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Home built volume Attenuator

post #1 of 9
1/31/12 at 7:48pm
Thread Starter 

Hi,

 

I recently bought a pair of 1964 custom IEMs and I love 'em.

My problem:

Because I can hear really quiet sounds with these I often get annoyed at at least one of these things:

  • The portable music player that I have has ~30 volume steps but I want to listen to volume 3 maximum. and I often want a volume between 0 and 1.
  • The portable music player that I have has a faint background hiss. (this does not scale with volume thankfully)

 

My solution:

Rather than spend $$$ on better player that potentially has the same problems just build a small Attenuator in form of a resistor. the resistor would be on the ground cable between a male and female audio jack which in turn go between the music player and the headphone-cable jack.

 

My question to you:

Will this produce any (unwanted) effects other than just scaling the volume down?

 

This is the electrical schematic that I'm thinking of building Iit from.

schematic.png

 

 

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post #2 of 9
1/31/12 at 8:29pm

the circuit you show is not so useful - it will cause mixing of the channels signal due to the common gnd impedance - at useful 10x attenuation you would be essentially listening to mono

 

you probably want a "hiss buster" arrangement of 2x 2 resistor dividers per channel

 

the "best" alternative for noise reduction of a iem drive signal is a custom stepdown audio output transformer/channel - but no one makes a standard part that is really good for this use

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post #3 of 9
2/1/12 at 4:45am

Exactly, you need 2 resistors per channel, e.g. a simple resistive voltage divider: Lout - R(15 ohms) - R(1 ohm to ground) - left speaker. Should result in ~24 dB attenuation.


Edited by xnor - 2/1/12 at 4:48am
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post #4 of 9
2/1/12 at 2:04pm
Thread Starter 

Hi, Thanks for your responses.

Just a note: I'm not a electrical engineer (esp. not with AC) but I know the basics.

xnor: My interpretation of your description is this:

schematic.png

Would this do what I described? If I want to change how much this circuit attenuates, would I change the 15ohm or the 1ohm resistor?

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post #5 of 9
2/1/12 at 2:31pm

Either, though it's better to change the top one.  Call Z_A the top resistor (labeled 15 ohms now) and Z_B the bottom one (labeled 1 ohm now).

 

CodeCogsEqn.gif

 

So for those values, V_load is about 1/16 of V_source, hence 20*log10(1/16) being around -24 dB.  Note that the resistors have more or less the same impedance at any frequency, while Z_load (your IEMs) may very well not.

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post #6 of 9
2/1/12 at 3:17pm
Quote:
Originally Posted by qwert33 View Post
Would this do what I described? If I want to change how much this circuit attenuates, would I change the 15ohm or the 1ohm resistor?

Yeah, actually you can change both resistors.

You can calculate the total resistance of the parallel resistor and your IEM with R1*R2/(R1+R2), so 1 ohm in parallel with a 16 ohm IEM would result in 0.94 ohms, just add the series resistor = 15.94 or about 16 ohms. I wouldn't go much lower than that because your player has to power that load.

 

Assume you want less attenuation and decide to increase the parallel resistor to 15 ohms. Now the total resistance (again, assuming your IEMs have 16 ohms) is 22.7 ohms. But the problem now is that the voltage is not split evenly between the resistors and your IEMs. The reason for this is that speakers usually do not have a flat impedance. Instead, the impedance usually rises at the resonant frequency and also at higher frequencies. The higher the parallel resistor value, the higher the output impedance the headphone "sees". Compared to a very low output impedance, this would result in a boost where the impedance of your IEMs is higher.


Edited by xnor - 2/1/12 at 3:20pm
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post #7 of 9
2/1/12 at 4:54pm
Thread Starter 

Awesome, Thanks for your feedback.

I'll get back to you guys once I've hacked a prototype together :)

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post #8 of 9
2/2/12 at 5:41pm

I would just get the koss vc-20, less than 10$ ; unless you want absolutely to play with electronics.

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post #9 of 9
2/2/12 at 7:46pm
Thread Starter 

The only problem with that is that I live in new zealand and the shipping cost would be 5x the item cost. thanks though :3

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