DIY cheap 9v preamp for dual source MIC

[rschultz]

I've always thought that Phantom Power was 48v... but turns out all you need is 9V for condenser mics in a T-powered scheme (2 conductor, not 3). So based on Doug's Adding an Internal Mic blog and the link within about Powering Microphones, I decided to try this. I also shared some R&D of how I wired up the mic in one of Doug Youngs threads called Adding an Internal Mic. Although that thread had the cap and resistor on the mic, whereas the box I built below had those components.

Here's the circuit:


I bought a Shure WL50 lapel condenser mic for $30 off Reverb. This was a $200 mic back in the day, probably 10-15 years old... but in great shape and worth it to experiment with. And I had a 10 mf cap and a 2k2 resistor in my stores.

I tried to power it with my 9v pedal board supply, most of us have one of those. Unfortunately the 9v power supply (daisy chained with my Helix) resulted in a 60 hz ground loop hum, so I had to use a battery. An isolated power supply (VooDoo, Cioks, etc) should also work.

I bought a Hammond 1590A box, drilled some holes and mounted four 1/4" jacks and 9v jack on top. The top left/right jack are for the mic, you can see the cap and resistor wired in according to the circuit diagram. The bottom left/right jacks are for the pickup, straight through to my tuner. Both then go out to the Helix where I mix them together.



Bottom of the preamp box:


It's all fed into my Helix:


The pickup has an IR on it and other EQ, reverb, mod, etc. The mic just has a preamp block with a HPF (350 hz) and LPF (12k) on it.

I haven't wired it into my guitar yet, but I did test it with the mic just draped over the guitar and hanging in the sound hole. I setup snapshots to turn the mic on and off. Once I got the gain/level right and mixed properly, it sounded great. I can really hear how the mic adds high end definition and sparkle that the pickup just doesn't have.

For reference, I did this both with a K&K+Ultratonic pickup as well as a Barbera Soloist. Both sound really good by themselves, but honestly sound even better with the mic.

[Doug Young]

Very cool! That should be useful for a lot of people

[Cuki79]

Nice!
Why not using a bigger box to include the 9V battery inside?

[rockabilly69]

Originally Posted by Cuki79

Nice!
Why not using a bigger box to include the 9V battery inside?

I was thinking the same thing? Or is he going to wire this circuit into the guitar?

[Doug Young]

He's powering it from the power supply on his pedal board. He already has power for his other pedals - so no need for a battery to go dead!

[rschultz]

No they are right. I can’t use the 9v power supply because of a ground loop. So I am planning just what they suggest, a bigger box for the battery. A 1590B or G should be big enough.

I bought this battery holder from Reverb.

I’ve got a set of rechargeable 9v I used on my Taylor that would work great.

[Doug Young]

Originally Posted by rschultz

No they are right. I can’t use the 9v power supply because of a ground loop. So I am planning just what they suggest, a bigger box for the battery. A 1590B or G should be big enough.

I bought this battery holder[/URL] from Reverb.

Hmm, that's odd. Are you using an isolated supply? I don't see why this would cause a ground loop any more than another other pedal.

[rschultz]

Originally Posted by Doug Young

Hmm, that's odd. Are you using an isolated supply? I don't see why this would cause a ground loop any more than another other pedal.

No I’m just using the wall wort PS that came with my Helix, with a simple splitter. So not surprised. Yes an isolated Voodoo or Cioks would work.

You can kinda see the splitter in that picture above.

[rschultz]

I finally had a chance to properly install that $30 mic into my guitar. It was rather long, so I cut it down to just what I needed and mounted it like Doug suggested in his blog.





Here's a picture of my Helix path:
Barbera pickup is on top, with custom IR, EQ, Mod and reverb. The volume block at the beginning just helps me turn it off when setting up the mic... I'll eventually get rid of it and just use snapshots for that.
Mic is on bottom, the preamp has the high and low pass filters. I put a noise filter (dyn) after it, seems to help filter the noise floor down to the same level as when it's turned off.

[rschultz]

I mentioned this before, but it is noticeably better. I thought the Barbera with the IR sounded pretty good. But after adding a mic... that makes it sound stale on it's own. Really brings out the high end presence.

My other guitar has a K&K+Ultratonic and custom IR. It probably sounds more natural than the Barbera/IR does. Can't wait to add a mic to that one!

[rschultz]

When I installed it in the guitar, I setup the endpin jack for both pickups using different jacks so I wouldn't get them flipped. On the TIP (barbera), I used a 2.5mm jack because it has very small wires. But the mic has bigger wires so I used the standard 1/8" jack. This way I can plug and unplug each one if I want.

[Doug Young]

Great, nice work. How does it sound?

[Cuki79]

Is the barbera active?

I see you use unshielded cables. I would try bringing a plugged power supply (ex: 1 spot) close to the guitar to see if I get hum…

[rschultz]

Originally Posted by Cuki79

Is the barbera active?

I see you use unshielded cables. I would try bringing a plugged power supply (ex: 1 spot) close to the guitar to see if I get hum…

No the Barbera is passive. I’ll check.

[rschultz]

Originally Posted by Doug Young

Great, nice work. How does it sound?

It is noticeably better. I thought the Barbera with the IR sounded pretty good. But after adding a mic... that makes it sound stale on its own. Really brings out the high end presence.

My other guitar has a K&K+Ultratonic and custom IR. It probably sounds more natural than the Barbera/IR does. Can't wait to add a mic to that one!