What happens when an audio JFET amplifier receives signals from two microphones?

Question

I have seen this JFET amplifier circuit in my notebook.

It is very possible that it isn't a practical amplifier and that they just tried to teach me about JFET applications, but I have two questions about it:

1. As it seems, the signals from the two microphones will superimpose each other in the gate. Is that a trivial thing to do? Will I be able to hear both microphones assuming that a speaker is connected to the output?

2. The output signal isn't linear, mostly because the JFET transistor is used here. Is it accepted to build such amplifiers? or will the quality of the sound decrease dramatically in practicality?

Answer 1

1. If you connect two signal sources together like this (with resistors in series), the signals will get added. This is commonly done when you want to mix together two separate audio signals. You will just hear both at the same time with no ill effects.

2. The signal looks distorted and it will sound just as distorted. It'll sound similar to a cheap speaker that's turned up way too much (so it starts clipping / distorting the signal).

In reality, you'd incorporate some kind of negative feedback loop into the circuit to linearize it and eliminate the distortion. The circuit's component values also don't seem to be chosen correctly (the output coupling capacitor is way too small, for example).

Here's the same circuit but with its component values and input voltage fixed.

Answer 2

JFET’s are used inside all electret mic’s because they can buffer with gain and low noise. The gain is controlled by an external drain R pullup to 3.3, 5 or 12V, which is chosen to centre the idle voltage for large swings based on the Idss current.

Mixing 2 signals from the same mic types is commonly done for redundancy for podium mics.

Electret mics have a wide span and if the rear is blocked will pickup very far away (too distracting) but are made with an open rear for differential far field attenuation. (Out of phase) This is a delicate balance and hampered by a dust filter and often works poorly in cheap lapel mics. But if done right , avoids feedback near a speaker monitor.

Here with 2 variable levels in phase, nothing exciting happens. But if one mic was inverted you could use dual magnetic mics as near-field mics and cancel out of phase.

To demonstrate this I made slider for V2 phase centered at 180 deg. shift from V1 with 0 to 360 full slide. So you can see the minor effect of cancellation with different pot gains.

This technique is also how they null voice from recordings for Karaoke.

You can measure the linearity by changing the low levels and computing then % difference in Vp- and Vp+ You can easily get 1% THD at 100mV out and useful to 1Vpp output. But beyond 10% imbalanced peaks or THD it’s pretty crappy.

So electret mics still need another amp to get to 1V line voltage.