A new mixer quest

[David]

It appears that I need a line-level mixer for my church rig because I want to run my guitars, bass and bass pedals through my on-stage power amp and speaker.  The power amp currently resides in my bass rig.  I'm going to strip this rig and put the components into a rolling rack I have.  I'm going to put all my rack equipment in there so everything is in one place and usable at a moment's notice.  Anyway, it looks like I need a line level mixer that has multiple output busses and only takes up one or two rack spaces.  I looked bloody hard, and nothing appears to be available that remotely resembles this configuration.  OK, DIY to the rescue, right?

In preparation for this quest, I pored over mixer-related postings here -- particularly Yano's illuminating spiritual mixer journeys.  I think he was a bit more ambitious than I am.  What I'm thinking about is a unit with four input channels (guitar, bass, bass pedals and a spare).  Since I can't predict easily whether these will be instrument- or line-level signals,  the input preamps will be GEO Adjusticators running at 12 to 15 volts and constructed with TL071s or 5534s.  This way, I get cut and boost on the gain control and I don't have to think about whether the input is instrument or line level.  I plan to put a mute switch on each channel before the Adjusticator.

Each input channel will feed three separate output busses, each one implemented with a GGG Mini Mixer.  This was explicitly chosen because its inverted inputs match the Adjusticator inverted outputs.  I will run these at 12 to 15 volts also and plan to construct them with 5532s.  I'm not going to use the stereo jack trick or use the indicator LED as I will put a LED power indicator on my power supply.

Finally, I'm going to include a headphone amplifier.  This decision gave me more grief than it should have.  I tried hard to figure out how to use the PAIA headphone amp, but failed miserably because it uses a bipolar power supply and the other two circuits don't.  Deciding to build on past successes, I chose to implement the headphone amp with the circuit in Alan Gary Campbell's "Not Just Another Headphone Amp" article at HeadWize.  I built one of these several years ago and used the heck out of it.  Besides, the PCB mask and population diagram were included.  Better yet, I've got some LM386-4s, so I can run this at 12 to 15 volts also.

Now, for power:  I'd like to supply power to this unit with a wall-wart that puts out approximately 18VDC unregulated and approximately 500ma.  Is this enough, or gross overkill?  Inside the unit, I'll have a board with the usual capacitor and regulator setup.  For each of my circuits, there will be a hum filter on the positive power rail consisting of a 100 ohm resistor and a 1000uf or a 2000us capacitor to ground.  I've used this trick with good results in the past.

I'd like to put parallel output connectors on each output buss.  You never know when you might have to feed two outputs...  Would a 5532 running at 12 or 15 volts have the headroom and oomph to be able to do this?

Does this seem like a good plan?  Are there any traps lying in wait that I've missed?  Any advice from those who've done it before?  Replies from Yano or John Egerton would be particularly appreciated!

Thanks, guys!  Looking forward to getting back to circuit-banging!

[Peter Snowberg]

It sounds like a good plan to me.

Your 18V supply should be just fine. I would recommend adding a 7812, 7815 or LM317 to regulate your positive rail at +12 to +15.

The 5532 would be THE opamp to use IMNSHO for output buffering. Good choice.

Best of luck your your build!

[grapefruit]

Hi David,

The old Passac Unity Eight mixer would do this. It has eight channels, a stereo output and two auxillary (pre fade) sends in 1RU. They haven't been made for some years now though. I would have thought there would be something around in 1 or 2RU.

Stew.

[David]

Stew:

There just happened to be one of those on Evilbay, so I checked it out.  I don't think that would quite do what I need.  Great idea, though!