lm1875 schematic

[obelix]

can anyone post the schematic for the lm1875 power amp? searched for it but can't seem to find it


thanks

[teemuk]

The datasheet has one, with google I found at least 10 more. Did you have a specific circuit in mind?

[obelix]

would the schematic in the data sheet be okay for use as a guitar amp?

edit:
just took another look at the data sheet and there are two schematics there one says single supply operation... would you know how that would differ from the schematic in page 1?

thanks

[teemuk]

The "single supply" uses unipolar power supply. That means it has only two supply terminals, which are positive (or negative) terminal and a common (ground) terminal. Generally, the common is not counted, which leaves only single terminal, hence the name. The terminal powering the circuits is called "rail" so the circuit is also known as single rail supply. The power supply circuit is analoguous to single battery.

However, acquiring maximum voltage swing of the output signal requires that the amplifier's output is biased to "half supply", which is mid-point between the supply terminals (Vcc and ground): You see how this is done by referencing the non-inverting input of LM1875 to voltage divider formed by R1 and R2.

However, the output will have the same, large DC potential as well and therefore must be AC coupled to protect the speaker. You see that a large electrolytic capacitor C6 is used for this task. This bulky component adds up distortion and cost.

Dual supply circuit (of page 1) has three supply terminals: Positive (Vcc), negative (Vee) and common (ground). Its power supply circuit is analoguous to two batteries in series having their interconnecting terminals grounded. Thus you are using the positive and negative terminals of the batteries.

Since the potential of positive and negative supply is often equal (aside opposite polarity) biasing the output to "half supply" results to zero volts: You see that the non-inverting input is simply referenced to ground. Consequently no AC coupling is required either.

A dual supply circuit often has better CMRR (common mode rejection ratio) and PSRR (power supply rejection ratio), which means it is less vulnerable to amplify the AC alternation that exists in the power supply (that ideally should be DC). This is because the power supply is basically "balanced" and common mode error in both supply terminals is "canceled". Single supply circuits lack this virtue and therefore they are lot worse in that respect.


Those are the main differences between the amplifier circuits. The Power supply circuits are different as well, of course. In theory, the dual power supply circuit (due to use of dual rails) would require two times the amount of components required by the unipolar power supply. However, in practice this is hardly the case: Since CMRR and PSRR of the single supply circuit are so poor the single supply circuit will need more filtering (regulation) than the dual supply to maintain a steady DC rail potential. Plus there are component requirements for the "half supply" bias arrangement. So basically there is nothing to gain.

Due to its superiority the dual supply configuration is most commonly used. The single supply mainly lives in applications that must be powered by a single battery that is, by definition, unipolar. Even those are sometimes converted to dual supply operation with some additional circuitry.

[obelix]

thanks...