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Question on attenuating a small signal

VoxRocker

Member
Let me start off by apologizing since this is not concerning a guitar amp.

What I'm trying to do is find the correct potentiometer resistance to control a signal coming from the speakerphone port on a Cisco IP phone. I'm building a device for work to power some intercom speakers but the level coming out of the phone (~2-4 watts I'd guess) is overloading the "MIC" inputs on my bogen amplifier. Once I turn the volume on the phone down, it works fine--no clipping. The only problem with that is the intercom software turns the volume back up on the phone to maximum; there is no way around this.

Here is the information I know:

The speakerphone output on the phone:

  • About 2 watts, no greater than 5w
  • Was originally connected to 8ohm 2" speaker
  • This is the signal I want to attenuate

The amplifier I'm plugging connecting the speakerphone leads to:

  • Going into a Low Impedence MIC Input
  • 35 Watts output to 70V horns
  • Has independent Vol/Bass/Treble (if that's important)


My proposed plan was to wire:

  • Speakerphone output to lug 1 of the pot
  • Wire lug 2 to the terminal block on the box
  • Wire lug 3 to ground

My questions:

  • What resistance potentiometer do I need? Will a 10k work? Linear ok?
  • Is the impendence differences going to give me problems? I know the MIC input in low impedence, will the 8ohm speakerphone output be ok?
  • Am I missing anything other than just attenuating the signal with a pot?
I'm fairly handy with a soldering iron so that's no issue.

Here's a semi-crappy phone picture of the project.



Let me know if you need anything else and a big THANK YOU in advance!

Matt D :phones
 

skipm45

Member
Hey MattD,
it sounds like you've already "done this" by turning the phone volume down. In theory it'll work. I would caution you to connect a resistor (guessing between 1k and 10k) from lug3 of the pot to ground. Otherwise with the pot turned all the way down, you are shorting the output signal directly to ground. Try about a 50k pot (another guess here)

Good Luck!

Skip
www.skipzcircuits.com
 

VoxRocker

Member
Thanks Skip.

The only problem with turning down the volume on the phone, is the intercom software turns the volume back up to full when it broadcasts. :jo What I really need is a trimmer that I can set-and-forget.

I'm debating on making a radioshack trip and just picking up a variety of pots. I've got spare phone if I melt this one.

Any more thoughts?

Hey MattD,
it sounds like you've already "done this" by turning the phone volume down. In theory it'll work. I would caution you to connect a resistor (guessing between 1k and 10k) from lug3 of the pot to ground. Otherwise with the pot turned all the way down, you are shorting the output signal directly to ground. Try about a 50k pot (another guess here)

Good Luck!

Skip
www.skipzcircuits.com
 

skipm45

Member
VoxRocker,
I'd use a 1 watt resistor. Not all of the power is going to ground unless you turn it all the way down. Also , this is 2 or 3 watts into 8 ohms. the tail resistor is on the order of 10k ohms. Again, this is basically "guesswork" on my part

Skip
www.skipzcircuits.com
 


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