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Old 30th December 2001, 03:41 PM   #1
ih is offline ih
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Default Planning to build Rod Elliot's Project3A, number of questions...

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
electronics...

Thanks for your comments.

Ian
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Old 30th December 2001, 04:51 PM   #2
paulb is offline paulb  Canada
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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:
Rod Elliot 100W amp suggestions?
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).
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Old 31st December 2001, 08:48 PM   #3
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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.
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Old 1st January 2002, 03:39 AM   #4
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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?

Thanks,

Ian
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Old 1st January 2002, 04:04 AM   #5
bawang is offline bawang  Malaysia
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Exclamation 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.

Enjoy!
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Old 1st January 2002, 06:29 AM   #6
ih is offline ih
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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?

Ian
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Old 1st January 2002, 06:35 AM   #7
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If you use a plastic case instead of a grounded metal case the amp will be susceptible to rf interference.
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Old 1st January 2002, 11:02 AM   #8
bawang is offline bawang  Malaysia
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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......
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Old 1st January 2002, 11:47 AM   #9
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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.
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Old 1st January 2002, 11:53 AM   #10
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Quote:
Originally posted by bawang
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.....
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