lm1875 fizzing away probelm

[PetruV]

It sounds like a ground is missing; in the stereo input plug perhaps?

im using mono and pluging in one chanell at a time and the two grounds are hooked up toghrthrt

 

[PetruV]

Now botch chanels work just have to get another rca plug for my computer and an usb cable,volume controle being made from pc
Here it is ,looks like crap just like all of the stuff i have,but hey it works
From radio just one chanel is used

 

[blu_glo]

well if you leave an input unconnected you can expect it to pick up hum from the surroundings. You can make a special splitter to split one output from your radio into two outputs for testing stereo gear - it's as simple as two wires!! then you can connect both inputs to the same radio output and get a dual-mono signal from the stereo amplifier under test....

 

[PetruV]

well if you leave an input unconnected you can expect it to pick up hum from the surroundings. You can make a special splitter to split one output from your radio into two outputs for testing stereo gear - it's as simple as two wires!! then you can connect both inputs to the same radio output and get a dual-mono signal from the stereo amplifier under test....

Hooked up to pc it now works but the gain is too high and it catches computer sounds

 

[picowallspeaker]

Then you can put some low capacitance ceramic caps where they're needed :
to make a suitable low pass filter at the input of the amplifier. What about 1-2 KΩ and 500p/1nF ?

 

[PetruV]

Is this heatsink big enough for two lm1875,if its not then the have to endure the heat

 

[blu_glo]

I can't quite figure the whole heatsink with the CD on top but it looks like a good start...

 

[blu_glo]

Hooked up to pc it now works but the gain is too high and it catches computer sounds

You will have to put an attenuator in the L and R inputs. Connect together a 10K and a 1K resistor. The input from computer sound card goes to free end of the 10K. The middle junction goes to the input of the chip. The bottom of the 1K should go to 0V. Do the same in the other channel.

 

[owenhamburg]

Fix the source not the amp.

Hooked up to pc it now works but the gain is too high and it catches computer sounds

I am afraid this is a common problem with most computers using most sound cards (Only seen one exception with a Pro sound card with balanced outputs did not have the same problem), my usual solution is to get an optical link from the computer to a cheap DAC form Ebay.

I personally hate the noises from computers, and even attenuated I still find them objectionable the external DAC issue has solved this for me every time I have tried.

Regards

Owen

 

[blu_glo]

Owen - You *could* also use the TI PCM2706 etc USB chips to extract the digital audio directly from the computer bypassing the soundcard. These USB chips are supported natively by windows (i.e the driver is recognised by windows itself from -XP through to -8).

However, the chips are not easy for beginners and the audio output is 1V or so, and would still need to be scaled back for the TDA2822M......

 

[blu_glo]

There's a very interesting application on P. 28 of the PCM2706 datasheet - it seems you can configure the chip to demand 500mA for your power amplifier.......

 

[PetruV]

aLtough it dosent look more than a bunch of coloted spaghetti,its actualy coming along nicely

 

[blu_glo]

- did the attenuator work ok?

 

[PetruV]

- did the attenuator work ok?

Made it and hooked it up but not tried it,since my mom was constantly using the other comp which has hooked up to the speakers an rca cable,when my mom is not usin it i caan steal the cable and play with it,the subwoofer on that computer is coincidentaly usin an lm1875t,but i had not built it it has bee bought like that,and the problem with it is that one of the chanel is full of his which temporaryly goes away when you hit it so i have nothing hooked up to that chanell,but im trying to fix it for years and u guys can help me now

 

[PetruV]

Finished !!! ,what should i do when powering it up the first time to make sure everithin is alright,also is it safe to have the center tap of the transformer as grounf for everithing and the case,btw the chips are isolatd from the heatsink

 

[PetruV]

It is still a good idea, especially with "chip amps" to leave the specified Zobel network in. This is because like in the old days, the easiest way to get a power output stage in a chip is using NPN transistors only. You can see this if you look at P. 2 of the datasheet. The bottom transistor is working with it's collector going to the speaker. This is inherently problematic and can result in instability with open circuit or high impedance loads. The Zobel network helps keep things under control - so leave it out at your peril!!

BTW you are unlikely to *hear* the oscillation - it's clattering away at way beyond human hearing, - perhaps at a Mhz or so!! The normal giveaway (without resorting to an oscilloscope) is a very hot chip, even when it's doing nothing...

The chip ran 10 minutes at full power at 5v with 4ohm load and didnt even get warm

 

[blu_glo]

aLtough it dosent look more than a bunch of coloted spaghetti,its actualy coming along nicely

SWITCH OFF AND UNPLUG IT FROM MAINS!!

TAKE the foil off the transformer - it is not needed and dangerous you are close to creating a shorted turn with the transformer top plate and base. Please remove ALL silver foil.......

 

[PetruV]

SWITCH OFF AND UNPLUG IT FROM MAINS!!

TAKE the foil off the transformer - it is not needed and dangerous you are close to creating a shorted turn with the transformer top plate and base. Please remove ALL silver foil.......

Thanks for reminding me i didnt even plug it in i will alwais ask you before doing something the foil wasnt touching anithing.is it safe to have the center tap hooke up to the casing

 

[blu_glo]

Forgive me for asking but are you * OLD ENOUGH * to be working on mains?

If not you must have a suitably qualified person supervising and check your work before plugging in!

I give out information on these boards on the assumption anyone following them is suitably qualified and competent in electronics.

 

[blu_glo]

Thanks for reminding me i didnt even plug it in i will alwais ask you before doing something the foil wasnt touching anithing.is it safe to have the center tap hooke up to the casing

The fastest and easiest thing to do is put a 0.1uF capacitor in parallel with 22-100 ohm resistor between 0V RCA inputs and the metal chassis. The metal chassis should be earthed at all times, please see my above post about working with mains.