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Tone control circuit critique

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Enzy

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does this circuit look ok I need it to control the tone of my speakers the signal to this circuit would come from my laptop.
 

Hi,

The circuit is a classical circuit.
I recommend:
* use high quality foil capacitors
* use high quality pots
* ceramic capacitors for the supplies

Some further improvements, if you like:
* maybe use a better Opamp
* use a low frequency highpass filter at the input to avoid DC
* and a high frequency lowpass filter at the input to avoit Opamp input overdrive
* a low value capacitor as direct feedback at the Opamp

Klaus
 

Hello Enzy,
I hope Output 1 isn't a real output, but your ground for Output 2?
LM833 Op-amps would be an ideal choice, as KlausST has suggested.
I've used a ton of these in many an audio project. They are excellent
low noise devices.
Regards,
Relayer
 

Relayer said:
Hello Enzy,
I hope Output 1 isn't a real output, but your ground for Output 2.
Regards,
Relayer
Click to expand...

Yes it's grounds for output 2, when I chose a connector in Eagle when I name one the other one gets the same name and a number

- - - Updated - - -

KlausST said:
Hi,

The circuit is a classical circuit.
I recommend:
* use high quality foil capacitors
* use high quality pots
* ceramic capacitors for the supplies

Some further improvements, if you like:
* maybe use a better Opamp
* use a low frequency highpass filter at the input to avoid DC
* and a high frequency lowpass filter at the input to avoit Opamp input overdrive
* a low value capacitor as direct feedback at the Opamp

Klaus
Click to expand...

I'm working on the extras now, I have OPA2604A which seems to be a newer low noise chip what are your thoughts

- - - Updated - - -

By the way will I actually use both high and low pass filter at the input or was that a typo, I understand that the low frequency high pass will block dc buy I don't understand the use of the high frequency low pass filter
 

KlausST said:
* and a high frequency lowpass filter at the input to avoit Opamp input overdrive...
Click to expand...

A very good suggestion that I have not seen implemented in many circuits. What should be the recommended cutoff frequency?
 

Hi,

Your new circuit:
* you need negative supply voltage
* C5 need to be higher value

********
Input low pass filter:
* it should not suppress audio high frequencies: fc > 20kHz
* it should attenuate input frequencies that can't amplified by the Opamp circuit: fc < 0.1 × fc_opamp

It don't read your Opamp's datasheet. But here an example:
If Opamp_circuit fc is 1MHz, then 0.1 x 1MHz is 100kHz.
Then choos fc to be somewhere inbetween 20kHz and 100kHz.
I often use the geometric center: sqrt(20kHz x 100kHz) = about 45MHz.
Doesn't need to be exact.

What's the benefit of it:
In normal operation: nothing, but it doesn't hurt either.
But if there is some HF influence of a nearby cellular phone, or when you plug/unplug the input with live amplifier...then this may overdrive the opamp's input. The Opamp gets out of regulation...and it takes some time
(maybe followed by some higher frequency ringing, not audible, but it may hurt the tweeters)
to come back to regulated state. This causes some lower frequency, nasty, audible sounds.

Klaus
 

Hi,

still missing the capacitors at the supply.

I doubt the input can drive R13/C16 properly.

***
By reviewing the design I recognized, that
* the sound control depends on input source impedance
* And the input (load) impedance depends on sound control setup.

To avoid this I recommend to use the second (unused) OPAMP to buffer the input. Try to get an input load impedance of about 5k..10k Ohms: Maybe like this:
470nF --> 10k (to GND) --> 10k (series) --> 470pF (to GND) --> OPAMP buffer

Klaus
 

KlausST said:
Hi,

still missing the capacitors at the supply.


Klaus
Click to expand...


I would build a dual pol. supply, I just used the +/- 12v to represent the supply.
Ill add it to the circuit soon and ill try to make the addition of the buffer stage, Voltage buffer?

- - - Updated - - -

I might have made a mess of adding the buffer stage, I guess I am only used to making a voltage follower. I added a 103 cap to the supply.

 

Hi,

EAGLE: There is a mess with the junction dots. I wonder why, because I usually don't need to set/delete junction dots on my own.
It's automatically done by EAGLE....just use an appropriate grid setup.

If you use a dual supply, then you need at least two capacitors.

Input filter and buffer:
I almost wrote it like the schematic should look like. From left to right:
Signal input, then:
470nF --> 10k (to GND) --> 10k (series) --> 470pF (to GND) --> OPAMP buffer
Click to expand...
Then comes the sound control stage.

Opamp Buffer = voltage follower

Klaus
 

Perhaps you should also consider shifting the volume pot near the op-amp input. Also put a resistor in the output- then the output can be fed directly into the amplifier.
 

KlausST said:
Hi,

EAGLE: There is a mess with the junction dots. I wonder why, because I usually don't need to set/delete junction dots on my own.
It's automatically done by EAGLE....just use an appropriate grid setup.

If you use a dual supply, then you need at least two capacitors.

Input filter and buffer:
I almost wrote it like the schematic should look like. From left to right:
Signal input, then:

Then comes the sound control stage.

Opamp Buffer = voltage follower

Klaus
Click to expand...

I added the junction dots.

not sure if my schematic is even correct now, if its wrong it would mean I have no clue where to put the part you mentioned, If I put them somewhere its just a guess.

- - - Updated - - -

update.

 

Hi,

compare this with my textual description.
ToneCoontrol.png

Klaus
 

Hi,

Volume control depends on your requirements.
If you have a rather low ohmic complex load, then I'd put the volume control just before the first buffer.(this ensures a more flat frequency response, independent of volume setup)
If you have a high ohmic resistive load, then I'd put the volume control after the sound control Opamp.( this ensures low noise, especially at low volume level)

Klaus
 
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    Enzy

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The amplifiers I build or buy I dont think any of them have a impedance higher than 20k at the input, I would say between 9k ohm and 15k ohm. So that means I am running a high resistive load so I would place it just as I had it originally.
 

Hi,

Maybe, maybe not.

Did you consider that a 500pF capacitance (cable plus internal capacitor plus ...) gives an impedance of 15kOhms at 20kHz..then there will be a parallel resistor lowering the total impedance. And it is frequency dependent.

You need to know your enemy.
With the pot at the output you have 0Ohms source impedance at very quiet and very loud position, but 2.5kOhms (corrected) at center position.
This will influence sound quality, don't be surprised if it sounds dead at center position but good at quiet/loud volume.

Klaus
 
Last edited:



I added a volume pot after the buffer OPAMP and then I added a gain of 10.

I added a resistor at the output of the opamp I assume thats to help reduce noise possibly from the potentiometers when they turn.

- - - Updated - - -

Im at work i had to redraw the circuit, for some reason the eagle files hasnt fully sync with y google so I couldnt retrieve the original files so I didnt redraw the power supply.
 

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