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Needing Help Making A Variable Talkback That's Stable

blake25537

Active Member
Mar 10, 2019
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Hi, I've been trying to make a variable talkback for a Cobra 29, 25 n others but none of the ideas I've tried worked out very good. I tried hooking it up on each side of the resistor, inline with the resistor but none are very stable. Could someone help or post a diagram on how to make a good one? Thanks in advance.
 

Well, what are you doing for Talkback?

Remember too that "Variable" and Microphone may produce together, but not always Squeal-Free.

Since 25 and 29 have a Mic Gain, that isn't the problem, the problem is in the output of the Talkback being AFTER the main amplification taking place. You can adjust Mic gain all day trying to make this work, but to have Talkback, you really should use a separate amplifier "box" for this.

It sounds to me that you're doing the PA and EXT SP trick and using that as the output.

What are you using to "tap" though?

Now, where you mount the resistor can be anywhere...in the line even back at the Handset / Microphone

And you really should not use "speaker" - instead - use headphones with a higher ohmic value in resistor to drop more volume across it to save your ears and leave more behind for the Driver and Final - or use a Monitor radio to hear what is actually being transmitted and received. Besides - the isolation you get in the ears helps reduce the feedback and improve the levels of what you hear.
 
yes I'm doing the ext speaker/pa with a resistor. getting feedback is not the problem, controlling the volume of the talkback that's what I'm having problems with. Thanks
 
Then you're better off using an external speaker with talkback volume control already in it.

Else you could do up a circuit that is not hard, just tedious because of what you'd have to do to install it.

Any particular control on the front panel you wanted to use...?

Here's the concept...
upload_2021-1-30_13-14-4.png
Try to get a 1/2 Watt or 1 Watt variable resistor to try this as an experiment first - to find values you need to make the radio talkback work properly so you don't burn out the control - I would use Resistors of 2-watt minimum to get an idea of how much loudness and heat they will produce.

The Buffer resistor is important as it is the one that will keep the pot form sinking all the current and burning out from the power being applied across it.

That is the purpose of the Shunt resistor - say 150 ohm, 2 Watt to start.

The Buffer Resistor is the one resistor you use now, to handle the volume from the Audio Amp.

The Shunt Resistor usually is other half of the OHMIC value you'll need to quiet the speaker down enough so you don't hear it thru the Pot. So if you use a 1K pot, with that Buffer resistor of say 27 ohms, you'll need a resistor of 120 to 150 ohms 1 Watt or more - to drop the power going to the speaker so the pot when it's fully down, you have quieted the speaker and not put the pot at a point where it dissipates all the power thru it.
 
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cobra 25 29? 100 ohm resistor on pa to a volume control salvaged from donor radio to external speaker then mount on back of the radio, have done that to a lot of cobras and it worked out ok,,,
 
Then you're better off using an external speaker with talkback volume control already in it.

Else you could do up a circuit that is not hard, just tedious because of what you'd have to do to install it.

Any particular control on the front panel you wanted to use...?

Here's the concept...
Try to get a 1/2 Watt or 1 Watt variable resistor to try this as an experiment first - to find values you need to make the radio talkback work properly so you don't burn out the control - I would use Resistors of 2-watt minimum to get an idea of how much loudness and heat they will produce.

The Buffer resistor is important as it is the one that will keep the pot form sinking all the current and burning out from the power being applied across it.

That is the purpose of the Shunt resistor - say 150 ohm, 2 Watt to start.

The Buffer Resistor is the one resistor you use now, to handle the volume from the Audio Amp.

The Shunt Resistor usually is other half of the OHMIC value you'll need to quiet the speaker down enough so you don't hear it thru the Pot. So if you use a 1K pot, with that Buffer resistor of say 27 ohms, you'll need a resistor of 120 to 150 ohms 1 Watt or more - to drop the power going to the speaker so the pot when it's fully down, you have quieted the speaker and not put the pot at a point where it dissipates all the power thru it.
OK I think I know what you mean.
 
i dont understand this stable stuff,, i turn it up to see how it sounds and set the echo,, then shut it back down i dont like to hear my self all the time,,,,
 
i dont understand this stable stuff,, i turn it up to see how it sounds and set the echo,, then shut it back down i dont like to hear my self all the time,,,,[/QU
I don't use it myself but a lot of people are wanting it in their radios. I put a on/off switch on them but didn't think it would be hard to make a variable talkback but I guess it is. I've tried all kinda ways n nothing works right. I'm going to try Handy Andy idea.
 
Then you're better off using an external speaker with talkback volume control already in it.

Else you could do up a circuit that is not hard, just tedious because of what you'd have to do to install it.

Any particular control on the front panel you wanted to use...?

Here's the concept...
Try to get a 1/2 Watt or 1 Watt variable resistor to try this as an experiment first - to find values you need to make the radio talkback work properly so you don't burn out the control - I would use Resistors of 2-watt minimum to get an idea of how much loudness and heat they will produce.

The Buffer resistor is important as it is the one that will keep the pot form sinking all the current and burning out from the power being applied across it.

That is the purpose of the Shunt resistor - say 150 ohm, 2 Watt to start.

The Buffer Resistor is the one resistor you use now, to handle the volume from the Audio Amp.

The Shunt Resistor usually is other half of the OHMIC value you'll need to quiet the speaker down enough so you don't hear it thru the Pot. So if you use a 1K pot, with that Buffer resistor of say 27 ohms, you'll need a resistor of 120 to 150 ohms 1 Watt or more - to drop the power going to the speaker so the pot when it's fully down, you have quieted the speaker and not put the pot at a point where it dissipates all the power thru it.
Thanks a bunch I'll give it a try.
 

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