Long ago, installed public addressing systems with the 'intercom' feature and performed well. Cannot remember details on its schematic.

What are the considerations to be taken in account to use a plain 8 ohm speaker as a distant microphone ?
Is a 70V transformer convenient or needed to use? Any special features for the front end preamplifier circuitry ?
-Some low level and airconditioning noises are expected in a large occupied room-

 

Hello,

I assume you mean a circuit as in the link:
http://www.electroschematics.com/590/intercom/

The low impedance of the speaker will reduce the influence from outside.
Use a shielded cable from the speaker to the intercom box.
Think of the 50 Ohms cables used in RF techniques.

Bertus

 

Long ago, installed public addressing systems with the 'intercom' feature and performed well. Cannot remember details on its schematic.

What are the considerations to be taken in account to use a plain 8 ohm speaker as a distant microphone ?
Is a 70V transformer convenient or needed to use? Any special features for the front end preamplifier circuitry ?
-Some low level and airconditioning noises are expected in a large occupied room-

One approach that avoids a transformer is to use a common base input sage for not too bad impedance matching.

There are examples online, but an easier search is the archive of Philips EE kit manuals - search "Philips EE20", its often the first hit. There are various examples of intercom projects. They used a 150R speaker - it will probably work OK with an 8 or 16R speaker, but the little transistor radio speaker transformers can still be got cheaply if you need a bit of step up ratio.

 

Why not use a $0.50 electret as a microphone and a speaker as a speaker?

 

What are the considerations to be taken in account to use a plain 8 ohm speaker as a distant microphone ?

One factor is the very poor sensitivity of the speaker, because its inertial mass is orders of magnitude greater than that of a normal mic. That obviously means the speaker output will require greater amplification than the mic. To what extent the greater mass disadvantage is offset by the greater sound-catching cone area, I don't know.
I agree with Gopher.

 

Thanks, guys.
No output to a speaker will be needed. The signal heard by the 'cone microphone' is to be processed, not reproduced. And an electret cannot be used by other reasons. The sensitivity from greater cone area against the greater amplification is a key decision which I equally do not know yet.

I could even explore rewinding the voice coil to a much finer wire with much more turns if 'transforming' becomes necessary. A fet should be at the front preamplifier end, I think, as sensitivity is key.

 

Thanks, guys.
No output to a speaker will be needed. The signal heard by the 'cone microphone' is to be processed, not reproduced. And an electret cannot be used by other reasons. The sensitivity from greater cone area against the greater amplification is a key decision which I equally do not know yet.

I could even explore rewinding the voice coil to a much finer wire with much more turns if 'transforming' becomes necessary. A fet should be at the front preamplifier end, I think, as sensitivity is key.

With a cheap cone speaker, the mechanical limitations of the speaker as a passive component picks up lower (bass) tones much better than higher frequencies. A tweeter designed specifically for high frequencies will be better at picking up the higher frequencies although at the sacrifice of mid-range (human voice) tones.

 

Thanks, guys.
No output to a speaker will be needed. The signal heard by the 'cone microphone' is to be processed, not reproduced. And an electret cannot be used by other reasons.
.

What other reasons? - You can put the electret capsule on the end of twisted pair, the load resistor simply goes at the equipment end. You can also put a simple amplifier on the end of the twisted pair - I had such an amplifier based on the TL431 published in Elektor a few years ago.

If a transformer secondary is best for your equipment, you could use a TL431 amp with a transistor radio audio driver transformer such as the Maplin LT44. That simply replaces the load resistor for the TL431.

 

Do you have the 70.V PA transformer? If so why, not use it? I can give you some detail on using these things if you want. Sounds like your application is pretty much of a no brainer. If you have the transformer, what are the wattages marked on the taps?