Diode between amps?

[jef531]

I am trying to installing an external amplifier onto a existing intercom system. When switching from the talk mode to the listen mode the unit uses the speakers as a microphone which is causing the system to feedback. I wanted to know if I installed a diode between the intercom and amp will that isolate the signal and eliminate the feedback by only letting the signal go to the speakers and not from them? If this will work does anyone have an idea of what type and size of a diode? Also should it be installed on the positive or negative side or both? Any help would be greatly appreciated.

 

[crutschow]

A diode will not work. Diodes can isolate DC, not an AC signal such as audio.

To eliminate feedback generally requires elaborate isolation type circuits or notch filters. These are sold by companies who make public address systems. An example is **broken link removed**.

 

[user_88]

Usually the problem is that some of the output sound frequency energy is being detected by the input port of the circuit. In this case, the input is the speaker utilized as a microphone.
One possibility is to reduce the gain of the system, but this is contrary to your use of the amplifier.
You might try to attenuate the higher frequencies at the speaker/microphone input.
You only need 2k or 3k ... maybe less .... of audio bandwidth frequency in order to have intelligible, normal speech recognition.
Try, placing a 1µF capacitor in series with a 100 Ω resistor at the intercom speaker terminal. .... Not absolutely sure about the values, but you should get some effect. .....
Do the wires going to the speaker/microphone have any sort of polarity?
Does one wire connect to the intercom ground?
If so, a polarized capacitor should go + to + when installing. However, a non-polarized cap can be installed without regard to terminal polarity.

 

[flat5]

No a diode will not help you. The system is an intercom. Have the "mic" close to the person speaking and keep the system gain as low as practical.

Is the feedback a high or low frequency?
There simple techniques to deal with the feedback. They may work.

 

[jef531]

I think I need to give some more info. The PA system that I am adding is a -24vdc system that uses a four wire setup:

1 -24vdc
2 Ground
3 Signal +
4 Signal -

The intercom system is a Nutone IMA-516

The existing horn is being used as a speaker and a mic depending on if the intercom is in listen or talk mode. The new amp and horn are using the -24v system. The signal coming from the intercom unit hooks to the old horn and the new horn, so when the intercom is put in listen mode is working as a mic that is then on the input side of the new horn which is causing it to feedback (it is amplifying the signal from one horn and outputting it on the other which is what is causing this issue). I need to figure out a way to isolate the signal from the old horn to the new horn. The feedback is faint at first and then gradually increases over a few seconds.

**broken link removed**

 

[audioguru]

Intercoms use a press-to-talk button that disconnects the local speaker or an electronic circuit disconnects the speaker amplifier when you speak. Then there is no acoustical feedback howling.
I don't know why you are adding more speakers.
If the microphone can hear the speaker then you will have acoustical feedback howling.