I have successfully connected an old rotary phone to an A7670 module everything works fine: dialling, ringing, and voice calls. During calls, I can hear the other person loud and clear through the speaker, but the outgoing voice from the microphone sounds very far. I have to speak loudly for it to be picked up properly.
Gain on A7670 is set to 7 (max).
From what I understand, old carbon microphones require a biasing voltage to function correctly. However, before I risk damaging the phone module, I thought it best to ask for help.
I tried connecting a 1k resistor from 5V to the positive mic line, with a capacitor in series on the same line, but all I got was buzzing. I might not be doing it correctly. I don’t have a schematic of what I tried, but I can upload one if needed.
Is this the right approach for applying bias voltage, or do I need something different?
I'll also include the schematic of the phone (even though I'm not currently using it), in case some of the original circuitry could help with proper amplification.
Carbon microphones require a current source to power them, not a voltage source. Carbon microphones act as if they have an amplifier built in, they don't need additional amplification. I suspect the microphone in the photo is an electret condenser microphone, which were designed as direct replacements for carbon granule microphones. If it really is carbon granule then I'd expect that it's so old that the carbon is degraded and not working very well.
Well not with a single transistor, but you could with a correctly designed operational amplifier or two, if you want the microphone to be sensitive.
By the way, I once modulated an AM transmitter to produce an FM signal by putting a carbon microphone in series with the grids in the local oscillator circuit. Using a thermionic valve, European name, or vacuum tube U.S. name.
As I said, carbon granule microphones behave as if they have an amplifier built in. The electret replacements are exact replacements, so they behave like carbon granule microphones so do not need amplification.
I'm still not seeing your secret circuit, without which I can't really help.
Note that in a real telephone handset, a certain percent of the microphone audio is fedback to the receiver part. test your microphone resistance. An electret mike will show a really high resistance but a carbon granule mike will be 100 Ohms or so.
Which secret circuit? If you refer to the resistor and capacitor is just something that I have tried but not actively using, as mentioned above I have connected the microphone leads directly to the M+ and M- of the module input without any external components. If the electret and carbon are interchangeable then it probably is cooked, it say 1984 on it I won't be surprised if it is!
This is what I have tried but only created buzzing.
I called it 'secret' to make the point that so far you had not provided it, your brief description of the circuit being useless. Now you have taken a small step in the right direction by providing a bit of it. Please provide your complete circuit. If you find this difficult please read this for help:
Based on what you have provided that's not going to work because although you have a 1k resistor feeding the microphone there's nowhere for the current to go after it's been through the microphone. From memory, and I could be mistaken, carbon granule microphones are designed to work with something like 100mA, their electret equivalent will be designed for similar current. The maximum current from your 1k resistor being connected to 5V is going to be 5mA. This is the point you need to find out what you actually have, as @Grumpy_Mike pointed out a carbon granule microphone is a variable resistor, so will probably be OK with current well below its design current. An electret microphone has built in electronics that need power. Without the design current I'd guess they don't work very well. There's a simple way to find out: shake it next to your ear, if it's carbon granule you will hear the granules moving around (hopefully). You now have 2 very different tests to find out what you have, one from me and from @Paul_KD7HB , try both and tell us the results.
In that case I assume that some make a noise and some don't. I'm confident to say the ones I saw while working for what was then Post Office Telecommunications made a noise when shaken.
Shaking the microphone does not produce any sound, so I guess as you guys pointed out could be an electret microphone.
The problem is that there is no circuit, I have drawn something for you but I doubt will be of any help, the module has 4 pins MIC+ MIC- SPK+ SPK-, and the cables for speaker and mic as mentioned above are soldered there directly without intermediate circuit. Speaker works very well, microphone is low and distant. No module schematics were given so I am not entirely sure.
Is there anything I can utilise from original phone circuitry do drive the microphone, I see that in parallel to the microphone there is a resistor and a capacitor but also one side of the microphone is connected to the transformer. I am not an expert of these old analog circuits and I am not sure how amplification was done in original circuit.
The transformer is an autoformer. Google that! The mike causes the magnetic field to vary in the autoformer. There are taps on the autoformer there the ear piece is attached. The autoformer. auto transformer, will send an higher voltage copy of the mike voltage. In effect it amplifies the mike signal by transformer action.
Well as I stated I have tried a different configuration with the same circuit, actually 100ohm works better. I though I would make it public in case someone will encounters the same issue.
