Coleman Regulators Part 2 - GM70

Rather a long time ago now I posted about Rod Coleman's filament regulators which I use for heating GM70s (and other DHTs). They're an excellent solution but they do require a decent raw DC supply which is worth writing about. I had intended to do this some time ago but audio has very much been on a back burner whilst other priorities (like work and the garden) have occupied me. Above is my implementation of Rod's suggested choke input supply.

At the time of writing Rod has a "preliminary" website for his filament modules, though it's due an update soon apparently. I guess I ought to say that I have no connection to Rod and have never even met or spoken with him. I'm very happy with his modules though, and his support and advice have been super. Indeed much of my scribblings here are a distilling of Rod's advice over the months and years.

So where to start? Fortunately Rod supplies suggestions for raw DC supplies for his modules, both his usual capacitor input power supply and a choke input power supply. They both do a similar job, but each has its pros and cons.

Here's Rod's basic circuit for the capacitor input supply, with thanks to Rod for permitting me to use it.

Note the trafo only needs to be rated for 21V but a whopping 7.1A because it's capacitor input. Here's a handy page on Sowter's website on rectification. (And here's another on Hammond's website.) Rod recommends we should be shooting for a nominal 25V raw DC to feed the modules (24.3V minimum, 28V maximum).

Looking at the second circuit on the Sowter page, in simplistic terms for 25V on the secondary we need a trafo rated at 25 x 0.71 = 17.75V. Great! That's less volts than we need. Well, sadly, the laws of the universe (and physics) dictate that we don't get something for nothing. We need a maximum DC current rating of 3.3A, so the current rating of the trafo needs to be at least 3.3 x 1.61 = 5.31A. Ouch. So in ball park terms the GM70 will consume 17.75 x 5.31 = 94W (or 94VA). A general recommendation is to derate by at least a factor of 2, so the trafo for each GM70 should be in the region of 188VA. Nobody said big SET amps were cheap!

Here's Rod's basic circuit for the choke input supply.

The trafo now needs a higher voltage rating, but lower current rating. Using the fourth circuit on the Sowter page, voltage = 25 x 1.11 = 27.75V, current = 3.3 x 1.06 = 3.50A, power = 2 x 27.75 x 3.50 = 194VA.

So perhaps unsurprisingly the trafo does the same amount of work whether capacitor or choke input is used. So which to use? Well, it's personal choice, but there are some other factors which are worth considering.

Following the trafo are the diode rectifiers. As the trafos have different secondary voltage ratings depending on whether capacitor or choke input the diodes will be at different potentials. Here are the outputs from Rod's PSUD files with voltage across the diodes (yellow) and final raw DC supply to the filament reg modules (red), first for capacitor input

and second for choke input

Unsurprisingly the choke input supply will need diodes with a higher voltage rating. Fortunately diodes aren't hugely expensive in comparison to the other items. Schottky types are preferred as they don't have the switching noise that other types have meaning there's less noise on the supply, which is a good thing.

Next in the circuit is a capacitor or resistor, depending on the supply. Let's concentrate on the capacitor input supply first. Here's Rod's power supply, this time showing current.

There's a LOT of ripple in the first capacitor, between -4A to 12A, so 16A. So the capacitor needs a big current ripple rating, and might even need to be shared by two or three parallelled capacitors.

Rod pointed these 15000uF 40V 9.5A ripple BHC monsters out to me - they look a good option to simplify the number of components in the power supply if the ripple is high. And they might even be a visual match for any Cerafines, Mundorfs, JJs or WKZs you might be using for your HT power supply.

Here's the choke input version

Okay, the curves showing current through the diodes and first cap aren't as pretty but look how much smaller the peak current is: 6A rather than 16A. That's a huge difference when it comes to specifying capacitors.

The smaller current in the first capacitor of a choke input supply is because the choke has had a beneficial effect on smoothing the ripple (it actually stores charge). Technically, the conduction-angle is longer as the choke stores energy during the lower amplitude portions of the mains sinusoid waveform.

Rod has shown a resistor before the first capacitor, and the choke before the second capacitor in the second filter stage and I presume this is because the choke would need to have a rather large current rating. Chokes with (relatively speaking) large inductance, high current rating, and low DC resistance are very large, very expensive, and very difficult to find. By using a resistor first a sensibly sized choke can be used, in this case a Hammond 159ZJ. The choke also has the benefit of filtering some of the spikes that can be present on a supply.

When I first saw the 0.001 ohm resistor I was a little surprised as I thought it was so small that the supply might act as a capacitor input anyway, but it didn't. The only 0.001R resistors available at my usual suppliers were crazy money so I parallelled two 0.01R to give 0.005R. Sounds like quite a difference, it is a factor of 5 after all. The 0.005R resistor drops 0.0272V more than the 0.001R resistor. There are bigger issues to concentrate on.

Having written all the above, there is another way to provide a raw DC supply and some of my friends have used SMPS power supplies to good effect. If you can find the right voltage and current rating, and they're reliable, they can be a cost effective solution. Indeed some of my friends just use SMPS supplies as a DC supply, and to be fair sound better that I thought they should. But SMPS are known to have a lot of (radiated) noise which can affect the sound unless they are an appropriate distance from the signal part of the amp (>2m).

There is a further issue Rod warned me of. Rather than risk diluting his thoughts by paraphrasing these are Rod's words:

"The "off-line" (i.e. mains input) SMPS have loads of capacitance bridging primary-to-secondary, in order to meet statutory EMI regulations. This leads to leakage current from primary (mains) to secondary. Regardless of any regulator, the leakage current, which has LF and broadband noise components, will pass to safety earth through the filament, through the cathode resistor (if present) and into the B+ supply circuit and onward to earth.

This leakage current varies is size wildly according to the quality of the supply. Only in Medical-grade SMPS is it really small (1uA level). The current should be compared with the Anode Current not the filament current, since it will mix directly with the returning current in the cathode. So a noisy 100uA can wreak havoc if you consider that the music signal might only be a few mA typically. Also, the HF noise-current will return to earth through a low impedance path, and this risks re-radiation into signal wiring etc.

Next, the SMPS is usually built to a cost, and the stress on parts is high. Filament supplies run 100% load, at all times, so de-rating is needed. But even then, I expect the lifetime is not great, and the degradation is the electrolytics may worsen noise before they expire."

So you can try them if you feel lucky but for my money Rod's filament modules give our DHTs the best chance. And mine sure sound good - they're fit and forget.

So, taking all the above into account I went with a choke input supply. Using the PSUD circuit below, these were my component choices.

The trafo uses a Hammond 185F28 which is 14V or 28V depending on whether the secondaries are connected in parallel or series. I got mine from everyone's favourite Hammond supplier Philip Ramsey at Bluebell Audio.

Next are the diodes and I used 100V 10A rated Schottky. Don't be tempted to parallel diodes with smaller current ratings as they will switch on and off at fractionally different times which could lead to premature failure. The heat dissipation of the diodes is marginal without any heatsinking so I mounted them on a small piece of aluminium angle, but the amp chassis would probably be more than adequate. Don't forget to isolate each diode if they're conductive. A benefit of choke input, with its lower RMS current, is lower stress on the rectifiers so they should run cooler too.

Then comes the 0.001R resistor, or in my case two 0.01R resistors parallelled. Looking at PSUD again, the resistor passes nearly 7A so each resistor could pass 3.5A. Therefore the power in each is 0.01 x 3.5 = 0.035W. Even derating by between three to five times is not a lot, around 1/8W, so I used whatever I could find that was cheap! Which turned out to be a massive 3W!

And now the first capacitor, C1. I didn't want to split an order across more than one supplier so the best compromise I could find was to parallel two 4.7uF 160V MKP 1839 polypropylene capacitors.

Next the choke, a Hammond 159ZJ, again from Bluebell Audio. A reassuringly heavy unit, though it is only rated at 10mH.

Finally the last capacitor. I parallelled two Panasonic TS-UP 22000uF 35V electrolytics.

And finally, finally. I added a snubbing network across the secondary of the trafo, a 100nF capacitor and 47R resistor in series. It doesn't matter which way round they're connected. The optimum values need to be evaluated on test with a scope and test equipment, but Rod reckons 100nF and 47R are good enough in most applications so that will do for me.

And it works well. The voltage range of the reg is only just above 20V so in an ideal world I would prefer a few more volts out of the raw DC supply, so maybe a 30V secondary rather than 28V would be better. But then you'd probably need to get a custom trafo wound.

And this is what GM70s look like when heated. On the left a graphite plate, and on the right a copper plate. The photo doesn't really do the beauty of the copper full justice...

And here's a close up of the copper plate GM70

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