Large Scale Central

Can I use a potentiometer to reduce sound level in a speaker?

Electronics Power and Sound
#1

Hi everyone,

So I’m using a Mylocosound card for a while, but I’ve been unhappy with it’s lack of a Master volume control. (And from what I understand is not possible to programe or necessary to most at thos point in time.)
The volumes can be changed, but you need to do it for each seperate sounds like static hissing, whistle, and bell.
Which to me is a little time comsuming and hard to keep sounds at reasonable levels each time I need to change.

What I was thinking was maybe adding a potentiometer somewhere along the speaker cable line to manual change the volume down.

Diagram[I found this diagram via google to demonstraight my question]

Considdering that I have no clue on electronics, I’m worried that this might not be a good idea.
I know there are things like watts, ohms and such to think about, but the maths confuses me.
And if it does work, what size of pot would I need in terms of K, ohms or such?

I am using an 8 ohm - 5 watt speaker if it helps.
Any advice would be highly gratful :slight_smile:

Matt

#2

Yes. Assuming the MyLocoSound likes an 8 ohm speaker, your will need an 8 ohm 5-10 Watt potentiometer or L-Pad. They used to sell these as Radio Shack as a Speaker Level Control. That diagram is however quite wrong. It will not work very well that way.

The best way to do it is with an L-Pad

Some Options…

#3

The one on the left is the way we used to control the volume on the PA systems at school. I thought that we used something a bit larger then 8 ohms though. At full volume the sound board would “see” 4 ohms if you used an 8 ohm pot.

#4

In times past, vaccuum tubes drove a transfromer and it was important to match the transformer output coil to the speaker coil.

With modern outputs it is an electronic devide driving the speaker, not a transformer and the match is not as critical.

Zimo MX695 decoders are rated at 10 watts but only for a 4 ohm speaker, the 8 ohm speaker would reduce the output power rating to less than half. And a 16 ohm speaker would limit this even more.

So, I added a 100 ohm pot in series with the speaker and it works fine for me and does not take up a lot of space. I used the 1 watt version. Diagram above shows parallel.

#5

Daktah John said:

Yes. Assuming the MyLocoSound likes an 8 ohm speaker, your will need an 8 ohm 5-10 Watt potentiometer or L-Pad. They used to sell these as Radio Shack as a Speaker Level Control. That diagram is however quite wrong. It will not work very well that way.

The best way to do it is with an L-Pad

Some Options…

ok awesome, I might go with the option on the left, the first one, as it seems the easiest.
That and I’m not sure how the wires would go on the mylocosound card if I tried the other 2.

I’m having trouble finding a 8ohm potentiometer.

All the ones I can get in a local electronics store are in M’s or K’s (not sure what that means) , so would a 5 or a 10 K be ok?
10k Pot Log
Or are they rated to high?

#6

I would suggest the 5k ohm pot. Having too large of a pot for the diagram I suggested using wont hurt anything, but the volume would go from full volume to real quiet really fast. So a 1 k pot would be better, but if 5k is the lowest you can find, then that would work.

#7

They do have 1 K pots, I’ll go for that.
I thought since 8 is between 5 and 10 in the numberline, I needed to go for one of those.

Thank you. :slight_smile:

#8

Again, a 100 or 200 ohm pot with a 1 watt rating in series will do the job and at a low cost.

So one input line to the end of the pot, the middle of the pot to the speaker.

The other output line to go directly to the speaker.

#9

Matt,

Do a search for a “wire wound potentiometer”. Usually, the low resistance pots carry a lot of current [your speaker is using a lot more than typical electronics.]

I got half-a-dozen wire-wound pots from a surplus shop for almost $0. They range from 20 ohm to 100 ohm.

#10

It looks like the only one I can find locally is a 5-Ohm 15-Watt Wirewound Potentiometer.

I’m guessing I’ll need to add some ohm resistors? somewhere along the wiring to bump it up to 8 ohms?

#11

Matt, I don’t think that is quite going to work. Resistance, or impeadence since its AC, would be half of the value if you put 2 same sized loads in paralell. So if you HAD, an 8 ohm pot, and an 8 ohm speaker, at the full volume setting the sound board would “see” 4 ohms. If its designed for 8, 4 ohms may draw too much current and damage the board. As stated a 100 ohm would be a good choice. At full volume the sound board would see an impeadence (load) of 7.407 ohms. That is close enough to 8 to not matter. Then as you turned down the volume, the load the sound board “sees” would go up in resistance (impeadence) and that would be ok.

#12

Matt: when you get your potentiometer , be sure to get one that is also called an audio taper pot and not the Liener taper version. As the Audio Taper version are used for sound systems. A linear taper version would be more suited for controlling the brightness of a LED or small micro bulb.

There is also another solution to your problem and that is to get a small Amplifier board and wire your Pot to it and the speaker and then wire the output of the Sound board into the input of the amp. You would have all the contol of the volume you could want. I use a small 2watt stereo amp board with my Mp3 modules and drive a 3" round 8 ohm speaker and you can hear the sound at least 100 feet away.

Here is a link to one of the amp modules I use: http://www.ebay.com/itm/Digital-DC-5V-Amplifier-Board-Class-D-2-3W-USB-Power-Mini-PAM8403-Audio-Module-/221507479907?pt=LH_DefaultDomain_2&hash=item3392dfe963

Dan

#13

Thanks everyone, man I didn’t think a simple idea would be this complex. (Well I had some idea, but now I got more to try and understand)

Think I might hold off for a moment and have a ponder on what to do next.

#14

A 100 ohm pot in series would make the DC resistance of an 8 ohm speaker go from 6 ohms to 106 ohms.

And this is due to the DC resistamnce of the speaker is usually around 6 ohms. Impedance and resistance are not the same.