Hi all,
I recently put together my first tube/hybrid guitar amp (hooray!) and I can sufficiently say that it doesn't work as I hoped so. It's extremely quiet and behaves exactly like a solid state amp (lots of 3rd harmonic distortion, etc) when the speaker is connected, which is a dead giveaway to me that there is an impedance issue from power amp to speaker. I was originally using only one side of an LF353n chip as a buffer/power amp stage, and after looking up the data sheet, I found that the buffer characteristics of the LF353n aren't really that great. After a quick load resistor test, I found the impedance to be at around 150 ohms (coming from a 10,000 ohm signal). The speaker in question is two watts, 8 ohms. I do have another LF353n chip (amounting to a total of 4 op amps).

So the question is this: What is the closest I could possibly get to 8 ohms with 4 of these op amps?

Schematic:

 

Welcome to AAC!

I'm afraid that you have the totally wrong approach.
An instrument amplifier usually consists of (but not limitied to) the following stages:

1. Preamp, impedance matching stage.
2. Gain and tone control stage.
3. Driver stage.
4. Power output stage.

A hybrid tube/solid-state amp tries to replicate the sound of tube amps by using tubes in stage 4 - power output. The remainder of the amp can be designed with IC amplifiers.

LF353 is an IC amplifier which fits in well with stage 1 and 2. It is woefully inadequate for stage 4.

 

Hi all,
I recently put together my first tube/hybrid guitar amp (hooray!) and I can sufficiently say that it doesn't work as I hoped so. It's extremely quiet and behaves exactly like a solid state amp (lots of 3rd harmonic distortion, etc) when the speaker is connected, which is a dead giveaway to me that there is an impedance issue from power amp to speaker. I was originally using only one side of an LF353n chip as a buffer/power amp stage, and after looking up the data sheet, I found that the buffer characteristics of the LF353n aren't really that great. After a quick load resistor test, I found the impedance to be at around 150 ohms (coming from a 10,000 ohm signal). The speaker in question is two watts, 8 ohms. I do have another LF353n chip (amounting to a total of 4 op amps).

So the question is this: What is the closest I could possibly get to 8 ohms with 4 of these op amps?

Schematic:
View attachment 153654

I have no experience with the LF353, but nothing in its datasheet makes me think it should be driving speakers. If you want a compact IC audio amp, maybe try the LM386? Still, I have to imagine a guitar amp needs more juice than any IC amp can put out!

http://www.ti.com/lit/ds/symlink/lm386.pdf

Doh! MrChips beat me to it, and with a much more thorough answer.