Question about using an external speaker with volume adjustment....

[KCOEM9103]

Hey everyone, I tried searching for an answer, but had little luck. I currently use a PRO-163 in my car, which is mounted in the console. See the attached pic for reference. I am using an external speaker since the scanner speaker "fires" inside the console. The issue is the way the scanner is mounted in the console, it puts it behind me while driving, so I thought about placing a remote volume adjustment within easy reach. I picked up a volume pot. from radio shack, and put it in, but it is not working very well. The pot. knob has like 35 positions of volume, but the only ones that do anything are the last 3. It goes from full volume to 1/2 volume to off, regardless as to how high I have the factory volume knob set to.

Does anyone have any thoughts?

Here is the pic:

The blank space in the console was replaced with a storage box with armrest. So I cant really move the scanner.

Thanks in advance!

 

[rfradioconsult]

How do you have it wired?

 

[KCOEM9103]

The - side is continous from the speaker jack to the speaker, the + side passes through the pot. knob then to the speaker.

 

[iMONITOR]

Is this how you wired it?

 

[KCOEM9103]

This is how I wired it.

 

[kb2vxa]

There are three terminals on a potentiometer as in the first graphic which seems to match the description. You have it all wrong, the load impedance changes with rotation and the power output with it. If you want a remote VC to work properly you need an L pad (constant input impedance), preferably a T pad (constant input and output) to keep the impedance constant throughout rotation.

Such is the drawback with direct coupled solid state output stages; with a pot as you lower volume the load impedance goes up and the output transistors draw less current, the power output goes down dropping the volume far below what you expected. That's why it only works on the high end where the load impedance is low enough for the stage to work in a more linear fashion.

 

[KCOEM9103]

Okay, I'll work on de-coding your response when I get home. Is the 1st diagram correct, or is it showing the wrong type of pot?

 

[slicerwizard]

It's clear that you bought the wrong part. Let me guess - the RS guy helped?

What's the part number?

 

[K8PBX]

Here's an L-Pad type volume control that maintains a constant 8 ohm impedance.

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=260-250

$5.58 + Shipping and comes with wiring instructions. It's wired just like SIG-INT illustrated in above Post #4.

 

[KCOEM9103]

This is what I bought:

http://www.radioshack.com/product/index.jsp?productId=3012598

 

[slicerwizard]

A 500k ohm pot for an 8 ohm circuit. It's only off by a factor of over 50,000...

You need the L-pad. Get it and you'll be good to go.

 

[kb2vxa]

Sorry Steve, that's an ordinary pot with an on-off switch on the back. An L pad has two interconnected ganged pots and a T pad has three. Here are two partial schematics of fixed attenuators I cropped out so you can see how the pots are wired, the only difference being the pots are connected as variable resistances, the slider connected to the "cold" end. Sorry I couldn't find variable attenuators but such are my technical resources.

 

[Mike_G_D]

audio L-Pads

Here is a link which explains the design and function of an L-Pad as it relates to audio volume control fairly well - plus it even has calculators to help one construct their own if desired: http://www.bcae1.com/lpad.htm.

And it has links to sources for L-Pads. If you Google using "volume control" and L-Pad you will find many more.

The one K8PBX illustrated is described as a mono L-Pad at least on the hyperlinked site. A link from the aforementioned article shows many more options from the same supplier: http://www.parts-express.com/webpag...ectGroup_ID=196&CFID=3328281&CFTOKEN=48292645.

Again, by searching you will find many more.

-Mike