By Darren Yates
Sick of hearing that "hiss" during the quiet passages of your favorite music cassette? Then remove it with this tape noise limiter. It uses only three common transistors and can be built in a couple of hours.
Tape noise is one of the major drawbacks of audio and video cassettes. Even though it is always there, it is only noticeable during the quiet passages of music.
Even on many video cassettes, the audio quality can be quite bad with that "hiss" easily heard on many tapes.
With many "passive" noise removers (i.e. ones that use no transistors or ICs), capacitors and resistors are used to make simple filters which remove most of the high frequencies that make up most noise.
The problem with these, however, is that they act all the time and reduce the clarity of the speech or music, even when the noise cannot be heard through it.
The circuit we present here is also known as a voltage-controlled filter and only works when the signal level drops below a preset level where noise becomes noticeable. It adjusts itself to cut the high frequencies, when there is no sound to be heard (other than noise) and then turns itself off when the speech or music, we want to hear, returns allowing all those juicy high frequencies to pass through again.
AUDIO TAPE NOISE LIMITER
A-D: RESPONSE WITH INCREASING INPUT SIGNAL
FIGURE 1
How It Works
The main part of the filter is the 4.7K resistor and the 0.01uF capacitor. Transistor Q3 operates as a voltage-controlled switch.
When the quiet passages occur, Q3 is turned on and the filter is brought into action. Figure 1 shows its response. Notice that all the frequencies below 3.3kHz are allowed to pass whereas the frequencies above 3.3kHz are progressively cut, the higher in frequency we go.
This type of filter is known as a low-pass filter because it allows low frequencies to pass unimpeded, and progressively blocks the higher ones.
Now, let's have a look at the circuit diagram in Figure 2, and see how we make the transistor switch work at the right time.
At the input to the circuit, some of the signal is taken by the 1uF capacitor and coupled via a level control VR1 to the base of transistor Q1. VR1 allows you to set the signal level at which the filter should come on. It also sets how hard the filter works, whether it reduces the high frequencies either a little or a lot.
Transistor Q1 is set up as an amplifier with a gain of about 180. It amplifies the signal so that we get a signal level of a few volts.
The signal is then coupled via a 4.7uF capacitor to two diodes and another 4.7uF capacitor. These components form a voltage rectifier and doubler. It takes the AC signal and turns it into a DC voltage twice its original value. Since it works on the principle of the more signal input, the more DC voltage, we have the means now to control the switch and make it cut in at the right time.
We now feed this DC voltage into the base of transistor Q2. This transistor acts as an inverter, so that when we get a voltage at the base of Q2, we get no volts at its collector, and when we get no volts at the base, we get volts at its collector.
The base of the transistor "switch" Q3 is connected to this collector point of Q2.
If we get a DC voltage at the base of Q2, we know that we have a reasonably loud signal at the input, so we don't want the filter to cut in. (This is because we won't hear the noise anyway!) This voltage causes Q2 to turn on and the voltage at its collector drops to about 0.2 volts. Because the base of Q3 only has 0.2 volts across its base-emitter junction, it doesn't turn on and so the filter stays off, just as we want it to.
FIGURE 2
When the voltage at the base of Q2 disappears, we now have little or no signal coming through. This is when we are likely to notice noise. Because there is no voltage at its base, Q2 turns off and its collector now goes to nearly 9 volts. But because we can only have 0.6 volts across the base-emitter junction of Q3, it now turns on hard, bringing the filter into play-just as we want.
When you build this, use either a piece of veroboard or if you have the facilities, make your own printed board. Make sure that your 9 volt power supply is well regulated otherwise AC hum can leak through into the circuit and be heard at the output.
When you install the circuit, fit it between the cassette deck or video recorder and your audio amplifier using either phono plugs or small jack plugs. Now play a tape or video that has a number of quiet passages. When they occur, adjust VR1 to give what you think is the best result.
All you need do now, is enjoy the fruits of your labor and listen to relatively noise-free sound.
PARTS LIST FOR AUDIO TAPE NOISE LIMITER
3-BC549 NPN Transistors (2N5818 or Equiv.)
2-IN914 signal diodes
1-10uF electrolytic capacitor (16VW)
2-4.7uF (16VW) electrolytics
1-1uF (16VW) electrolytic
1-0.01uF metalized polyester
1-50K ohms linear pot.
1-82K ohms (1/4W) 5%
1-18K ohms
2-4.7K ohms
1-1K ohm
Also see:
adapted from: Electronics Handbook 1992