stereo volume limiter

[adambookman]

Hi all! I hope this is the appropriate place to ask my question.

Here's the overview of my project:
I'm building a "listening station", basically a car cd player connected to a chair that has spearkers built into it. I wired a 12v ac adapter to the stereo to give it power and that seems to work. I also wired the speaker leads to the speakers, and they also work. The problem I have is that the stereo can get too loud if turned all the way up. This isn't an application where I can trust the user not to turn the thing all the way up, so I need to build something to reduce the volume, so I can set the max to a level that I'm comfortable with. My plan is to build a box that has inputs for the negative and positive leads coming from the cd player, run each of these thru some kind of potentiometer, and have outputs that go to the negative and positive connections on each speaker. I purchased a Radio Shack Stereo Volume Control (271-1732C), which is listed as having a resistance of 100k ohms with a 20% tolerance, rated power of .05 watts, and max attenuation of 100dB min. This kind of works, but only when the knob is fully on, or barely backed off (maybe a degree or 2).

Here are my questions:

1) I think I've got the wrong rating/type of potentionmeter. Does anyone know of a better choice?
2) Do I need to connect the positive AND negative wires to the potentionmeter, or just the positive? Connecting only the positive seems to work, but I'm not sure.


Any advice would be greatly appreciated. Thanks!

Also, I'm using a Pioneer DEH-1700 CD player, if that makes any difference.

 

[indecided]

Firstly, what you need is a "gain" control, to prevent the pot from introducing more distortion into the sound. But it seems like you're utilizing the Pioneer's internal amplifiers to power the speakers directly, and a "gain" control won't be really feasible in this case.

While I never used the 1700 series, I have used the 6/7/8xxx series players before. There should be a menu in the setup that allows you to set the attenuation level. likewise, the option should be labeled ATT. But i'm not sure if the 1xxx series has that control. Either way, this can reduce the gain by a fair bit, but I don't think it is enough for your application. It's usually used to prevent peaking.

Anyways, try not to buy stuff from RS - they tend to be overpriced, and their quality varies like delivery chinese food. You should start with a lower value pot, perhaps 10-20k instead? Try a online distributor if time isn't of the essence (perhaps 2-3 days) and a bit of shipping should net you better parts. You should be able to find the parts at Alltronics/Digi-Key/Newark/etc. Make sure the pot is a log (logarithmic) type.

Also, the wiring for the pot should be : pin 1 should be connected to ground, pin 2 (wiper) to the output, pin 3 to the input.

Hope that helped you. Don't take my word on the pot values, i'm just speaking generally based on the number of amplifiers i've cracked open - a lot seem to use 10k pots.

 

[audioguru]

You are connecting a pot in a 4 ohm circuit with 20 watts in it. Therefore you need a 4 ohm L-pad that is rated for 20W per channel.
4 ohm L-pads are rare so in your case where you want more than a slight reduction then an 8 ohm/20W L-pad will be fine. They are sold in home entertainment products stores for mounting on walls to control the volume of sound in rooms away from the amplifier. More efficient ones have volume steps intead of being continuous. Stereo L-pads are common but 4-channel ones are not.

 

[adambookman]

Thanks very much for the help! It's going to save me quite a bit of time and money. I'll let you know how it goes.

Thanks again for your advice.