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gianmarco 23rd March 2013 05:26 PM

Re-design stage monitor after a big failure.
Hello everybody, as someone read before, I tried to repair my broken stage monitor, but with no success.

So, the thing I want to do is to re-design a new active stage monitor, from scratch (or copying some projects here and there LOL).

Now I have a blank wood edge monitor, even with no holes. I have the transformer, and I have to buy the rest.

Can you please help me in construct something good to use in live?

second, I have two wood case, I could even create a passive monitor to use with an amplifier, but I don't know what component to use or if is cheaper to create one by myself than buying one.

Waiting for your answers.

JMFahey 23rd March 2013 09:31 PM

Start by posting some data about your transformer.

gianmarco 24th March 2013 02:38 AM

The transformer is this:

I can't find many other infos, than the ones written onto the label.

I wrote an email to comelit srl to have some infos. But still no anwers

JMFahey 24th March 2013 04:45 AM

Ok, 43+43VAC mean +/- 60V rails.
To know the VA, please measure and post the EI core dimensions.
I'm only interested in full width, height and stack thickness.
In mm .

picbuck 24th March 2013 03:18 PM

You're speaking of a gainclone or chipamp, same difference. Both Google and YouTube are interesting--although YouTube tends toward uninformative videos of speakers playing music.

The good news is that yes, you can solder up some very good equipment at large--huge--savings over retail. And as a matter of fact it's not difficult. Or that's not difficult if you have some idea of what you're doing. Not that you have to be an electrical engineer (you don't), only that some familiarity with the subject helps a lot.

The bad news is that things can get very frustrating until you reach that "some familiarity" point. The first one is the hardest, of course, but no kidding, this stuff can become all-consuming. A musician might be better off spending the time and effort practicing. Nobody ever got to Carnegie Hall by soldering wires together...well...except the techs, of course.

In any case, the electronic parts for a gainclone/chipamp might run some $10. But that's a lowball price.

Potentiometers (volume and tone controls) with knobs run around $3 each. Switches around $2 each. These don't much matter with just an amplifier, but once you get into mixers and effects boxes those dollar bills add up.

Usually the main cost item is the transformer for the power supply, which is in the $20-30 range, or can be much more. The one you have might or might not work. Transformers are rated in voltage, and amps/wattage/VA (same difference), and whether they're CT (center tapped). "Center tapped" is pretty much the standard. CT means it's actually two transformers in a single housing. This is how you get the "plus or minus" power supply, and the center tap is ground (zero volts).

Generally what's used is in the 15-30 volt range, CT. A transformer must (not should) be something like 30% oversize at the very least. That is, a 50watt amp would require something like a 70watt power supply. More is better.

The physical size or shape of a power supply transformer doesn't matter. Many will use only toroids (expensive), but I personally see no point in that.

The tiny black chip that does the amplifying requires a relatively huge heat sink (that heat is why you need an oversize power supply). For a largish amp (50ish watts) these can be hard to find and expensive, which is why Google Images shows some bizarre setups.

However, a 50mm fan/heat-sink combination made for a computer CPU (Central Processing Unit--the chip) is used by many. These run $15ish. Of course, you have to be willing to run the fan. But for an onstage environment fans would probably be a very good idea anyway. By the way, buying just a heat sink is more expensive than buying the combination.

The housing/chassis is, for many, the greatest expense by far. Then again, you can put the whole shebang into a soup can if you want to. Literally. Drill some holes for ventilation.

There's also the matter of the circuit board. Usually a printed circuit board (PCB) that simply supports the components. This can be a noticeable expense. Or you can use perfboard (that's its name) from Radio Shack.

None of these gloomy predictions tell you anything about how to build one, of course. Possibly this page might be of interest.

DIY TDA2050 Hi-Fi Chip Amplifier (chipamp)

gianmarco 24th March 2013 04:08 PM

thanks both for your answers!

@JMFahey - the transformer is 107x90x52mm (LxHxD)

@picbuck - I am actually studying Electronical Engineering at university, I know about circuits, physics laws, few digital and few microelectronics... but I am still studying.
I think I have the basics for make my own amp.

I have a big heatsink (about 250x80x50mm) where there was the stk4048V now burned.
I can solder with a pre-drilled PCB (the ones with many holes) if necessary.
The chassy (as said) is this:

now I am reading the link you gave me.

JMFahey 24th March 2013 04:34 PM

Ok, you have a core of 18.7 square centimeters, which is good for 350VA.
It can easily power a 200/250W RMS amplifier.
+/-60V rails will give you about 150/160W into 8 ohms, or 250W into 4 ohms.
I very much doubt you can fit such a big amp into that tiny cabinet.

I would consider 2 options:

1) forget that big power transformer for now, and build a 50/60W chipamp (with the proper transformer), mounting it inside that cabinet without much trouble.
Anyway remember thar Eminence guitar speaker can't handle more than that.
You'll have a very good active monitor for your keyboards or acoustic guitar.
I would do that and be happy. :)

2) later, when you are more experienced, build a "big" power amp, such as apexaudio QUASI amplifier

Don't be misled by the "beginners" label ;) , it's a big mean loud amplifier and perfect for your transformer.
He even gives you the PSU, how's that?

And it's not for beginners, but for people who has at least 3 or 4 amps previously, following something like the classic ladder: TDA20xx (Pickbuck's suggestion) > LM3886/TDA7294 > discrete around 100W amp > Big amp.

Do not skip steps in that ladder, each one prepares you for the next step.

Good luck.

gianmarco 24th March 2013 04:40 PM

the eminence cone is gone, i have to buy a new one by ciare, is about 300W why do you think the amplifier can't be fit in the cabinet? There was a 4048V with that transformer and the big heatsink before. Maybe I can build it external with a speakon connector, if the cabinet is too small... there aren't 12" speakers under 300W power, a 60W amp is too small...

JMFahey 24th March 2013 05:12 PM


Originally Posted by gianmarco (Post 3425204)
the eminence cone is gone, i have to buy a new one by ciare, is about 300W why do you think the amplifier can't be fit in the cabinet?

Mounting there limits heat dissipation big way.
No problem with, say, a 60W RMS amplifier, maybe 100W might be the realistic limit.
Yes, JBL/RCF/(insert "big" name here) can fit 300W RMS or higher amps in (much larger than yours) powered speakers, but "they" use Class D or at least class H amplifiers, place heatsink fins in the tuning port so bass notes cool them, have sophisticated thermal protection systems, etc.
Please be realistic.


There was a 4048V with that transformer and the big heatsink before.
True enough ..... and it kept burning :rolleyes:
Big heatsinks mean nothing (or very little) if fresh air can't freely flow around them.


Maybe I can build it external with a speakon connector, if the cabinet is too small...
Excellent idea


there aren't 12" speakers under 300W power,
:confused: :confused:


a 60W amp is too small...
Maybe, but that's what the original Laney used ;)
For a powered monitor, it's enough.

What you have there is the unpowered version of Laney CP12
Which, by the way, is already larger than yours and has an 80W power amp.:eek:

gianmarco 24th March 2013 05:17 PM

ok so let\'s try the 60W, is there any cheaper cone to fit this amp? and not only the 120€ cone 300W from ciare?

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