POWER AMPLIFIER WITH IC LM4780

(16 January 2006)

Croatian version

LM4780

This IC is, in fact, a double LM3886 in one package and its properties are similar to those of the LM3886. The advantages of the LM4780 are its compact design and a better match between two amplifiers, which is of special importance in parallel and bridge circuits. The amplifier’s schematic is shown in Figure 1.

LM4780 schematicFigure 1

Parts list:

R1, 2 1 kΩ
R3, 4 47 kΩ
R5, 6, 8' 4k7
R7, 8 100 kΩ
R9, 10 0.1 Ω/3-5W
R11 10 kΩ
C1, C2 2200uF/50V
C3, C4 220nF/50V
IC LM4780

Our design of the LM4780 incorporates jumpers (CON 4) whereby any of the three available modes of operation can be selected quickly and easily: stereo, parallel-mono and bridge-mono.

In the stereo mode, the jumpers should bridge pins 5-6, 9-10 and 11-12. This will set both amplifiers in the same mode of operation, resulting in two non-inverting amplifiers whose gain is equal to 1+R7/R5 or 1+R8/R6, respectively. The input of the right amplifier will thus be connected to the right input pin (CON 2-3) whereas the speakers should be connected to outputs OUT L and OUT R.

In the parallel-mono mode, the jumpers should bridge pins 3-4, 9-10 and 11-12. Again, this will set both amplifiers in the same mode of operation, giving two non-inverting amplifiers whose gain is equal to 1+R7/R5 or 1+R8/R6, respectively. However, the input of the right amplifier will now be connected to the left input pin (CON 2-1). The speaker should be connected to output OUT L whereas outputs OUT L and OUT R should be paralleled.

Finally, in the bridge-mono mode, the jumpers should bridge pins 1-2 and 7-8. This will set the right amplifier in the inverting mode of operation, with the gain equal to (R8 + R8 ') / R6. The speaker should be connected between outputs OUT L and OUT R.

The choice of the power transformer and heat sinks will depend on the desired power and mode of operation. The comments given in the LM3886 article regarding the power transformer and cooling also apply here.

Figure 2 shows the bare LM4780 printed circuit board.

LM4780 pcbFigure 2

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Related articles:

LM3875

LM 3886

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Comments

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Kenneth Scharf · Sept 15, 2015

Why are R5 and R7 not equal to R2 and R4? This would appear to give one channel a lower input impedance than the other.

moxtone (mod) · Sept 15, 2015

@Kenneth Scharf,
there was a typo in the parts list (relating only to R2 and R3; R4, R5 and R7 are OK), which is now corrected. Input impedance of each channel is 47K (equal to the values of R3 & R4). This applies to the stereo mode whereas in the mono parallel mode, input impedance is half that value. In the bridge mode, input impedance is slightly below 4K7. Thanks for visiting!

moxtone (mod) · Mar 28, 2015

@pat,
This was covered in the article on the LM3875 that precedes this one (moxtone.com/3875_en.html). There, I wrote that the resistor "at the output of the amplifier improves stability and, in parallel circuits, helps to split the load between two amplifiers. This resistor can also be omitted in most cases, which is best determined experimentally."
Thanks for visiting!

pat · Mar 28, 2015

Why did you add the 0.1ohms resistance at output?

moxtone (mod) · Aug 12, 2014

@Colin Purkis,
I'm afraid the PCB is not available for purchase. Thanks for visiting!

Colin Purkis · Aug 12, 2014

Good morning - Can you please advise if your LM4780 PCB is available for purchase ? I am looking for a small footprint bridge mono amp to replace the blown boards in two of my powered foldback speakers.
Cheers
ColinP
Australia

moxtone (mod) · July 19, 2013

@audiosensecd,
I'm sorry, no PDF is available. All necessary information is right here on this page, anyway. Thanks for visiting!

audiosensecd · July 19, 2013

hi there moxtone i'm interested of your design in fact i'm currently with tda 7293/94 but i'm gain interest in LM4780 how could i download the PDF file of your design email me please just reply is enough for me thanks...

moxtone (mod) · Apr 28, 2013

WELCOME!

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