quite simple volume control

[broenni]

Hello diyAudio

I'm looking for some help with my electronics project.
Since I'm quite new to electronics and especially to audio I cant get it to work the was I want.
The Problem is this:

I have a low power audio source form an audio player on one side and a small 8 Ohm 0.25 W speaker on the other.
The signal is almost loud enough so I don't need much amplification. But I would like to an a volume control.
I have a 10k slide potentiometer I would like to use. It works if I just connect it to the signal. The Problem is that the signal goes from 100% to 0% in the first copple of mm in the potentiometer. I need it to fade from 100% to approximately 20% over the howl range of the potentiometer.

I tried it with a simple 741 op-amp but I must have something wrong in my circuit.


Does anybody have some advice for me?
Is the 741 something that can work?

tanks
Roland

[Nrik]

Hi Roland.
You need to put the potmeter in the input side of your circuit.
The Speaker should be connected directly to the amplifier.

Technically speaking the 741 is not powerfull enough to drive an 8 ohm speaker, but for such a small one, it might work for you.

[Nrik]

This will work much better with the same components.

[gruni]

seems the potentiometer you used is a linear type. for volume control you need a log type.

[broenni]

Thanks for the suggestion. I will try that as soon as possible.

@gruni: I read that log potentiometers are better. Unfortunatly I didn't find any in the size that I need. 2 to 3 cm long and no higher then 1 cm.

[cliffforrest]

Normally a log pot is used to avoid crowding the adjustment range to one end of the travel.

However it is possible to do a good approximation of a log pot by adding a ressitor to your lin pot.

See

ESP - A Better Volume Control

Good luck!

Cliff

[broenni]

Thx Nrik, it pretty much worked. I did this:


I added two resistors. One to limit the signal going to quiet.
And one to limit the signal strenght a little because it is slightly distortet.
But I think it will work like this if I reexamine the resistors values.

[Frank Berry]

The 741 will have a problem driving an 8 ohm speaker. The 100 ohm resistor on the output will certainly limit the output current but will severely limit the available output power. Have you thought about using an LM386 chip rather than the 741? It's designed to drive an 8 ohm speaker and requires fewer parts.

[AndrewT]

The 741 and similar opamps running on +12Vdc single polarity supply will have ~+1V to +11V output range, i.e 10Vpp.

This is roughly equivalent to 3Vac at the output.

3Vac into an 8ohm load requires an output current of 3V/8r = 0.375Aac.
This is equivalent to 530mApk into the load.
The opamp cannot provide this.

Now we add in the 100r resistor.
3Vac into 108ohm load requires 3V/108r = 0.0278Aac
This is equivalent to 39mApk into the load. Some opamps can provide this most will have entered limiting and/or protection mode.

The 741 datasheet will specify a range of load resistances/impedances that it can drive.
Expect around 1k to 1M not 108r.

You need to look for a high current type opamp or a high current buffer to drive speaker loads even 64ohm speaker loads.

The input volume control is strangled by the series 1k0 resistor in it's tail.
A volume pot can always get to -60dB ref max signal and many will go to lower than -80dB ref max signal.

Adding the 1k0 to the 10k pot limits the minimum volume to ~-20dB ref max signal.
This will act more like a sensitivity control rather than a volume pot.

The two inputs +IN & -IN have different resistances. -IN sees 10k/10k = 5K
+IN sees 10k.
This will result in an output offset that is bigger than the opamp is capable of.

Since the amplifier is AC coupled at both input and output a small output offset is of no consequence, neither to the speaker nor to the sound quality.

However, a big output offset will reduce the maximum signal that can pass without clipping the signal (=more distortion).

with a 10Vpp output and zero output offset the maximum clip free signal is +5Vpk and -5Vpk.
add in a +1V output offset and the maximum output is limited to +4Vpk and -6Vpk (still 10Vpp)
The higher the output offset the worse the clipping of the signal.
Check that the output sits at ~ half the supply voltage for maximum signal transfer.
Send a maximum audio signal through the opamp that is at least 10dB (~1/3 of the voltage) below that 10Vpp capability, i.e. use <3.1Vpp and preferably <1Vpp for 20dB of overhead.

[Mooly]

You could add an NPN and PNP transistor to make it workable driving low impedances. Using a 741 or any opamp like this isn't any good for driving a speaker.

The caps circled need increasing in value by a factor of 10 really.

The 470 ohm is to minimise crossover distortion... try it without to hear what this sounds like.

The transistors can be any general purpose types of at least 500 milliamp collector current rating.