# I'm constructing a audio amplifier in LT spice, some good starting points?

Thread Starter

#### ZimmerJ

Joined Dec 9, 2020
58
Hello, i'm new here on AAC. I've seen alot of interesting topics, questions and answers here, so i would like to be part of it.
Question:
It is for a school assignment, and i'm choosing to build a multistage amplifier (Pre-amp and Power-amp). What is a good starting point when choosing values for components and such?
Instinctively, my first thought is to start from the output, thinking that i want to achieve for example a certain power for my speaker, and then work from the "results" that i want from this audio amplifer. Frequency spectrum should be between 20 Hz - 20 kHz and say i wanna power a 30 W speaker. Where do i start off?
And what other things should be considered when constructing an audio ampifier? I know there is alot if it's suppose to be professional work, but if there is a way to keep it somewhat basic and just achieving the frequency spectrum and output power, that would be great.

I have done a layout, with the connections and the components.

I have basic knowledge of circuitry and components, but calculating the values of each doesn't seem too easy when doing a more complex circuit like this. I feel like i don't really know where to start. I maybe i should make it a single-stage amp for my level of experience?

Appreciate any answers, eager to learn. Thanks

#### Papabravo

Joined Feb 24, 2006
20,383
I would start on the input side. Take a low level source and amplify it with one or two stages. This building block can then be used as a standalone pre-amplifier. Then look at a power stage for driving the speaker.

#### Audioguru again

Joined Oct 21, 2019
6,152
Starting from the output:
Did you select a 4 ohms or 8 ohms speaker?
Calculate the RMS voltage across the speaker that produces the needed 30W.
Then calculate the peak-to-peak voltage which a DC power supply is needed to produce. Add a few volts tor the amplifier output voltage loss.
Calculate the maximum peak output current then select suitable output transistors.
The calculations use simple multiplying. 2 x 3= 6.

#### ronsimpson

Joined Oct 7, 2019
2,748
From a man that has reinvented wheels....look at other people's 30W amplifiers.
Note: most internet schematics have errors.

Thread Starter

#### ZimmerJ

Joined Dec 9, 2020
58
Starting from the output:
Did you select a 4 ohms or 8 ohms speaker?
Calculate the RMS voltage across the speaker that produces the needed 30W.
Then calculate the peak-to-peak voltage which a DC power supply is needed to produce. Add a few volts tor the amplifier output voltage loss.
Calculate the maximum peak output current then select suitable output transistors.
The calculations use simple multiplying. 2 x 3= 6.
Neither, but i guess i will start with that. Thanks for the advice, i'll give it a try.

Thread Starter

#### ZimmerJ

Joined Dec 9, 2020
58
From a man that has reinvented wheels....look at other people's 30W amplifiers.
Note: most internet schematics have errors.
Makes us two, or at least i've tried. Will do!