Mic + Guitar Mixer

[Wighty]

Hey all,

After wowing my fellow band members with my diy stompboxing skills [tonebender, bazz fuzz + amz boost] they said that a mixer would be handy for our amp starved practices, so that multiple microphones and maybe extra guitars could be used. So i had a quick look around and started on GGG's mixer. However i managed to screw that up as am hopeless with circuits using IC's. So i had another look around and found one of aron's old schematics:

http://www.aaroncake.net/circuits/mixer1.htm

Would this be suitable for mics and guitars?
Would i be able to modify it to suit mics?

Many Thanks,

Wighty  

[Noplasticrobots]

By "modify it to suit mics" do you mean adding an XLR input? I'm sure you could. But I don't know how.

[Wighty]

I'm quite new to this, but if that would make a mic work through it i guess so!

Wighty

[Wighty]

Just found out what the xlr input is and yes ill probably have maybe 2 of those and then 2 1/4" sockets does the schematic need any extra mods for this..?

[Mark Hammer]

Balanced mic lines don't HAVE to plug into balanced mixers.  They won't be as hum free as possible in that case, but they will work just fine if one of the "hot" leads is also treated as ground along with the ground pin on the XLR plug/socket.

Ah, for the good old days when a "good" amp could be identified by how many input jacks it had!    Mine had one channel and four inputs!

Just note, however, that a mic and a guitar put out vastly different signal levels so you can,t just stick a mic into one jack, the guitar into the other and expect music magic from the amp.

What might work better is if you build the GGG mixer and do the following:

• Replace the 100k feedback resistor in the first op-amp with a 220k unit.
• Replace the 100k input resistors for two of the 4 inputs (let's say R5 and R6) with 10k units, and two of the other inputs with, say, 150k units.

What you end up with is two inputs that have a fixed gain of x22 (good for boosting those mics), and two inputs with a boost of about x1.5.  That should alow you to use the volume pots for the various inputs in a roughly comparable way.  IN other words, you won't find yourself struggling to find the "right" mic setting in the last 10% of pot rotation, and struggling to find the right guitar seting in the lower 10% of pot rotation

Note that dropping the 100k value to 10k with a 0.1uf cap will produce a low-end mic rolloff around 160hz, which (unless you're in an all Barry White material revue that does "Elvira" by the Oak Ridge Boys for an encore) will atually help reject a lot of dangerous bass content from the mics.  The other two inputs will roll off around 16hz.

I certainly don't want this to run away with you in terms of complexity, but my sense is you will be in a small rehearsal space too, right?  IN which case, feedback becomes an issue.  A single-band parametric section for feedback rejection might be a good idea and could be easily integrated into that second op-amp section so you can get a little more mic power in that small space without having to hear screeching along with it.  See the document on simple parametric EQ at GEOFEX: http://www.geofex.com/Article_Folders/EQs/paramet.htm

[Wighty]

Okay, thats all great advice! But could i apply it to the jfet mixer [link at top of thread] ?? Its much simpler, so chances are i wouldnt mess it up, but would it be as effective?

Thanks again.

[Mark Hammer]

One way or another, you will have to contend with the immense difference in output level between voice mics and instruments that have passed through pedals, and will have to find a way to get them close enough in amplitude that the volume controls on the mixer are usable.  The suggestion I offered above is intended to situate the needed gain in the mixer itself.  Of course there is nothing to stop you from throwing together a little booster box (remember the LPB-1, and its soon-to-follow cousin the "Ego" were pushed by Electro-Harmonix as a way of bringing mic level up) to plug a mic into and run a cable from there to your mixer.

The other thing is that I have no idea what sort of amp this is feeding.  If its a two channel amp with separate volume and EQ per channel (like many older Fenders) then a 2-input mixer for voice on one channel, and a 2-input mixer for guitar on the other could work out very nicely.  You would just have to crank the volume control on the mic channel and turn it down a bit on the instrument one.

All told, though, I think my earlier suggestion probably works out better a a generic solution that can be used with just about any amp, even single channel ones.

[Wighty]

Well... Due to ease, ill try assembling the simple mixer at the top of the thread, maybe inserting this simple mic preamp i found after the input stage?

Although its probably not ideal, will it still mix okay!?

http://www.epanorama.net/circuits/micamp.html

Thanks once again.

[Mark Hammer]

If your two inputs are both mic, then stick it after.  If at least one of the inputs is NOT mic, then you'll need to provide that preamp for the mic but not the instrument signal.

[Wighty]

Okay i will do! thanks for all your help.

Wighty