Yesterday I discussed using op amps for buffers in the pre amp and effects loop stages of a small LM386 based amplifier. I had lamented that I had thus far been unsuccessful in working with the MC4558 dual op amp chip which was confusing me since I was having great success with the LN741 single op amp. I have now resolved those issues, which I believe stemmed from incorrect biasing of the signal due to my own ignorance – the biasing circuits I used on the 741 were something new to me and I correctly anticipated they should work the same (albeit with different pin connections) on the 4558. Previous tests did not have this implemented correctly.
Some further research and examination of the two amplifiers I discussed reveals that what Peavey and Fender both did with their dual op amp chips in the input stage was to use a voltage divider on the signal and cascade the signal through a pair of inverting amplifiers. They use a potentiometer to control the first op amp’s negative gain level and then invert the signal with positive gain in the second op amp within the tone circuit. Both the Studio Pro 110 and the Squier 15 amplifier do this, although they have different circuits to accomplish the task – primarily due to the Fender having it’s high gain control integrated with a single pot in the pre amp circuit.
My next challenge will be to attempt to reproduce such a scheme for the “clean/normal” pre amp stage on my design. This will be a more complex circuit than my previous input stages shown on the op amp design and the transistor design. I like the idea of including a proper pre volume control which won’t destroy the sound – my hope is this will allow the small amp to behave more like it’s bigger cousins.
I have not included a schematic here because the dual op amp test circuit I’m using so far is just a simple unity gain setup like before.
