using a 18volt transformer

ballfire · Post by **ballfire** » 19 Feb 2020, 04:33

hello ..good afternoon

i have this transformer with primary 220 volts and the secondary output is 18volts and its 3 amperes ..i want to ask anybody what kind of diode bridge i will use?? how many ampere of a diode bridge need for this 18 volts 3 ampere transformer..???
im gonna use this power transformer to power up the power section of a LM1875 power amp with 20 watts output on the speaker...any advise??
by the way are the other components right?? epecially the two plate resistors??

phatt · Post by **phatt** » 19 Feb 2020, 05:35

The drawing is missing the centre tap common/ground.
Ctap of Tx should join the Common of supply rails.

Any bridge rectifier that is close to 4 x 1N4004 will suffice.

With an 18VAC Secondary you will only get about 12-0-12 VDC maybe 13Volt at best. Which will easy run the LM1875 *BUT* you will never get 20Watt @8 Ohms,, proly closer to 10Watts.

With 12Vrails you won't need the Zener diodes just insert a 10R resistors and add two 100uf filter caps after the drop resistors.
Other here might have better ideas,,
hope it helps.
Phil.

ballfire · Post by **ballfire** » 19 Feb 2020, 07:46

phatt wrote:The drawing is missing the centre tap common/ground.
Ctap of Tx should join the Common of supply rails.

Any bridge rectifier that is close to 4 x 1N4004 will suffice.

With an 18VAC Secondary you will only get about 12-0-12 VDC maybe 13Volt at best. Which will easy run the LM1875 *BUT* you will never get 20Watt @8 Ohms,, proly closer to 10Watts.

With 12Vrails you won't need the Zener diodes just insert a 10R resistors and add two 100uf filter caps after the drop resistors.
Other here might have better ideas,,
hope it helps.
Phil.

i thought if theres a rectifier bridge like that it will increase the voltage after the diode bridge say 18v x 1.4 =25volts to power up lm1875 to produce a 20 watts..by the way that is a dual power rail...

because i saw a chinese amplifiers actually mine..because i CANT afford a real marshall amp i bought a cheap amplifier ..there power section looks like that and they put a lm1875 there a power transformer with primary 220v and secondary 21 volts with 2 amperes ..and a 4 ampere diode bridge the result is loud it says 30 watts and its really 30watts on the output but i found out a problem the power section gets really hot and the plate resistors damn they put a 1/4 resistors there and they burned gradually but it takes 5 years before the amp busted totally ...but the sound of it its really good especially on the clean channel...

im confused here???? i will only get 12volts ??? how..

plush · Post by **plush** » 19 Feb 2020, 08:45

ballfire wrote: im confused here???? i will only get 12volts ??? how..

Your transformer is capable of delivering 18 volts AC. When secondaries are connected in series with a center point tied to ground, it means the voltage is swinging from -9 to +9 in regards to ground.
Rectified, positive peaks go one way, negative - the other. You'll get maximum of -12v and +12 DC (|9 x 1.41 minus diode drop|) in regards to ground in such configuration.

Also, using zener diodes for stabilizing is very old and ineffective approach.

ballfire · Post by **ballfire** » 19 Feb 2020, 09:45

im confused man...

why is it that chinese amp so loud ??so i must use a 21 volts transformer to achieve a 20 watts on speaker output??? i thought that lm1875 can deliver 20 watts output of speaker sound???

Ben N · Post by **Ben N** » 19 Feb 2020, 11:06

Maybe it's worth clarifying: When you say the secondary is 18v, is that across the whole secondary, or is it 18-0-18?

phatt · Post by **phatt** » 19 Feb 2020, 11:12

ballfire wrote:im confused man...

why is it that chinese amp so loud ??so i must use a 21 volts transformer to achieve a 20 watts on speaker output??? i thought that lm1875 can deliver 20 watts output of speaker sound???

Yes it can IF you run it from 50Volt supply,, OR a split +25/-25volt rails.

What plush said is right,,You need to take your time and read up a little more before you build.

The confusion might come from the fact that your "Chinese Amplifier" amp might be running from a single supply.
LM1875 can be run from a single rail supply as well as a dual rail +/- supply.
In both cases the higher the voltage (Up to the limits of the device) the more wattage is produced.

If that Chinese amp is single rail supply then you have drawn the supply *incorrectly*,,maybe go back and check.

Most of the data sheets show an easy to read schematic on how to run it from single rail supply if you want to go that way.
But split rails is the better option.

I quote from the data sheet;
"The LM1875 delivers 20watts into a 4Ω or 8Ω load on ±25V supplies.
Using an 8Ω load and ±30V supply over 30watts of power may be delivered."

The max single rail supply voltage is 60VDC
The max split rail supply is +30 -30 VDC.
so anything less than those volatges means you get less output power.
As said with an 18Volt Tx the chip will only be capable of about 10Watts into an 8 Ohm speaker
Which in a small room is Loud. (as you have noted)
Phil.

ballfire · Post by **ballfire** » 20 Feb 2020, 01:02

now you understand people yes thanks god thats what i mean my trans former 18-0-18volts

i want to put a 18-0-18 transformer because i dont want to overheat the power amp because based on that chinese amp inspite it delivers
a 30 watts speaker power the power amp gets hot very hot and busted gradually..i want to minimize the heat so maybe i can put a lower voltage like a 18-0-18volts??

do you mean guys it will not achieve a 20 watts atleast even it recieve a 25volts after those two big 2200 capacitors??

can you have other solutions or suggestion what power supply rating can i use in order to get a 20 watts speaker power??

man i want it a loud amplifier on my room so i want to nailed a 20 watts loud..i want it so noisey..

ballfire · Post by **ballfire** » 20 Feb 2020, 01:03

now you understand people yes thanks god thats what i mean my trans former 18-0-18volts

i want to put a 18-0-18 transformer because i dont want to overheat the power amp because based on that chinese amp inspite it delivers
a 30 watts speaker power the power amp gets hot very hot and busted gradually..i want to minimize the heat so maybe i can put a lower voltage like a 18-0-18volts??

do you mean guys it will not achieve a 20 watts atleast even it recieve a 25volts after those two big 2200 capacitors??

can you have other solutions or suggestion what power supply rating can i use in order to get a 20 watts speaker power??

man i want it a loud amplifier on my room so i want to nailed a 20 watts loud..i want it so noisey..

phatt · Post by **phatt** » 20 Feb 2020, 06:28

ballfire wrote:now you understand people yes thanks god thats what i mean my trans former 18-0-18volts

i want to put a 18-0-18 transformer because i dont want to overheat the power amp because based on that chinese amp inspite it delivers
a 30 watts speaker power the power amp gets hot very hot and busted gradually..i want to minimize the heat so maybe i can put a lower voltage like a 18-0-18volts??

If the chip is overheating (i.e burn fingers in less than a second)
Then it's a fair bet the heat sink is to small.
In which case you replace the heat sink with a much larger unit.
Some of these small amps only use a tiny bit of alloy as heat sink and claim 30watts. ha ha of course they fail.

In case you don't already know,,,Never run a power amp chip with no heat sink as you will likely blow it.

ballfire wrote: do you mean guys it will not achieve a 20 watts atleast even it recieve a 25volts after those two big 2200 capacitors??

AGAIN I quote from the data sheet;
"The LM1875 delivers 20watts into a 4Ω or 8Ω load on ±25V supplies.
Using an 8Ω load and ±30V supply over 30watts of power may be delivered."
I will add a caution to that; IF you want the 30 Watts then DON"T try and drive a 4 Ohm speaker as that will definitely burn out the chip.

ballfire wrote: can you have other solutions or suggestion what power supply rating can i use in order to get a 20 watts speaker power??

As above, but you will need to add those zeners as in your first post to drop the +/-25VDC rails for the opamps.
try 220 Ohms /1Watt. I doubt they will over heat but will become warm,, if too hot use 2Watt resistors.
For the zener to work as a regulator the voltage AFTER the 220R should be at least 2Volts higher,, or even 3~4 volts.
Cheers, Phil.

ballfire wrote: man i want it a loud amplifier on my room so i want to nailed a 20 watts loud..i want it so noisey..

Yes always sound better when loud.

ballfire · Post by **ballfire** » 21 Feb 2020, 02:17

hello good morning everybody...

thank you for helping me and understand this topic better americans are really great..especially in amplifiers i salute everybody here..

im targeting a 18volts to the amp and 20 watts of power speaker is okey for me..

what should i used in bridge diode?? how much ampere i will use if my transformer is 18-0-18v 3 ampere??can i use a 4 ampere bridge diode?? or 25 ampere bridge diode???

i will test it this weekend...

phatt · Post by **phatt** » 21 Feb 2020, 07:35

All good,,
You can make the bridge with four 1N5404 if you want, they are a 400V, 3A device,,,
or if you want a BR64 is a 400V, 6A full wave bridge.
Or anything with similar specs will do the job.
Phil.

ballfire · Post by **ballfire** » 21 Feb 2020, 08:09

phatt wrote:All good,,
You can make the bridge with four 1N5404 if you want, they are a 400V, 3A device,,,
or if you want a BR64 is a 400V, 6A full wave bridge.
Or anything with similar specs will do the job.
Phil.

i have a 25 ampere diode bride its a full wave bridge can i use it??

phatt · Post by **phatt** » 21 Feb 2020, 10:41

Yes you can, there might be a little more voltage drop across the rectifier due the to higher rating but it won't be much.
Phil.

ballfire · Post by **ballfire** » 26 Feb 2020, 01:06

i tested the lm1875 power amp last weekend the result it was failed !!..i found out the chip was fake and behave improperly.i followed all the advices here but
it was failure i run many test and debugged but nothing happends..i bought the chip from online from a china website dumb fool of me..what a waste of money ...i thought it was original its not...

anybody suggestion where to find a original lm1875 may be a tda 2050 chip???

Manfred · Post by **Manfred** » 26 Feb 2020, 05:04

The original LM1875 is available from Mouser Electronics.

phatt · Post by **phatt** » 26 Feb 2020, 06:42

Was the LM1875 mounted on a heat sink? If not it will likely fail.
Did you run it through a limiter at first power up?
If not make a simple light bulb limiter it will save you from such disasters. google it,, easy to make up one.
Also what schematic did you use? Maybe post it or link to it.
Better still post a pic of your actual circuit you made.

Yes there are fakes out there so don't waste the time on junk.

Phil.

ballfire · Post by **ballfire** » 26 Feb 2020, 07:43

phatt wrote:Was the LM1875 mounted on a heat sink? If not it will likely fail.
Did you run it through a limiter at first power up?
If not make a simple light bulb limiter it will save you from such disasters. google it,, easy to make up one.
Also what schematic did you use? Maybe post it or link to it.
Better still post a pic of your actual circuit you made.

Yes there are fakes out there so don't waste the time on junk.
Phil.

yeah never forget those heatsink those aluminum thing...i have lot learned on this freestompbox i learned a lot from the master here..i know how to debugged atleast...its the lm1875 chip is the problem i try other chips like tda2050 it works!!but the lm1875 its busted i hear like motoboating sound..but when i change it with tda2050 it works...this tda2050 works only with my 12-0-12 volts transformer only i never tried to the 18-0-18 v because i feel the chip will gonna blow up...i read some forum that using 18-0-18v to the tda2050 it exceeds noe to the required voltage which is 50volts only voltage ...

if i use 18-0-18v transformer to the tda 2050 its over 50volts ..

phatt · Post by **phatt** » 26 Feb 2020, 11:56

If the LM1875 is motor boating then it's passing signal which means it's working but unstable. Your PSU layout might be a problem.
If you have long leads from Filter caps to LM1875 then that can cause issues.
Judging by you first post showing the PSU schematic you wanted to use it's likely you have missed something.
Without a picture of your layout there is little we can do to help you.
So pictures will speed up the fault finding. hint!!!!!
Phil.

george giblet · Post by **george giblet** » 28 Feb 2020, 06:02

Do you have the 1 ohm resistor and 0.22uF capacitor on the output of the LM1875?
They help prevent oscillation. They aren't optional you should have them.

Look at the LM1875 datasheet.