by Joe O' Connel
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ABOUT THE AUTHOR
The author just received his B.A. in history and philosophy of science, graduating from the University of Chicago with highest honors, and plans to work as a professional historian or inventor. He has electronic experience as a hobbyist and is the author of 20 Innovative Electronic Projects for your Home (TAB Books).
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Previously I discussed stepped volume controls (TAA 4/88) as superior alternatives to standard potentiometers for many reasons. In my article I included the computer programs and resistor tables I used-my attempt to spare you the tedious math normally required to construct these devices.
FIGURE 1: One channel of a preamplifier, showing the author’s approach to volume control design.
But I suspect many audiophiles still hesitate to add stepped volume controls to their systems because the switches are hard to find and the tradeoff be tween wide range and fine resolution is inevitable. In this article I address these and other problems with an new approach that uses readily available 12 position switches to build stepped volume controls that provide:
- Wide range (80dB or greater)
- Fractional resolution (0.5dB)
- Precise balance control
- Convenient muting
- The ability to turn off one or both channels
- A theoretical increase in signal-to noise ratio (S/N) over standard volume controls.
I doubt whether any other approach can attain these features of volume control design, except perhaps a digitally con trolled electronic attenuator.
My approach is characterized by two separate stepped attenuators in different parts of a preamplifiers circuitry (Fig. 1). The first control is inserted in the signal path before the second stage of the preamplifier (the traditional position for a volume control), where it attenuates the signal in coarse steps of 5 or 10dB each. With steps this large, this first control can easily have a very wide range. Another stepped control follows the preamplifier's second stage and permits fine adjustment in 0.5 or 1dB steps. The total range of the second control is just 5 or 10dB because its only function is to interpolate between the first control's positions.
Using two separate switches allows very fine resolution even with fewer than 23 positions. Figure 2 shows how three commonly available 12-position switches can cover a 55dB range in 110 steps of 0.5dB each. Using separate switches for left and right fine control also lets you control the balance or turn off either channel for testing. For changing records, answering the phone or scanning the tuner dial, the ‘coarse ’ volume control becomes a muting switch by turning it a few clicks to the left.
FIGURE 2: A volume control made with three 12-position switches.
The 12-position switches required are readily available and offer high quality contacts. But if 23-position switches are available, you may prefer them instead (Fig. 3). This alternative provides a wider range and its coarse control would rarely require adjustment in nor mal use. Using two separate attenuators allows high-quality, high-resolution control over a wide range, as well as providing incidental features such as balance and muting control.
FIGURE 3: A volume control using one 12-position and two 23-position switches.
A Secondary Virtue
My design also reduces noise at the preamplifier's output. Although much of the distortion produced by an amplifier circuit depends on the signal passing through it, a certain portion is generated regardless of the input. In the standard approach, the second stage of a preamplifier feeds this noise directly to the power amplifier. This noise is attenuated in my approach by the control that follows the second stage.
As an example, consider a situation in which an incoming signal needs a gain of 15dB from the preamplifier to obtain a desired listening level. Assuming the gain of the preamplifier's second stage is 60dB, a loss of 45dB must be inserted somewhere in the signal path to ensure the correct output level. A normal preamplifier would attenuate the signal 45dB with its volume control. Assuming the second stage creates a -80dB noise level, the S/N of the output would be 15dB - (-80dB) =95dB.
In my approach, however, the two controls share the job of attenuation so that the first might reduce the signal by 10dB and the second by 5dB. The signal level at the output would be the same in both cases, but the noise level would be 5dB less for the preamp that followed my approach, for a S/N of 100dB. Of course, all the attenuation in a preamplifier can't be shifted from its position prior to the second stage or that stage would overload and create more distortion. But for 5 or 10dB the approach is valid.
This approach to volume control design is completely compatible with the computer programs discussed in my earlier article. If you don't have access to a computer, Tables 1 and 2 list resistor values corresponding to Figs. 2 and 3, respectively.
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TABLE 1
11 Position ‘Left and ‘Right ’
0.5 dB per step
Source Impedance: 1k
Load Impedance: 75k
Total Control
Impedance: 14k-o Value in ohms 8639.106 501.098 525.174 549.380 573.527 597.402 620.768 643.369 664.946 685.229
CQWONOUVEBWN=- - 12 Position ‘Coarse’ 5 dB per step source
Impedance: 1k-ohm
Load Impedance: 75k-ohm
Total Control impedance:
14 k-O Value in ohms
43.753 34.087 60.692 108.162 193.071 345.551 620.972 1121.615 2029.858 3604.022 5838.216 SO WONONEWN
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TABLE 2
9 Position ‘Coarse’
10 dB per step Source Impedance: 1 k-O Load Impedance: 75k ohm
Total Control Impedance: 50 k-o
Value in ohms
15.611 33.777 107.011 340.388 1096.063 3652.301 12787.976 31966.875 evenness] 21 Position ‘Left ’ and ‘Right ’
0.5 dB per step
Source Impedance: 1k
Load Impedance: 75k
Total Control Impedance: 14ka Value in ohms 4834.210 292.881 310.154 328.277 347.248 367.057 387.679 409.083 431.209 453.987 477.322 501.098 525.174 549.380 573.527 597.402 620.768 643.369 664.946 685.229
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For those who have implemented my computer programs, follow these instructions to generate the resistor values:
1. Treat the three attenuators as if each were a completely separate volume control in a normal preamplifier.
If you know or can measure the input and output impedances of the second stage, these will become the load and source impedances for the first and second control, respectively. Then you must use the source impedance of the ‘coarse ’ control and the impedance of the power amplifier for the ‘left ’ and ‘right’ controls. You can measure these per my previous article, use the manufacturer's specifications or estimate them.
2. ‘Zero’ on the knobs actually corresponds to a few decibels of attenuation. Some loss is inevitable with any volume control, but you can ignore it when labeling the knobs since it has the same effect on every setting. Thus, for the volume control in Fig. 2, set tings of 30, 40 and 47.5dB actually represent 31.42, 41.42 and 48.92dB attenuation. Their absolute numbers are off by the same fixed amount (1.42dB), so differences between them are exactly as they should be.
3. Use the program option to increase the total impedance and increase the computer values of the ‘coarse ’ control's first two or three resistors, which gives you a better chance of obtaining suitable resistors (especially if the range of your control is wide). As Table 1 indicates, I increased the total control impedance from the computer's. initial suggestion of 14 k-O to 50 k-O to get satisfactory resistor values.
The disadvantages of two volume controls are the extra contact in the signal path and the inconvenience of using three knobs to change volume.
These aren't serious problems. You can obtain the equivalent of a volume, a balance and a muting control for the price of two contacts. This seems a good bargain, especially using 12 position switches with silver or gold contacts. To reduce the amount o fumbling between switches, carefully select the range and resolution of each switch so that most volume adjustments require moving only the ‘coarse’ control. You can set the ‘left ’ and ‘right ’ controls for channel balancing and mostly leave them alone.
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Also see:
A MULTI-TONE INTERMODULATION METER, PART 1
FINE TUNING YOUR FORD, By John Buschmann