DIY 3 Channel Active Mixer

[daleykd]

Hey guys!

Though it's not exactly a stomp box, it would be part of my pedal board.  Plus, I know that when it comes to DIY circuits for audio, this is the place to ask.

I'm trying to build a "personal monitor system," based largely on the Rolls PM351 (a schematic can be found by doing a quick Google.  I don't want to break any forum rules by posting a link, unless I'm allowed to).  I have also looked at various other simple active mixers, trying to grasp the concept (like http://www.all-electric.com/schematic/simp_mix.gif).

It will have three channels: Mic input, instrument (guitar/keyboard) input, and monitor mix (speaker level OR line level input).  Then, it'll be mixed and output to set of headphones for IEM.

Since this is going to be my first DIY project involving a more complex schematic and a PCB, I'm trying to simplify the Rolls schematic by making it a mono signal all the way.  I have no need for anything stereo.  Also, the Rolls input for the instrument seems to use a transformer and a lot of work to make it's bus also output to a XLR and mimic a DI.

Here are my first sets of newbie questions:
1) If I don't create a DI circuit in my box and instead, use an external DI (use a paraout for my instrument in which goes to the DI), is there a chance I'll have a feedback loop when the DI goes to the board and comes back to my mixer via the monitor mix line?
2) The Rolls looks like it uses 1 uF caps after the pot on the monitor mix circuit, while I've seen many other schematics use .1 uF before the pot.  What's the deal with the different placements and different values?  How do I determine the appropriate value for these caps?
3) Rolls uses 100k ohm pots, which seems awfully high to me.  The simple mixer above only uses 10k pots.  Thoughts?
4) Rolls uses a 4560 op amp for the mic input channel.  Do dual op amps still move your signal out of phase 180 degrees, or does the dual mean that it flips it back?

I'm sure I have a bunch of other questions, but when I've been learning SO much over the last couple of months, my brain is about to explode with no one I know to ask.

Thanks in advance for any answers you can provide.  I'm looking forward to designing the circuit and posting it here for your input.

[tombaker]

Howdy,
I wont go into too much detail as my knowledge base isnt that concrete but a dual opamp means there are two opamps in the one dip8 IC. Both have an inverting and non-inverting input, output and theres a VCC and VEE input.
It means that if your circuit calls for two opamps you only need (presumably) one Dip8 IC to complete it.

[GibsonGM]

1) If I don't create a DI circuit in my box and instead, use an external DI (use a paraout for my instrument in which goes to the DI), is there a chance I'll have a feedback loop when the DI goes to the board and comes back to my mixer via the monitor mix line?
2) The Rolls looks like it uses 1 uF caps after the pot on the monitor mix circuit, while I've seen many other schematics use .1 uF before the pot.  What's the deal with the different placements and different values?  How do I determine the appropriate value for these caps?
3) Rolls uses 100k ohm pots, which seems awfully high to me.  The simple mixer above only uses 10k pots.  Thoughts?
4) Rolls uses a 4560 op amp for the mic input channel.  Do dual op amps still move your signal out of phase 180 degrees, or does the dual mean that it flips it back?

1) Don't know exactly what you mean; if you attenuate the signal correctly and do this right, it shouldn't feed back - but your setup may vary or have something set up which would alllow this.  Gotta try it, I think!

2) Input caps block any DC from getting into your circuit, and also from coming OUT of it (via bias networks that carry DC in many circuits).   The value determines how much bass your circuit sees.  1uF is 'generic', allowing most bass freq's thru.  .1uF would trim some.  The formula for half-power cutoff is 1/2pi*R*C (keep studying!).   You can choose these by ear most of the time using an educated guess as a starting point.

3) The pots are chosen partly because they help set the input impedance to the opamps.  Their value, in conjunction with the input impedance of the device used, determines total impedance.  Higher value pots also introduce more noise to the circuit.  I'd try to design around a lower pot, see if it does what you want.

4) Dual opamps are like Tombaker said...2 separate ("discrete") opamps in 1 package, that are independent of each other.   They can be set up to work inverting or non-inverting.  If you're going to be running your signals out of boxes and back INTO them for PA use etc., you do need to understand and consider if your signal is being inverted or otherwise phase shifted!  Capacitors will add phase shift, too! 


I would plan to build 2 input "types": line and instrument level.  The difference is basically a resistive voltage divider (simple).  Keep this simple, and you'll probably do alright given the tons of stuff to read up on to get you there!  Don't overdo it...and make sure you learn the concepts as you go along 

[daleykd]

Hey guys.

Thanks for the response.  That's exactly the info I needed to get past my first hump, and it made absolute sense, too!

Here's my first revision of my mixer.  Again, I've used the Rolls PM351 as the main schematic, tweaked slightly for mono signal and no built-in DI.  I'm missing the power circuit because honestly, I don't understand why Rolls' is more complicated than http://www.generalguitargadgets.com/diagrams/mixer_sc.gif.

1) How is it so far?
2) If the opamps are correct, and I only need two, then I could get away with a single dual opamp.  Rolls uses the 4560 and 4556 in their products, and I've got three samples of the RC4560 on order from TI.  Could I use only this one opamp?
3) (The picture is missing pin 8 from the opamp going to Vcc.)
4) The Rolls PM351 and PM50s has a 1N400X diode in the power circuit immediately after the DC +.  What's the purpose of this diode, since the Mini/Simple Mixer doesn't seem to have one?
5) I thought VBIAS was 4.5V, but that it required equal resistors to obtain that.  The Rolls PM351 doesn't use equal resistors in the power circuit.

Thanks again for your help.  I feel like I'm ALMOST there!

[GibsonGM]

1) at first glance, looks good!  I didn't trace it, tho (time...). Someone will shortly I bet!  

2) Yes, all you need is one dual opamp to have a stereo setup (2 separate channels).  One caveat...J.D. designed the Mini thing with a tL072 in mind, and knows it WILL work with that unit.  If you sub a different opamp in, get the DATA SHEETS for BOTH types.  You have to compare the pinouts for each type of opamp, and be sure they're the same, or that you build the circuit around the difference, or it will not work!  The TL07X series is the standard for most DIY stuff, so be sure to get a bunch for later use!    Now, I'm not sure what you're really getting at...the Rolls may not work right if you don't use the unit they call for with the biasing they want (without tweaking it).   You should use the parts listed wherever possible.    That doesn't mean don't try, but you will have to think about what's going on as you go forward (or risk frying something or having it just not work).  My gut says to go with Jack's circuit as-is, and get some 072's!  THEN, tweak it to your liking.  

4) a diode right after the "+" is generally there to protect the circuit from being connected to a power supply (wall wart, etc) with the opposite polarity (ie, backwards).  If current flows in the opposite direction, as it will if you plug the wrong 'wart' in, then the diode blows before the circuitry is harmed (ideally, not a guarantee!).   I like to use them in parallel with the power supply rather than in series - they will not conduct unless power is applied reversed.  Look up "protection diode" for more info.  

5) Some circuits do use very odd biasing...please, stick to the simple version, the 2 equal R's, for now!  It can get really tricky getting the 'odd' ones right, ha ha!    

[tubegeek]

A) Without looking up the 4556, I'll bet it is a higher power output device than the typical small-signal op-amp types, because it's used to drive headphones. So it's probably a good idea to keep that part if you're adapting that circuit. Google the data sheet and see what the suggested applications are for that chip, that will tell you what its SUPPOSED special purpose is.

(For all fans of The Jerk, that was lower-case "special purpose," not upper-case "Special Purpose.")

B) Why the two paralleled XLR connectors for the mic input? They won't work well to mix two different mic signals, FYI. I guess you could use that hookup as a passthrough to feed something else but I think there are better ways to do that trick.

C) Feedback will only happen if you do one of these two things: either send sound coming from a mic back into that mic (which isn't likely to occur using IEM's) or, send a signal from the output of a mixer back into the input of the same mixer (either the project mixer or the house mix.) I don't think that's going to occur with this setup, correct me if I misunderstood.

[GibsonGM]

I would build the Mini, as drawn by J.D.    Then add an "out" tapped off R14 to an LM386 and drive the headphones that way.   Why do the extra work when it's already been done for you?    All that needs doing is to fix the XLR's to the circuit.   Good thoughts, tubegeek. 

Daleykd: What's often helpful is to draw up a "black box" type of flow chart...show your input and what it is (guitar, CD player, BOTH, etc)....mixing, amplification, signal conditioning (tone controls), recovery, output.   What you've got up there seems incomplete, like a snippet.  To set you up right, it would be nice to be able to look at the whole plan!    You can't really just pop in other opamps and expect to end up in the same place....a well-defined idea of exactly what the circuit should take for an input, what it does to it, and where you send it are the real first steps to take!  )

[daleykd]

Wow guys.  Thanks for all the feedback and help.

I may be too far down the rabbit hole to go back to the TL072/LM386.  I've already ordered my parts (since they'll be coming from Thailand).    

* For simplicity and cost reasons (I realize it's only cents, but I'm building this for our church and they aren't paying for any of this; plus, I need to build like 5 of these things eventually), I do hope to utilize the 4560.  After all, it was free.  Some Googling seems to indicate the 4556 may be a better output opamp, but others have used the 4560 as the output, too.
* I wasn't sure, originally, if I could use the TL072 as a preamp on the mic circuit.  Again, that's why I started with the 4560.
* I figured the diode was to prevent opposite polarity, but wanted to double check.
* I'm using the parallel XLRs as a parallel out.  I forgot to add XLR M vs. XLR F on the schematic.  This is so the user can do a "more me" mix of themselves and still send the unmodified mic signal to the snake.
* It is a snippet, but I feel as though the only thing I'm missing is the power circuit.

I was trying to use the concepts of the mini mixer, but it seemed as though it was for guitar pedals and not the three different inputs (mic level, instr. level, and line level/speaker level).  That's why I was digging deeper into the Rolls.

If we use the Mini Mixer, the TL072 would be the recovery amp, the LM386 would be the output amp, and the 4560 would be the mic preamp?  Seems complicated?

Also, on the Rolls, do we use the 1000UF cap on the DC+ (going to GND) to prevent popping?

I'll try to draw up a block diagram to give a better idea of what I'm trying to do.

[daleykd]

I haven't had a chance to make the block diagram (only so much I can do in my free time at work).  But here's the latest schematic with the power line.  I updated the opamp to be a single 4560, and labeled the XLR inputs.

[GibsonGM]

Well, ok Daley, you have to do what you have to do!  I get my stuff from Newark electronics...they're cheap, good, and not sending from Thailand, ha ha!   

The 1000u cap on the power supply is a reservoir cap...it acts like a small "battery" to supplement the power supply is current draw should momentarily jump up.  Keeps things smooth, you might say.

The circuit I described is not complicated! It is basically what you have, using one TL072 and one LM386 to drive your headphones.  Down the road, if an opamp dies...you can get the 72 ANYWHERE, pretty much.  I believe in easy-to-get parts.  If the 4560 are a better chip, well, then ok.   Not one I'm familiar with, though.   Guess it can drive a little better, so there ya go.

The concept looks fine; I think you should start putting one on the breadboard!  What are you going to use to power it?  (wall wart...how much ampacity?)  It's not a beast or anything, but will use some current driving the phones...

[tubegeek]

OK, I have a better idea what you're looking to do now, thanks.

I would make  the following changes:

Take the junction of R7 and R9, as well as the right-hand end of R5 all to the same place, the - input of U1A. There will now be nothing connected to the + input of U1A, connect that to VBias via a 20K to 50K resistor (for a guess.)

You will then be using a "virtual earth" mixer (which will prevent interaction between the 3 sources) and all your inputs will be phased correctly with each other. They will all be inverted at the headphone output, but that'll be easy enough - just swap the headphone ground with the headphone "hots" at the headphone jack (common the L & R hots and ground them, drive the headphone ground with the output - you did say this was going to be mono?) and that will fix the polarity of the headphones. Make sure the headphone jack is not connected to the case of the device if you do it this way.

You can use a 100K pot for the 100K resistor in the feedback loop of U1A, and that will allow you to adjust the gain of the headphone amplifier, while you can adjust the relative mix levels using the existing VR1, VR2, VR3.

Another not-bad idea: use an LM386 amplifier chip to do the polarity inversion, and then drive the headphones as normal, by running your schematic as shown into the - input of the LM386. That way you'll be using a power amp for power purposes instead of the 4560 but you'll get the proper polarity back.

The reason I suggest the "virtual-earth" setup is to isolate the sources, which is a good feature of the virtual earth mixer. Since I want you to use a power chip anyway on the output  the inversion doesn't much matter.

You could use a pot to control the volume of the cans (instead of the 100K pot in the feedback loop I mentioned above) if you were to add the second chip - send the 4560 to the clockwise end of the pot and take the wiper to the - input of the LM386. The other end of the pot would go to VBias.

Fun project, I'll be interested to hear how it turns out. Be sure to build it robustly if you're going to be depending on it for performance - remember Michael Corleone in The Godfather? You don't want to be coming out of that bathroom with just ....

[daleykd]

First off, let me apologize if I accidentally came off rude or insulting.  Everything you guys say, I read at least three times, and try to absorb as much as possible.  I have already learned SO much just from this short thread.

First... is Newark Electronics in Plano, TX?  That's just down the road from me (my in-laws live there, about 25 mins away), and would be ideal to pick up items if I need it ASAP.

When I get the parts, I will DEFINITELY breadboard it.  I was planning on using a wall-wart to start, then add in a backup 9V battery.  The wall wart that I have right next to me is a Boss PSA-120S (9V, 4.5W, 250 mA).  I have a few other 9V's, but I don't have them in my room right now.

tubegeek, first read-thru, I got 80%.  Second read-thru, I got 95%.  It almost all makes sense!  I'm just a bit confused on the LM386.
1) Would I use both the 4560 AND the LM386?
2) Would the LM386 be before or after 4560?  I understood you to say it was between the 4560 and output.  With pot, it'd be 4560 --> pot --> LM386 --> output.  Is this not too much gain?
3) I think I might be missing a preamp for the instrument input.  I don't foresee it having enough gain before going to the output amps.

Again, the main purpose of this is a personal monitor system for a worship leader who plays acoustic, sings, and wants to hear the rest of the band.  (It'd also work for other vocalists who also play an instrument... like me, the bassist.)  Therefore, we have a mic-level input, a guitar pickup-level input, and a speaker-level input (or line-level input if we come straight off the soundboard).

After a response, I'll draw up a new schematic for review.

[tubegeek]

First off, let me apologize if I accidentally came off rude or insulting.

I haven't noticed that, but then, nobody is ever very nice to me so I have a pretty thick skin.

Everything you guys say, I read at least three times, and try to absorb as much as possible.  I have already learned SO much just from this short thread.

Well, see, that's why there are still people hanging in here. The threads where someone asks a question and then acts like a know-it-all shut down pretty quickly, I've noticed. Give-and-take usually stays pretty lively here. You're doing fine as far as I can tell.

First... is Newark Electronics in Plano, TX?  That's just down the road from me (my in-laws live there, about 25 mins away), and would be ideal to pick up items if I need it ASAP.

Lucky! BTW, I don't think I've mentioned a single part you can't drop into Radio Shack and get, even in these impoverished times for retail DIY.

When I get the parts, I will DEFINITELY breadboard it.  I was planning on using a wall-wart to start, then add in a backup 9V battery.  The wall wart that I have right next to me is a Boss PSA-120S (9V, 4.5W, 250 mA).  I have a few other 9V's, but I don't have them in my room right now.

All good. Be sure to include a little filtering if you use a wall-wart for DC, they are usually a bit noisy but we can deal with that pretty easily. One resistor, one cap, done. In your schematic I'd add a small (100 ohms to 1K) resistor connected between D2 and C9 (cut the left-to-right wire that's there, install the resistor instead of the wire.) You will have a very small reduction in voltage but you will also have very effective smoothing of any crap on the wall-wart's DC feed. If we keep the new R small, we won't lose much if you switch to battery. The wall-wart is probably going to be a little high so that's less of an issue on the wall-wart supply, but we can still use a pretty small R value in that scenario so that's why I suggested it. C9 is already in there, we're just making it a little more effective this way.

tubegeek, first read-thru, I got 80%.  Second read-thru, I got 95%.  It almost all makes sense!

Wow - you're reading these things multiple times!? Damn, now I GOTTA keep helping you!

I'm just a bit confused on the LM386.
1) Would I use both the 4560 AND the LM386?

Yeah: 1/2 of the 4560 for preamp for the mic, 1/2 of the 4560 for the virtual earth mixer, LM386 to give the headphones a little kick in the pants (and as a free bonus, to re-invert the signal.)

2) Would the LM386 be before or after 4560?  I understood you to say it was between the 4560 and output.  With pot, it'd be 4560 --> pot --> LM386 --output.  Is this not too much gain?

Good question. You have the setup correct, yes, it's in that order with the LM386 after the 4560.
From your roundup below, it seems like we have a pretty wide variation in gain levels to deal with. A mic input might need 50 dB of gain, an instrument input maybe, I dunno, 10 to 20 dB? And a speaker-level input would probably need to be cut some, maybe -10 dB.

For a performance setup (in other words, not studio-quality, highest-possible performance) the idea will probably be to have plenty of extra gain and then throw some away - that's why I've been looking to add some kind of overall level control for you. Once we get the three wildly-different sources balanced properly, then we'll adjust the level going into the headphones.

Any one of the stages can be configured to add gain or not - the feedback loops are responsible for setting the gain of each stage.

Have you seen Rod Elliott's ESP pages? There are some very detailed explanations there, of many of the things we are talking about here. A lot of what I'm telling you I got a much better understanding of/learned outright, from those projects and general educational articles.

A couple of links:

The three-part "designing with op-amps" article is just gold. Straight-up gold. Look under the section "Beginners' Luck - The beginners' Guide to ..." for that series, and a good article on mixers.

http://www.sound.westhost.com/articles.htm

And amongst the projects there are mic preamps (some very fancy) and mixers and headphone amps - all sorts of relevant circuits, with excellent, detailed explanations of what they are and why they are the way they are.

http://www.sound.westhost.com/projects.htm

In about a month and a half, you'll know a whole boatload of stuff about your project, but I have a feeling that time frame is a little long for what you want to accomplish. However, I also have a feeling you will end up reading up on this stuff and at that point you'll be on your way to being a hopeless geek with no alternative but to haunt these forums forever. Ask me how I know....

A super-duper reference book for this subject is Small-Signal Audio Design, by Douglas Self. I think a 2nd edition is in the works, but the current one is already a very well-thumbed item around my house. It costs more than your project though - maybe a local library?

3) I think I might be missing a preamp for the instrument input.  I don't foresee it having enough gain before going to the output amps.

Yeah, you're probably right. Maybe a TL072 might have to go on your shopping list too, for sticking behind that jack.

Again, the main purpose of this is a personal monitor system for a worship leader who plays acoustic, sings, and wants to hear the rest of the band.  (It'd also work for other vocalists who also play an instrument... like me, the bassist.)  Therefore, we have a mic-level input, a guitar pickup-level input, and a speaker-level input (or line-level input if we come straight off the soundboard).

Try and get a soundboard feed: more control, and better level for your purposes. Does your board have any free AUX out buses for a monitor mix?

After a response, I'll draw up a new schematic for review.

Well, I'd be working with a pencil and an eraser at this point. As you can see, we're still in the evaluating-our-requirements phase, so the suggestion above to work with a block diagram first is probably the way to go. Also, from an educational perspective, I think the next step of putting actual circuits into each block will make really clear sense as a follow up.

Sorry to slow you down. In fact, there is a LOT to be said for "slap a trial circuit together and see what it does well, and what it doesn't do as well." If you go ahead that way there are two things to expect:

1. It'll do some things well, and you'll be thrilled with the fact that you can make that happen. This is the very best part of DIY fever. And there's only one cure: more cowbell.
2. It'll do some things less well, and you'll be faced with a desire to improve the gizmo. This is another really good part of DIY fever, but it can disappoint you of you're looking to have The Perfect Gizmo Right Away. There is NO cure for this part.

A couple more things:

I mix sound in a church every Sunday. Full choir and an ace band. It's a blast! I have a feeling you're kinda busy Sundays from 9 to 1, but if you're not, you can listen in on the webcast I mix, I'll give you the link if you think you'll be able to use it.

Also, I thought of another link, and I'm pretty sure there are articles on this site that deal with the kind of setup (musical worship leader) that you are working on. I wrote a guest post a month or so back on mic'ing up the worship leader so skip that article, it's crap.

Chris Huff's Behind The Mixer:
http://www.behindthemixer.com/

.... OK, that's quite enough for before-breakfast....

Best,

-j