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Author Topic: Yet another newbie-looking-for-project-advice post  (Read 5803 times)

smadin

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Yet another newbie-looking-for-project-advice post
« on: May 24, 2008, 10:59:10 PM »
Hello all, and apologies in advance for the high degree of redundancy to follow...

I've built a Ruby and am quite happy with it (modulo the limitations of the cheap little speaker it's driving currently), but I'm interested in moving on to a larger project.  So far, I think that's about the same as the majority of intro posts here :-)

I'm only in the very initial concept stages right now for what I'd like to do next -- after all, I'm so out of practice playing that I won't be able to make any amp sound its best just yet, and flubbed notes will only sound more jarring coming out of a big amp and a quality speaker.  I've been reading through a lot of stuff here, at runoffgroove, and occasional other places that come up on google, poking around GGG, AMZ and GEO, and reading through power- and op-amp IC datasheets, but although I know my way around a soldering iron and am beginning to get a feel for the functions of the various bits, my overall grasp of circuit theory is shaky at best.

What I'm thinking I'd like to do, roughly, is build a 1x10 cabinet, probably with a Jensen MOD10-50, and ideally design it so that the amp electronics are very modular: I'm thinking of this cab as more or less a platform for trying out different amp circuits as I build them, so I'd like to be able to swap them in and out without much trouble.  It needs some amp to start with, though; based on reading here, I'm inclined to think an LM1875, or a pair of them, would be a good approach.  (Unfortunately the pictures and schematic links for the LM1875 chipamp on the wiki appear to have gone 404, but there seems to be a good amount of information available 'round the net anyway.)  However, I'm also intrigued by the LM2876, which looks to have some better protective features built into it than the 1875.  Nothing on the 2876 came up with the search function here; has anyone had any experience with it, or does anyone know any particular reasons not to just work from the app note schematic?

Choice of IC isn't really my main concern, however; the part of the task I find most daunting is the power supply (a great advantage of battery amps...).  To drive an 8ohm speaker at 40W, it looks like the 2876 wants a +-30V supply, while for the 1875 (and presumably likewise for a dual 1875 setup?) I'd want +-25V; both chips have a pretty wide supply range, though.  Is building an adjustable PSU, so I could swap in, for example, an 1875 board at +-25, a 2876 board at +-30, or even a 3886 at +-35 (or a 386 at +-12), without changing out the power supply, liable to be prohibitively complex or difficult?

Thanks very much, and thanks also for all the information I've already gotten reading through these boards.

teemuk

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Re: Yet another newbie-looking-for-project-advice post
« Reply #1 on: May 25, 2008, 12:19:58 PM »
Well, it’s not just an issue of rail voltage but also current since you need both current and voltage to create power (P = UI). So, in addition to proper rail voltage, your power supply must also be able to provide the current that the power amplifier circuit will draw. When you go shopping for a mains transformer you will find out that they quote a “VA rating”, which stands for Amperes x Voltage, hence the amount of power the transformer can provide. If you know the secondary side voltage you can therefore calculate the current and make sure it is sufficient. The VA rating should be higher than the amp’s output power to make up the efficiency (or lack of it) of the power amp circuit. Rule of thumb (for class AB amps) is to pick a VA rating that is two or three times higher than the amount of output power.

You can always reduce the rail voltage with regulators but those circuits will naturally introduce additional complexity and you need to build them so that they tolerate the high current that the power amp draws. With high power amplifiers that draw a lot of current this is a big issue, which is why you hardly ever see those using regulated supplies. More current draw means higher losses over the regulator circuit, therefore more heat. The regulators those amps would need would be pretty equal to building another power amp, requiring heat-sinked power transistors and stuff. It’s generally not worth it, though definitely plausible. I’d rather save the use of regulators for low current circuitry where demands are less strict.

You could likely build a basic supply but swap in different mains transformers depending on the required rail voltage and VA rating. The rectifier and filter capacitor circuitry is pretty universal anyway so you only would need to rate those components for the worst-case scenario (highest rail voltage and current demand). Conversion between single- and bipolar supply of course requires a bit more planning and rewiring. Fusing should, of course, be done according to circuit but fuses are easy to swap so that’s not a problem.
« Last Edit: May 25, 2008, 12:22:27 PM by teemuk »

smadin

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Re: Yet another newbie-looking-for-project-advice post
« Reply #2 on: May 25, 2008, 01:21:08 PM »
Quote
You could likely build a basic supply but swap in different mains transformers depending on the required rail voltage and VA rating. The rectifier and filter capacitor circuitry is pretty universal anyway so you only would need to rate those components for the worst-case scenario (highest rail voltage and current demand). Conversion between single- and bipolar supply of course requires a bit more planning and rewiring. Fusing should, of course, be done according to circuit but fuses are easy to swap so that’s not a problem.

Thanks!  That sounds likely to be the best option.

I forgot to mention current in my initial post, but I was thinking about that.  Based on the equations in the LM2876 datasheet, at 40W output I'd need +-30V supply rails at about 3.16A.  On the other hand, the 2876 can run on +-25V, which looks like it would mean about 27W output and 2.63A.  Assuming I did the math right, that is.  So looking at those numbers (and considering my budget!) perhaps I should start with +-25V, and I think I should then be able to swap in either an LM2876 circuit or an LM1875 without changing the power supply, is that right?  Would a 250VA transformer be about what I'd need for that?

Thanks again for the help!

smadin

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Re: Yet another newbie-looking-for-project-advice post
« Reply #3 on: May 27, 2008, 06:28:12 PM »
Ah, with a bit more reading it looks like my math was wrong.  I misunderstood the conversion factor going from AC to DC (and so was rather confused by some of the PSU schematics I'd been looking up...).  So to supply +-30VDC I should need a 21-0-21 transformer, and with an LM1875 160VA should be plenty -- and now that I think about it, the formula in the LM2876 data sheet is for peak current.  So even with a 3886 in there, a 200VA transformer should be enough.  Does that sound reasonable?  (I'm sure this is really basic stuff for most of you, so I apologize for undoubtedly covering the same ground as countless other newbies...I feel like I'm getting a much better understanding, though, so hopefully I'll become rapidly less burdensome.)

J M Fahey

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Re: Yet another newbie-looking-for-project-advice post
« Reply #4 on: May 28, 2008, 09:29:50 PM »
Short reply: if possible go straight to an LM3886, powerful enough to play live with a drummer.
Transformer: Volt rating: .707 x rail VDC. Example: for +-30V rails, 30x.707= 2x 22V (assuming a small diode loss). VA rating? : at least 1.5 x RMS power output, preferably 2x. Your 160VA units should be fine for a 60/70W power chip. The Jensens are great.

smadin

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Re: Yet another newbie-looking-for-project-advice post
« Reply #5 on: May 29, 2008, 09:49:35 AM »
Well, the chances that I'll ever actually need an amp powerful enough to play live with a drummer are probably slim, but I'll keep that in mind!  For the time being this is just a fun hobby and a way to learn interesting things, for me -- and I enjoy making things.

I'll start looking around for 160ish transformers.  This is turning out to be a sizable project, so to tide me over I've picked up supplies to build a wall-wart powered Little Gem MkII with the buffer replaced with a Fetzer, a la the Fetzer-Ruby.

Thanks for the input!  I'm constantly impressed by what a great resource this forum is.

teemuk

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Re: Yet another newbie-looking-for-project-advice post
« Reply #6 on: May 29, 2008, 11:00:25 AM »
In my opinion, the difference between building a 30W chip amp or a 60W chip amp is pretty minimal. The other will cost a bit more but you’ll likely end up spending the same amount of work effort to both (which is an issue that at least to me makes a far greater difference than a small difference in cost). Then, whereas the 30W amp is mainly good for private practicing only (some could argue on that), a 60W amp can adequately handle gigs and band practice situations.

Loudness of a higher power amp is not that big issue either: In terms of loudness 30W and 60W have little difference (3dB = 1.23x difference) – both are definitely loud enough to get you kicked out of an apartment if you keep cranking up. Yet a 60W amp has more clean headroom, which in many situations becomes more important than the perceived “loudness”. For quiet practicing, volume control can always be turned down. The speaker system’s efficiency will make the final – big – difference.

So, as long as we are not talking about discrete designs my opinion is that one should just built that higher power design right away.

smadin

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Re: Yet another newbie-looking-for-project-advice post
« Reply #7 on: May 29, 2008, 11:53:43 AM »
That makes sense, Teemu.  I'm still inclined to build a few different circuits, because I'm interested in comparing how they sound as well, and once I have a little more experience, trying out different designs, but starting out bigger is probably a good idea.  Thanks again, and thanks also for making your excellent book available; I haven't had time to read very much of it yet, but I expect I will be referring back to it frequently for as long as I'm building these things.