LM1875 Audio Amplifier - Component Selection.

This "Instructable" helps component selection when using the LM1875 Audio amplifier.

A simple amplifier was needed for a Woofer loud speaker and as I could not buy the component parts for the same price I decided to purchase the LM1875.

The data sheet gives example circuits but for some reason offers different values for the single supply version and the Zobel network is quite brutal.

Data sheet here:

https://www.ti.com/lit/ds/symlink/lm1875.pdf?ts=1699541175231

To configure a single supply version then biasing resistors are needed to set mid-supply and so an output de-coupling capacitor is needed to stop the voltage offset getting to the loud speaker. The values given on the data sheet are good but the feedback resistors have caused some confusion (somebody in the Drawing Office might have lost a decimal place) so use the same values as the dual supply version i.e 1k ohm and 20K ohm (I used 22K ohm as it is the same as the bias resistors).

Please note that the minimum gain may be set to 10; the given values are for 20.

Required Gain is the output voltage divided by the input source voltage.

Note that the minimum supply voltage is 20 volts or +/- 10 volts and 50 volts or +/- 25 volts is needed for maximum power output.

The resistor and capacitor across the output to the loud speaker is known as a Zobel network but is in fact derived from a Boucherot cell (probably because Zobel is easier to remember!).

https://en.wikipedia.org/wiki/Boucherot_cell

The network is added to counter-act the inductance of loud speaker and associated cable which will have an increasing value with frequency.

All you need to know is that the "Zobel network" limits the maximum impedance by being the highest value on the line i.e the RZ value.

I use the following formula to give suitable values:

RZ = Re × 1.25

CZ = Le / RZ²

For a nominal 4 ohm loud speaker, I used 5 ohms (3 x 15R ohms = helps with power dissipation) and 10uF.

Please note, by implication, that RZ will have to dissipate the power not going to the loud speaker!

As given you will have a very capable audio amplifier though I modified the arrangement for sub-woofer operation with an additional network to limit the band-width to a nominal 150Hz.

An item to note from the data sheet is that the Amplifier is stable for (A.C) gain above 10 so this has to be the lowest gain when changing the suggested values. I added a 22K ohm resistor in series with 0.47uF both in parallel across the existing 22Kohm feedback resistor and a further 4nF capacitor also in parallel to sharpen the cut-off.

If you are building this project then this site might be helpful:

https://www.sound-au.com/project72.htm

Dual supply, ready built modules are available very cheaply but if you wish to build a single supply version then note that the LM1875 pin spacing does not match 0.1" pitch.

The pins can be bent to fit 0.1" pitch if the outer pairs are first flattened and then twisted at 90 degrees so that the bent portion can be aligned to suit.

The most important tip is to arrange a "star" zero volt common return at the power supply. There will be large supply and output currents and minute signal currents so do not mix them as the device will oscillate!