It Goes To 11

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Comment by petey twofinger on November 11, 2011 at 6:41pm

tough one but i would say first do you have a regular amp to test your piezo in the cbg ?

 

a preamp may do the trick , do you have a pedal or a booster you could try ?

 

is the megaphone circuit a chip amp , does it use a chip or transistors ?

 

 

Comment by Habanera Hal on November 11, 2011 at 6:32pm

Ok, I'll admit I can't read schematics, but I can figure out inputs/outputs and power on a circuit board.  I picked up some of those 10w megaphones from Harbor Freight today (BTW - they're on sale for $4 right now so I don't mind if I fry one or two) and tore one apart.  I hooked up a 9v to replace the C cells and a 4" speaker to replace the bullhorn, so far so good, everything still works.  I then replaced the microphone with a 1/4" jack. I plug in one of my piezo CBGs and hardly hear anything.  Check all my connections, all are ok.  The siren and music switches work and the thing screams but nothing from the jack.  I try a  different guitar and a different cord, still nothing.  Then I hook a piezo straight to the mic conection on the board, and I get loud tapping like it should be when I tap on the piezo.  Why am I getting so much resistance through my cords?  What can I do about it?

Comment by Mickey Sadler on November 10, 2011 at 10:09am

For a good, free set of lessons in electronics go to http://openbookproject.net//electricCircuits/index.htm .

 

It's a 6 volume set, but some parts are incomplete as it is a work in progress.

 

1) DC

2) AC

3) Semiconductors

4) Digital

5) Reference

6) Experiments

 

Comment by Ellwood T. Bear on November 9, 2011 at 7:20pm

Hmm teaching basic electronics, start with the math first....???

Comment by John Wilson on November 9, 2011 at 6:44pm

So... read it again and let me know:  where did it stop making sense?  :)

Comment by Habanera Hal on November 9, 2011 at 9:45am

mmmmm........brain hurt. me not get john say.  john smart.  hal dumb.  must make hurt go away.  hammer on head takes hurt away.   mmmmm.....better now.

Comment by John Wilson on November 9, 2011 at 8:39am

Arg!  I started writing a little tutorial on how to interpret schematics but overnight Microsoft rebooted my computer for a patch update and I lost it!

 

I guess for now I'll just leave you with this:  anything that someone has figured out, you can figure out.  I know *exactly* how frustrating it is to look at this stuff and not understand it.

 

Those triangles are called operational amplifiers.  An operational amplifier is simple, but not simple at the same time.  It's not simple because it's not like, say, a resistor.  A resistor is simple because it resists the flow of current, and it's easy to imagine current flowing like water and the resistor like a narrow little tube that restricts how much water flows through the pipe.

 

Operational amplifiers don't have a simple real world equivalent, but what they do is simple.  An operational amplifier is also known as an opamp.

See the + and - symbols on that triangle?  Those are the inputs.  And the lead coming off the point of the triangle is the output. 

The opamp first figures out the difference in voltage between the inputs (it subtracts the negative input from the positive input), and then it MASSIVELY amplifies the difference, and tries to put that amplified voltage on the output.

 

Like, if the difference is .1 volts it will amplify that hundreds of thousands of times and then try to put tens of thousands of volts on the output line.  (Of course, in real life the practical output voltage is limited by the power supply.)

 

That does not sound very useful at all!  It's like that super angry drunk guy at the end of the bar:  you tap him on the shoulder and he hauls off and smashes you in the nose!

Ah, but imagine tying a wire from the output to the - input. 

 

Put a single voltage, let's say 4.5 volts, on the + input.  If the - input is at zero volts, it subtracts zero from 4.5, and gets 4.5, then it massively amplifies it by thousands and thousands and tries to peg that output at infinity volts.  The voltage goes up, incredibly fast, and then, the output voltage goes past 4.5 volts.  Suddenly everything goes in reverse, and it tries to massively amplify a negative voltage and go to zero.  Once everything settles down (it usually settles down way faster than we have to worry about with audio circuits) then this op amp circuit will output any voltage that is given to it!

 

So we mix in an audio signal with our 4.5 volt bias signal.  When the audio signal goes up, there is a difference between the audio signal and the current state of the output.  Because the output is tied to the - input, it forces the output to follow the signal at the + input. 

 

Hey look, a preamp!  And it's way easier than designing a transistor based one where we have to worry about device variation and temperature effects and all that nonsense. 

 

With just a few resistors we can set the gain of our preamp to a manageable level.  Or with a capacitor or two we can create a tone control.

 

In real life there are all kinds of complications with supply voltages and how fast the op amp can actually move (called the slew rate) and so on.  But that's the basics.

If I was implementing this particular circuit I would use an MC33202PG and I would probably have to muck about with it a bit to make it work with a single supply voltage.

Comment by petey twofinger on November 8, 2011 at 9:36pm

or you could hack a cheap pedal into the amp . a danelectro or a behringer . behringer has a nice cheep reverb / delay pedal , both at once on a few settings , the echo is up to 2 seconds . it would cost a lil more , but it has all the controls on it . i have a mini amp here that has 3 pedals in it .

 

the simplest aproach would be to buy a pedal , and use it on the floor ! but ... i was plesantly surprised with that behringer , plus i got it for 15 bucks on CL .

Comment by John Wilson on November 8, 2011 at 8:48pm

Hmm.  Well, I understand it well enough to understand it but I'm not sure I understand it well enough to explain it.  Let me think about it.

Comment by Habanera Hal on November 8, 2011 at 7:27pm

Another thank you, John!  I admit that this is all greek to as I still have trouble putting batteries in the right way.  When I look at wiring diagrams, I just go, "huh?".  I have no idea what those symbols mean (what's a triangle?) and I'd need a GPS to follow a circuit route.

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