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fr1s 25th February 2009 03:06 AM

Noob Question -.-
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

pacificblue 25th February 2009 07:03 AM

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.

sandyhooker 25th February 2009 07:12 AM

I am runing a stereo 1875 with split rails +-12volts into 8 ohms ...2amps is way more than is required.

sandyhooker 25th February 2009 07:17 AM

Correct that post...I'm using 12-0-12 tranny...15volts +- on the rails.

pacificblue 25th February 2009 07:59 AM


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?


Originally posted by fr1s
I'm goning to run 8ohm spkr and maybe 4.

fr1s 25th February 2009 05:32 PM

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.

pacificblue 25th February 2009 06:47 PM

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.

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