Hey all! We have to build a simple audio mixer circuit for my electronics class. I've designed the circuit and tested it using EasyEDA.com and it appears to work fine. However, when I built the circuit (rebuilt it twice now) on a breadboard it seems to be outputting a much lower voltage than I am expecting. I've uploaded a pic of the simulated circuit. I used an AC source for each "mic" right now and replicated that in person when I tested it today. With the current settings (0.07V amplitude @ 5kHz), in the sim I get around 10V. In real life I am getting a little under 5V. Does anyone see anything wrong with the schematic in the picture? Is there something I am not seeing? Thank you for helping!

 

Are the opamps in your breadboard circuit powered by ±15V or some other voltages?

 

They're powered by +-15V from this myDAQ. http://www.ni.com/en-us/shop/select/mydaq-student-data-acquisition-device

 

The maximum current specification for those supplies is only 32mA and you have three LEDs with only 50Ω current limiting resistors so you are probably overloading the supplies. Measure the actual supply voltages.

Increase the LED resistors to 10k. This will keep the LED current at about 1mA each - quite bright enough to see.

 

The maximum current specification for those supplies is only 32mA and you have three LEDs with only 50Ω current limiting resistors so you are probably overloading the supplies. Measure the actual supply voltages.

Increase the LED resistors to 10k. This will keep the LED current at about 1mA each - quite bright enough to see.

What are you measuring the output with. You don't mention whether the output value is RMS, Pk, or Pk/Pk.

 

What are you measuring the output with. You don't mention whether the output value is RMS, Pk, or Pk/Pk.

Sorry. Should be 140mV peak to peak input for each channel and I’m getting around 8Vpp in the output

 

With the pots shown, the input amps could have a gain of anywhere from zero to 50. the summing amp could have a gain anywhere from zero to 1. Trying to define the expected gain without specifying the set point of those pots is impossible. Beyond that, get rid of the LEDs or at least bump the LED series resistors to a much larger value - 5K to 10K would be good.

 

With the pots shown, the input amps could have a gain of anywhere from zero to 50. the summing amp could have a gain anywhere from zero to 1. Trying to define the expected gain without specifying the set point of those pots is impossible. Beyond that, get rid of the LEDs or at least bump the LED series resistors to a much larger value - 5K to 10K would be good.

The measurements I took were with the pots at max. So 100k for all of them. Thank you all for your responses! We have to have the LED’s there, so I’ll change the resistors before them out with a 10k!