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 Chip Amps Amplifiers based on integrated circuits

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 25th February 2009, 04:06 AM #1 fr1s   diyAudio Member   Join Date: Feb 2009 Location: Van City Noob Question -.- Hi. Im almost done my LM1875 amp, but today I got hit by one question. I have 25v 2A centre-tap transformer and I'm goning to run 8ohm spkr and maybe 4. Can I run my amp(stereo) with only 2A and also I'm planing to put in there LED. I saw that Brian GT uses 10k resistor on his power supply, I tried one and it didnt drop much. So is it regular resistor or like 2w or somthin? Thx Fr1s
 25th February 2009, 08:03 AM #2 pacificblue   diyAudio Member   Join Date: May 2008 25 V center tap as in 25-0-25 or as in 12,5-0-12,5? The former leads to too much rail voltage for the LM1875 in any configuration. The latter would be okay for an 8 Ohm mono amplifier, a bit tight for an 8 Ohm stereo or 4 Ohm mono amplifier and too small for a 4 Ohm stereo amplifier. The resistor for the LED can be calculated. If you use standard LEDs they usually have forward voltages between 1,6 and 2 V and forward currents of 20 mA. So you calculate according to (Usupply - Uf) / If = R E. g. 18 V supply voltage -> (18V-1,6V)/0,02A=820Ohm. Using a higher value resistor leads to lower current, lower light intensity and longer life time. For other LEDs, e. g. blue, white or high intensity types you should search for the datasheet and look up the forward current and voltage. The resistor rating is (Usupply - Uf) * If E. g. (18V-1,6V)*0,02A=,0328 W You should choose the power rating with 100 % safety margin. In the above example that would be 0,656 W and would lead to a practical value of 1 W. __________________ If you've always done it like that, then it's probably wrong. (Henry Ford)
 25th February 2009, 08:12 AM #3 sandyhooker   diyAudio Member     Join Date: Dec 2005 Location: Arizona I am runing a stereo 1875 with split rails +-12volts into 8 ohms ...2amps is way more than is required.
 25th February 2009, 08:17 AM #4 sandyhooker   diyAudio Member     Join Date: Dec 2005 Location: Arizona Correct that post...I'm using 12-0-12 tranny...15volts +- on the rails.
pacificblue
diyAudio Member

Join Date: May 2008
Quote:
 Originally posted by sandyhooker I am runing a stereo 1875 with split rails +-12volts into 8 ohms ...2amps is way more than is required.
.. if your speakers are efficient and don't form a too difficult load for the amplifier, and if your listening habits are civilized.

1 A is exactly, what the amplifier can deliver into an 8 Ohm load with that voltage per channel. That is 2 A for stereo, and the voltage raise through the smoothing capacitors leads to a corresponding current drop. So a 12-0-12 V 2 A transformer that leads to ±15 V rails, would give you rails with 1,6 A plus a little from the capacitors. More than required?

And
Quote:
 Originally posted by fr1s I'm goning to run 8ohm spkr and maybe 4.
__________________
If you've always done it like that, then it's probably wrong. (Henry Ford)

 25th February 2009, 06:32 PM #6 fr1s   diyAudio Member   Join Date: Feb 2009 Location: Van City I'm not sure what I have 12-0-12 or something else but when I attached the multimeter to it, it said that the transformer is 25.5 v I believe it says that it's 25v vct or something like that, but I definitely know that's it's 2A.
 25th February 2009, 07:47 PM #7 pacificblue   diyAudio Member   Join Date: May 2008 If you have a center-tapped transformer, there are three wires on the secondary side. If you measure each wire against each other your meter will indicate in two cases the same voltage and in one the the sum of the other two. __________________ If you've always done it like that, then it's probably wrong. (Henry Ford)

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