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Mic Dual FX Loop Design Help
Mic Dual FX Loop Design Help
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Old 7th July 2018, 11:44 AM   #1
arlester is offline arlester  United States
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Default Mic Dual FX Loop Design Help

Hello All & Thank you in advance for any help I may receive.

My goal is a stomp box with 2 mic inputs / outputs & one switch that toggles between either channel - one channel always on.

When the channel is selected, it is sent to the FX. When not, it is by-passed & sent out.

Only one channel is sent to the FX at a time.

Could this box be built following the same design as a looper for instruments, or is there some other ingredient needed for a low impedance signal?

Mille Grazie
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Old 11th July 2018, 02:36 AM   #2
leadbelly is offline leadbelly  Canada
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If you expect to plug in stage mic's directly to it, then you'll need to use a mic input stage and provide phantom power as well.
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Old 12th July 2018, 08:18 PM   #3
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by arlester View Post
Could this box be built following the same design as a looper for instruments, or is there some other ingredient needed for a low impedance signal?
I think you will need to bring the mic level signals up to instrument level (or line level) before feeding them to the FX loop. This would involve using either two external mic preamps, or building them into your switching box.

This also means you will, at a minimum, need a gain control for each mic; these will need to be in your switch box if the preamps are built-in.

Do you also want tone controls for each mic? Most of the time, the answer is yes. So now you also need to build those into your switching box.

How about phantom power? It's not necessary for most dynamic mics (like the awful Shure SM58), but if you plan to use condenser mics, etc, you will need to build that into your box as well.

By now, your switching box is starting to look like a small audio mixer, and we haven't even come to the actual FX loop switch itself.

For the switch, I think the same concept will apply as the A/B switches you mentioned. Once you have a line-level or instrument level signal coming out of the mic preamps, you can then switch that to the FX send and FX return, or bypass both. That will take two poles for each mic signal, so I think you'll need a four-pole, two-way switch.

I don't know if you can find a four-pole stomp switch, so now you may be looking at having to use a relay or two inside the box, and using a stomp switch to control the relays.

To me, this is starting to look like a rather large and complex project if the entire thing is DIY. It's not going to be the easiest thing in the world to build a pair of quiet mic preamps, along with tone controls for each.

On the other hand, a small audio mixer with insert jacks already contains much of what you need, except for the foot-switch.

If the foot-operated aspect is essential, you can probably build your switch box to interface with the small audio mixer - two of the usual TRS insert cables run from the audio mixer's insert jacks to your foot-switchable box.

-Gnobuddy
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Old 13th July 2018, 12:12 AM   #4
MAAC0 is offline MAAC0  Portugal
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If You are switching live mikes beware of the thumps on the speakers also....
I would use a small mixer like Gnobuddy says.
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Old 14th July 2018, 01:21 PM   #5
arlester is offline arlester  United States
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wow... i had to laugh at myself reading through that explanation. thx!
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Old 14th July 2018, 08:04 PM   #6
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by arlester View Post
wow... i had to laugh at myself reading through that explanation. thx!
I try to remember to laugh at myself frequently. The alternative is to start taking myself seriously, and that is usually a runaway vicious circle that eventually leads to one becoming a self-important twit.

So - how are you planning to proceed? Small mixer and external box for the switching?

-Gnobuddy
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Old 14th July 2018, 09:10 PM   #7
leadbelly is offline leadbelly  Canada
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Another option, and the one that I would do, is build as 2 boxes.

1 box would be a buffered blender/looper/ABY. I would pick a good DIY design for guitar, or make my own, and have both high impedance and low impedance inputs.

Box 2 would be a DIY mic preamp with phantom power. I would use INA217's for this box.
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Old 15th July 2018, 01:02 AM   #8
arlester is offline arlester  United States
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thx for all the advice. i really enjoyed it. i don't think my "simple" little pedalboard-friendly box is going to get past the drawing board phase... not the 1st time : )
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Old 15th July 2018, 03:20 AM   #9
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by arlester View Post
i don't think my "simple" little pedalboard-friendly box is going to get past the drawing board phase...
Well, sometimes that happens. But...if you're already using a mixer for the mics, and it has insert jacks, then the actual switching box won't be very different from what you originally envisioned.

It seems you can indeed buy a four-pole, two way foot switch ( Switch - Footswitch, 4PDT, Blue, 12 Pins | Amplified Parts ), and that means no need to futz about with relays or power sources for the relays. A single stomp on the switch will put the FX loop ahead of one mic, or the other.

-Gnobuddy
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