Need to use op amps as an audio buffer.

Thread Starter

NameET

Joined Jun 4, 2018
14
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:
TubeAmpSCHM1.PNG
 

MrChips

Joined Oct 2, 2009
25,060
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.
 

ebeowulf17

Joined Aug 12, 2014
3,282
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.
 
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