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Colin's circuit with the LM386 amplifiers has both of them missing the very important 10 ohm resistor in series with a 47nF capacitor to ground at the outputs as shown on every circuit in the datasheet.
Colin's circuit with the LM386 amplifiers has both of them missing the very important 10 ohm resistor in series with a 47nF capacitor to ground at the outputs as shown on every circuit in the datasheet.
I think your head's stuck somewhere on "hi-fi", McDuck. That's what, a Zobel network on the output? How is that "very important" on an intercom, for chrissakes? We're talking about a "squawk box", not a piece of high-fidelity equipment here.
The Zobel network stops the LM386 amplifier IC from oscillation at a high frequency which causes severe distortion to audio and over-heating. That is why it is shown on all schematics on the datasheet. The LM386 little amplifier is not hi-fi but it sounds pretty good when it is connected properly as shown on the datasheet.
Well, I guess it wouldn't hurt (turns the load impedance essentially non-reactive), but is it really necessary on such a low-power device? What chances are there for oscillation in this circuit?
A speaker is an inductor and its reactance is a fairly high value and it causes phase shift at the frequencies where an audio amplifier will oscillate.
Don't argue with me about leaving the important inexpensive zobel network off. Instead you should argue with the manufacturer.
I will probably do this, but two questions: would this work with pn2222's (I have a bunch), and would this work on a 5 volt supply? Thank you for the help, this should work perfect for my purposes.
Yes (for sure) and yes (probably), if you're referring to Colin's 3-transistor circuit in this post. Simple circuit, easily breadboarded for testing. Plus no separate mike (electret) needed (uses speakers as microphones).
I notice one thing missing from that circuit that would be nice to have: a volume control. At least a trimmer adjustment so the user doesn't get blasted in the ear or have to strain to hear.
Colin, how about turning that 220K resistor on Q2's base into a potentiometer? Would that work OK?
If you change the value of the 2nd transistor's bias resistor then you change its operating point and it changes the operating point of the 3rd transistor. A bad idea.
Oh lordy, the phase-splitter intercom; first one I ever built, waaaaay back... An alternative is to drive the intercom line with a Howland current source. You don't have the problem with the collector resistors paralleling up as you add stations on the line, and the sidetone cancellation is much better because there are no reactive components introducing different phase shifts and frequency responses into each station. Built a bunch of them back in the 70's.
There's another intercome style floating around that you can consider. It uses the LM386, so better audio and more volume for less power than Colin's second circuit. Also, it has a common base amplifier for the speaker-as-microphone. Both stations listen to the common signal line all the time, and each station is push-to-talk. No local feedback, no phase-spliter to adjust, but also not full duplex. If anyone's interested, I'll fish through the archives for the schematic, and I'm sure I don't remember where I got it so I can't credit the source.
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