Microphone Level Mixer

Hey,

I am looking to make a small mixer for use in recording applications.
Heres what I want to do:

-I want it to have two XLR inputs, and one XLR output
-Ability to flip the phase of one of the mics
-have a potentiometer control the mix between mic 1 and mic 2 (where 12 o clock is 50% mic1 and 50% mic2)
-the output being at MIC level


I probably cant do this passively since if I change the level balance the load impedance will change and mess with the output level (and sound quality).

I have found many DIY microphone preamps, but none that would output at mic level.

This is going to need an active circuit, but I only want the active circuit to be able to compensate for the impedance shift and the level change so the output is at mic level

Heres why:
A friend and I run a recording studio. Every time we record drums, we run out of inputs on our interface. We could just buy a set of 8 channels, but it would only work with sample rates of 44.1 and 48k (when we run all of our sessions at 88.2). This box would make it possible to mix two mics together before going to the mic preamp (ie. top and bottom of a snare drum), effectively increasing the number of inputs

If you guys could help me figure this out it would be greatly appreciated!
 

wakibaki

Banned
2008-01-08 11:51 pm
You are looking for a sub-mixer. Why not simply buy a small conventional mixer with a couple of mic preamps and feed it into one of the inputs on your existing setup? It will be as cost-effective as spending your time sourcing and buying the components, building, debugging, boxing and adjusting some Heath-Robinson device. The reason you can't find one off-the-shelf is that nobody else takes this approach to solving this problem. If you have to go into a channel with a preamp you can just cut down the signal level with a resistive divider.

w
 
I built a totally passive mixer that works fine. But the only way you can get phase inversion passively is with a transformer.

My mixer doesn't have a blend control, simply a volume control for each input.

Really? How did you go about doing that? That would be pretty much exactly what I'm looking for.


The reason why I don't want to just buy a mixer is because I'm not sure if I can trust the preamps to be that good (or quiet), and also I like to use outboard preamps with certain coloration to them (like a tube preamp or a transformer-based Neve style of preamp)

Could I also build two small, op-amp based preamps; sum them and then use a trim pot at the output so that it doesn't overload the subsequent preamp?
 

bob91343

Member
2010-03-11 10:43 pm
Use a potentiometer as gain control for each input. Sum all potentiometer outputs through resistors. Job done.

The tricky part is selecting resistor values. The individual volume controls will interact to some extent; if you judiciously pick the resistances you will minimize said interaction.

In my case, I added a microphone input to a guitar amplifier. I use the volume control on the guitar for that and one control on the mixer box for the microphone, which has the required connector. The metal box is about 1.5 inches cube, big enough for the parts and no more. Requires no power and nobody can detect interaction. It's mounted on top of the guitar amplifier.
 
I built a totally passive mixer that works fine. But the only way you can get phase inversion passively is with a transformer.

The phase inversion is easy. Mics have balanced outs, a switch right after the xlr input to swap the + and - is all you need. ( or a mic cable wired out of phase )

But why do you need this if you already have good mic pre amps? They already do what you want without the extra noise a pot befor the pre will add. (especially with mics that dont have built in amps ie. dynamics)
 
Really? How did you go about doing that? That would be pretty much exactly what I'm looking for.


The reason why I don't want to just buy a mixer is because I'm not sure if I can trust the preamps to be that good (or quiet), and also I like to use outboard preamps with certain coloration to them (like a tube preamp or a transformer-based Neve style of preamp)

Could I also build two small, op-amp based preamps; sum them and then use a trim pot at the output so that it doesn't overload the subsequent preamp?

And you think that manipulating millivolt signals before a preamp will be _quieter_ than mixing line level signals? If that's the case, a LOT of manufacturers (like all) have it wrong. Use 1 preamp of whatever your preference is per mic and sum together the high level signals.

With my AKG C451s setting the preamp gain to 46 dB covers most of the bases. Now and then you may need to reduce it. Brass instruments get LOUD but most 'normal' situations are good with the 46dB setting. BUT, that preamp can deliver 25 V p-p which is pretty danged loud. If you're going into a PC sound card with that, you may blow out the input amps. Peak limiters or cautiously set output level controls (better yet both) can keep everything OK.

My preference is for transformerless because I'm too cheap to spend the bucks on a better transformer. The noise level with the AKG condenser mics and my transformerless preamp is lower than the ambient level in whatever room I've ever used them in so I'm not going to change it.

 
But why do you need this if you already have good mic pre amps? They already do what you want without the extra noise a pot befor the pre will add. (especially with mics that dont have built in amps ie. dynamics)
So what you're saying is that if I try this passively, there will be more noise introduced into the signal due to the pots?




And you think that manipulating millivolt signals before a preamp will be _quieter_ than mixing line level signals? If that's the case, a LOT of manufacturers (like all) have it wrong. Use 1 preamp of whatever your preference is per mic and sum together the high level signals.

I understand that it would make more sense to sum the two line-level signals, but unfortunately I only have one preamp per channel on my interface (4 on the Interface, and 4 Outboard pre's)

In the future (meaning when we have even more outboard preamps), I could just get a line mixer with multiple outs and do exactly that.



What I want this for is a cost-effective way of increasing the number of inputs to my interface even though I would not be able to manipulate them separately after being recorded (committing to the sound before going into the preamp). This is just a project I'd like to do to see if I can do that and make it work.
 

wakibaki

Banned
2008-01-08 11:51 pm
Take a look at Rod Elliott's site: Low Noise Balanced Microphone Preamp. As you can see the link shows a balanced input mic preamp, I presume that since you're using XLRs you want balanced rather than single-ended.

This link: 48V Phantom Feed Supply for Microphones shows how to feed phantom power.

This circuit has 3 inverting stages which can be switched in and out to invert either mic or the summed output. You can't depend on it being right, I just drew it, it's completely untested. Only the summer has any gain.

[IMGDEAD]http://wakibaki.com/images/pre_summer.jpg[/IMGDEAD]

I had to split the diagram to make it legible. Only 1 mic circuit is shown, since they're identical. The first stages are balanced -> single-ended. The second stages are inverters. The third is a summer. The last is another inverter.

[IMGDEAD]http://wakibaki.com/images/pre2summer.jpg[/IMGDEAD]

[IMGDEAD]http://wakibaki.com/images/pre3summer.jpg[/IMGDEAD]

There's a gain of ~2 in the summer, this will give you something to turn down. The summer is inverting, which is why I included the 3rd inverter. The 6k8s in the phantom supply need to be matched. The 22uFs at the input are non-polarised. Read the stuff on Rod's site for PSU details. I still think it's all a bit much.

w