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I'm recently getting into learning about designing circuits as I'd like to pursue this as a career. I thought I would start off with a simple guitar amp, currently what I have is a preamplifier using a 2N3904 outputting to a power amplifier using an lm1875, however, for some reason I keep getting this error regarding my design. I wasn't getting this before I added my power amplifier, any ideas? The goal of this project is to just get a signal from an electric guitar, amplify it with the preamplifier, and then just output it to a speaker using the lm1875, R9 is used to substitute the 8ohm load from the speaker. Also, I have R8 set to a value of 1 as it is shown in the datasheet for the lm1875.

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    \$\begingroup\$ Sounds like you don't have a .SUBCKT set up for your opamp. \$\endgroup\$ Oct 16, 2023 at 4:28
  • \$\begingroup\$ Wat is the purpose of the C2-R4-C3 network? Also, what does R3 do? \$\endgroup\$
    – AnalogKid
    Oct 16, 2023 at 4:47
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    \$\begingroup\$ A few tips if you are starting out. Set up a bench with a couple of power supplies, meter and scope. learn how to prototype and test physical circuits. Modelling is very useful but in the end you will need to build something, and this is a real skill that does not come easy. A model is only what it says, a model. Also if you are going to do audio stuff, learn to use opamps. \$\endgroup\$
    – danmcb
    Oct 16, 2023 at 6:14

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There are problems at many levels.

You have added and used a component, the LM1875, which doesn't have a simulation model, so while you can draw it in, you can't simulate it.

Also I don't know if this is intentional for artistic or sound tone reasons, but your preamp stage feeds a bias current to the guitar from supply and loads the guitar output with too low impedance.

A proper guitar input stage would not feed current to guitar and would use a much higher input impedance.

What it does look like is that you have possibly copied an electret capsule mic preamp stage instead of a guitar preamp.

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  • \$\begingroup\$ Thanks for pointing out the LM1875 issue! I was having trouble figuring out what preamp config to use so I found this one in a textbook I'm looking at,,, i figured the mic config would operate the same for a guitar but I was wrong! haha i recently found that a common emitter config is pretty good for preamp circuits for guitars so rn I'm just trying to figure out how to select my resistances and capacitances. Sorry for the amateur work, just trying to learn!! Again, thank you for the feedback! \$\endgroup\$
    – aaroniza0
    Oct 18, 2023 at 19:30
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  1. The minimum power supply voltage for the LM1875 power amplifier is 16V as shown on its datasheet. Its output is zero with your 5V supply.
  2. Your preamp was designed for an electret microphone, not an electric guitar pickup.
  3. I agree that the simulation program does not have a model of an old LM1875. A few newer power amplifier ICs have simulator models.
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  • \$\begingroup\$ Thank you for the feedback! I will definitely look into newer power amplifier models, have any you recommend that are easy to use and understand? Yeah this circuit was from a textbook I'm looking at the moment,,, I was wrong to think it would work for a guitar haha. Have any advice for a newbie? \$\endgroup\$
    – aaroniza0
    Oct 18, 2023 at 19:32

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