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I've got a (pretty dumb) radio which increases volume by digital dB, and not some analog device. Thus it goes something like { Mute, -75db, -65db, -56db, -49db, etc }. Now I use this radio basically only at night, and the problem is, -75 is too silent, it's hard to hear that, while -65 is too loud again. The radio is a stereo setup, with the speakers having 6 Ohm impedance. If I introduce for each speaker lets say a 6 Ohm resistor, would that half the volume, and just that, or would it have some sort of effect on the sound I am getting? I only found (http://www.tdpri.com/forum/amp-central-station/200037-changing-impedance-speaker-resistor.html) as reference, and did not really find an answer to that.

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How much power (wattage) does this stereo put out? Is it a nice home amplifier or more like a portable all in one unit? Either way, adding a resistor in series with the speaker won't hurt, and shouldn't effect the sound quality much. –  Garrett Fogerlie Nov 24 '12 at 16:35
    
A good audio amp being a "voltage source" has very low impedance often <1% of rated load to handle any speaker 4~10 ohm without attentuation as well as low distortion for bass. Since your radio pad is not accurate here and you desire very low levels, a simple L pad is what you need to attenuate your output level by 20 dB. Consider adding a series R1 and parallel R2 to speaker to make this work. R1=5 , R2= .6 ohm for example. –  Tony Stewart Nov 24 '12 at 19:20
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5 Answers

up vote 3 down vote accepted

If you wanted to over-engineer the problem, you could look up "resistive pad". These are combinations of resistors that will present the same impedance to the amplifier and the load, and therefore would not alter the sound quality. In your case, it's not likely to matter. But there is one other effect. Many amplifiers put out a fixed amount of noise regardless of where the volume control is set, and it's quite audible at low sound levels. Adding an attenuator (such as you want to do) will cut down that noise as well as the volume. Then, when you turn up the volume, the noise doesn't come with it. In that case, your sound quality will have actually improved.

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It'll change the sound quality a little, but at such a low volume, not enough to worry about. 6 ohms would probably be about right.

I would add a shorting switch across the resistors for daytime use.

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I have the impression that the step size decreases at higher volumes. If you don't need the max volume, why not use a higher resistor and aim for the part of the scale where you have finer control?

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Something I have not seen addressed yet is the fact that sound is logarithmic not linear so doubling the impedance of the load will not cut the volume in half. Just a side note...

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Adding a resistor in series with the speaker will not increase the impedance of the speaker.

Well, yes, from one vantage point, we can regard the resistor plus speaker as being an output device, and that has a higher impedance than just the speaker.

However, the fact is that we are not obtaining any output from the resistor (other than heat). The output device is the speaker, and the resistor is more properly regarded as being part of the source. What it does is it raises the source impedance of the amplifier with regard to the speaker.

Raising the source impedance of the amplifier will change the frequency response, emphasizing treble, and the speaker's resonant frequency somewhere in the bass. Experiment will show to what extent. This is because the speaker has a higher impedance at high frequencies (since the voice coil is an inductor) and also at its resonant frequency, due to counter-EMF. Frequencies where the speaker's impedance are high are affected less by this source impedance increase than frequencies where the speaker's impedance is low. But you might like this, especially for quiet listening at night. "Loudness" buttons on stereos for quiet listening boost bass and treble, to compensate for the Fletcher-Munson effect (ear being less sensitive to low and high frequencies at lower volumes).

This mention Fletcher-Munson leads to another point: doesn't your radio have some tone controls? At the -75dB setting, maybe you can turn up the treble and bass a little bit to hear better.

Another idea is to relocate the radio. Place it so that the speakers are aimed at your head. This way it is easier to hear the higher frequencies, which radiate directionally from speakers.

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+1 for mentioning that it will change the freq response. Although I'd claim that you're just being pedantic on the resistor changing the source impedance vs. changing the load impedance. –  user3624 Dec 7 '12 at 20:38
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