Calculate db attenuation of volume pot. in amp

[faultfracture]

May I ask how to figure out the volume attenuation caused by a volume potentiometer in an amp circuit please?

 

[Dulalala]

May I ask how to figure out the volume attenuation caused by a volume potentiometer in an amp circuit please?

Attuenation (dB)= 20Log(Vout/Vin)

 

[faultfracture]

That is a helpful start, but could you (or others) help me with a example?

Let's say I have a Sansa Fuze and a Headamp Pico and a HD800...

So, the source output is .75 Volts into 50k ohm input impedance of the pico, I believe the amp gain section of the Pico in low mode is 5db. The output impedance of the amp section should be 1 ohm or less and the impedance of the headphones is 360 ohm with 98db sensitivity. What i don't know is how the volume control of the pico effects this... I'd like to calculate the attenuation at quarter volume knob, half, three-quarter, etcetera... Thanks,

 

[MindsMirror]

What i don't know is how the volume control of the pico effects this... I'd like to calculate the attenuation at quarter volume knob, half, three-quarter, etcetera... Thanks,

You can't determine what the effect of the volume pot at 1/4, 1/2, 3/4, etc. will be unless you've made some measurements and calculated it. The volume pot will most likely follow a semi-logarithmic curve, and there's not a formula you can use to calculate what each position is unless you've measured and mapped it out already. It will also be different from pot to pot, half way on one amp will be different than half way on another. The only volume positions you can be reasonably sure of are 100% which should be the gain of the amp with 0dB attenuation, and 0% which should be no signal out.

If you want to know what 1/4, 1/2, and 3/4, etc. on your amp are you need to measure it. You could do that with a multimeter or a sound card's line-in.

 

[faultfracture]

What if I found a PN of the pot or got info from the manufacturer?

However, a bigger concept question for me maybe should be how does the volume pot work in a amp like the pico, is it a gain control? If I theoretically knew the resistance at, for example, 1/2 of the volume control, how would i calculate the overall gain through the whole system?

 

[MindsMirror]

A volume pot is basically a variable voltage divider.

Turning the knob changes the position where the output taps off the resistor. When the knob is at 100%, the output is all the way at the top, it connects directly to the input and there is no attenuation. When the knob is at 0%, the output is shorted to the bottom so there is no signal. Anywhere in between will depend on how they designed the resistance curve. You might be able to find a datasheet which shows a typical curve, but I think it would be more accurate and easier to just measure it.

 

[Dulalala]

f I theoretically knew the resistance at, for example, 1/2 of the volume control, how would i calculate the overall gain through the whole system?

Use the same formula I gave above. Though as @MindsMirror mentioned, wouldn't it be easier to measure the output voltage of the DAC, then the output voltage of the amp instead and then use the formula above?

 

[faultfracture]

But the volume pot does act a gain control in a headphone amp such as the pico?

 

[MindsMirror]

But the volume pot does act a gain control in a headphone amp such as the pico?

Yes and no. It's like a gain control in the sense that it changes the level of the signal. The difference is that gain is done by an active circuit and it increases the signal level, volume pots are passive and they only decrease the level. Gain controls on headphone amps are not continuous like a volume pot. The net result in dB at the amp output is the gain of the amp minus the attenuation of the volume pot.

 

[milkdudd]

Not sure if this is the right place to ask my question but I have to start somewhere. My Marantz HD-DAC1 dac/headphone amplifier doesn't have any analog inputs except the auxiliary that they describe for phone or iPod. Stupid or not, I connected my sacd player to this input. Volume turned all the way down at first. I knew the voltage coming in would probably be a mismatch but since it's all analog I didn't think I could damage anything. It actually sounds okay except for a crackling sound which I assume is because the voltage coming in is too high. The manual for my sacd player says it outputs two volts at the RCA's. Other than voltage issues it should work shouldn't it? I mean analog is analog correct? I know connecting this way isn't ideal but it's the only way I can think of to listen to my sacd's on my headphone amplifier. For those of you that know a lot more about this than me, do you suggest I use attenuators with the RC interconnects? If the answer is yes, how many decibel reduction should I shoot for? Attenuators don't cost a whole lot but I'd still like to buy the right ones the first time. Thanks in advance