DIY Passive Line Mixer?

[FALKEN]

Disclaimer: I can solder but I don't know anything about wiring diagrams and op amps and resistors and whatever else you guys use to build this fine stuff we call gear.

Question for Frederic or anyone else:

Would it be possible to build a line mixer that does not use any amps or anything? It would be like the fulcrom but it would have volume and pan pots.

or more simply,

would there be an easy way to connect 16 input jacks, 2 output jacks and 32 pots (and I guess some caps and resistors?) to make a line mixer? I can use an ouboard pre for makeup gain like the fulcrom. its just that I want something like the fulcrom that I can use with my tape machine, which means I need volume and pan controls. thanks!!!!

oh and if this is plain stupid or doesn't make sense just let me know!!

like I said I don't really know much about this stuff. I've made some cables and wired a few guitars (following diagrams!) and that's about it.

 

[frederic]

Here's a schematic...

http://www.tkk.fi/Misc/Electronics/circuits/linemixer.html


The drawback of a totally passive mixer is that the "load" on the outputs of your synths and such is pretty heavy, meaning you have to crank their outputs.

Passive means it doesn't amplify at all... it only reduces. Even with all the controls set for "full volume" the output will be less than the inputs themselves.

But, that's a basic schematic, assuming you have stereo in, and stereo out, without panpots.

I have one with panpots somewhere in my file cabinet, I'll try to dig it out for you.

But at the moment I have to go back to tacking carpet on the stairwell

 

[FALKEN]

Thanks frederic!

I was actually just googling and found that. It is very useful! I also found this: http://www.forsselltech.com/summing buss.htm which is pretty cool. I would be using this (if I can build it) with my tape machine which is 16 outputs and unbalanced (although I will interface some balanced gear). my thinking was that instead of having a line amp, I would just plug it into a mic preamp at the Hi-Z inputs. it looks like it can be built. the questions remaining are: would it be too much noise with 16 channels (assuming I am using a good preamp at the output), how do I make the pan pot, and can I make it balanced and passive? I am psyched!

 

[frederic]

Welcome.

Except that schematic you posted isn't passive

You said passive, so I offered passive.

If you don't mind an op-amp or two, thenwe can design something significantly more useful.

questions remaining are: would it be too much noise with 16 channels (assuming I am using a good preamp at the output),

Passive mixers depend on the outputs of the gear plugging into it... so they will be noisy if you have ground loops and so forth. Some synths/modules/outboards don't mind each other, others don't play nice.

how do I make the pan pot, and can I make it balanced and passive? I am psyched!

Pots (variable resistors) have three terminals. 1, 2 and 3. 1 is typically leftmost, 2 is the wiper, and 3 is the rightmost looking at the knob on the pot.

So... use a 5k linear pot...

Pin 1 goes to the resistor feeding the op amp in your schematic.
Pin 2 (wiper) goes to the input jack.
Pin 3 goes to another resistor feeding a second op amp in your schematic.

You build that schematic twice, one for left one for right.

 

[FALKEN]

Passive mixers depend on the outputs of the gear plugging into it... so they will be noisy if you have ground loops and so forth. Some synths/modules/outboards don't mind each other, others don't play nice.

Do you think that the average commercially available option is any more complicated than the diagram you posted? I am thinking passive just might work for my needs. ?

my tape machine specs say: line output: -10dBV/0.3V nominal unbalanced, load imp. 10Kohms or higher

I would also be running some channels through some compressors.

You build that schematic twice, one for left one for right.

is this to accomplished a balanced signal chain?

I was looking at the link you posted and the guy says that you can build it with 4 resistors but it looks like there are 8 in the diagram? Am I missing something? It seems like he is putting an extra resistor between the volume pot and the ground. ?

 

[frederic]

Do you think that the average commercially available option is any more complicated than the diagram you posted? I am thinking passive just might work for my needs.

Active is always more complicated. More parts, more op amps, power supply, and so on.

my tape machine specs say: line output: -10dBV/0.3V nominal unbalanced, load imp. 10Kohms or higher

Then they would probably not like a passive mixer.

is this to accomplished a balanced signal chain?

No, its to build a stereo mixer. Balanced is difficult to do with passive only.

 

[FALKEN]

hmmm...

the fulcrom is passive and people seem to like that one okay...is it something about the addition of the volume and pan pots that screws up the design?

I was also thinking of building a 2 channel balanced passive sum and difference mixer with a volume pot at each input. it would use 2 stereo pots (for the balanced connections) and then split the signal from each input there, and combine each signal into each output, one using both tips and both sleeves, the other using a tip/sleeve, and sleeve/tip.

I think that for this to actually work all input and output connections would have to be balanced or else I'm not actually doing the "difference" side. whaddaya think?

 

[frederic]

In a nutshell, every op amp you put into the circuit, is an opportunity to add noise to the signal. Electronics, especially bits of silicon, aren't perfect.

Every resistor you add to the circuit, reduces the signal.

The "magic" in any audio design is to balance the two.4

You can make a passive, balanced mixer, but it requires more parts. One set of parts for the + side, one set for the - side, and double of the above if you want stereo (well, not quite double, but close).

This is where op-amps make life easy. They accept balanced inputs by design. And, you can use a pair of them (one a buffer, one an inverter) to give a balanced output on the end.

 

[FALKEN]

This is where op-amps make life easy. They accept balanced inputs by design. And, you can use a pair of them (one a buffer, one an inverter) to give a balanced output on the end.

I see. do op-amps change the circuit to an active circuit?

Here is my dilemma. I am sick of these small mixers. they don't sound good to me. you can try to convince me otherwise but what I really want is that chandler TG mixer. holy god that thing looks nice; exactly what I want, too, as far as the routing and I even think the built-in talkback mic looks cool too. I think that thing was made for me.

I was hoping to try to "clone" it but more along the lines of the fulcrom, where I could use a mic pre as the output gain and get different flavors of that (this is how a passive mixer would work, right?) .

So that is why I am afraid of op-amps. I am afraid if I use them it will just sound like any other small mixer. whereas the fulcrom (passive) is supposed to sound like whatever you use for makeup gain.

Am I wrong?

I'm not afraid of more parts. In fact, I would be more confortable making this a ptp project rather than trying to use a circuit board. I am most concerned about the sound quality. I am pretty sure that these new line mixers made by neve, api, and chandler are passive mixers that use a pair of lineamps for makeup gain. Thanks a lot for taking the time to answer my questions. You are a very cool dude.

 

[frederic]

I see. do op-amps change the circuit to an active circuit?

Yes.

Generally "active" means a power supply is necessary. Op-amps require a power supply. Resistors, pots, capacitors, diodes, inductors and chokes do not.

What I really want is that chandler TG mixer. holy god that thing looks nice; exactly what I want, too, as far as the routing and I even think the built-in talkback mic looks cool too. I think that thing was made for me.

If you want to clone that, acquiring the schematic is a good thing to do. Then you can evaluate what the parts would cost, how much time it would take, and how nice your version of it will look when complete, as compared to the of a used one if you can find one.

YOu're talking about this?
http://www.soniccircus.com/store/merchant.mvc?Product_Code=N8546&Screen=PROD

Both that and the smaller version, are active mixers.

So that is why I am afraid of op-amps. I am afraid if I use them it will just sound like any other small mixer. whereas the fulcrom (passive) is supposed to sound like whatever you use for makeup gain.

Am I wrong?

You are correct in that a passive mixer will sound like whatever you feed it, assuming there is enough level to drive through the passive mixer.

You are less correct in your statement about sounding like "every other small mixer", only because that statement isn't qualified.

If you use 15 cent op-amps not designed for audio applications, then yeah, whatever you make is going to sound like absolute crap.

If you use more expensive op-amps, with good specifications and at least a reasonable high impedance input then you have a chance of building yourself a nice sounding mixer, with minimal colorizing of the audio coming in.

If you think about my statement, this is why the old mackie 8-bus consoles had a specific "sound" to them, and why Soundcraft has a specific "sound" to them as well. And Neve. And Amek. And Trident. And SSL.

They all use active stuff in their designs, whether individual transistors, op-amps, or even vaccum tubes in the really old stuff.

It's about specing quality parts, and making a really good design. And that's a tough thing to do without experience, knowledge, and the time to test your circuits and make changes to the breadboarded design, and update your schematics and documentation.

It's how the "big boys" do it.

The smaller mixers use components that allow them to sell them in a lower price range. If a Mackie 4-channel, stereo mixer were $2500 instead of $99.95 because it used top quality components and were built like a tank, would you buy one?

That's why the cheaper stuff sucks, phonically. They use 15 cent op amps trying to make something affordable. Nothing wrong with it, people buy them of course, and they serve a purpose.

about the sound quality. I am pretty sure that these new line mixers made by neve, api, and chandler are passive mixers that use a pair of lineamps for makeup gain. Thanks a lot for taking the time to answer my questions. You are a very cool dude.

Some of them are passive, some of them are not. Sometimes they have op-amps (or transistors) before and after the passive mixer so they can sell "passive mixing" to passive purists, and technically they are, but to me they really aren't.

You can design a good op-amp based mixing console. I've done it. I breadboarded each section one at a time and monitored the ins and outs with an oscilloscope to see how much colorization of the audio signal my circuits were doing.

THough once you tweak and EQ knob, you're coloring the audio anyway

 

[FALKEN]

yes, that's the one. ok; so I won't be able to "clone" it; but I think I can make something similar in function that would be passive, by wiring each pot to the output busses point-to-point, using some resistors. I estimate I will need about 30-40 db of makeup gain for 16-20 channels and I have a few pre's that should be able to do that nicely. do you think this would sound as good or better than a mackie? or is opamps the only way to go?

This is where I got the idea from: http://www.mercenary.com/rmfo16chpasu.html

I just need volume and pan on it. Actually, I don't need pan. A 3-position switch would work just as well, if not better than the chandler (people have complained that the pan pot does not 'lock' into center position). Thanks again!

 

[Ethan Winer]

Frederic,

> The drawback of a totally passive mixer is that the "load" on the outputs of your synths and such is pretty heavy <

Yes, and this affects more than just volume. The main failings of passive designs are the volume of one channel changes as you adjust the volume or pan of another, and the output wires must be very short to avoid losing all the high end. I can't think of even one advantage of a passive mixer (other than it's portable with no batteries), and there many obvious problems.

--Ethan

 

[FALKEN]

Ethan,

How do you think the fulcrom avoid this problem?

From the ad copy:

The absence of faders, EQ, aux sends, pan knobs, or any other superfluous features allows for an entirely passive circuit. There are no amplifiers, ICs, transistors, capacitors, or transformers in the signal path whatsoever. The simplicity of the fully balanced, symmetrical signal path allows the use of no-compromise passive components for ultimate fidelity. The unit passes signals "from DC to daylight" without coloration.

I can't imagine the effect could be noticeable if this is how its done in the fulcrom? would the fulcrom then be hurting your DA converters by "loading them down" ?

 

[FALKEN]

Here's a schematic...

http://www.tkk.fi/Misc/Electronics/circuits/linemixer.html


The drawback of a totally passive mixer is that the "load" on the outputs of your synths and such is pretty heavy, meaning you have to crank their outputs.

I was looking at that link again. he says

If you need more than few inputs (more than three or four) then it is a better idea to use some more advanced active mixer circuit or at least add an amplifier to the signal output.

so I think it will work. but you comment about the "load" on the outputs is concerning. is there any way around this? I can't adjust the volume of my outputs at all.

 

[mshilarious]

I think you should overcome your aversion to opamps by using some of the better ones available. Burr-Brown has lots of nice ones, if you are spendy, play around with building a circuit out of OPA627s. Dan Lavry posted an input stage using that opamp one time, and he don't make no junk

I'm not that spendy, so I use OPA134 series. But I guarantee you wouldn't think my BB-modded A&H MixWizard sucks.

Watch the capacitors, they can dramatically affect tone too. Finally you need a good robust power supply and a good grounding scheme. Add those things together and you've got yourself a great mixer.

Really I think you'd want to use frederic's design, I don't think he's gotten to the busses yet though, so you'll have to wait with the rest of us.

Another thought for a semi-active unbalanced line mixer would be to use a simple JFET buffer in front of the mixer stage.

 

[frederic]

Really I think you'd want to use frederic's design, I don't think he's gotten to the busses yet though, so you'll have to wait with the rest of us.

We're getting closer... I keep getting distracted with other stuff.

Earlier last week was a new clutch in the truck... later in the week was a roof repair (slate is a PITA!), did some emergency plumbing yesterday... and today helped my cousin change the oil pan in his hyundai.... he drove over a large rock, oil pan first

Though today I did spend some time in my studio, tacking carpet to the stairwell, which makes walking up and down barefoot that much nicer. Also installed the new locks to the outside so I have one key for my studio for the door to the outside, and the double-door set going to the rest of the 2nd floor of my house. Trying to make the key ring a little lighter.

But we'll get there... I had a small snafu with my schematic software... have to reinstall, and just been procrastinating

 

[FALKEN]

I think what I need is a guide explaining some of these simpler audio circuits and what each component is doing and why the values were chosen or calculated. I now see why my idea wouldn't work - I guess it is because the pots change not only the volume of the material but the load on the circuit.

 

[mshilarious]

We're getting closer... I keep getting distracted with other stuff.

I feel your pain . . . I have three circuits in various stages of design and completion, one of which I started a thread on a month ago

roof repair (slate is a PITA!)

I use to read Fine Homebuilding articles about slate roofs and drool. Of course now that I live in hurricane country, it would be a bad idea for me. I'll stick with cheap, ugly, replaceable asphalt.

Although my last house had a standing-seam copper roof. That is nice to have, except for the TV antenna in the attic

Anyway, if you cut your own copper nails, I'm officially nominating you for DIY-er of the year!

 

[frederic]

Although my last house had a standing-seam copper roof. That is nice to have, except for the TV antenna in the attic

I made an attempt at wireless internet, for roaming outside with a laptop - and the slate blocks it. I could have mounted the antenna outside I guess, but I had it in the attic and didn't get squat anywhere except in the attic.

See, all the walls in the house (except the studio) is plaster, slathered over a metal mesh, which the metal outlet boxes go through, thus grounding the metal mesh all over the place.

Even the repairs I've done to various walls were redone in the same way, so it looks the same and has the charactoristic 1941 slight wavyness.

Anyway, if you cut your own copper nails, I'm officially nominating you for DIY-er of the year!

ha ha.

I bought the nails. But, I cut, and folded, my own copper flashing!

 

[Ethan Winer]

> How do you think the fulcrom avoid this problem? <

I'm sure they don't!

> I can't imagine the effect could be noticeable <

I can't imagine it would not be noticeable. Hey, there's nothing wrong with active electronics. I mean, duh! There are transistors and op-amps at every single point in the chain between microphone and loudspeakers. So where's the value in avoiding active circuitry in this one place, when mixing signals?

--Ethan