Party Line Intercom

[col_b96]

Hello,
Has anyone ever built/attempted this

ComClone2

I've just bought the parts and had no luck. I've seen some other posts on various forums of peoples attempts, but no one seems to conclude if it works or not ( and in my case..not)

Look forward to your replies.

 

[audioguru]

The mic preamp has an extremely low input impedance of only 520 ohms that shorts the few kohms output impedance of the electret mic. STUPID!
The mic preamp should be non-inverting that has a high input impedance. SMART!

 

[col_b96]

So your saying it will work badly if not at all. Thing is I have it working ok, but sidetone cancellation does not work. Ideas ?

 

[audioguru]

So your saying it will work badly if not at all. Thing is I have it working ok, but sidetone cancellation does not work. Ideas ?

The "duplex hybrid" circuit has an adjustment to null sidetone. The null is important to stop local acoustical feedback howling.
Does your null pot do anything?

 

[col_b96]

No.I'm feeding a 1KHz audio test signal into it ( with R30 electret bias removed) Can see signal going into U2B from audio bus. Can see signal from RV2 varying, but when it also hits U2B there is no change in amplitude anymore, just a slight rounding off of the signal.

I've made two of these, both have the exact same fault. Else i would have blamed my construction.

 

[audioguru]

The instructions talk about a Bus Termination Resistor but I don't see it on the schematic. The hybrid null depends on matching the termination resistance.

 

[col_b96]

Hello!

It's included in the power supply schematic - specifies at 2K2 ohm between audio and ground.

 

[col_b96]

As a test i dropped the ermination down to 500 Ohm - sidetone started to functon - but it ate my DC call level. Have you any more thoughts, because i am at a loss.