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Cory D

Connecting a 5w, 12vdc amp to a 12vdc telephone intercom system

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Hi all, haven't been in here for a while. I have built a 12VDC telephone intercom system that works fine. What I'm trying to achieve is connecting a amplifier to the phone service line so that the conversation can be heard without using the phone handheld. I have built a 6w amp from a schematic I downloaded using a TA7222AP amp chip (Schematic Below). I'm a little lost with this: For optimum performance input and output must be separately grounded. How can this be? What am I missing here? How can you have 2 different grounds?

 The input: I tried a 600ohm TA and tried hooking it direct...no go. What am I doing wrong here? Help! Suggestions...!


Cory D

6W amplifier using TA7222AP

Here is the circuit diagram of a 6W amplifier using the TA7222AP from Toshiba.TA7222AP is an excellent integrated audio amplifier which can deliver 5.8W to a 4 Ohms load at 12V supply voltage. The IC has very good features like, muting function, low distortion, high ripple rejection, short circuit protection, thermal shut down etc. This amplifier can be operated from 8 to 12V and this makes it ideal for car radio applications.














  • The circuit can be assembled on a Vero board.
  • Use 12V DC for powering the circuit.
  • The IC must be heatsinked.
  • Speaker can be a 4 ohms one.
  • For optimum performance input and output must be separately grounded.

Below is the amp schematic...the pic didn't download on the last post, sorry

Cory D


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Of course the input ground pin 7 and the output ground pin 8 both must connect to 0V on the power supply. But if the pins are connected together and a long wire connects them to the power supply then the variable high current from pin 8 in the resistance of the wire affects the 0V on pin 7 which adds distortion and might cause the amplifier to oscillate. Use separate ground wires.


How will you prevent acoustical feedback howling? If the handset mic hears its amplified output signal from a speaker then you will have feedback howling. A speakerphone uses a special circuit that cancels the signal from the handset mic from being amplified. If both ends of a telephone call have a simple speaker then you will have long distance feedback howling. That is why most cheap speakerphones use voice-switching (half-duplex): I speak and my speaker is muted. You speak and your speaker is muted. Then we both cannot speak at the same time unless there is a complex full-duplex circuit.  

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Thank you for responding, Audioguru. So the ground on pin 7 and 8 should be separate to the source 0V? So if both grounds are separate, does the distance still have an effect with distortion and oscillation?

Acoustical Feedback: I'm not sure what "Half-Duplex and Full-Duplex is. I'm not using speakerphones. Just 4Ohm-6w speakers. The speaker will only be on one end of the line. The speaker will be close to the handset, though. Another speaker will be approx. 10 to 12 feet away.

I'm going to try this circuit (Below) to signal the amp from the phone line. I don't have the components yet to build or even try it. I have the 600 ohm TA and the diodes...but the 620K resistor is not in stock at any of the local electronic stores nor the CAP. Both are on order. Do you think this will work. I have 3 other schems that is used for this application. This one has less the components. Your input?


BTW, Audioguru, thank you so much in the info on my Phone intercom system. The 300 ohm did the trick! Later, after you helped me get it going, I found a You Tube video showing how to make a 2-telephone intercom system for your house. Thanks again for the help. Now if I can get this amp and phone system to work....I'll be good to go!

Edited by Cory D

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Acoustical feedback howling occurs when the microphone (in the handset) can hear its amplified output from the speaker and the sound goes around and around making a feedback howling sound. It is stopped in a cheap speakerphone by using half-duplex which is allowing the sound to be transmitted or received but not both at the same time.


Half-duplex in a speakerphone mutes my speaker when I speak then my voice does not come from my speaker but the other end's speech also does not come from my speaker until I shutup. If the other end has a speakerphone that uses half-duplex then their speaker is muted when they speak and at the same time they do not hear me until they shutup. If there is noise or one person always speaking on one end then it forces the direction so that the other end is not heard unless they scream louder than the noise.


Polycom makes full-duplex speakerphones that use a digital echo-canceller circuit to prevent acoustical feedback howling and both ends can speak and be heard at the same time. Their speakerphones cost close to $1000 for the cheapest ones.

Polycom speakerphone.png

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