Exicon MOSFET Eval Amps?

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Ad in July's Electronics World has these MOSFET amp modules on "sale" as evaluations. 100pounds including VAT and UK Postage. No idea as to US price yet.

But before I go digging too much into it... anyone know of these? Worth playing with? (especially for sub amps?) Quoted specs of 100w to over 500w into 4ohm. THD less than 0.01% at 1kHz, slew rate of more than 100v/us and bandwidth greater than 100kHz.

Given todays exchange rate, this could be a good deal. Or not. Any ideas?
What you get for your ~$120 is an amplifier and heatsink. This is about half the price of a 310W BK amplifier from Old Colony (BK has created a reputation for being a good-sounding Class-AB MOSFET amp). It's also almost three times the cost of a 400W Symmetrical Power MOSFET Amplifier PCB from Anthony Holton. Is it a good deal? It depends on your point of view.

The amplifier circuitry is discussed below. The PCB appears to be well made, and well layed out. The heatsink is adequate, but only barely (I wouldn't want to push the amp hard for long, especially into a 4-ohm load).

The specs look nice. I checked the datasheet for additional information. There are two schematics in it. The second is 'similar to' the design of the modules.

The module appears to use an opamp (unspecified make/model) as the input stage. Usually, this includes lots of stuff that people say they can hear, and don't like.

The output MOSFETs have no load sharing resistors. In theory, they aren't needed, but I've never seen a MOSFET amp schematic without them. In Randy Slone's book, he found that as the load sharing resistors get smaller, the distortion lowers. This might be an extreme case of that.

There is no overcurrent protection. This is offset by the fact that MOSFETS are usually robust enough that they don't need overcurrent protection, in normal use. Be sure of two things: Never short the outputs, and Use rail fuses.

Finally, there are none of the standard safety features. By this, I mean things like speaker protection relays (delayed on, immediate off), DC voltage monitoring and heatsink temp monitoring. On the other hand, these items are rarely in a DIY design, and rarely added by a DIYer when they're not there.

Remember, this is an evaluation PCB. It was never designed to handle the day-to-day abuse that occurs in an average home stereo. It was designed to show, in general, how the EXICON MOSFETs can be used in an amplifier, and how they sound.

Because of the complete lack of safety features, and the small heatsink, I'd say that this design is not appropriate for a permanent construction. YMMV.

Good luck.

[Edited by thoth on 07-23-2001 at 11:27 PM]
Exicon mosfets

I have built a amp (design from june 2000 electronics world, A new class B topology) using exicon 10P20's and 10N20's and it sounds fantastic. The price is a bargin for the modules. The mosfets and caps alone costs more then 100 pounds if you bought them seprately, let alone the heatsink.
True that if you were to run them at high power for extended times you will have to increase the cooling. and you should build a little antithump board, if you were fussy you could biuld DC protection on this board too, but not really needed.
I think because they are V-FETS they aren't prone to thermal runaway and will shut down abit when they get too hot, though i have never tried it. they should also srug off a short circuit, don't try this with bipolars!
re: Toths comments


I'm commenting Toths on his remark that "It was never designed to handle the day-to-day abuse that occurs in an average home stereo"

From my perspective "the day-to-day abuse" is not related to reconnecting the amplifier unit to various loudspeakers, but to the music played on the amp.

Even DIY'er like me build themselve an amp, hook ik up to their speakers, and stick to it until their children leave home (and there is money to spare).

Most commercial amp's lack features like nice-on / off speaker control.
Most of the commercial amplifiers don't like being used at max performance for more than one day continouis is my observation.

However some basic safety measures (fuses on the mains etc) must never be discarded.

Might just go for it..

I agree with the reasonable protection measures like turnon delay and fuses on rails (and tempting on outputs). Tempted to use a temp and fan set up - If I build using these, they are gonna go for the subs. MOSFETs in general should help in this area, being less prone to thermal runaway. I figure to use several modules, maybe 4. Serious powersupply since I have a found the "right" surplus transformers from a pile of broadcast equipment that have been made redunant.

My DIY thought was to build a voltage gain section with the active crossover and then gang up pairs of these as current drivers - just keep adding till I have enough. These would make that approach seem alot easier.

Really my other choice here is a true commerical amp - Crown or Crest. I have an old bias about from my "Commericial" days. I could beat the hell out of these amps and they always worked. But they are not in the DIY budget range at all..
First, I'd like to clear up my earlier statement about 'day-to-day' abuse. I've seen MANY people who just put an amp on a shelf, without considering clearance for heat. This was the primary abuse I was refering to. When combined with the dust, lint (carpet) and hair (from animals and people) that seem to quickly coat heatsinks and close vent holes, you soon end up only half the cooling you need.

Now, on to the rest. If you're willing to put protective circuitry around this amp, you'll probably end up with a good sub amp. Special things to consider in this application are the DC Servo capacitor may need to be a little larger, and the input DC blocking capacitor may need to be larger. In both cases, this is to decrease the LF rolloff.

I like the idea of a fan for safety purposes. I'd use a small (60MM-80MM) fan, attached to the front of the heatsink (like a CPU cooler), and have it triggered at between 60C and 80C, depending on the detailed specs of the module (at least 10C below absolute max temp). I'd also have a speaker cuttoff that triggered at max temp.

Finally, I tried to place an order with Profusion. They were smart enough to figure out that I was 8 timezones away, but were too stupid to E-mail me anything but a WORD document, even though I asked them to send it again, as plain text. They also couldn't tell me what their normal working hours were, so that I could call them. I guess I'm spoiled by dealing with people whose goal is to keep the customer happy. If you're local to them, you might not have any problems. Otherwise, if you can find another supplier for these modules, I'd consider using them.

Good luck

I notice that you have built Russel Breden's 'A new 100 Watt Class-B topology' amplifier which appeared in EW-June2000. I too tried it, but since the output devices were not specified (the clarification regarding Maplin selling these devices appeared in the next issue), I tried IRF540 and IRF9540. This was done more due to ignorance and I realised that this was a wrong choice of substitutes. The amp worked at low volumes but the output devices fried up when the volume was turned up.

I checked the Exicon site and the equivalents chart shows that the 2SK1058 and 2SJ162 are equivalent to Exicon's 100volt 16amp devices. Can the Hitachi devices be used in the amp referred to? I have about 12 pairs to hand.

In the original schematic of the amp, Tr12 is specified as BC337 as well as MPSA42. Which is the correct one?

This amplifier should be of interest to everyone, with the Exicon or Hitachi devices (the latter if suitable) since the author writes that the sound is indistinguishable from commercial Single Ended Class-A amplifiers. There are many single ended class-A buffs on this site and if a new class B topology can render the same sound quality at a lower cost and complexity of construction, why not? I am certain that many would see this as a welcome and viable alternative.

Helix, your comments please and everyone else too!
Thoth said:
The output MOSFETs have no load sharing resistors. In theory, they aren't needed, but I've never seen a MOSFET amp schematic without them.

Hi Thoth,

I believe the load sharing resistors you are referring to are actually current sensing resistors for short circuit overload protection circuitry or else the designer put them in because they are use to seeing them and they make the designer more comfortable. Since the Exicon amp doesn't include any overload protection then these resistors are not required.

The reason load sharing resistors are not needed with MOSFET
amps is that a MOSFET's impedance has a positive temperature coeeficient. This means that if a MOSFET starts to carry more than it's share of the load current then as its junction temp goes up it's impedance goes up also. The increased impedance then reduces the current flowing through that transistor. This behavior is the reason MOSFETs don't suffer from secondary breakdown or thermal runaway problems. This is probably the main reason why really high power amp designers like MOSFETs so much.

Load share resistors

Load share resistors are actually local feedback devices that decrease gain, increases stability, increases freqency response, decreases distortion etc. This is assuming all other things are equal. I foresee that others will disagree on distortion, but consider this. If the gain is lower, you need more gain at the driver stage. Beware of what you measure. Also consider non-linear effects of paralell units with non-identical (gain) properties. Also consider what load you are measuring against -- is it high/low impedance? Is it reactive? Does it vary with frequency?

Such feedback is good and is usually referred to as degeneration.

Such resistors are usually used even in bipolar designs where matching is much tighter. For MOSFETs where Vgs is highly variable, they are even more important.

Most MOSFET's have a positive tempco (as temp increases current increases) up to a certain point. After this point they usually have a negative tempco ("so you don't get thermal runaway").

What Exicon claims is that the devices used have a "true" negative tempco which means you don't have to worry about load sharing in paralell devices. This is analogous to saying that 100m runners will all have the same time when they run 1000m since the one who runs fastest will get tired and the others will catch up etc.

I have personally not tested these units, but my gut feeling says -- get out of here!


[Edited by Petter on 08-18-2001 at 09:25 AM]
"I have personally not tested these units, but my gut feeling says -- get out of here! " If you would take the time to read the data sheets you will find the Rds has a shift from negative to positive at 100mA.This means that if you run 100mA bias per output device they will share current with no resistors.Hexfets on the other hand take about 5A before the Rds goes positive.This is fine for switching applications but will never be reached in audio as the SOA will be exceeded at less than 2A in most circuits.Also of interest is the Vgs of the hexfet.At high temperature the Vgs decreases faster than the Rds increases.Lateral mos does not have this problem.Hitachi, Xicon, and Semelab make lateral types.They also make vertical types so be careful.International Rectifier does not make a lateral type.Linear Technology makes a bias servo IC for IR type fets.
Hi guys - for what it's worth, my Zen Revisited uses the Exicon devices - the 250W 160V versions. My own judgement on the results is very favourable, but I must admit I've not compared them with anything else. I'm running at 4amps bias with a 50V rail. My only other comment is they were not cheap at around GBP8 each. No trouble obtaining small quantities.

No problems dealing the Profusion, though (not unsurprisngly) when asked about the suitability of these for class A use, they couldn't help and pointed my to the tech notes being discussed here. I also obtained PSU caps and thermal switches from them.

I took the trouble to measure them - they do appear to be pretty consistent - mine were all from the same batch. With a VDS around 6V, biased to nominal 4amps DS current. Holding the VGS constant across the sample of 5 N and 5 P types, the results (DS current) were: (the VGS was slightly different between the N and P)tests.

P type: 3.95, 4.07, 4.12, 4.07, 4.05 amps
N type: 3.98, 3.95, 3.97, 3.97, 3.97 amps

Further queries suggest the matching not too important with the Zen - I was only did it because I was paralleling stages.


Samuel Jayaraj,

I made the amp, then had a few problems wit hit, and since then it has lived in a state of little use. I have now come back with a bit of money, time and test gear to have another go.

I should have a new board up and running in a week or so but the question still is :

"what is the correct value of Tr11 and Tr12 ?"

At first i tried a MPSA14 in at Tr11 and a MPSA42 at Tr12.

can you post a schematic of that amp, please ? (Russel Breden's 'A new 100 Watt Class-B topology' amplifier, from EW-June2000)

because i can't find it anywhere and i'd like to build a power amplifier based on mosfets (output stage) for my own, something in ther 2x50w - 2x100w range. this one seems interesting.

thank you,
samuel jayaraj

hi dear, i m also from bangalore.i m very much interested in building amplifiers.i have made many amplifiers .now i want to make a powerful mosfet amp.i m new in bangalore so i dont know that where to go for parts.at my native place lucknow these mosfets are not available.kindly send your e mail so that we can communicate our ideas. thank yoy
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