Microphone Stompbox Blender/Looper

[moosapotamus]

I did a search and came across a couple threads on this subject. None of them really boiled it down to an actual design or schematic. So, I'm bringing it up because it seemed like there was at least a little interest, and I think I could use something like this...

It's an effect loop/bypass box with a wet/dry blend that allows you to put a dynamic microphone (i.e. SM58) through one or more stompboxes. Might also be interesting to house it all in a wah pedal shell using the rocker pot as the blend control.

Input and output need to be low-Z with XLR jacks. Input also needs to provide gain (mic preamp). Effect send and return need to be hi-Z with 1/4" jacks.

I'm thinking of using the following building blocks...

Input -
Balanced input mic preamp like maybe the INA217
http://focus.ti.com/docs/prod/folders/print/ina217.html
But, I'm not sure about the output impedence... too low???

Send/Return & Blend -
Splitter Blender, Buff n Blend, B Blender, Paralooper... take your pick.

Output -
DI box circuit... here's one...
http://sound.westhost.com/project35.htm

Put 'em together in that order and... whatdy'allthink? Any potential problems or concerns? If nothing major, I'll draw up a scheme and start working out the nitty details.

Thanks!
~ Charlie

[boogietube]

I don't know, but I 'd really be interested in your paralooper project with an added  D.I. output (3 pin xlr) and parallel input.(1/4")
Sean

[jmasciswannabe]

YES YES YES!!!! Can't wait to see what you come up with! The vocalist in my band would love something like that. The blend idea is excellent!

[R.G.]

Oughta work. Unless you're doing studio work with this, the fancy input gain preamplifier may not be needed. You can get performance suitable for live performance from ordinary low noise opamps.

Once you get it up to a suitable level, you can run it into both the effect and the blender.

For a blender, I suggest you read "Panning for Fun" at GEO (http://www.geofex.com) as that shows a particularly useful form of blending.

After that, a DI ought to do it.

Just shuffle the blocks around!

[moosapotamus]

For a blender, I suggest you read "Panning for Fun" at GEO (http://www.geofex.com) as that shows a particularly useful form of blending.

Yes! I almost forgot about your panning write-up, R.G. That looks like the best way to go for the blender section. Thanks (again!), R.G.!

I guess the only part of this idea that I don't really know enough about is impedance matching. Should I be concerned about this? Shouldn't the output impedance of the preamp section match the input impedance of the blender section?

I like the fancy input preamp. I'm thinking if I build this, I might as well make it so it can stand up for studio work as well as live.

Thanks!
~ Charlie

[R.G.]

I guess the only part of this idea that I don't really know enough about is impedance matching. Should I be concerned about this? Shouldn't the output impedance of the preamp section match the input impedance of the blender section?

Impedance matching is important when you're transferring power or when you don't want reflections, or when you're trying for super-ultra low noise, like with moving-coil phone inputs (ulp... when was the last time you listened to a spinning record?).

Otherwise, if you're transferring signal as a voltage, you want the sending output impedance to be at least a factor of ten lower than the receiving impedance. Matched impedances *guarantee* a signal loss of 1/2. Mismatches of sender to receiver of 1:10 or more guarantee not loading down the sender.

For generality's sake, if you were sending a signal best described as a *current* (we don't in general do this, but strange things have been done) then you'd want the sender's impedance to be at least 10 times greater than the receiver, so the receiver did not block part of the current from flowing. We don't normally do our signals this way, so forget I said this.

[moosapotamus]

Thanks! OK... Here's a first pass at a schem for this idea...



Anyone see any potential problems?

The mic input is a female XLR and mic output is a male XLR jack.
Send and return are 1/4" closed-circuit jacks.
TR1 is an internal trimmer to set the max gain.
Gain to the effect send and the blend section is controlled with VR1.
VR2 is the blend pot.
VR3 controls output volume.

The DI box could be any DI circuit. I'm sure Rod Eliott's (linked above) would sound good. But, a transformer would be much simpler. And, since the purpose of this box is to mangle , a cheapo from mouser might be just fine.

I'm not totally sold on the inverting output buffer. Maybe just add the resistors and make it non-inverting, or throw in a phase invert switch somewhere like at the input, maybe, or not?  What do y'all think?

Will this circuit will work well? Anything else look like it might need fixing?

Thanks, again!
~ Charlie

[petemoore]

  Wonderful and practical thing to have, I should be building and using something like this.
  When I compare what my guitar has 'on it' [6+ effects, 2 or 4x12''ers etc.] and what my vocals have 'on 'em', [a JBL Eon, 15'' and a horn], what stands out is that the vocals sound dry...and could use 'attention'...something like a pedalboard for mic's is a fantastic idea.
  No sound man to not have that doesn't know what you want.
  Consistant effects that can be gotten used to.
    RS doesn't have but the small overpriced little pieces of perf for sale these days around here...
 

[petemoore]

  A momentary on long echo would be quite the useful item...like the "Hello' s in Comfortably Numb...hold the switch down for a buncha long preset repeats.
  Variable amount of Reverb would be nice to be able to easily control when singing.
  *Mic Off [when not in use]...I've seen designs for this ranging from a sensor pad where your feet are..'enabling' the mic when you stand in front of the mic...to mic input level sensors...anything below a set level of input turns the mic off...this can REALLY clean up 'stage mess sound' from getting into the PA. It keeps 'crap' from being re-amped and adressed to the audience...try turning the vocal mics off and notice how much the mix can clean up when it's not amplifying all the added [non-vocal] 'junk' sounds of guitar and bass amps and drums etc. that the vocal mics pick up when left on. Also notice how much more volume before feedback is available when  only one mic is predisposed to create unwanted feedback...clearer signal of the 'wanted to amp source', less feedback/less mush =improved performance.
  Distortion on vocals is an area I don't know much about except by trials...putting a Dist+, FF, BMP etc. between mic and input...and that tends to be hard to work with other than putting a nasty grind [cool sometimes] on the vocals...a "Tube-Like' distortion I don't know of might be cool...I used a Dynakit Tube Amp driven hard into 4x12's and that was quite a 'nice' mild/usable distortion tone for vocals I used to use often...it's what I had and I liked it alot...would like that again, or reasonable facsimile...
 

[moosapotamus]

Thanks for the comments, Pete!

The momentary switch idea is great. Smallbear sells a momentary DPDT stompswitch that I think might be perfect... one side is normally open, the other side is normally closed. You can use a toggle switch to select which side you want to use, so when you stomp on it you are either engaging or disengaging the effect(s), depending on how you have the toggle switch set.

A mute switch is a great idea, too.

Thanks!
~ Charlie