Hi guys,
This is my first post here and also my first DIY audio build. Looking forward to the challenge.
I am looking to build a relatively simple mixer that takes 2 x stereo line level RCA inputs, run them through trim knobs, gain knobs and an LED for each input and then run them through a rotary crossfader and out the other side as a line level RCA output.
The most complex bit in the whole thing is the rotary crossfader, but I figure I need a 4-gang log potentiometer somewhat like this: http://www.mouser.com/ds/2/54/TD90-777943.pdf
I'll also need the following:
3 x stereo RCA PCB connectors
2 x 2-gang trimpots
2 x 2-gang potentiometers
2 x LED
I want the unit to be able to take inputs from DJ mixers or iPhones or iPods for instance and send the output to a power amplifier.
I have the following issues:
- I don't know what value pots I need to get for any of them. Do I need 100k Ohms, 20k, etc? What is going to get me good quality audio output?
- I don't know which brands to buy?
I hope I can get some help here.
Thanks,
Ryan
This is my first post here and also my first DIY audio build. Looking forward to the challenge.
I am looking to build a relatively simple mixer that takes 2 x stereo line level RCA inputs, run them through trim knobs, gain knobs and an LED for each input and then run them through a rotary crossfader and out the other side as a line level RCA output.
The most complex bit in the whole thing is the rotary crossfader, but I figure I need a 4-gang log potentiometer somewhat like this: http://www.mouser.com/ds/2/54/TD90-777943.pdf
I'll also need the following:
3 x stereo RCA PCB connectors
2 x 2-gang trimpots
2 x 2-gang potentiometers
2 x LED
I want the unit to be able to take inputs from DJ mixers or iPhones or iPods for instance and send the output to a power amplifier.
I have the following issues:
- I don't know what value pots I need to get for any of them. Do I need 100k Ohms, 20k, etc? What is going to get me good quality audio output?
- I don't know which brands to buy?
I hope I can get some help here.
Thanks,
Ryan
Attachments
Last edited:
For driving the led you need some sort of circuit. There must be some opamps or transistors. Why don't you just get a line mixer and modify the crossover or gain controls ?
I want to use the audio input to drive the LEDs. But yeah - I will need something like a resistor right - so that the LED's light up at approximately 90% of full input voltage.
So if consumer line level is -10 dBV, and peak amplitude is 0.447 volts, then 90% of peak would be .4023 volts. Is that correct? Am I going about this the right way?
No. I'm not going to do that - for a variety of reasons including that I want this to be my own design and not a mod and also that it be a passive mixer.
Input and output impedances
Just doing some more research now.
So, as I understand it, the input impedance needs to be much lower that the output impedance for current to flow through the circuit. Is my understanding correct?
And then, the input impedance needs to be lower than then output impedance of whatever is going to be plugged into this mixer, for instance an iPod. But that shouldn't be an issue should it? It will be quite easy to make the impedance of my mixer as I want it.
The issue seems to be: The output impedance of my mixer needs to be lower than the input impedance of the power amplifier that my mixer is going to run to.
How do I make the output impedance of my mixer as low as possible but still keep it passive?
Just doing some more research now.
So, as I understand it, the input impedance needs to be much lower that the output impedance for current to flow through the circuit. Is my understanding correct?
And then, the input impedance needs to be lower than then output impedance of whatever is going to be plugged into this mixer, for instance an iPod. But that shouldn't be an issue should it? It will be quite easy to make the impedance of my mixer as I want it.
The issue seems to be: The output impedance of my mixer needs to be lower than the input impedance of the power amplifier that my mixer is going to run to.
How do I make the output impedance of my mixer as low as possible but still keep it passive?
No need to think about how I am going to power it. I want to use this design for sound system installations in restaurants where - the less cables you can run, the better and easier the installation.
Thus far this thread appears to be an attempt to get us to do the design!djryanash said:
You have two ways forward:
1. find something suitable and buy it
2. learn enough about electronics to do it yourself
We can assist with both of these.
As this is DIYaudio.com, let us assume for the moment that you want to do it yourself. Do you understand potential dividers, and volume controls? I mean understand, not just know that they exist. You know how to calculate input and output impedance of a volume control? If not, that is where to start your reading.
Your understanding of impedance requirements is mixed up. Generally you want the output impedance of a stage/device to be much lower than the next stage/device's input impedance. The higher the output impedance, the more the interconnect cable's parasitic capacitance and the next device's input capacitance will interact to form a low pass filter. This could lead to rolled off highs.
So, yes, the output impedance of the mixer needs to be low to drive your amplifier's input. The issue with using your passive trim pots, master gain and cross fader (why do you want three controls for adjusting level of two sources?) is that each one effectively increases the source impedance seen by the amplifier. A better engineering solution would be to insert an active buffer after each pot, and depending on the length of the cables between sources and mixer an input buffer. A buffer is a circuit that typically has a high input impedance and a low output impedance. It can be as simple as a single transistor emitter/source follower or an op amp set for unity gain. There are also integrated circuits designed specifically as buffers.
Of course this gets you into power supplies and active circuits, but you need that anyway. That's because to be bright enough to be useful the LEDs would need too much current to be driven passively and they would impact the waveform. You need a high impedance buffer to drive them.
Take a look at Rod Elliot's site for circuits that you can use to meet your design goals. High Quality Sound Mixer
So, yes, the output impedance of the mixer needs to be low to drive your amplifier's input. The issue with using your passive trim pots, master gain and cross fader (why do you want three controls for adjusting level of two sources?) is that each one effectively increases the source impedance seen by the amplifier. A better engineering solution would be to insert an active buffer after each pot, and depending on the length of the cables between sources and mixer an input buffer. A buffer is a circuit that typically has a high input impedance and a low output impedance. It can be as simple as a single transistor emitter/source follower or an op amp set for unity gain. There are also integrated circuits designed specifically as buffers.
Of course this gets you into power supplies and active circuits, but you need that anyway. That's because to be bright enough to be useful the LEDs would need too much current to be driven passively and they would impact the waveform. You need a high impedance buffer to drive them.
Take a look at Rod Elliot's site for circuits that you can use to meet your design goals. High Quality Sound Mixer
Hold your horses pal!! I'm not trying to get you to do the design. As the thread says, "Help to design and build a passive input mixer".
1. I want to do it myself so that I can get my head around how it all works. I don't understand a potential divider (I haven't heard of that) but I think I understand how a volume control works: It's two resisters that change their resistance with a knob or slider and therefore output the same or less of the original input voltage. That's the best way I can explain.
2. That's exactly what I'm trying to do - learn about electronics. That's why I'm here. I was hoping some helpful souls can point me in the right direction. And I'm also going back and relearning the maths and science I missed in school and well as concurrently taking an online electronics course. I figured the best way to learn is to actually make something and I figure this is a good start because it's something that I want to make and it seems relatively simple.
I didn't study any of this at school as I am sure many of you did. I had a crazy childhood that didn't afford me the opportunity to finish high school so gimme a god damn break here!!! I'm trying my best to get this stuff but I don't even know where to start because I'm not eve sure what I don't know and there is so much information out there that I'm trying to separate the wheat from the chaff with all this.
Glad you're eager to learn. I'm one of the people here who learned about electronics on my own. It's a fun hobby. The Elliot Sound Products site I linked above is a good place to start the process. He does a good job of explaining how things work and introducing the math involved.
Another place I learned a lot was https://www.passdiy.com/project Mr. Pass is a master of simplicity, and loves to teach. You'll notice that Rod Elliot doesn't agree with Nelson Pass on the Zen concept, with his own "Death of Zen" project. Build both and see which philosophy sounds better to your ears. I've built a number of amps. Pass designs tend to be my favorites, although it's about subtle differences.
Once you get a basic understanding, read the troubleshooting threads. For me that was where I really started to understand what was going on. How would I fix that? What was recommended? What worked?
If you look at a professional mixer you'll see that it is a bunch of modules mounted to a backplane. Rod Elliot's project gives you blocks you can string together to meet your needs. You can usually omit some of the functions in each block if you don't want it. Post your ideas here and there are lots of us ready to help.
Happy soldering!
Another place I learned a lot was https://www.passdiy.com/project Mr. Pass is a master of simplicity, and loves to teach. You'll notice that Rod Elliot doesn't agree with Nelson Pass on the Zen concept, with his own "Death of Zen" project. Build both and see which philosophy sounds better to your ears. I've built a number of amps. Pass designs tend to be my favorites, although it's about subtle differences.
Once you get a basic understanding, read the troubleshooting threads. For me that was where I really started to understand what was going on. How would I fix that? What was recommended? What worked?
If you look at a professional mixer you'll see that it is a bunch of modules mounted to a backplane. Rod Elliot's project gives you blocks you can string together to meet your needs. You can usually omit some of the functions in each block if you don't want it. Post your ideas here and there are lots of us ready to help.
Happy soldering!
Ah right. Thanks for clearing that up for me. 🙂 You're a great help!!! They should promote you to General. 😉
I wanted the trim pots to set a good overall level for the inputs (at the back of the unit); the master hair for adjusting the level for like "Hey, it's too loud in here. Could you turn it down please?"and the crossfader for - well - fading seamlessly between the two inputs. All restaurants/bars I'm worked in, they have to turn the music off before the DJ starts playing i.e. no smooth transition between iPod and DJ and customers notice these things.
I read about this 'active buffer' thing when I was reading about op amps. With the 'single transistor emitter/source follower' type, would that still allow me to maintain this whole circuit as passive? Or does that mean going 'active'? I might have to concede that active is the way to go if that is the case. Is this the type of thing you're talking about: https://wiki.analog.com/university/courses/electronics/text/chapter-9
The cable length we're looking at for running to the input mixer could be anything up to 30 or 40 meters. Best to plan for 50 meters of cable in a "worst case scenario". Is that considered a "long" cable run? It's kind of standard when doing sound system installs in bars, etc.
Right...okay. So active it is then. So what are my options with that? I suppose a 9 volt battery would be a bad idea? Hahaha. I'm looking for the easiest and most hassle-free way to supply power.
Great. Let me dig around that site for a bit and see what’s happening.
Thank you sooooo much for your help. I feel like I’m stumbling around in the dark here so this kind of information and feedback is just so critical to my learning process. I’m gonna get this done and man - I’m excited. 🙂
30m to 50m of audio cabling in a commercial environment where paying customers will not be too happy with unwanted interference coming in through speakers, virtually demands a balanced impedance connection system.
Don't go chasing customers away with a cobbled together DIY attempt at a musak system better suited to a kid's bedroom.
Don't go chasing customers away with a cobbled together DIY attempt at a musak system better suited to a kid's bedroom.
I can see your concept for input trim. What are your sources? Most modern sources are similar levels. While we are at it, why a mixer rather than source selector? Play along with your favorites?
The single transistor buffer is what is shown in 9.4 on your link. That's active. What's your objection to active?
You could power most opamp circuits from a pair of 9V batteries if you wish. There are also regulated power supply kits or bare boards available from a number of sources. The Super Regulator in the DIYAudio Store is probably overkill, but any of the EBay +/-15 V supplies will do for learning.
The peak indicator fig 8 Audio Mixing Console - Part 1 is a pretty basic circuit. It uses any dual op amp as a comparator - the resistors R1-3 provide a reference voltage. Once the signal exceeds that (either positive or negative) one of the opamp outputs goes high and turns Q1, which lights the LED. Q1 can be any general purpose NPN
The single transistor buffer is what is shown in 9.4 on your link. That's active. What's your objection to active?
You could power most opamp circuits from a pair of 9V batteries if you wish. There are also regulated power supply kits or bare boards available from a number of sources. The Super Regulator in the DIYAudio Store is probably overkill, but any of the EBay +/-15 V supplies will do for learning.
The peak indicator fig 8 Audio Mixing Console - Part 1 is a pretty basic circuit. It uses any dual op amp as a comparator - the resistors R1-3 provide a reference voltage. Once the signal exceeds that (either positive or negative) one of the opamp outputs goes high and turns Q1, which lights the LED. Q1 can be any general purpose NPN
A normal volume control is a variable potential divider. That is why I said you need to start by learning about potential dividers. I am trying to be helpful! You want to design it yourself, so first you need to learn enough to ask the right questions then you need to learn about the answers. There isn't a short cut.djryanash said:
So learn about potential dividers, which are an application of Ohm's Law. Then learn about cable capacitance and first order filters. That will tell you whether a passive design is possible for your application. No point in designing a perfect passive mixer if it simply can't do the job you want done. My guess is that 50m is far too long for a passive design, but that might depend on what audio quality you can accept.
Yeah - I was hoping to get away with the minimum permissible, but everyone here is making it clear that I need to think bit bigger.
I understand what balanced means - from a sound engineering viewpoint, but I have no idea the theory behind it or how to implement it into a circuit. Do you know of a place I could do some reading online to get me started?
The Rod Elliot link that @BobEllis posted gets going on it, but even that is over my head at this point.
D.Self, ESP, Jensen, Whitlock, Rane and many others give sufficient information to get you up to speed on balanced impedance connections.
But note it is the impedance that is balanced, not the signal.
But note it is the impedance that is balanced, not the signal.
So I guess I should really just get into building stuff now and take it out of the realm of theory and into practicalities. So that would mean building some passive elements such as resistors, capacitors, etc and maybe some op amps, etc. I live in Hong Kong and shipping from part-online will take awhile, so I'd like to do it in one batch.
What do you suggest I order to get me started with building stuff? Would be greta if I could order a few things that would all work together but can be easily rearranged so I can try out many different circuit ideas.
I work out of a shared space called MakerBay and they have loads of resistors and capacitors and a few different types of integrated circuits - but not much else - and I think it's all pretty cheap stuff. Would I be able to get started with that? In which case, what do I need to unseat in? I I got my own DMM and I'm borrowing a Fluke oscilloscope from a colleague. There are breadboards there and clips, etc.
Anything else I need?
I think I just build some schematics that I find on some of the websites you suggested above?
Or maybe I should stay in the theoretical domain and just try out building some of these schematics in CircuitLab [https://www.circuitlab.com] for now?
Well, as this thing becomes more complex, my concept is changing. Originally I just wanted to keep it as simple as possible so as to get an idea of how it would work. But as things progress, it is becoming more complex (passive to active) to accommodate long cable runs, impedance differential, signal quality, etc.
So originally I just wanted 2 x stereo unbalanced RCA inputs. Now I'm thinking 2 x balanced TRS inputs - which would be able to accept unbalanced as well.
One input will more than likely be a DJ mixer, so that's +4 dBu and the other might be pro level or an iPod i.e. -10 dBV. I've read these definitions many times and still don't fully understand what they mean. But I know they sound - in terms of level - comparatively.
So how do you make that distinction? For me, "source selector" means a push-button unit that selects each source discretely so that no two sources can be heard simultaneously. A mixer on the other hand "mixes" two our more sources together. I'm not being facetious - just making sure that we are on the same page as it were. So I have called my unit a "mixer" for that reason.
I no longer have an objection because I've become aware that it will have to be the route to go. I originally wanted to not have to supply a power source because that would make installing the unit sooooooooo much easier on-site. But that's clearly not gong to be an option.
I wonder how long the batteries would last for?
Wow - so now I need to add a power supply kit and a new circuit board into the equation? This is going to take me forever to build!!! I don't think I'll ever get it done. I have to add so many more components - and while it may seem elementary to you gentlemen, every new thing that I add, I have to research how it works and oftentimes that means going right back to high school maths!!! It's just an endless road to travel on.
I didn't even know there was a DIYAudio Store. Hah - good to know.
What's a "comparator"?
Why do I need a "reference voltage"? I kind of get what's going on - I think. I mean, I just thought that you pass a voltage along a wire with a resistor on it and if the amplitude of the voltage exceeds the rating of the resistor, the excess voltage get passed on to an LED further along the wire which lights up. I though it would be waaayyyy simpler - voltage source (audio signal), resistor, LED, ground. But this has like 2 x op amps, 6 resistors, capacitors. Man - this is crazy. And this is just trying to light the LED.
Maybe I'm in the wrong forum. The more I read here, the dumber I feel.
- Status
- Not open for further replies.