mixer and vu meter

Posted on Feb 6, 2014

The 10k resistors in question are R15, 17, 18, and 19 (R16 is somewhere else). Their purpose is to separate the input channels, because otherwise turning the volume control for one channel down to zero would also connect the output signal to ground, cutting out both channels as I discovered when I excluded them which breadboard testing). The origi

mixer and vu meter
Click here to download the full size of the above Circuit.

nal plans called for a 10k resistor on each channel, so I included one. The catch (as I figured out this morning) is that if a significant current is required (such as that needed to power non-amplified headphones), the voltage drop across these resistors increases in accordance with Ohms law. This causes a large drop in the output voltage, which is the reason for both the weak signal output AND the apparent decrease in VU sensitivity. With no output, or when connected to an amplified output such as a stereo, the current draw is significantly lower, and the VU meter functions normally. With no way to compare the signals between an amplified and non-amplified output, this took a while to click. Additionally, because I tested the VU meter and the mixer circuit separately on the assumption that one would not impact the other, I didn`t see this effect until I assembled the full board. I`ll do a bit of research, see if I can find a simple solution. Can`t use diodes, because it`s an AC signal (also tested during breadboard prototyping). Probably just use a lower value resistor in the end. As an aside, I really don`t care if people copy anything from this, so I`ll post videos, schematics, anything. If someone wants to rip off the entire project though, send me a message, I`ve got a spare PCB. This was linked to from some other guys site, who was building his own mixer. As such he excluded the footnote that it was not intended for...

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