Hello,
I am trying to build a simple active mixer which has 6 stereo inputs and two outputs. This is my first schematic ever so I really need some feedback.
The idea is to make it portable, designed as a kit so it can easily be assembled by beginners, and fairly cheap to build. The target cost for all the parts should be under 20$. It should have pretty low noise and I would like to use it for playing live music with the Korg Volca range.
My first design uses only one OpAmp and a summing circuit.
The project is listed HERE.
So far this is the schematic:
Please give me some feedback on this design.
R1-R24 are all 10K resistors. By choosing 33K for R35 and R36 I think I should have a 10-13% gain for the signal. Is this math correct?
Would it make a lot of sense to use an OpAmp on each channel or would this be good enough?
What other things should I consider?
I am trying to build a simple active mixer which has 6 stereo inputs and two outputs. This is my first schematic ever so I really need some feedback.
The idea is to make it portable, designed as a kit so it can easily be assembled by beginners, and fairly cheap to build. The target cost for all the parts should be under 20$. It should have pretty low noise and I would like to use it for playing live music with the Korg Volca range.
My first design uses only one OpAmp and a summing circuit.
The project is listed HERE.
So far this is the schematic:
An externally hosted image should be here but it was not working when we last tested it.
Please give me some feedback on this design.
R1-R24 are all 10K resistors. By choosing 33K for R35 and R36 I think I should have a 10-13% gain for the signal. Is this math correct?
Would it make a lot of sense to use an OpAmp on each channel or would this be good enough?
What other things should I consider?
Hi,
Feedback ? You simply have no idea what you are doing.
What is the purpose of R29 to R34 and what should be
for consistent layout the even numbered R2 to R24 ?
rgds, sreten.
Feedback ? You simply have no idea what you are doing.
What is the purpose of R29 to R34 and what should be
for consistent layout the even numbered R2 to R24 ?
rgds, sreten.
@sreten: You are absolutely right. The purpose of this is both learning and making a portable mixer.
Thanks for the feedback.
I corrected the numbering of the resistors and I added an annotation that specifies that the purpose of R29 to R34 is to allow balancing left and right.
Please let me know what other things I got wrong.
https://s32.postimg.org/3ws8y4lzp/mixer_1a.png
Thanks for the feedback.
I corrected the numbering of the resistors and I added an annotation that specifies that the purpose of R29 to R34 is to allow balancing left and right.
Please let me know what other things I got wrong.
https://s32.postimg.org/3ws8y4lzp/mixer_1a.png
I would add DC blocking capacitors on the inputs (<1uF) and maybe diodes to the +/- rails at each inverting op-amp input to protect the op-amps from excessively large input signals.
The outputs of your mixer will be inverted from the inputs, you will need another inverting stage to correct that.
I would increase the supply voltage to a minimum of +/- 9VDC. This is required for output swing, also many op-amps do not do well with such low supply voltages unless designed for it. E
The outputs of your mixer will be inverted from the inputs, you will need another inverting stage to correct that.
I would increase the supply voltage to a minimum of +/- 9VDC. This is required for output swing, also many op-amps do not do well with such low supply voltages unless designed for it. E
You want to go into the kit business with your first ever circuit? Have I misunderstood you?
The output needs ground leak resistors.
The balance control may introduce L-R crosstalk.
Attenuating a signal just to amplify it again introduces noise.
No need for output volume controls?
The output needs ground leak resistors.
The balance control may introduce L-R crosstalk.
Attenuating a signal just to amplify it again introduces noise.
No need for output volume controls?
Hi,
What is the point of balancing L+R on each input if you cannot
balance the levels between the inputs ? That as a minimum
is what a mixer does, far more important than balance.
What you have designed is not what most people want at all.
Possibly someone with lots of keyboards or old midi sources
might want something like it, but it is pretty hopeless IMO.
As it is, its a fixed level combiner with pointless balance controls.
20$ is woefully underestimating the cost of building it.
FWIW the parallel double outputs are worthless.
You have a lot to learn about music equipment.
rgds, sreten.
What is the point of balancing L+R on each input if you cannot
balance the levels between the inputs ? That as a minimum
is what a mixer does, far more important than balance.
What you have designed is not what most people want at all.
Possibly someone with lots of keyboards or old midi sources
might want something like it, but it is pretty hopeless IMO.
As it is, its a fixed level combiner with pointless balance controls.
20$ is woefully underestimating the cost of building it.
FWIW the parallel double outputs are worthless.
You have a lot to learn about music equipment.
rgds, sreten.
Last edited:
Lithium ion batteries are expensive and require special arrangements for charging, not in scope for the stated budget.
Not sure if your input is an attempt at constant power panning, and unless your sources all have output level controls you may have an issue with level from input to input.
Don't forget you need decoupling on the op-amp supply pins.
Not sure about your choice of op-amp. $20 per board with PCB included puts it in the realm of difficult if not impossible unless the volume is high.
Not sure if your input is an attempt at constant power panning, and unless your sources all have output level controls you may have an issue with level from input to input.
Don't forget you need decoupling on the op-amp supply pins.
Not sure about your choice of op-amp. $20 per board with PCB included puts it in the realm of difficult if not impossible unless the volume is high.
I just bought 5 off mixer pcb's for £73, so that's £15 just for the pcb.
Your mixer really needs volume control pots on all the inputs.
Needs at least +/- 9 volt rails.
Needs coupling capacitors on the input.
It will need an enclosure which aren't cheap.
I bought in a USB mixer for £50.
I am now having a go at making my own simple USB/mic mixer and cant see me getting change out of £75.
You cant compete with mass produced items.
Last edited:
Thank you all for the feedback.
Just to make it clear, I don't want to go into the kit business. This is a hobby for me and designing it as a kit might benefit other who would like to make the boards and put this together at home if we manage to make it decent. The main purpose is to learn stuff along the way.
Now I have more questions than answers because I don't understand the reasoning behind it all and the only way to learn is to get why it's like this. I started making the changes but before posting the next revision I would like to get some clarifications.
1. Why add DC blocking caps on the input if I already have some on the output?
If I add them on the input should I also add them on the output?
2. The output is inverted but in this case it shouldn't be an issue. Right?
3. Why does the output need ground leak resistors?
4. How do I figure out if the balance control introduces L-R cross-talk?
5. Why do I need decoupling for the OpAmp supply?
6. Why are the parallel double outputs worthless? What is a decent way to outputting to both speakers and headphones in the rare situations where that would be preferred?
7. What is the problem with using LiPo batteries here, considering that LM833NG works with voltage that low? Don't I need less than 2V on the output?
Also I expect the PCB to be the most expensive part. If the PCB will work OK, I will also make a plexiglass enclosure design that can be easily laser cut pretty much in every city.
All the input sources will be Korg Volcas or similar so they will all have their own volume controls. Do I really need an additional volume control on each input in that case? I am aware that I need at least a master volume control and I will add it in the next revision.
8. As far as I know, all commercial mixers have an OpAmp on each input and then an additional OpAmp on the output, after the summing circuit. How would it help if I also add individual OpAmps here? Would it be a big difference in practice?
Just to make it clear, I don't want to go into the kit business. This is a hobby for me and designing it as a kit might benefit other who would like to make the boards and put this together at home if we manage to make it decent. The main purpose is to learn stuff along the way.
Now I have more questions than answers because I don't understand the reasoning behind it all and the only way to learn is to get why it's like this. I started making the changes but before posting the next revision I would like to get some clarifications.
1. Why add DC blocking caps on the input if I already have some on the output?
If I add them on the input should I also add them on the output?
2. The output is inverted but in this case it shouldn't be an issue. Right?
3. Why does the output need ground leak resistors?
4. How do I figure out if the balance control introduces L-R cross-talk?
5. Why do I need decoupling for the OpAmp supply?
6. Why are the parallel double outputs worthless? What is a decent way to outputting to both speakers and headphones in the rare situations where that would be preferred?
7. What is the problem with using LiPo batteries here, considering that LM833NG works with voltage that low? Don't I need less than 2V on the output?
Also I expect the PCB to be the most expensive part. If the PCB will work OK, I will also make a plexiglass enclosure design that can be easily laser cut pretty much in every city.
All the input sources will be Korg Volcas or similar so they will all have their own volume controls. Do I really need an additional volume control on each input in that case? I am aware that I need at least a master volume control and I will add it in the next revision.
8. As far as I know, all commercial mixers have an OpAmp on each input and then an additional OpAmp on the output, after the summing circuit. How would it help if I also add individual OpAmps here? Would it be a big difference in practice?
Hi,
I am not sure if I am the right person to answer your questions since I am not an expert myself, but I'll try to help you as far as my limited knowledge allows...
Now to your questions...
1. What I learned on this forum is that even a small DC across the potentiometer wiper would result in scratching sound when you turn it. Input capacitors also help to protect your mixer from the faulty sources or those which have excessive DC offset on their outputs which can then be summed up and amplified by your mixer.
2. I don’t know how important this is for the audio output, but when it comes to the mixer side it may be a problem. You can read more about it here.
3. I guess it is needed to prevent charging of the output capacitor if the output is floating (when nothing is plugged into it).
4. No idea at the moment but am interested to learn too.
5. The answer is too long and it is better if you read it by yourself, e.g. here. Also in the op-amp datasheets manufacturers typically give design guides and recommend proper decoupling for their op-amps.
6. I do not see the problem in having dual outputs, the problem is in their implementation.
7. Most of op-amps perform best if they are given a lot of voltage headroom. In many cases it means using +-12V rails and higher for line level signal processing. Even if the op-amp is specified to run from as low as +-2.5V supply it does not mean that it will perform well at such a low supply voltage.
8. Trust it. Add those op-amps to all inputs. You have to isolate inputs from each other and from the following stage. Using op-amp buffers at the inputs will help you doing it. In your case (as it is shown in your schematic) all inputs are connected together and then to the inverting input of the op-amp which means that any impedance change on any of the inputs (e.g. you plug something in or turn your balance potentiometers) will change the gain of the inverting amplifier that you have. I am also curious how did you arrive at 10-13% gain in your configuration!? I strongly recommend you to have a read on how to use op-amps and how they operate. Even if they look like lego bricks to you they are actually not… You really need to get some understanding before you can advance further with your project. And even if you succeed to come up with a good schematic, designing a PCB for it is another challenge. Just connecting the points together on the PCB following the schematic would not work. Maybe you should start from a similar existing design and try to understand why it is done the way it is.
I hope that more knowledgeable forum members would correct me if I misinform the OP in my answers.
One advice, if you have a question and you don’t know where to start looking for answers, just type it in google and see what it gives. You’ll be surprised how often you see the answer to your question on the first page of the search results…
Regards,
Oleg
I am not sure if I am the right person to answer your questions since I am not an expert myself, but I'll try to help you as far as my limited knowledge allows...
Now to your questions...
1. What I learned on this forum is that even a small DC across the potentiometer wiper would result in scratching sound when you turn it. Input capacitors also help to protect your mixer from the faulty sources or those which have excessive DC offset on their outputs which can then be summed up and amplified by your mixer.
2. I don’t know how important this is for the audio output, but when it comes to the mixer side it may be a problem. You can read more about it here.
3. I guess it is needed to prevent charging of the output capacitor if the output is floating (when nothing is plugged into it).
4. No idea at the moment but am interested to learn too.
5. The answer is too long and it is better if you read it by yourself, e.g. here. Also in the op-amp datasheets manufacturers typically give design guides and recommend proper decoupling for their op-amps.
6. I do not see the problem in having dual outputs, the problem is in their implementation.
7. Most of op-amps perform best if they are given a lot of voltage headroom. In many cases it means using +-12V rails and higher for line level signal processing. Even if the op-amp is specified to run from as low as +-2.5V supply it does not mean that it will perform well at such a low supply voltage.
8. Trust it. Add those op-amps to all inputs. You have to isolate inputs from each other and from the following stage. Using op-amp buffers at the inputs will help you doing it. In your case (as it is shown in your schematic) all inputs are connected together and then to the inverting input of the op-amp which means that any impedance change on any of the inputs (e.g. you plug something in or turn your balance potentiometers) will change the gain of the inverting amplifier that you have. I am also curious how did you arrive at 10-13% gain in your configuration!? I strongly recommend you to have a read on how to use op-amps and how they operate. Even if they look like lego bricks to you they are actually not… You really need to get some understanding before you can advance further with your project. And even if you succeed to come up with a good schematic, designing a PCB for it is another challenge. Just connecting the points together on the PCB following the schematic would not work. Maybe you should start from a similar existing design and try to understand why it is done the way it is.
I hope that more knowledgeable forum members would correct me if I misinform the OP in my answers.
One advice, if you have a question and you don’t know where to start looking for answers, just type it in google and see what it gives. You’ll be surprised how often you see the answer to your question on the first page of the search results…
Regards,
Oleg
Last edited:
You probably want to protect your circuit from whatever feeds it. You ought to want to protect whatever follows it. Only if you can be absolutely certain that adjoining items have DC blocking caps, and you will never do any 'hot' switching, can you remove coupling caps.rbt said:
Up to you. Some people like to maintain absolute phase.
So you don't get loud clicks when plugging things together.
If there is any resistance in the slider-track interface (which there almost certainly will be) then this type of balance control will introduce crosstalk. For an uncritical application this might not matter too much.
You want your opamps to remain stable?
Hi,
If your using the volume controls of all the inputs, then
your box is really not a real mixer, its more a combiner.
So all your stereo inputs have no pan/balance option ?
Why would you want a headphone output ?
That goes on the main mixing desk.
TBH all you need for 6 stereo in / 1 stereo out
is an entirely passive resistive combiner/mixer.
It doesn't even need a PCB, a doddle to hardwire.
rgds, sreten.
If your using the volume controls of all the inputs, then
your box is really not a real mixer, its more a combiner.
So all your stereo inputs have no pan/balance option ?
Why would you want a headphone output ?
That goes on the main mixing desk.
TBH all you need for 6 stereo in / 1 stereo out
is an entirely passive resistive combiner/mixer.
It doesn't even need a PCB, a doddle to hardwire.
rgds, sreten.
Hi,
Adding in practice the entirely passive resistive combiner/mixer
will not need any coupling capacitors, its just a box with all the
sockets and a bunch of resistors and solid wire hardwired.
rgds, sreten.
Adding in practice the entirely passive resistive combiner/mixer
will not need any coupling capacitors, its just a box with all the
sockets and a bunch of resistors and solid wire hardwired.
rgds, sreten.
Hello again,
Thanks for all the feedback!
I took some of the advice and added coupling caps on the input and decoupling caps to the OpAmp, grid leak resistors and a main volume control.
I would like to have more than just a passive resistive combiner because that attenuates the signal too much and it's not able to drive headphones enough.
I want a headphone out because the point is not necessary to have this mixer board in a professional environment. I am looking more to have it as a mobile solution for summing the output of several Korg Volca synths. Still, I would be able to pan each of the inputs and not attenuate or distort the signal at a level that is audible.
I also considered adding 6 more OpAmp chips for each stereo channel but I am not sure the benefit is worth adding the extra complexity and wasting that much more power.
I know that laying out a board is not easy especially since I never did it but I need to start somewhere.
What else should I fix before attempting to layout the board?
Thanks for all the feedback!
I took some of the advice and added coupling caps on the input and decoupling caps to the OpAmp, grid leak resistors and a main volume control.
I would like to have more than just a passive resistive combiner because that attenuates the signal too much and it's not able to drive headphones enough.
I want a headphone out because the point is not necessary to have this mixer board in a professional environment. I am looking more to have it as a mobile solution for summing the output of several Korg Volca synths. Still, I would be able to pan each of the inputs and not attenuate or distort the signal at a level that is audible.
I also considered adding 6 more OpAmp chips for each stereo channel but I am not sure the benefit is worth adding the extra complexity and wasting that much more power.
An externally hosted image should be here but it was not working when we last tested it.
I know that laying out a board is not easy especially since I never did it but I need to start somewhere.
What else should I fix before attempting to layout the board?
- Status
- Not open for further replies.