Op Amp Mixer 2.1 version 1

2.1 mixer preamp board that allows two inputs mixed to a single Stereo output and Subwoofer output. For example, great for computer audio input. Has sub output and bass and treble boost cut. I have now tested the sub output and have added that video to this project and my SqueezLite Rover project here. Based on TI OPA1654 op amp but should work with any good opamp. Overall, I am happy with this project result! However, improvements are always welcome. Oh, and video's may be LOUD so turn down your sound before playing!

Also, I am using PTD902 and PTD901 series potentiometers.

J1 and J2 are 50k such as PTD902-2015K-B503. Also, you can just use PTD902-2015K-B103 if your inputs can tollerate a 10k load and most things are just fine with 10k loads. 

VR1 is 100k PTD902 such as PTD902-2215K-B104 with Detent.

VR2 is 10k PTD902 such as PTD902-2015K-B103 

VR3 is 200k PTD902 such as PTD902-2215K-B204 with Detent.

VR6 is 20k PTD902 such as PTD902-2215K-B203 with Detent.

VR7 is 10k PTK901 as it only has one output such as PTD901-1015K-B103.

VR8 is 50k 4 gang pot PTD904-1215K-B503 with Detent.

I think I have completed this project now.

The good: it works and has really good sound.

The bad: After many attempts to isolate digital noise crated by the ESP32 I have decided that DAC onboard of the pre-amp creates some digital noise that is picked up by the preamp opamps. So, after many attempts to clean this up I have decided to remove the DAC and place it on the ESP32 board directly. I have updated the schematics design to reflect this change. Also, I have found that even powering the DAC via this preamp introduces to much noise into these sensitive preamps.

I would go as far to say do not even attempt to run an ESP32 via the same power supply. I have spent hours of time redesigning the power supply of this board and nothing has been able to prevent the ESP32 from injecting digital noise into the preamp. So, this works great for very small and simple I2S micro controllers. However, if you plan on using with an ESP32 wait until I provide the ESP32 Rover and DAC on my Squeeze Rover DAC project I will be posting by Friday of this week. I have spent much time on both of these already and am waiting for the final boad designs on both of these to give an overall review of a really capable preamp with SqueezeLite cababilities. That project is awsome so go check it out here. Also, check out my blog here there is also a post on this project there and another more simple project based on ESPHome here. 

Overall, I am almost done with this project. I am proud of how far I have taken it to get to this point. I know I called it a simple preamp but after much testing it has grown into this project. I concider it a well rounded project that delivers excellent sound when you take into concideration about power supplies and isolation of digital from the sensitive analog preamp stage. So, with proper power supply conciderations this delivers very high quality sound with very little background noise or distortion. 

I have also added this project here for the ESP32 SqueezeLite client. This project adds Bluetooth and SquezeLite to this preamp project. I have also added this post to my alshowto.com site.

Added Extra Features

So, this addition allows one to add other outputs that get the same feed as the final output stage. This allows one to do things like add midrange controls as the overall board is as big as I really want to make it. For example, I am working on a center channel element that will allow center midrange speakers to be added. This is also a great spot to add future filtering or further sub woofer outputs tuned to specific amps and speakers. Very much control can be had with this and I will also be adding input 

modules in the future. For now I am testing the center channel concept out and will add that to the project list once I have ironed out all the bugs.

• GRD - of coarse this is ground for the extra features.
• 15+ - Positive 15 volts to power extra features.
• 15- - Negative 15 volts to power extra features.
• LP - Left Pre Output. This is right before output of final stage that is the final HP filter AKA: LPre in schematic. 
• RP - Right Pre Output. THis is right before output of final stage that is the final HP filter AKA: RPre in schematic.