It is possible to make something similar to a Russ Andrews Mega Clamp or the Super Clamp at a lower cost. The purpose of any clamping device such as this is to clamp very high voltage spikes on the mains to limit them entering equipment. A typical spike protector power strip or plug in box is built using Metal Oxide Varistor's (MOV). The cheapest kind use just one clamping device, usually a single varistor between L-N. A better protector will use at least three MOV's, between L-N, L-E and N-E. However, varistor's degrade over time with repeated clamping of spikes, therefore rather than using just 1 or 3 components I have used a bank of 10 in this build, you can use more if you wish.
The 2 types of MOV's used here have different specs, the red Harris type being high energy rated. I have also included a X-Class cap in the circuit to provide broadband noise suppression and the resistor (around 300K, 1W or higher) is needed to discharge the circuit if the unit is unplugged from a live feed.
Spike Protection vs Surge Protection
Many commercial protection devices are labelled surge protectors but you should be aware that a 'true' surge protection device is slightly different. Most of these simple plug in units or trailing sockets should really be called spike protection devices or mains filters. In order to prevent a very large surge from say a lightning strike we really need to include a Gas Surge Arrestor. These use a spark gap inside a sealed component containing either air or inert gas. Surge arrestors can handle much higher currents in the order of 20KA were-as a single high energy varistor is rated around 4.5KA. Gas surge arrestors are much slower to act than a MOV so both are normally used in combination when you need real surge protection. Having said that both are significantly faster than any fuse and even a fast blow fuse can take 100mS to act were-as a MOV is 8uS. So don't expect a fuse to blow and save your equipment in any surge situation, the fuse blows far too late, including any internal fuses.
Also do not confuse a mains surge protector (MOV and Gas Arrestor) with an anti surge component used on a power amp transformer for example, preventing switch-on thumps and power switch arcing. This is another type of anti-surge device and usually connected in series with larger power transformers to stop the massive in-rush of current at switch on when the transformer initially appears as a dead short to the mains. Ouch.
Circuit and Build
By using a bank of MOV's in parallel and if possible 2 different types such as 10 in this circuit we can increase their rating and limit the degradation that occurs over time and greatly extend the unit life. I used varistor's rated at 4500A for 8/20uS. I used 6 across L-N, 2 between L-E and 2 between N-E. The varistor's from the lines (L and N) to earth have a small capacitance of a few pF, therefore using 2 in parallel gives the same capacitance as a Y-Class cap so these act as such and provide the RFI filtering to ground.
This unit could be built hard-wired, assembled on tag strip or vero-board. If using vero-board you could parallel some of the copper tracks for the L-N-E lines. You can omit the X-Class cap if you wish or add 2 x Y-Class caps for some extra RFI filtering to ground. No hard and fast rules. These are easy to make and not exactly rocket science. You can add a mains neon on the case if you wish or a neon mains plug as an easy indication option but be aware that mains neons can add noise on the line so not really recommended.
** Main circuit download below inside Zip file **
The pictures below show show 3 types of cases I have used to hold the circuit. You can use a box with a separate lead and a plug or a psu style case with the plug built in.
It is possible to add a Transient voltage suppression diode, sometimes called a Transorb to the circuit. Some transient voltage suppression diodes can be useful for protection against fast spikes but the commonly available mains rated types have very limited power capability compared to a MOV and most are only specified at 8/20uS, the same speed as a good MOV so not worth fitting unless you can find a very high speed type.
Parallel / Shunt Use
The unit is a parallel (shunt) type device since it does not use any series mode chokes like other mains filters nor have a 'clean' output socket. Some people dislike such series style mains filters on HiFi due to the series chokes but it is a matter of personal taste I guess. Since this filter does not use any chokes it can simply plug in parallel with your HiFi or studio gear on the same wall socket or ideally distribution block. It only needs the input lead and no output lead or socket but you could add one if you wish and make this part of a trailing distribution block. It will clamp any nasty mains spikes and also shunt any broadband noise on the mains providing a cleaner mains supply to your Audio setup. Since it does not use chokes in series with the supply there is no dynamic compression or 'sat on' sound that you can get with other types of designs esp when used with amplifiers.
Wideband RFI Noise
I have made a few of these now and you can increase the size of the X-Class capacitor to 1uF which is about the largest single cap value available from RS. I always use at least 0.47uF or put 2 smaller in parallel to give you close to 1uF. This then provides a much wider (lower point) frequency range of noise suppression. These are fair cheap to make so it is worth putting one on the TV setup, HiFi and maybe Computer for example.
** Main circuit inside zip file download below **