Pads and how they help fight feedback
In the electronics world, a “pad” is a circuit that lowers the amplitude of a signal while providing a fixed impedance, or load, in both directions. There are, of course, other ways to lower signals, such as a voltage divider, which is a series of resistors to ground (think of a fixed volume control). The problem with this circuit is that it is directional, one way in and one way out. If the direction of the signal is reversed, the impedance of the network changes. What makes a pad different than a voltage divider is that a pad has the same impedance no matter which direction the signal flows; it is bi-directional. The Mojo Pad, for instance, maintains 1 megohm of resistance in either direction. The 2 most common types of resistive pads are the “T” pad and the “H” pad. The T pad is used in unbalanced networks such as a high impedance instrument cables and the H pad is used in balanced networks such as XLR cables.
When a harp player chooses to play amplified he will typically cup a microphone and plug the microphone into a guitar amplifier. As soon as the musician begins to turn the volume up on the amplifier, the monster that is feedback is awoken and begins its hideous howl. Everyone turns to give the harmonica player the “evil eye.” So, why is this happening? Well, that guitar amp is not designed for the relative high output of a microphone; it is designed for the much lower output signal of a guitar pickup. As a result, the amplifier has an abundance of preamp gain (exactly how much varies from amp to amp). This problem can be dealt with in a few different ways such as replacing the preamp tubes in the amplifier with lower gain tubes, lowering the volume on the microphone, or placing a pad in line with the microphone cable. The first solution requires the purchase of the tubes and the knowledge of which tubes to replace; this is a good solution if the performer will always have his amp available. The second solution works but it is a little harder to manage with any accuracy. The third solution results in a very small device that is easily placed in line with the microphone cable and can be used with any amplifier where excessive preamp gain is causing an early onset of feedback.
So, now that we have established how a pad may benefit us, there are still two important factors to address. One is the value of the pad, or how much signal is attenuated across it, and the other is frequency response, or tone.
The optimal pad value will allow you to place the amplifier’s volume control in the upper quadrant of its range before feedback becomes a problem, say from 8 to 10. How much padding is needed to achieve this? Well, it depends on two things: the output level of your microphone and the amount of preamp gain in the amplifier; both can, and do, vary wildly. On average, a 20 db pad will be appropriate; however, for a particular amplifier and microphone combination, 10db may be better. It depends on the particular pairing of microphone and amplifier.
Next is tone. Because tone means different things to different people, I will define tone as a musical sound with reference to its pitch, quality, and strength. When you use resistive padding for an audio signal, there will be a slight roll-off of the high frequencies resulting in a “darker” tone, and this is often desirable to harmonica players. There is, however, a negative to high frequency roll-off. With a darker tone, it will be harder to cut through the mix and be heard by your audience. The solution is a bypass capacitor that will allow some highs to bypass the pad. The result is a balanced and natural harmonica tone where the highs are now reduced by the same margin as the lows.
So, if a pad is for you, here are some considerations. A pad should be passive device (not requiring power), it should have at least 1 megohm of resistance in each direction, and it should be small and convenient to use. Before you buy, be sure that the pad is bi-directional and ask what the impedance value is; also consider a pad with a high bypass cap.
My name is Randy Landry, I am an electronic technician by trade. I own and operate Lone Wolf Blues Company, where we make effect pedals and amplifiers for harp players.