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 (5) Soft start protection and main power supply.        

(A beginner’s take on choosing a soft start, and using the 41Hz PSU1-PS power supply)

41Hz soft start, PSU1-SS

Introduction

So you need a soft start: Of course you do, if your transformer is greater than 350VA. If you are dubious and omit a soft start, save up for the replacement of damaged components on the main power supply, and for fuses, to get the “beastie” working again. Substantial household fire insurance would also be a good idea, if you want to live on the edge like this!

Even high power diodes (or rectifier bridges) and top quality capacitors can be fried quite quickly on your power supply, without a soft start. I tried some “accidental” experimentation, without a soft start, and these components only lasted for 30 to 40 power-ups (switch-on of main power). The capacitors suffered firstly, and as they slowly deteriorated, diodes began to blow. Replacing the blown diodes resulted in a diode exploding! At this point the poorer quality capacitors started to fail completely, and the diodes always failed after this point, at least two at a time. Using the better quality capacitors (the best I can find, Panasonic or Nikon Chemi-con), the diodes failed one or two at a time without exploding. This exercise was very destructive and costly!

I might add that I was using a 1A slow blow fuse with a 1KVA transformer, and the fuse did not blow until damage occurred! See my later document which includes fusing of amplifiers.

The 41Hz soft start

The 41Hz soft start PSU1-SS is particularly useful for beginners as it has the following features:

• The mains connections are made using a screw header rather than soldering, which allows the unit to be interchangeable; for example by using it on a test rig and in the final amplifier. Incidentally, all the other 41Hz kits mentioned in this document use PCB mounted screw headers for external connections, thus enabling numerous connections and disconnections, without damaging the PCB by soldering and de-soldering.

• The main power outlet is provided using the delay circuitry attached to its relay, and there is another relay controlled direct supply (non delay), for the ancillary units which are necessary to provide the 5V and 10V for the TP.

• Threre is a screw header connection which can be used for a power on / off switch on the front of the case.

• Also, there is a screw header connection which supplies suitable power to light a mains on / off LED, which can be mounted on the front of the case beside the above mentioned switch.

The soft start kit is similar to all 41Hz kits, as it has a top quality PCB and top quality components.

However, I have had trouble with the type and value of the ballast resistor, and would recommend that builders calculate their own values of ballast resistor, not copy mine blindly. Small details in this world can have a dramatic effect on outcomes; and sometimes lead to divorce!

In other words, do not build and operate any of these electronic devices, unless you understand the theory, and are competent in carrying out the physical construction work. If you do not possess these skills, pay Audiophool or V-bro to do all this for you. In any case, the table below shows what I have been using as ballast resistors. This can only be described as a starting guide, prior to proceeding with the choice of resistor, far less the construction.

  HB’s table of ballast resistor values

HB’s choice of ballast resistor

I have used the 41Hz soft start, for all sorts of situations over the last year and a half or so, and none of them have missed a beat. According to Per-Anders Sjöström, the original designer of the soft start, on his web site, vast numbers have been produced and used successfully for over twenty years or so!

However, the world is not that simple, and it is possible to buy all sorts of modules and kits on the web, purporting to act as soft starts, for high power applications. No doubt many will be perfectly adequate, but I would need proper endorsement of such products from experienced Forum members, before I could accept them as ideal for beginners.

Never the less, both V-bro and Scratchy have provided us with suitable circuits on the Forum, tested and ready to be copied by us, the general “camp followers”. However, I have not included hyper-links to these posts on the Forum, leaving new readers to access the Forum, and accustom themselves to searching the Forum for information; “the swine that I am”!

Beginners, please ignore these matters, and only experiment with such circuits, once you have a TP working for its living, and you have gained enough experience to safely deal with these matters. Then you will be able to properly assess their value within your system.

Danger!

Soft start devices operate at mains voltage, and great care must be taken. I refer everyone to my warnings in red at the bottom of each of my documents.

This is a hobby to be enjoyed, and I quote my father-in-law (circa 1985) who was involved at the “coal face” in electrical power engineering and electronics, and who lived to the ripe old age of 87 years: “Always give electricity a lot o’ respect”.

Time for a distraction!

Britney in a dress

Main Power supply for the TP

The linear power supply which I suggest that you should use, requires a transformer, which must be selected to suit the power requirements and rail voltage of the TP. Power is  specified in VA (volts times amps, potential times flow), as transformers operate in an alternating current. environment. In direct current mode the power in watts is calculated as voltage times current, and is a universal law courtesy of Mr Watt. In alternating current the VA definition is a way of delineating that the power is available, and is not simply the product of potential times current, but is also dependent on phase.

The beginner does not need to further understand these matters or the use of imaginary numbers to explain and solve alternating current theory. Indeed, VA is equivalent to power in watts, as far as we need to consider for the soft start capability.

I like to use a transformer at 50% higher than this minimum, and have used double the VA rating without any misgivings. The law of Diminishing Returns does set in however, not in terms of cost for basic toroidal transformers, but in terms of the size and of the weight of the transformer and, of course, more mechanical hum as the beastie gets bigger and bigger!

The reason for this required headroom is that the TP is a constant voltage type amplifier with varying currents, and if the transformer is too weak to cope with sudden power “demands” (current demands in reality) then the rail voltage will be compromised (lowered). The filter capacitor charging current flows for a very brief time, only when the transformer output voltage exceeds the DC voltage of the filter capacitors. This current is a multiple of the standard state current, and this has the effect of reducing the VA rating of the transformer. Although this situation might only pertain for a tiny amount of time, during the reproduction of music the noises coming out of the speakers can be “fairly” uncomfortable on the ears!

OK we understand all that? No, it does not matter; use the transformer with VA value which exceeds the power requirements of the addition of both channels (left plus right), by at least 20%, and please keep reading on!

I repeat, the input and output voltages of a transformer are in an alternating current environment. In order to choose the correct transformer output voltages, to suit your chosen rail voltages in direct current, a good approximation is obtained by multiplying the AC output voltages by 1.414 (the square root of 2). As an example, I now use a 500VA transformer, with output voltages of 35Vac which is easily capable of servicing the TP which can produce 2 x 150W = 300W total. These cool running amps run at over 90% efficiency!

Thus 35 x 1.414 = 49.5Vdc.

So I would suggest that my Vrail value would probably be around Vrail = +/-50Vdc.

The +/- signifies that there are two separate supplies to the amplifier, and the manner in which they are connected to the amplifier. This will become obvious when building the power supply, and connecting it to the TP.

There are all sorts of transformers available from the traditional basic type, the toroidal, modified toroidal, r-core, L-frame, C-frame etc. I have used a good quality basic toroidal transformer so far, and would advise beginners to take this route. Over the years, I have read all sorts of reports how “this type of transformer or another” is better than what someone else believes. I have not compared different types, as these special types are usually very expensive, and if I get time, (or money!) I will have a go at comparing them. By that time FFF’s wishes might have been realised, and we would all be using economically priced switched mode power supplies, and trying not to electrocute ourselves with them!

Anyway, here is a link, kindly supplied by Scratchy, which describes the basics about linear power supplies, in a much better way than I could ever attempt!

41Hz main power supply PSU1-PS

I recommended the use of this power supply in the previous documents, as a suitable kit for the beginner. It is very straightforward to build using the sharp photographs supplied by 41Hz, and the PCB and components are of the highest quality available. Before ordering this kit, check that the 8A current handling of the diodes suits your set-up. In my case at +/-50Vrail into 8 ohm speakers, they are perfectly satisfactory. If their current rating is not sufficiently large for your set-up, there are other power supplies available. Or you could build your own, perhaps copying my basic design, included in a later document!

Power supply PSU1-PS

However, there are no discharge resistors, sometimes called bleeder resistors, included with the kit; hence:

Discharge resistors.

I consider that these resistors are absolutely essential when using a test rig, and indeed even when all the electronic gear is safely ensconced in a case. Without this discharge facility, the energy stored in the capacitors will remain for some considerable time, waiting to “bite the unsuspecting finger”.

This is a serious matter, and these large capacitors are dangerous!

There are mounting pads for SMT resistors of jacket size 2512, on the bottom of the 41Hz power supply PSU1-PS, and I used 10K at 0.5W resistors. These have worked successfully for 18 months or so with no problems. These discharge resistors take quite a while to discharge the capacitors, but all these electronics are inside an earthed case, so no delicate fingers can touch anything nasty before  the discharge is complete!

On all my recent power supplies, I now use lower resistance with a lot more power handling, as I prefer a much quicker discharge time, particularly when trouble shooting on my test rig. Currently, on my test rig, I use two 2.7Kohm 6W power resistors in series for each power output, of the power supply. Also I have a voltmeter permanently connected across Vpp and PGND, and carefully watch the voltage decaying until it is well below 1V, before even thinking of working on anything down current of the power supply.

The power resistors on my test rig, are mounted across the power supply outputs in “fresh air”, for best cooling, with the leads electrically insulated using small bore heat-shrink tubing. The leads are fitted into ferrules which enter the screw headers. On all my next power supplies, I intend to mount the resistors through the fibre glass board and hard-wire them on the bottom of the board. Care must be taken when soldering to use the minimum solder necessary for the joints. As mentioned elsewhere in my documents, if “meltdown” (catastrophic failure) occurs during a high temperature failure, when solder could drip down, it should not be able to short to earth on the case bottom.

You should calculate and test your own values of discharge resistors, as the cost of the fire insurance risk, is very pertinent to this issue! I am certainly no expert in these matters. Please take care to understand what you are doing with discharge resistors, as it is a potentially dangerous situation. Please read up all you can on these matters, prior to carrying out any physical work etc. We do not want any fires or shorts across power supplies, if we like staying alive! Please take these safety considerations, very seriously. If in doubt, ask an expert not me, I am just a kit builder.

The 41Hz kit the PSU1-VR is straightforward to build, and is a tough little “feller” and has easily survived some serious misdemeanours by me! The outputs, V10 and V5 voltages are easy to adjust, and the kit consists of all the top quality components associated with 41Hz, the “best of the best”. (“Men in black”?)

If you think I am becoming biased towards 41Hz kits, think again and read my other documents. I have absolutely no commercial connection what so ever, with Jan (or indeed Audiophool and MAEDOS); which is not surprizing, given my lack of knowledge and experience, and general insanity! I intend always to be totally independent and perhaps accidentally, slightly odd?

Haggis Basher.

Tributes: I am indebted to my mentors on the 41Hz Forum, for all the kind help, and the general support and encouragement they gave me, during construction of my TPs. Also, I am indebted to my mentors on the 41Hz Forum for their generous help in editing all my documents, for accuracy and formatting, without which it would not have been possible, to produce these documents.

Opinions: The opinions expressed and any conclusions reached in this document, are mine and mine alone, except when I have quoted someone directly; I am an individual with no commercial connections to anyone or any company.

Disclaimer: I will not in any way be held responsible for any damage to any audio equipment or for any damage to any other electrical equipment or for any damage to any other property or for injury to any person or persons. I will not in any way be held responsible for what you do with electric power. Electrical power can be lethal! If you are not professionally qualified to handle mains power, then do not! Get help!

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