A simple AVC
Posted: Thu Jan 11, 2024 10:35 pm
I found that I had a lot of the old NE571 (and SA571) in stock. Years ago, I lashed up a simple AVC, which proved very useful when taking programmes on tape (in those days) that could have wildly varying levels.
The simple AVC reduces the dynamic range somewhat, bringing up the levels of quiet programmes, and crushing the levels of over-loud ones. I included a simple clipper into the circuit, and if you find that you're hitting the clipping point much, you really do need to follow this with a good 15 kHz filter, to get rid of the harmonic crud that the clipper generates.
There is a 1µ capacitor connected to pins 1 & 16 which is the timing of the AVC action. You might want to increase its value if you want a smoothly acting long-term level adjuster. The one I have on the bench here has a 2µ timing capacitor, but does tend to hit clipping more easily..... Don't be tempted to go below 1µ for faster action - it'll give nasty bass distortion. As usual, it's all a compromise!
The 33k resistors between pins 2 and 15 to ground reduce the range of action. Both resistors have to have the same value, but you could go as low as 15k to reduce the range of action at very low levels. I found that 33k worked well enough to prevent very low level inputs being accompanied by lots of noise! Again, it's open to experimentation.
The back-to-back LEDs can be separate devices or - as I used - bi-directional green LEDs. These conduct at just the right point to clamp the output level. The 10k resistors in series with them "round" the point at which they conduct, reducing the distortion they'd otherwise create. The 10k resistors can be reduced to almost 0Ω for absolute clamping action, but the distortion they create will be much more obvious.
It's a fun circuit - it's no universal panacea, but it can help a bit, and it costs nearly nothing to build. Give it a try!
The simple AVC reduces the dynamic range somewhat, bringing up the levels of quiet programmes, and crushing the levels of over-loud ones. I included a simple clipper into the circuit, and if you find that you're hitting the clipping point much, you really do need to follow this with a good 15 kHz filter, to get rid of the harmonic crud that the clipper generates.
There is a 1µ capacitor connected to pins 1 & 16 which is the timing of the AVC action. You might want to increase its value if you want a smoothly acting long-term level adjuster. The one I have on the bench here has a 2µ timing capacitor, but does tend to hit clipping more easily..... Don't be tempted to go below 1µ for faster action - it'll give nasty bass distortion. As usual, it's all a compromise!
The 33k resistors between pins 2 and 15 to ground reduce the range of action. Both resistors have to have the same value, but you could go as low as 15k to reduce the range of action at very low levels. I found that 33k worked well enough to prevent very low level inputs being accompanied by lots of noise! Again, it's open to experimentation.
The back-to-back LEDs can be separate devices or - as I used - bi-directional green LEDs. These conduct at just the right point to clamp the output level. The 10k resistors in series with them "round" the point at which they conduct, reducing the distortion they'd otherwise create. The 10k resistors can be reduced to almost 0Ω for absolute clamping action, but the distortion they create will be much more obvious.
It's a fun circuit - it's no universal panacea, but it can help a bit, and it costs nearly nothing to build. Give it a try!
