Saturday, 6 September 2014

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

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