Modding a pedal design for microphone

[solarplexus]

Hi there!

Long time no see!  I have a quick question for you guys:  Is it possible to mod a guitar pedal to match the impedance of a microphone?  Asking because I need a delay for my vocals live, and would love to have the Madbean pedals (love his stuff BTW) Cave Dweller, (which I also have in my pedal board for guitar), as a vocal delay.  Thing is, we all know that mics are low impedance and guitars high impedance.  Is there a way to modify the input impedance without have to go the mic -> preamp -> guitar pedal -> DI -> board, or rather just mic -> pedal -> board with some TRS cables? 

Any input would be greatly appreciated, and if the only solution is to go from mic to preamp to pedal to DI to board, well... so be it.  But you know how it is with DIY:  We always want to make it simple by making it super complicated! ahaha

Cheers!

Matt

[composition4]

You have to think of a few things... first the two minor things... if the mic is phantom powered, it will still need power sent to it somehow. This only really applies to condensor mics. Second minor point is that most microphones will be balanced, so you either need to have the next stage accept a balanced input, or just convert balanced to unbalanced.  Conversion is easily done by grounding pin 2 or 3 of the microphone xlr cable in your circuit input, i.e. joining it with pin 1.

The two main things you really need to think of though are the output impedance of the microphone as you have mentioned, and also the level. Microphone level is very low, which is why preamps usually offer a lot of gain.  Because the output impedance of microphones varies all over the place, the safest thing to do is have your circuit offer a very high input impedance to cover all bases.

So in that light, I'd probably build an opamp stage into the front end of the delay pedal with variable gain via a pot in the opamp feedback loop up to 60dB-ish, then have that stage feed the rest of the circuit.  I'd then put an opamp output buffer at the end of the circuit followed by a pot in a voltage divider configuration so you can match the output level of the delay pedal to your mixer input.

Also make yourself up a cable that is XLR on one end, an the other end pins 1 (GND) and 3 (COLD) both join to the sleeve of a guitar jack plug, and pin 2 joins to the tip.

So that way you could plug microphone into pedal with your xlr cable, then pedal straight into a mixer insert.

The only caveat with this solution is that you lose isolation so you should probably power the pedal with a battery to avoid ground loops, and also you lose the balanced output of the microphone. You could fix this by using a balanced input as the first stage of the pedal, but this adds complexity.

Anyway sorry for rambling it's late here, if you need more detail or help with drawing up a schematic I'll give you a hand tomorrow

Jonathan

[wavley]

Or you could just buy a used Tube MP for like $20 and use that as your mic pre/line out.

[solarplexus]

You have to think of a few things... first the two minor things... if the mic is phantom powered, it will still need power sent to it somehow. This only really applies to condensor mics. Second minor point is that most microphones will be balanced, so you either need to have the next stage accept a balanced input, or just convert balanced to unbalanced.  Conversion is easily done by grounding pin 2 or 3 of the microphone xlr cable in your circuit input, i.e. joining it with pin 1.

The two main things you really need to think of though are the output impedance of the microphone as you have mentioned, and also the level. Microphone level is very low, which is why preamps usually offer a lot of gain.  Because the output impedance of microphones varies all over the place, the safest thing to do is have your circuit offer a very high input impedance to cover all bases.

So in that light, I'd probably build an opamp stage into the front end of the delay pedal with variable gain via a pot in the opamp feedback loop up to 60dB-ish, then have that stage feed the rest of the circuit.  I'd then put an opamp output buffer at the end of the circuit followed by a pot in a voltage divider configuration so you can match the output level of the delay pedal to your mixer input.

Also make yourself up a cable that is XLR on one end, an the other end pins 1 (GND) and 3 (COLD) both join to the sleeve of a guitar jack plug, and pin 2 joins to the tip.

So that way you could plug microphone into pedal with your xlr cable, then pedal straight into a mixer insert.

The only caveat with this solution is that you lose isolation so you should probably power the pedal with a battery to avoid ground loops, and also you lose the balanced output of the microphone. You could fix this by using a balanced input as the first stage of the pedal, but this adds complexity.

Anyway sorry for rambling it's late here, if you need more detail or help with drawing up a schematic I'll give you a hand tomorrow

Jonathan

Wow!  Thanks!  That's a good start!  I could basically build 2 boosters one for input and one for output to control the input gain and output gain?  Running on a battery is actually prefered:  Less cabling. 

[solarplexus]

Or you could just buy a used Tube MP for like $20 and use that as your mic pre/line out.

It is a good idea, but I want to keep the cable clutter as minimal as possible.  With the pedals and 2-3 guitars, it's already quite a lot!   

[Ben N]

two points:
1) If you're doing stuff with opamps, then it should be no biggie to make a balanced input stage using both inputs on the opamp, rather than grounding one line. Look at the third schematic here.
2) The place to bring your signal to line level is after your guitar-level pedal, not before it. Look for effects loop buffer circuits for the concept--they pad down the hot signal from preamp to guitar level for your pedals, then boost it back up to feed the return.

[composition4]

Agree with Ben on both points. Although considering you'll be buffering the front end of the delay pedal, I think you may as well use it as a gain stage to bring microphone level up to guitar level.  If you have a weak dynamic microphone like an SM57 which will put out 1.6mV max, 60db gain will give 1000x that which is 1.6V peak. That's probably an ideal voltage for a PT2399 to work with.

[solarplexus]

Thanks for the help guys!

A little confused though... should I build a booster to put before the delay or after?  Or both? 

Would a simple LPB-1 would do?

[Hatredman]

Both.

And in an electronical point of view, there is little difference between a mic preamp and what you call "a booster", besides the different working impedances.

I also doubt that the cave dweller has room for both "boosters", so you will need to put them on external boxes anyway.