Electronics 6 Channel Mixer [Design & Building] Project

Hello folks, I'm new to this forum, but a long time viewer of both the site and of the Custom PC magazine. So I thought my first thread would be a project thread of some description. I've had an interest in how things work and more specifically an interest in electronics and electricity from since around 5. I often have ideas that are never realised more often than not because I can't be bothered with the more difficult stages of design and so give up. Hopefully, however I will curb that trend with the production of this project. As they say:

"You get out what you put in"​

Brief: A friend has asked me to build for him a 6-Channel mixer for around £35. I already have some of the circuitry down and initial designs laid out in my head, but here is the basic brief that he has asked me to comply with!

• There should be 6 inputs, each with it's own independent volume control.
• Each input should have treble and bass controls.
• Each input should have an LED or other indicator to show if the signal goes into saturation.
• The mixer will only have 1 output.
• There should be a master volume control, with stereo VU meters setup in a 3dB logarithmic scale.
• It should be strong and able to take some abuse.

That's about it for this initial project post folks, nevertheless I plan to update this thread soon with design details and circuit schematics! Yay! Thanks for reading folks, Bladeware.

 

Update: 15/02/2012 - Initial Schematics & Circuit designs

Ok, folks got a small update today, got the schematics done for the input stage, drawn by hand an scanned in so you'll have to excuse any marks etc...

The red section above handles the initial amplification and volume controls for the first channel, notice how they're are two of these pre-amps, one for the left channel and one for the right channel. Volume will be controlled via VR1 and VR2.

Next the blue section acts as a Baxandall tone EQ, where the top of the two pots controls the bass and lower one the treble. Finally the signal is sent to the purple section, where an op-amp is set up as a comparator, this compares the voltage set by the voltage divider (R5 + R6 for the right channel) with the signal coming from the tone control.

The output will then be sent to the output stage which I will post later.
Because I need 6 inputs the circuitry you see above will be repeated 6 times in all.

Please stay tuned will be updating soon, also if you have any comments or criticisms, please feel free to leave suggestions/advice

 

I'll follow this one dude. I'm working on a valve amp (but 2 channels only) design. Nice start and great challenge when it comes to keep price down .

Take a look at Ti / Burr brown chips, they have great stuff and their website is fully documented.

http://www.ti.com/lsds/ti/analog/audio/audio_overview.page

Good luck mate !

 

Update: 15/02/2012 - Output Stage & VU Meters

- I'll check it out mate, thanks
Ok so, below is the circuit diagram for the output stage:

All the Resistor values are default, and will not be the same in the final schematic. Firstly, the red section is where all of the signals are mixed together, there are two mixing op-amp circuits, one for the left channel and one for the right. Then the combined signals are fed into a second op-amp, this is where the master volume control will be. There are two of these as well, one for the left and one for the right.

Then that mess of resistors, is the resistor network for the VU Meters, the resistors are set-up in parallel so that the correct value can easily be calculated, in order to get the correct voltage for a 3dB step. I might use an IC for these VU meters, because at the moment I'm using 36 Resistors just to set up reference voltages :/

Finally the green section contains all 18 op-amps, these compare the signal with the reference voltages set-up in the section before, and if the signal is larger than the reference voltage the LED lights up, simple right??

I'll post back soon with a parts list and hopefully some photos of the parts too. I'll also hopefully have some sort of enclosure design down by then, thanks for reading folks, advice/tips are always welcomed Bladeware...

 

A few things to take note of:

1) You do not have coupling capacitors on the inputs. This means that your amplified signals are not circuit ground referenced. This causes issues with your clip detector circuit because it references the mid-supply but the AC signal may not necessarily be centred at 0v. Use input capacitors to turn the amplifier into an AC coupled design.

2) You do not need to use resistor ladders to reference the mid-rail. Your circuit ground is the mid-rail. Just connect the inputs of the comparators to circuit ground. See point 1 on AC coupling the input amplifiers.

3) Not all opamps support gains less than unity. You circuit uses an inverting configuration with the pot adjusting the gain of the opamp loop where the voltage gain can drop to below 1x. You can simply use the pots as attenuators in front of a classic non-inverting amplifier design instead.


Edit: I almost forgot, your clip detection circuit only works on the principle that the signal is symmetric and will only flash the led on the positive clip and only for a short period of time. For it to work properly, you actually need 2 opamps. One to compare the compression waveform and another to compare the rarefaction portion. Both should output to capacitor via diodes to allow short clip impulses to be detected by a human eye.
See this circuit: http://www.redcircuits.com/Page132.htm



Here's what I came up with in about 10 min as an alternative to the front-end amplification:

I just used the OPA2134 as an example but cheaper opamps like the NE/ JRC/ NJM 4556 or 4580 or 5532 will work as well.
Voltage gain is set at 3.2X (10.10 dB) which should be sufficient since your supply voltage is fairly low. A typical 2Vrms input will give you ~6Vrms on the output. Just barely under the output saturation limits with a 9v rail without opamps that are rail-to-rail capable.
More gain can be added in the subsequent stages where necessary.




Board layout:

 

Update: 19/02/2012

I'd like to say that I've been busy over the last few days, but that would be lying - Lovefilm has been distracting me . Anyway, I've got round to ordering the parts from Rapid, looks like my budget might be blown unfortunately Components add up, the potentiometers were especially expensive!
Also, thanks to the advice from Dreamslacker I'm in the process of re-drawing my circuit diagram and hopefully will have the new schematics up here shortly. I'll make sure to keep any thread-watchers updated as soon as the parts come in, although I'll probably update it tomorrow with the enclosure designs. So stay tuned folks!

 

Buy the same pots but in bulk, it should be cheaper. Do take due care to make sure you get Audio Log taper pots or sliders!
Most pots are linear and will not offer proper volume control. You can only get away with linear pots if they are used in an inverting configuration and only if the pot is connected as both the Rf and Ri resistance.

Also, it will probably blow your (friend's) budget but having a pot on each channel's output but before the main mixer amplifier is probably a nice touch.


Edit:
A mixer channel ideally comprises of the following:

Input -> Volume control with amplification/ buffer -> Eq with bypass -> Output volume control -> Output panning -> Left & Right inputs of master output stage.