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Registered Member #105
Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408
I'm building a little amplifier for my laptop for a new plasma tweeter, measured the output of my sound card at about .3V (thats pretty conservative), so I chose a non inverting op amp with a gain of ~23 to get an approximately 6V p-p signal. My question is where should i put the volume control? Should i replace one of the gain setting resistors on the op amp with a variable resistor, or should i use a potentiometer right at the output of the computer with the center tap connected to the input of the op amp? The op amp is feeding the rest of the circuit through a 330k resistor and a 10nf capacitor in series, so it appears as a high impedance output, mostly voltage gain.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
All the audio amp circuits ive seen attenuate the input signal and have a fixed gain. Not 100% sure why. Anyone know the reasoning behind that?
There's three places where you could put the volume control pot:
1. Before a fixed gain amplifier stage 2. As the feedback resistor in a variable gain stage 3. After a fixed gain amplifier stage
They all have their advantages/disadvantages. For instance... Putting the pot before the gain stage allows you to turn the volume right down to zero and prevent clipping in the amplifier if the input is very strong. Using the pot as a variable feedback resistor in a non-inverting amp won't let you turn the gain down to 0, only down as far as +1. Putting the pot after the amp won't let you attenuate strong signals before they clip the amplifier, but it can attenuate noise generated within in the amplifier itself.
The arrangement with the pot in the feedback path is common where you need to adjust the gain over a large range (like +1 to +1000). When you want a gain of +1 the amplifier gives a gain of +1 with this arrangement, instead of first attenuating the signal by 1000 times then amplifying it by a fixed 1000 times which adds a lot of noise. However, I think it needs a pot with a special resistance taper to work properly if I remember correctly.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Fixed gain followed by a volume control as an attenuator is a popular solution, but it causes a bottleneck in headroom, as GeordieBoy explained. For situations like that, Peter Baxandall designed an active volume control which you can read about here:
As a bonus, it gives a log law from a linear pot.
Another popular circuit is a microphone preamp made with a differential pair, and the gain pot strung between the emitters, so it adjusts the gain through negative feedback. This one needs a reverse log pot to get a log gain characteristic. On the mixing desk I use in my home studio, the gain pots all do nothing until the last 20 degrees or so, when the gain goes through the roof! Something tells me the manufacturer ordered log pots instead of reverse log by mistake
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Op Amp Volume control Placement
