DIY Passive Line Mixer?

[FALKEN]

well i asked some fulcrom users and they say that as they change volume (in the daw software) of a track the mix level does not change. the problem you have described must have to do with the "load" changing via the volume pot. of course I am a newbie so I don't know for sure.

I wonder should I pursue this line mixer but in an active design?

-even if I bought a line mixer I would still want to make some rack-mounted "busses" to comp say vocals with or process two bass tracks through a compressor together.

-I also want to make this sum and differnece mixer to do MS processing.

easily done? Where would you buy a blank rack case and how would you punch the pot-holes in it?

 

[mshilarious]

well i asked some fulcrom users and they say that as they change volume (in the daw software) of a track the mix level does not change. the problem you have described must have to do with the "load" changing via the volume pot. of course I am a newbie so I don't know for sure.

I wonder should I pursue this line mixer but in an active design?

-even if I bought a line mixer I would still want to make some rack-mounted "busses" to comp say vocals with or process two bass tracks through a compressor together.

-I also want to make this sum and differnece mixer to do MS processing.

easily done? Where would you buy a blank rack case and how would you punch the pot-holes in it?

http://www.partsexpress.com/pe/pshowdetl.cfm?&PartNumber=262-430&DID=7

Pot holes are easily drilled, especially if you get an aluminum front like in that case. For larger holes, like for XLR connectors, score a Greenlee 15/16 chassis punch, you can find them on eBay or I think Markertech carries them.

 

[FALKEN]

question!!!!

Say you had a box with 16 "fader pots" that was "passive" meaning each fader had an input and an output and this fader pot and that's it. so you could use it between your tape machine (or what have you) and your line mixer which doesn't have faders. would this work?? or would it just sound like ass??? (for instance, on an electric guitar, the volume pot affects the amount of high end loss)

 

[mshilarious]

question!!!!

Say you had a box with 16 "fader pots" that was "passive" meaning each fader had an input and an output and this fader pot and that's it. so you could use it between your tape machine (or what have you) and your line mixer which doesn't have faders. would this work?? or would it just sound like ass??? (for instance, on an electric guitar, the volume pot affects the amount of high end loss)

It should work fine, again presuming your signals are unbalanced. You won't have the same high-end loss issue you have with guitars, because a line level signal is much lower impedance.

My only reservation would be that nice faders are expensive, so once you bought 16 of them you might wish you had just saved that for a good mixer.

 

[FALKEN]

It should work fine, again presuming your signals are unbalanced. You won't have the same high-end loss issue you have with guitars, because a line level signal is much lower impedance.

My only reservation would be that nice faders are expensive, so once you bought 16 of them you might wish you had just saved that for a good mixer.

nah; I'm done with mixers. heh. First I was done with computers and now I am done with mixers. I am quickly digging myself a hole that I can't get out of.

q: why would the signals have to be unbalanced?

 

[Ethan Winer]

Falken,

> I wonder should I pursue this line mixer but in an active design? <

I'm a DIY'er from way back, but these days you can buy a Behringer mixer for less than $50 so why even bother? The parts alone will cost more than that, and you still have to make the thing!

--Ethan

 

[FALKEN]

Falken,

> I wonder should I pursue this line mixer but in an active design? <

I'm a DIY'er from way back, but these days you can buy a Behringer mixer for less than $50 so why even bother? The parts alone will cost more than that, and you still have to make the thing!

--Ethan

Ethan,

I don't think you can get a 16 channel behri for $50. I don't want to say that behringer mixers suck (I do own one and its served me well for over 5 years) but at this point I am looking for something a bit higher quality.

I think that some of this quality will come from eliminating unnecessary features (for me) such as monitor sends, mic preamps, equalizers, faders, auxiliaries...

I am still trying to figure out how to do a passive design. I found this. Its a passive "volume pot" of sorts. I guess its a 20 position switch with different resistors at each position. It isn't cheap though. I might be able to use two of them in a passive M/S decoder. for a 16-channel mixer it would cost over $3000 just for the pots, and they're "stepped" anyway. I dont see the difference between one of these and a "normal" pan pot. To use a "normal" pan pot is it just a matter of using input and output transformers or what? I am going to go to the bookstore after work and see if I can't find a book on this. I drew up a schematic for a control section. just a bunch of switches and inserts. and I have the idea for the passive M/S encode/decode with an insert (if I can figure out how to do the volume pot correctly). if I can figure out the line mixer part I'll have everything I need.

 

[mshilarious]

q: why would the signals have to be unbalanced?

Because a single fader can't attenuate both sides of a balanced signal. So you would need input and output transformers, or some sort of (really well matched) stereo fader. Or you could just toss one side of the signal to ground, but I assume if you wanted balanced sends, you wanted balanced returns.

I dont see the difference between one of these and a "normal" pan pot. To use a "normal" pan pot is it just a matter of using input and output transformers or what?

Stepped attenuators are popular for "critical" applications where the designer wants to use specially selected and perhaps matched resistors. In other words it provides a higher quality part than a standard pot, but basically it does the same job. You can find stepped attenuators for a LOT less than $145, and stuff them with resistors of your choice.

They don't solve any of the issues with passive mixers discussed above though.

I'm still rooting for the opamps. Get yourself 9 OPA2134s for $20, that's two channels per chip on the input, and one chip for the stereo makeup gain, 18 100mm faders for around $250, some pan pots (don't know the price there), and build a +-15V power supply, add a few resistors and high-grade caps and I really doubt you'll be disappointed.

Personally I would be tempted to scavenge a low-grade mixer just for the box and the faders, you'll save time and money that way.

 

[frederic]

Because a single fader can't attenuate both sides of a balanced signal. So you would need input and output transformers, or some sort of (really well matched) stereo fader. Or you could just toss one side of the signal to ground, but I assume if you wanted balanced sends, you wanted balanced returns.

At the cost of double the parts count, you can actually.

 

[FALKEN]

I'm still rooting for the opamps. Get yourself 9 OPA2134s for $20, that's two channels per chip on the input, and one chip for the stereo makeup gain, 18 100mm faders for around $250, some pan pots (don't know the price there), and build a +-15V power supply, add a few resistors and high-grade caps and I really doubt you'll be disappointed.

Got a link to a schematic?

 

[Ethan Winer]

Falken,

> I don't think you can get a 16 channel behri for $50 <

Pretty close to that I bet!

> I think that some of this quality will come from eliminating unnecessary features (for me) such as monitor sends, mic preamps, equalizers, faders, auxiliaries... <

I don't see why. If you don't use those features and keep the Returns all the way down, they shouldn't contribute noise or distortion. In the larger picture, the noise and distortion from even the cheapest modern gear is 10 times lower than what your microphones pick up in your room, and the distortion is 100 times lower than your loudspeakers.

> its a 20 position switch with different resistors <

The main advantage of these is channel matching, as MSH explained, and they're also useful for being able to recall a settng exactly. This is less valuable these days when any DAW can recall a mix exactly.

Explain once again why you're not just mixing in your DAW.

> for a 16-channel mixer it would cost over $3000 just for the pots <

I can think of a dozen things you could buy for $3000 that will make a much larger improvement in your situation.

--Ethan

 

[FALKEN]

I am not mixing in my DAW because I am not recording to my DAW.


>I can think of a dozen things you could buy for $3000 that will make a much larger improvement in your situation.<

This is why I am now looking into the opamps as an option. building this thing will a) save me money to buy more neve pres b) be fun c) give me exactly what I want in a mixer and d) look cooler than anything else on the market (to me).

anyone have a schematic?

 

[FALKEN]

aaaaaaaaarrrrghhhhh!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[FALKEN]

I have done nothing but at schematics for the last few days




my brain will soon go numb






I have been researching the burr-brown idea.

I have found a few differnet schematics on the net but they all are either too complicated (mic inputs, tone controls, etc.) or not complicated enough (line amp with no volume control and no summing section).




ahhhh!!!!!!

I want to use the opamp as a buffer, to separate each channel. I don't need it to have any gain. just to take a balanced signal and pass it to a summing buss with a volume control and possibly a pan.

then is the question of where do you get the PCB's??

Can you get a per-chip PCB that just gives you the pinouts so you can do PTP wiring all-round?

THEN is the question of the power supply?????????????????

aren't there standard power supplies you can use with these opamps???

why do you always have to build your own?



I want to build this thing so that it sits in an 8-space SKB type rack on top of my desk. there will be no faders (I might add them later). it will have (ideally) on each channel a volume knob, a 3 or 5 position pan switch (haven't decided yet), and a 5 position bus switch (1,2,3,4,M). because all volume controls would be on knobs I feel it would be necessary to work with busses to do any sort of fades, etc.

I want it to accept balanced and unbalanced connections and I want it to produce an output that can interface with both balanced and unbalanced gear.



any help would be greatly appreciated.

 

[mshilarious]

Before we get into this, read nearly everything here:

http://www.prodigy-pro.com/forum/viewtopic.php?t=2779

I have done nothing but at schematics for the last few days

OK I have some time and will try to help a bit. My first reaction is take frederic's line input stage and wait for his mix and assign stages.

http://www.homerecording.com/bbs/showpost.php?p=1788168&postcount=12

You can substitute OPA134, or if you want to get crazy, 627 for the 5534 he used to perhaps get higher performance. You could also increase the supply to +-18V to score a bit more headroom, but make sure it is regulated since you can't exceed that with those opamps.

A simpler design just uses a single opamp, something like this:

But then you are still left with the problem of a fader, without causing the same issue as the line mixer. That can be fixed with another opamp stage following the fader as a buffer:

In that schematic, replace R1 with the fader, with one side to the input signal, one side to ground, and the wiper to the + opamp input. Use a dual opamp like 5532, OPA2314, or OPA2604 for each channel.

Another way to do it is to put the fader in the feedback loop of the input stage opamp to modify the gain, but I don't think it's possible to create gain of less than one in that case. However you could account for that in the makeup gain stage by simply using less makeup gain. I don't really know if one approach is better than another, as I am no expert at opamp design. In fact I struggle to fully understand frederic's design.

Anyway, whatever you do, follow the passive mixer schematic from there, and add another opamp in the master section for makeup gain and to have a balanced output:

Or even this:

Note that is unity gain, but I don't believe it has to be in that application, you would change the resistor values in the feedback loop (and add them to the positive side) to add gain. You would need closely matched resistors in that section, I would think.

Sites referenced above:

http://sound.westhost.com/project51.htm

http://sound.westhost.com/project87.htm

http://www.muzique.com/lab/buffers.htm

then is the question of where do you get the PCB's??

You'll have to design it, and maybe make it too. If you are going for a single board per channel design (probably wasteful, for such a simple circuit), at the 16 channel point it becomes economical to get PCBs professionally made from somebody like PCB Express or Express PCB (poor trademark registration there!). I've never used either one, but Express PCB has some cool PCB and schematic design freeware

THEN is the question of the power supply?????????????????

aren't there standard power supplies you can use with these opamps???

why do you always have to build your own?

Probably because there is no market for it. Anyway the power supply for a small number of opamps is really pretty simple to build. Figure on a 30V 1A center tap transformer, a few diodes and caps, into 7815 and 7915 regulators should get the job done.

Kickass site in general, but especially on power supply design (with calculators!):

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/rectct.html#c1

 

[FALKEN]

mshilarious,

thanks for taking the time to show all of this to me.

truth be told, I had printed out and read twice already some of the sites you mentioned.

but you put it all together into a circuit I could actually use. I think I could build this. I understand it and could draw the schematic. (more like trace it)

But the power supply part scares me; I am no physicist.

Forgive me for sounding lame, but why couldn't you use a standard wall wart?

 

[frederic]

then is the question of where do you get the PCB's??

If you're using schematic capture software (eagle, orcad, or 20 other things), after you design the schematic, you can use the parts layout portion of the programs to draw the PCB, then "auto route" a board. You can take the output of this stage, and either send it to a place that makes PCB's. AP Circuits, EaglePCB, ExpressPCB are a few of about a zillion. Google for "printed circuit boards" without the quotes.

Here are some DIY links....
http://www.lvr.com/pcbs.htm

Can you get a per-chip PCB that just gives you the pinouts so you can do PTP wiring all-round?

Sure. Companies make little boards to solder DIPs, SIPs, TOC's, SMD and a variety of other chips with larger solder tabs around the perimeter of the boards. Another option (for DIPs anyway) is Radio Shack. They sell 1"x1" through 6"x8" through-hole copper clad circuit boards where each hole in the board has a copper pad under it, to solder the chips and other parts to, as well as wires. This is my preferred prototyping method. It's fast, it's soldered, no wirewrapping blah blah blah.

THEN is the question of the power supply?????????????????

My design, like most designs, will require a +/- 15V power supply. Not very difficult to make.

why do you always have to build your own?

Because wall warts absolutely suck. You can use two 15V wall warts and make a split power supply that way, however current will be limited and, well, they're wall warts.

You can buy commercially made power supplies for anything, you just have to research and hunt them down.

I have on my workbench a +/- split switching power supply that is very small, compact, and provides 8A for the +15V as well as the -15V. It's made by "Cosel", I think, and it's very small. But it's a switching power supply, switching in the 150Khz range... good enough for testing, not something I'd use in a completed design. For audio stuff, I prefer linear power supplies even though they are less efficient, just because I have a few working designs. As I finish sharing my console design on here (see link Ms. Hilarious was kind enough to give you) you'll see it.

I want it to accept balanced and unbalanced connections and I want it to produce an output that can interface with both balanced and unbalanced gear.

If you look at my line/tape input circuit (again, above link), you'll like that circuit quite a bit. It allows you to plug in balanced, or non-balanced no problem without attenuating the signal. Required a few extra parts but the end result is really useful.

A lot of the balanced designs out there really don't work well with non-balanced inputs - you lose some of the gain which of course, is an opportunity for more noise.

I was going to say "take a chill pill" and wait for me to get through the compressor, EQ and buss select modules of my design (on thread posted above) as the summing part would be after that... but I've been quite slow in keeping that thread going full speed.

The good news is I'm going to be posting the compressor and EQ modules in a few days... stripping out the digital control stuff was more difficult than I thought

In hindsight I should have started the thread off with "lets build a digitally controlled, analog mixer" rather than go through all this aggrevation. But oh well

 

[mshilarious]

Here's a basic power supply that should be good for a simple 16 channel design. I pulled this quickly out of something I am working on, you wouldn't need the dual primary transformer shown (unless you wanted to provide for 120/240V operation). A basic 120:30 center tap 1A transformer will do. There is also probably more capacitance than necessary, but I like big capacitors

 

[FALKEN]

wow.......

thanks a lot for the responses.

quick brain fart -

I have a couple of passive DI boxes that use transformers. would it be possible to buffer each signal using something like this? basically put one of those transformers at each input and output and wire it like a passive mixer from there, with no power? is this just stupid? I want to get this one idea out of the way before I start buying parts.

 

[mshilarious]

wow.......

thanks a lot for the responses.

quick brain fart -

I have a couple of passive DI boxes that use transformers. would it be possible to buffer each signal using something like this? basically put one of those transformers at each input and output and wire it like a passive mixer from there, with no power? is this just stupid? I want to get this one idea out of the way before I start buying parts.

In one of the Lab Meta links, NYDave posted a passive mixer schematic using balanced transformer inputs and then opamps for makeup gain. The major downside is that transformers are expensive.