Tiny 30 watts OTL amplifier using LM1875 Reviewed by on . Rating:
You Are Here: Home » Amplifiers » Tiny 30 watts OTL amplifier using LM1875

Tiny 30 watts OTL amplifier using LM1875

If you need to have a high power amplifier in small size. I suggest this project , It have powered drive output up 30 watts, by low noise and Low distortion. And easy to bulid because use a few devices. But is good sound energy worth the cheaper.

The circuit principle.

The LM1875T is Amplifier IC model of National Semiconductor. A body on Supplier Package : TO-220, Pin Countto use is 5 pin. This IC is buit by High technology monolithic type makes very low Total Harmonic Distortion Noise less than 1% (0.015%@8Ohm@20W) the quality of sound.

They can be use with DC power supply wide range between 16 volts to 60 volts, and can use both dual and single voltage. In this project use the single voltage supply is Positive and Ground. As Figure 1

Figure 1 Tiny 30 watts OTL amplifier using LM1875

The voltage of power supply will has effect to power of IC that is if we enter DC voltage supply of 50 volts, the IC have power up to 25 watts. When use with impedane load between 4 ohms to 8 ohms.

But we use the maximum signal supply voltage up to 60 volts, and power output up to 30 watts. Which impedance load 8 ohms.

This IC model IC have the protection system with the Maximum Operating Temperature in range -65 to 150°C, So widly use, but if you use fan to IC heatsink to reduce heat of them so very well.

In circuit diagram form in signal characteristics is non inverting input signals. in to the input. There are R4-resistor adjust input impedance and coupling through the C1-capacitor in to amplifier section inside the IC1. Thus R1 and C1 will set the frequency of input signal, by R1, C2 filter the noise (D-coupling), from power supply to protect noise through the input.

Both R5, R6-resistors are connected in feedback form, and act as adjusting gain of circuit. But R7 and C5 are used to bypass or through high noise frequency to ground. To To ensure the quality of sound output improves. And there is C6 coupling signal out to the output ( Load) The value that use to good sound quality even more. and also protect DC voltage leak to load.

Figure 2 the graph shows various features of the LM1875

Figure 2 is graph to show the comparin various feture of LM1875T-IC as table 2 (A) show compere between Distortion percent of signal (THD) and power watts at output. Which IC drive over 20 watts. The resistor load of 8 ohms. To the harmonic distortion less than load 4 ohms.

Table 2 (B) show compare between the values of harmonic distortion and frequency of signal at output. By in range frequency about 80 Hz up to about 10 KHz Harmonic distortion in the stable and have lower average between both load about 0.025%.

In table 2 (C) show compere output power and voltage of power supply at load 8 ohms. Which if we use voltage up to 30 volts will be watts power output about 32 watts.

This amplifier use high voltage power supply and highe current up to 3-4 Amperes. This helps improve the stability of the circuit. Resulting in better sound quality as well.

How to build its.
The amplifier circuit as Figure 1 that is single side or mono system only. Thus if build on stereo system you have to builds two sets.

To begin with find all components before, so make an Actual-size,Single-sided copper PCB layout as Figure 3. Then assemble all parts to PCB as Figure 4 so easy.
Figure 3 Actual-size,Single-sided copper PCB layout

Figure 4 the components layout.

For who not have DC power supply voltage : 50-60 volts. Current around 3-4 amperes. To try circuit in Figure 5


Testing and aplication

First, apply good signal (CD, tuner) to input, If have the tone control and low volume with it better,Connected input signal is applied to the project.and speaker of 4-8 ohms sized about 30 watts to output.

The components list
Resistors size : 0.5 watts +5%
R7__________1 ohm s
C1__________1uF 50V___Electrolytic
C2, C3_______10uF 50V___ Electrolytic
C4__________100uF 50V___ Electrolytic
C5__________0.22uF 50V___Mylar
C6__________2,200uF 50V___Electrolytic
C7__________0.1uF 50V___Ceramic
The speaker size of 4-8 ohms 30W
PCB, terminal,wires, etc.

Comments (3)

  • Ali Nasir

    Kindly inform me that do you suply kits and/or printed circuit boards on request?
    Ali Nasir

  • mess

    im also interested in buying 100watt amp kit with power supply for my custom made speaker cab.please mail me UR charges if U can send.

  • mess

    I made this amp for the first time and I felt like a genius cause Im not really an electronic expert….it work on my initial test with 12v DC 1000mA. Cause I want to be in the safer side before I fully understand how much voltage required to produce 25 to 30 watt output.It dosent sound loud though in my 8ohm speaker.but that being said its power by 12v DC only.So I bought a 48V DC 2A readymade power supply hoping to get atleast 20.watts output from the actual target of 30watts.Now I dont get any change in the output volume and its still the same loudness as using 12V.it still sound like portable mobilephone speaker.besides,worst thing is one of my capacitor 100uf near the power input expload after testing for 30 seconds.I dont know if ill be able to make this one with the proposed 30watts target.please please help anybody…. Im guitarist and nothing would please me more than playing my guitar in my own made amplifier.please help.

Leave a Comment

Copyright © 2015 www.eleccircuit.com

Scroll to top