Planning to build Rod Elliot's Project3A, number of questions...

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I'm planning to build Rod Elliot's 60-80W amp (project 3A) to hook up to my computer, since the sound card amp, well, sucks. It's going in my bedroom, so it doesn't need to be *that* loud, but I want decent quality (but again, since it'll be mostly fed from MP3's, it doesn't have to be that great, again).

Where can I get decent metal cases in Sydney, Australia? I'd really like to to be strong enough to support my monitor, too, but that's just a bonus. So far, the best I've come up with are plastic instrument cases... but since thing is going to be running in close proximity to my computer/monitor/network, I'd really, really like shielding...

If I'm not running at full power, will I be ok with the 160VA 25-0-25 toroid? Or should I spend the extra money on the 300VA? Will it make any difference at all? What about recommended filter capacitor sizes? If I buy the bigger transformer, can I get away with smaller capacitors (keeping the same amplifier, anyway).

I mainly ask because the 50W control amp in EA June-July 1996 only used the 160VA, but I see a lot of designs recommending more.

How can I estimate the power handling of my speakers? They're old 'PYE Model SP 3' ones, inner volume is about 30 litres, sealed box, chipboard. Woofer is about 8 inches, but the midrange and tweeter have some funny cover over most of them, so I can't measure them without disassembling the box. They're the next job for rebuilding, once I've done the amp :)

I might do the speaker wire anyway, since it's pretty pathetic -
insulation outer diameter is about 1.5mm.

Component mounting - can I get away with stripboard/Veroboard? Or should I spend the extra money for the PCB's? I can manage stripboard layout without any problems (I used to prototype a lot of digital circuits), but will the tracks handle the current needed by an amp? Or should I do dodgy things like solder copper wire along the length of the track? I was considering wiring it point-to-point, but I think it's getting a little complex for that to be practical...

Any layout guidelines for preventing oscillation or other problems? Will there be any problems with the +/-35V rails on stripboard (apart from having to re-drill holes so that the cable actually fits into the board)?

I'm planning to use MJE3055/MJE2955's - will this be a problem, so long as I keep the output power down? I really do like the idea of $1.70 output drivers - I can kill them and not care.

There's a good chance that whatever hardware I use for this amp will be reused in my next amp anyway, so I'd like to get it right now...

Final question: transformer inputs are non-polarised, right? I don't see any reason why they would be, but I'd just like to make absolutely sure...

High-power amps are a nice change from low-voltage digital

Thanks for your comments.

I can answer some of your questions:
You can use lower rated transformers, but you can also run the amp at lower voltage. I suggest you ask Rod directly about a particular transformer - he's very amenable to answering questions, especially if you buy boards from him.
I highly recommend buying the ESP boards. It makes building the amp a snap, and the circuit is complex enough to make it worthwhile.
I'm using the 2955/3055 drivers, and Rod is using them in his own version of the amp. Better transistors will apparently be more linear and are not that much more money. There was a thread on this topic a few weeks ago with suggestions on other transistors:
Transformer inputs are non-polarized in the way that you connect them to the power line. However, if you have dual primaries and have to wire them in series or parallel you have to watch the phasing (AC term for polarity).
I'd second Paul's comments.

160VA should be fine (see Rod's article on Linear Power Supply Design, this answers your filter cap question too). The ESP boards are definitely worth buying. This is one way of reducing the chances of oscillation. I only ever use veroboard for small circuits that aren't layout critical - also it's very easy to make expensive mistakes with high current circuits.

Have you considered the Project 19 amps? These are great, and small, so when you do your speakers, you can easily bi-amp, add a subwoofer, etc without too much cost. They'll do about 70W cont into 4R. They also have the benefit of thermal, DC, short-circuit and power up/down mute protection. Quiescent current is about a fifth of Project 3A boards. Project 3A is probably higher quality, but not discernably higher if you're using MP3 as a source.

You won't be able to find out anything useful about your speakers unless someone has some data somewhere.
Thanks for your help!

I went back and read articles off Rod's site for another few hours this morning, so I've got plenty to think about...

I think I'll stick with Project 3A, rather than 19 - originally I was going to go with 19, but 3A is significantly cheaper for me to build, and I won't care if things burn out.

Biamping sounds interesting, but I think it's overkill for my purposes. At the very least, though, I think I'll open up the speakers and rebuild the crossover. When I get time, anyway.

I didn't really want to go with the 2955/3055 drivers, but they're about all I can find. And they're really cheap, anyway. Again, I think they'll do for what I want.

Am I right in guessing that R1/C1 in the power supply given at the end of the article are for smoothing out transients?


Burning out ESP Project 3A?

Having built a couple of Rod's project 3A, I can certainly vote for it as an excellent sounding and robust amp. But be careful of 'blowing' it though, as you may as well kiss your (expensive) loudspeakers goodbye.

Built it properly, and use it conservatively. You'll appreciate it in the long run. Buy the boards from Rod and it'll be a snap to build. I stuffed my board in less than 3 hours, excluding enclosure works.

Be careful about the power supply to the amp though. If one of the fuses (there are fuses for the negative and posotive rails) blow, the output of this amp will swing (nearly) to the voltage of the other rail.

But for being simple, good sounding, relatively high powered and because I have a phobia for IC's, this amp is second to none.

Well, obviously, I don't intend to blow it up! It's sort of a 'just in case' when I test it or load it for the first time (I've got plenty of expendable speakers around here to test with).

In any case, what I think I'll do in the long run is replace the current speakers with a pair of bookshelf speakers and a subwoofer. This is meant to go with my computer, and at the moment the speakers are sitting on the floor on either side of me, which is hardly ideal.

I still can't find a decent metal case anywhere, though (well, there's rackmount cases, but they're too big and expensive for what I want). If I got a plastic instrument case, is there anything I have to watch out for safety-wise? Any special shielding precautions I should take? I have tons of shielded cable around here that I was going to use, anyway, but should I build aluminium covers or anything for the boards?

Will the amp be fine running from the line out from the sound card? I can't see any reason why not, but you guys know much more about this than I do.

I'd also like a headphone socket - the EA article I read used a switched headphone socket with 680 ohm resistors in series with the headphone socket inputs. Again, I don't see any reason why the amp would have a problem with that, but can anyone see one? Is 680 ohms enough?

Yep, you certainly can use the line out of your soundcard to drive the amp. In fact, I am driving one of them from my old and trusty Soundblaster 16 right now, listening to MP3s. Just be sure that your soundcard have the capacity to drive the amp as the amp needs about 1V for full power into 8 ohms. Otherwise, you may not be able to drive the amp to full power. But then again, as long as the volume is sufficient, right?

Personally, I am using 19" steel rack mount enclosures. A pain to work with. Aluminium will be much better....... but then the magnetic shielding (from your monitor which may be sitting on top of the amp) may not be as good........ And the idea of having loads of expendible loudspeakers aren't a bad idea......
For me, a 19" rackmount has just enough space for 2 300VA transformers, 2 boards, 2 medium sized heatsinks, and a fairly large capacitor battery. Don't skip on the enclosure, get it as large as possible, you'll thank yourself when it comes to fitting in the boards. I'm using one thats 3 rack units high, aluminium front panel, steel sides.
Plus, you have more space to move the boards around to prevent RF interference from the transformers, if it happens.
bawang said:
Personally, I am using 19" steel rack mount enclosures. A pain to work with. Aluminium will be much better....... but then the magnetic shielding (from your monitor which may be sitting on top of the amp) may not be as good

For good shielding the enclosure must be made of a ferromagnetic material and grounded... unfortunately most ferromagnetic materials have rather non-linear magnetic properties so they can have undesirable effects on low level signals near by.....
I've started figuring out exactly what I'm going to buy so I can get an idea of costs, and of course, more questions have come up.

Should the 5A fuses for the amp boards be fast, or slow blow? I'm leaning towards fast, since even fast is (comparatively) slow, but Rod's article doesn't say either way (or at least, I haven't spotted where it says, yet).

I can imagine some pretty huge peak currents through there, though, so maybe slow is better - or is that what C+ and C- are there for? I just can't imagine 100nF making a significant regulation difference against a few amps at 35 volts, although I guess there is a small amount of resistance in the fuse, plus the wiring for the power supply which would likely be mounted away from the amp board... am I sort of on the right track here? As I mentioned, my electronics experience is mainly digital interfacing and switching, and these sort of questions generally don't come up much...

Is there any reason I should choose between M205 or 3AG mains fuses?

On that note, I spotted in my Jaycar catalogue a combined IEC socket, switch and fuseholder (M205), which I will most likely get. Can I reasonably expect this to have the internal wiring done, so it'll have switched active (fused), neutral and earth pins on the back? I'm talking about cat. no. PP-4003, for those who have the catalogue.

Transistors, again - now that I check the catalogues thoroughly, I've found that only SOT-96 (and TO-220 2955/3055 pairs are available (I thought I was going to use TO-3 ones, until I realised that there's no 2N2955 available). I've used TO-220's before, and given their size, I wouldn't like to try conducting a few dozen watts of heat away from them - are SOT-96's much better?

Would it be possible to get away without using mica washers, so long as my heatsink is anodised? It sounds like a bad idea to me, but I can't see any reason it wouldn't work, so long as I don't scratch the heatsink while installing the transistor (and the bolt is insulated). Is this practical to do, or just too unsafe to bother with?

Thanks for all your comments.


Use slow-blow fuses for the supply rails and ac due to the high capacitor charging currents and toroidal transformer inrush current.

The TO3 complement to the 2N3055 is the MJ2955 from ONSemi. Do not use the TO220 altenatives (MJE3055/MJE2955 etc) as the thermal perfomance of these devices is significantly inferior. Stick to TO3 (or TO264 if you must have a flat-pack device). There are many suitable complementary pairs in the ONSemi product range.

Do not rely on the anodizing for electrical isolation. Whilst in theory this is possible, in practice it will only lead to disaster.

Yes, the fun is beginning..
You should use fast-blow fuses on the DC supply rails, and a slow-blow fuse on the AC power line. Those 3-in-1 power entry modules are "prewired".
The complement of the 2N3055 in a TO-3 is a MJ2955. I thought SOT-96 was a surface-mount package.
You still need mica insulators, even if the heat sink is anodized. As soon as you tighten the screws it will make contact.
You can probably parallel another set of output transistors if you want to cut power dissipation down. Rod's original project 3 shows how to do this (and it might be worth asking him about it if you decide to do it).
[edit: just noticed Geoff posted while I was writing this. Geoff, the DC fuses are on the amp, after the filter caps. I think they should be fast blow.]

I suggested slow-blow for the supply rail fusing to ensure reliabilty and freedom from false failure due to ageing. A fast-blow fuse will not protect the output devices, a semiconductor will always fail first. The rail fuses are provided to prevent further damage from occurring once the output transistors have failed. This being the case, they can be slow-blow and rated with a generous safety margin to increase their life-span.

Fuses ?

I used a slow-blow fuse for the mains input and fast blow fuses for the DC supply on the amp PCB. These fast-blow fuses nay not save the transistors but they may save the speakers.

I think this is a better arrangement given the fact that this amp (ESP Project 3A) has no output protection whatsoever, which may be it's greatest weakness (or strength), depending on which way you look at it.

Still, I've been running this amp 24 hrs a day (it's NEVER OFF) since it was constructed over a month ago. Given a big and properly ventilated heatsink (better than 1 degrees C/W) and a good transformer, it should never overheat under most conditions.
"Can I use 2N3055/MJ2955 together? I mainly ask because the data I've got shows that they're quite different, whereas the other pairs have identical figures." There are many different versions of a '3055', the best way to deal with this is not to use any of them.Instead use the MJ15015/15016 pair.For USD$1.42/$1.52 you get 180W of safe area up to 60V, not the 50W at 50V that a TIP3055/2955 offer.You can also use them on +/- 50V rails.Carver used them in the M400 and Crown used them in the Macrotech MA600.
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