminium angle with the PCB connected by standoffs. Note the use of mica insulators as the higher current regulators need insulating. Make sure you use some thermal paste on both sides of the micas - I didn't have any to hand at the time but retrofitted later. Note also a zener fitted across the chip pins, a little fiddly to solder. This is some added protection in case the valve isn't plugged in when the power is turned on.

And here's the finished module with additional heatsink. As I breadboard I like to use them as self contained modules, which is why I mount them on the aluminium angle. If I ever do get around to putting them in a box they can then be mounted to the metal chassis for additional heatsinking. The finned heatsink does a very good job of taking the heat away from the regs and I intend to mount the heatsink above the chassis eventually. (The heatsinks I bought in Japan for about a quid each...)

Here are the two regs in service. Rod supplies instructions on how to test and commission the regs, and as you can see from the DMM it's easy enough to set the voltage. I like to set it just under the nominal voltage and backed it off just a little from the voltage shown.

You might just be able make out the additional capacitor across the raw DC supply - this is a 1000uF cap that decouples the reg from its power supply when connected by longer wires, if it's in a separate chassis for instance. This is a rather nice amp I have to say, one I should write about some time.

Quasars - Open Baffles Par Excellence

Okay, okay, it's time I wrote about some proper DIY audio, enough shirking and flattering to deceive. And if you're thinking "he's a bit full of himself - first blog about DIY and he's telling us how wonderful his stuff is" then you'll be relieved/disappointed to know that I make no claims to progeny of these speakers at all. No, speakers are not my "thing". These open baffles are the design of James Doddington and we are fortunate as a community that he has been happy to share his design. Mine are fairly rough prototypes, but these are James' own speakers

The drivers are fullrange AER Mk 1s which cross over around 150Hz to Supravox 285GMF bass helpers. Both are very nice drivers and not cheap either, though the AERs are no longer available.

James has spent a long time fine tuning the shape of the baffles. By their nature baffles need a large surface area for low bass notes, even allowing for the pi/2 effect of using the floor. The shape of James' baffles use the cardiod effect to improve bass response for the given size, down to 40Hz. Nevertheless these are still large speakers at 1350mm high and 420mm wide. The size is the downside of baffles, but the upside is the lack of boxy honky colourations that cabinets usually have, and well built baffles shouldn't.

Whereas I used yucky 18mm thick MDF for my prototype build James' finished articles use 20mm thick perspex which being see through minimises the visual intrusion compared to wood. The front baffle is shaped, the sides of varying depth and the base profiled as they all enhance the sonics. There's some discussion too as to whether the perspex enhances the sound too. Here's the dimensions of James' Quasars.

In comparison my prototypes are an easy build. The front baffle is flat and 450mm wide x 1350mm tall sloped back at 5 degrees, the base is 500mm deep, and the sides are a simple triangle. It's important that the joins are all air tight. Sadly two baffles won't come out of a single sheet 2400mm x 1200mm, but spare MDF is always handy. The bass helper is mounted approximately 100mm above the base, and the full range driver at 750mm.

The crossover is usually a series one as this offers benefits over parallel. It's fairly simple, or rather there are few components: an inductor, a large polypropylene capacitor, a much smaller paper in oil capacitor to bypass the PP and bring back a little tone, and likely a resistor to balance the relative efficiencies of the two drivers. The inductor wire benefits from a cross sectional area of 2.5mm^2 as this minimises the DCR of the coil, though smaller diameters will work.

Currently I have a pair of Alpair 12Ps mounted as they're new and I thought I might as well run them in in the new baffles. My intention is to make some MLTLs for the 12Ps and use my Fostex FE208E sigmas and T90A supertweeters in the baffles, but one job at a time. I'm waiting for the crossover components to arrive, it'll probably be in the new year now unfortunately.

According to the Mark Audio datasheet for the 12P the drivers should be treated gently for the first 100 hours, then slowly increase volume and bass content and the drivers will improve for up to 800 hours! So at the moment I'm trying to resist the temptation and turn the wick up and listen gently. But the speakers already show tremendous potential.

I wasn't expecting much soundwise as the 12Ps are new and I don't have any crossovers yet but even now they're really very nice, albeit quiet. The pedigree of the 285GMF is clearly evident; it's a wonderful driver - clean, fast and tuneful. A number of James' Quasar designs have used 15" Eminence Beta drivers to excellent effect and are substantially cheaper, but the Supravox are really nice. It's early days with the Alpairs, but initial signs are good; they sound a sweet driver and very capable.

So only another 95 hours to go and I can turn the volume up a bit :-). It will be interesting to compare the 12Ps with the 208s in the fullness of time.

Record Players and Flamenco

Oh come on, he's having a laugh now surely?! This is his fourth blog and he finally mentions record players, but he's going to talk about Flamenco! And they're not record players, they're gramophones. What does he know about Flamenco anyway?

Well truth be told not a lot. But the more I experience it (and I think you do experience Flamenco) the more fascinating I find it. Until my last couple of trips to Andalucia the little I knew of Flamenco came from TV and according to telly it's not easy to see the real deal y'know, most of it is for the tourists de diddly dee. Real Flamenco is squirrelled away behind closed doors. Now I don't know how "real" it is but Flamenco is pretty easy to find on the streets, bars and restaurants of Malaga and Cordoba.

Whilst walking into Plaza Constitution in Malaga a couple of weeks ago there was some live Flamenco mid-afternoon which the locals and a few tourists were enjoying.

 

And then a chap in the audience fancied an impromptu dance and joined in. Clearly Flamenco was in his blood. As it was the locals watching.

 

 

 

 

 

 

 

 

I saw him walking away down Calle Marques de Larios afterwards breathing a little heavily and drawing hard on a fag. And looking rather content.

Malaga is an interesting and vibrant city and as we had an evening flight home the following day we had plenty of time to take in the Flamenco Art Museum. But first breakfast and as a last-day treat it had to be churros con chocolate y cafe doble. General I find that food which is brown is unhealthy. And rather yummy. You really couldn't eat churros every day and live a long life, but as a treat it's fabulous! And I don't drink coffee so you can imagine what a double hit does to me...

Now buzzing from caffeine, sugar and fat it was off to find the Museum of Flamenco Art.

It's a relatively small museum in a couple of rooms above a bar on the ground floor, but has a number of old wind up gramaphones which were interesting. Flash wasn't allowed so the exposure isn't ideal in some of the photos.

This is Carmen Amaya. I'd never heard of her until a few days earlier when I saw an old film of her in the super Flamenco Centre in Cordoba.

This is a clip from the film Los Tarantos, shot in 1963, the year she died. I think it's extraordinary and compelling, and unlike any of the Flamenco I've seen myself. Perhaps because they can't match her speed? See what you think.

There's a colour version out there too but I prefer the black and white version, perhaps because it's grainy, perhaps because it's the first version I saw.

Back to the gramophones

Close up of the horn

And finally The Graphophone with presumably wax cylinders