Quick question

[newey]

Got the amp today- or rather, it was waiting for me when I returned home after being out of town for several days. Haven't had the chance to do anything more than eyeball it and read the minimal documentation included with it. There is a schematic, but it's copyrighted so I won't repost it here.

I was sort of hoping they might give some details on suggested mods, but the instructions are real basic. Essentially, it's "A- attach speaker, B- attach audio source, C- attach 12-18V/1.5-2A DC power source."

All of which I will do, but first I have to secure a power source.

[ashcatlt]

I'm more than jealous that you got to visit NOLA. Hope you had a good time.

I'll ask my wife where all my wall-warts are.

[newey]

Thanks, Ash! Although I'm liking SGs laptop battery and recharger idea, I may have some options there.

N'awlins' was a hoot- saw lots of good live blues and jazz, when I wasn't sitting in a conference room.

[newey]

OK, I've secured an enclosure and I'm ready to build this amp out. Still haven't figured out what to use for a power supply- My temptation is to just run it at 12V and accept the loss of a bit of power.

I'm making this into a "micro-mini" head. Can I use a DPDT switch for on/off and to power up an LED when "on"? (I think I can . . .) Also, I have a 12V LED- can I just use that as is, or does it need a resistor in series with it? I also have some 3.4V ones- how do you calculate the value of resistor needed?

[newey]

In other words, will this work?

[sumgai]

newey,

Your diagram looks good.

Yes, you always need a resistor when powering an LED. The reason is the resistor acts as a current limiter. Let an LED draw too much current, and you have a used fuse staring you in the face.

To calculate the needed value, you need to know only the current draw specification of the LED. Most of the older style, and still common as dirt, LED's are rated at 20ma maximum. That's fairly bright, but newer LED's can get much brighter, and with less current. Be sure of your figures here, or like I said about the fuse thing.....

Thanks to Kirchhoff's Laws (regarding current), the same amount of current that passes through the resistor will be what passes through the LED. Armed with your maximum current draw figure, and knowing the supply voltage, you can now apply Ohm's Law, to wit:

E = I x R

Or in this case:

R = E/I

12v / 20ma = 600Ω

A next higher, and fairly standard, value near that would be 630Ω. Even higher values will reduce the brightness, should you feel the need to do that.

If your LED is rated for lesser maximum current, then plug that value into the formula above, and see what you get. Most of the time, a standard resistor value will be pretty close. Today the standard values run pretty much like so:

100, 150, 180, 220, 270, 330, 470, 630, 820 and 910. Multiply or divide by 10, as needed. Other values can be had, but at more expense - they aren't in as high a demand, thus......

HTH



sumgai

[Teleblooz]

Dontcha love the irony when a thread entitled "Quick Question" ends up 3+ pages long?

[KIIMH]

Jun 28, 2010 11:23:17 GMT -5 Teleblooz said:

Dontcha love the irony when a thread entitled "Quick Question" ends up 3+ pages long?

i dont get it

[newey]

OK, I have an LED rated at 12V, 20mA. Applying the formula, I get 800Ω. OK to use a 1M resistor?

I also have some rated at 2.1V, 20mA- Am I correct that the resistor wouldn't save these, since 12V of power will be running?

[JohnH]

Do you mean a 1k instead of 1M (hoping you did!)

Should work at 1k buts it looks like a lot of current is running hrough that LED and resistor - better use a 1/2W or 1W rating. You could get the 2.1v LED to work, so the resistor has to be calculated to drop the remainder of the voltage

[newey]

Yeah, 1K. But then I read the back of the LED package. This is designated as an "LED assembly", and according to the packaging, it has a 680Ω resistor incorporated internally. So I'm guessing this should be good to go as is.

[sumgai]

newey,

LED's are not rated for voltage, only for current. As it happens, if the correct current is flowing through the junction, the voltage observed therein will be no more than a couple of volts DC. It goes down as the current goes down, when the resistor value goes up.

And yes, some LED's come with a built-in resistor. Likely the 680Ω version is meant for a 12vDC circuit. They are also available in other resistor values, but there's nothing stopping one from using them in other voltage situations. A lower voltage will mean the LED is dimmer, and you can't do anything about it. A higher voltage will require an external resistor (as before), but the value of such will be the total calculated resistance, minus the internal 680Ω.

HTH



sumgai

[newey]

Got it. So, using 12V, I should be good to go.

I got all the components fitted to the enclosure, so I'm ready to wire this up. Still not sure on what to use for the 12V supply, for now, I'm just running the wires out the back for testing purposes. Eventually, I'll need a 12V power jack.

[sumgai]

newey,

So how much power is being drawn to operate this little goodie?



BTW, it's not only legal, but it's considered de rigeur to share schematics and other design info. The deal is, any unique ideas and/or implementations and such are already covered under one or more patents. A copyright only covers the expression of an idea, not the idea itself. (Think in terms of a book - you can't copyright the words, only the idea(s) espoused by the words.)

If I take a Fender schematic and build an amplifier exactly like it, and try to sell it on the open market, Fender would have a case against me, not because of copyright violation (they haven't seen any schematics at this point), but because of patent violation(s). However, if I were to take a Fender schematic, change the name(s) in the title block, and then attempt to pass it off as my design, then Fender would have a beef over copright violations, as well as the patent(s).

The short story of it is, a schematic is intended to be publicized for two reasons - it carries needed information to those who are charged with the maintenance and repair of such items, and it also conveys to the public in general (who are interested) that these particular ideas have been used, and presumably there are patents where applicable. Failing to disclose that information in a general way is tantamount to failing the public trust upon which the patent system is based. (For further reading, it's interesting to delve into the history of trade secrets versus patents.)

Looked at that way, it's easy to see how all those boutique builders have been able to rip off Fender (going back all the way to Jim Marshall!) - they didn't publish the same schematics, even though they did nothing more than tweak component values, all the while using the same basic circuits. (Fender couldn't patent the basic Williamson design - that had been put into the public domain by RCA more than a decade earlier.)

There's never a really solid, cast-in-stone answer, but case law (as you well know) has given us some fairly consistent guidelines, over the years. But my guiding light in this is that if I can go onto www.ampwares.com and download a schematic for just about any amp ever made, then it's a certain bet that you posting the drawings for you new little pride-and-joy is not going to upset anyone. For that matter, how do you think Aspen Pittman (of Groove Tubes fame) ever published his Amp Handbook, with all those schematics from so many sources - most of which have the title blocks intact, complete with copyright notices? He'd still be trying to look up owners (many of whom are long dead), trying to get permission, if that were the law of the land.

No dice, son - post the thing and be done with it. Err.... pretty please? ;D



sumgai

[newey]

OK, SG, you're right, I was being a bit overzealous. Here's the goods:



© Cana Kit Corp 1990-2010

DC supply is 12-18V, 1.5-2A. Anyway, this isn't much help, as I notice no component values are included thereon- I'd have to read them off the board.

[sumgai]

newey,

Actually, it's my fault - you gave the needed parameters in your post above (#30). 1.5 to 2 amps, eh?

Well, that will give you about 18 to 24 watts, at full bore, minus any built-in wastage that goes up in heat....

Now, what to use for a power source?   I seem to recall that we once discussed cell-phone batteries, did we not? Or was that cordless phone batteries? No matter, you've still got a bit of figurin' to do.

Your first baseline parameter to consider is that you'll be drawing (requesting) about 1 watt, perhaps a bit more, for most of your playing. The maximum draw will occur only at peaks, for very short periods. So, your nominal is likely to be in the neighborhood of 250 - 500mA, for the greater part of your usage. Taking the battery out of my cordless phone (a Panasonic, about 3 years old), I see that it can deliver (when requested) about 830mA/H. This translates most easily to "you can use up to 830mA for up to an hour, or some arithmetical combination derived therefrom". When reading these figures, the time factor is variable, but the max current draw shouldn't be exceeded. (This goes for nearly every kind of battery.)

That's at 3.6vDC. You could, in theory, hook up three of these in series for a total of 10.8vDC delivered to the amp, and it would probably work. How it would sound, missing a volt or so, I couldn't say, but I tend to think it'd be OK.... possibly a barely perceptible distortion, but likely not.

Remembering that these are Li-Ion units, that means that the delivered voltage will tend to stay the same until the last 3 or 4% of the charge cycle - then it will drop like a stone. So using all these figures, I think that if this slightly lower voltage is sufficient, then you could expect the batteries to last for up to 3.5 or perhaps 4 hours, but I am being optimistic here - it could be as little as half that.

Even so, a couple of packs made up of these things, one for the amp and (at least) one on the recharger, and you're all set! ;D

My first thoughts. Unfortunately they probably won't be my last ones, either.

HTH



sumgai

[newey]

I'm well past the stage where cellphone batts could be implemented.

The amp is already fitted to its enclosure, which does not have sufficient room for onboard batteries. While cell batteries could of course be outboard, the whole point of using those is portability.

Since the batteries have to be outboard anyway, I'm going back to an earlier idea of SG's, using 2 6V lantern batteries in series.

I believe these will stow nicely in the bottom of the speaker cabinet, with a velcro hold-down strap. For now, two alligator clips on the power leads for testing, later some type of connector hard-mounted into the back of the amp chassis, with an accompanying cable to the batteries.

We'll see how much usage I get out of the 6V cells, and if it's squirrelly, I can look at a rechargeable solution.

No batteries for the camera, either, so pix will have to wait. . .

[chuck]

if you have room for the huge lantern batteries , why not room for the tiny cell phone batteries ?

[newey]

Here's sort of an exploded diagram of what I'm doing, showing front and rear panels and wiring connections.



The whole thing is about the size of 2 decks of cards side-by-side, and about 2 decks tall.

[sumgai]

chuck,

newey said that the amp's case won't have enough room for any batteries, so he'll now have to put them in the speaker cab. I had earlier suggested the lantern batteries, but they were rejected as being too big/bulky/heavy for an all-in-one box. Separating the amp from the speaker cab allows for a greater choice of batteries.

HTH





sumgai

[newey]

ARRGH! I centered the hole for the volume pot, and it's just a hair too low to fit the board into the enclosure.

Luckily, I bought 2 of the enclosures, so I may have to start over with a new enclosure. I'll have to offset the holes towards the top a bit.

BTW, the board has 2 heat sinks attached to it, each about 1" sq. When installed in the enclosure, they'll have about a half-inch airspace to the top of the box, and about an inch and a half airspace to the rear. But the enclosure is fully, well, enclosed . . .

Should I be worried about heat buildup? Do I need to be drilling some airholes?

Also, the base of the board is going to be just off the bottom of the enclosure, once I redo it. I have insulated the bottom of the inside with tape to avoid any accident contact of the bottom of the board with the enclosure, but there's very little airspace there.

[chuck]

ahhhh ... thanks for clearing that up for me Mr Gai

i was just thinking that the more compact and lighter the batteries the better ...

[sumgai]

chuck,

I would think that too, but take a second look at newey's avatar - he tends to "go on safari" at the wrong time of the year, so brawny batteries will help him keep playing during such moments of second childhood. ;D



sumgai

[sumgai]

newey,

This thing's cheap by any measure..... if you burn it down due to lack of ventilation, then you'll know to let in some air, next time.

BTW, why are you so concerned with making the case as small as possible? It's not like you'll get an award or anything, is it?





sumgai

[newey]

BTW, why are you so concerned with making the case as small as possible? It's not like you'll get an award or anything, is it?

Well, I had the enclosure(s). And, after all, Small Is Beautiful

[chuck]

for the 17th straight year , Brawny Batteries ... the number one choice of lumberjacks everywhere