Op-Amp Tone Control Problems!

[Mr Mark]

I needed a Tone Control section for a design I am doing, not being an
analogue man, I "replicated" the circuit from Elektor "302 Circuits"
book page 324. That is for a Microphone Amplifier with preset tone
control. The circuit looks like a typical Baxandall control with an
Op-Amp at each end. I replaced Op-Amp LM387 with a LM358 (I can't
remember the reason), I changed all the connections so that the Op-Amp
"looked" the same to the circuit. I cannot get the circuit to work,
like nothing getting through, not even past the first stage before it
gets to the Baxandall circuit. Any suggestions where I went wrong, I
have spent many fruitless hours on this now and I am getting to the
stage where I am considering redesigning the PCB, to suit the
originally suggested Op-Amp, help please???

Mark in the UK

[email protected] (remove the X to reply)

 

[John Woodgate]

I needed a Tone Control section for a design I am doing, not being an
analogue man, I "replicated" the circuit from Elektor "302 Circuits"
book page 324. That is for a Microphone Amplifier with preset tone
control. The circuit looks like a typical Baxandall control with an Op-
Amp at each end. I replaced Op-Amp LM387 with a LM358 (I can't remember
the reason), I changed all the connections so that the Op-Amp "looked"
the same to the circuit. I cannot get the circuit to work, like nothing
getting through, not even past the first stage before it gets to the
Baxandall circuit. Any suggestions where I went wrong, I have spent many
fruitless hours on this now and I am getting to the stage where I am
considering redesigning the PCB, to suit the originally suggested Op-
Amp, help please???

You aren't giving us enough to go on. Most of us don't know the Elektor
circuit and we certainly don't know how you've modified the overall
thing to use LM358. You could put YOUR circuit on alt.binaries.
schematics.electronics or on a web site and give us a link to it.

I suspect that you are using the first op-amp as some sort of buffer,
offering a low source impedance to the second op-amp, which has the
tone-control network in its feedback loop. Are you using a single-ended
power supply or + and - supplies? If you are getting no output from the
first op-amp, what is the d.c. voltage on its output pin? If you are
using a single supply, you must bias the + input of the op-amp to about
half rail voltage. The output pin should then be at the same voltage. If
it isn't, your d.c. feedback isn't working.

The LM 358 will give you bad crossover distortion unless you put a d.c.
drain resistor from the output pin to your most negative supply. See the
data sheet.