Topic: My Naiant littlekit INA111 Kit preamp project

[Chuck]

I have wanted to build a small, portable, stereo microphone pre-amp that provides 48v DC power to the microphones straight from batteries for a while now.

The opportunity to purchase the batteries and chargers for cheap http://www.skycraftsurplus.com/index.asp?PageAction=VIEWPROD&ProdID=2015
presented itself recently, so I started working on it. Around the same time, Jon, at Naiant offered an opportunity to buy the stereo pre-amp gain circuit for a great price too!  http://taperssection.com/index.php/topic,122713.0.html

I used a Hammond 1590BB enclosure.

Here are some photo's:

[Chuck]

I had the XLR and RCA connectors. It was just a matter of fitting it all into the box and make it work ergonomically.

A little hammered gold spray paint on the outside of the enclosure...

[Chuck]

... and a sticker from Jon and it's done.

I haven't had the chance to use it yet, because I found a small problem after it was totally assembled. Jon at Naiant  is going to help me resolve that problem.

I'll report back on how it sounds after I get it working perfectly.

[Chuck]

Some notes on the batteries...

Those batteries charge (on average) to 10.1 v each.
So, 10.1 x (5 batteries) gives me 50.5v DC. Which is within the 48v phantom power spec of 48v DC +/- 4v.
After the batteries sat for two weeks I measured them again. They averaged out to 9.4 v each so, 47 v DC.

I figure when they eventually get to 9 v DC (9 x 5 = 45v DC - still in spec ) and it'll charge them again.

[Chuck]

Here's the schematic for the gain circuit. I think Jon had to tweak it a bit, to work with my 48v DC power source.

[Chuck]

Cool, and no CNC work necessary what so ever.  It looks to me like there's no way to charge those batt's while in the box, correct?  Looks like you're having fun...

I haven't figured out how to charge the five 9v NiMH batteries while they are in the box.
There is enough room left on the enclosure for a DC input jack, in case I ever figure out how to charge them in the box, in the future. Since I bought six 9v batteries, I have three rechargers that charge two at a time. That's my charging plan for now.

I always liked the way that the Oade brick pre-amps used batteries and not a DC/DC upconverter circuit to provide very clean phantom power to the microphones. The 9v NiMH are not as heavy or bulky as the SLA's that are in the Oade pre-amps. When I saw how cheaply I could buy the NiMH's it was a done deal. I started collecting everything I needed to just build a pure battery powered phantom power box. Then, I saw Jon's offer in the retail section to sell his orphaned pre-amp circuit boards from his littlebox pre-amp product. They fit with what I was already doing, perfectly.

I drilled holes for everything with a standard 3/8" drill. I didn't even use a drill press. So, yeah, no CNC work either. I'm much more concerned with function, than cosmetics. But, I don't think it looks that bad. The paint didn't come out exactly the way I thought it would. I wanted a more raw yellow gold color. I'm going to leave it though. The special 'hammered' finish spray paint I bought helped cover some defects in my drilling and filling.

[Chuck]

I think I stuck pretty much to that schemo for yours, except I swapped out TIP112 for a FET into a power BJT--effectively still a Darlington, except with massive current gain (which isn't necessary here) and more importantly, lower dropout.  Again not necessary here, but that's the configuration I use in littlebox as it conserves about 2mA in the regulator and drops 0.7V less to its output, so I felt like testing it.  The basic goal is to keep the supply to the chip within its limits, so I need a reg to shed voltage above 36V.

Also, you have 6k8 supply resistors.

I have to thank Jon for agreeing to look at the box after I finished building it. I compromised his circuit somehow when soldering the leads or installing it into the enclosure. You'd have to see that circuit board in extreme closeup to appreciate how small the surface mount components are.
I can follow and sometimes diagnose through hole built circuits. But, with that surface mount stuff I'm in over my head.

[Chuck]

Do post some sound clips.  If the board is still available I think I'd like to try this as well.  Nice looking box in my view. 

I have a spare of the Hammond case I used and doubles of some of the other components... like the RCA jacks, switches etc... PM me and we can work something out if you end up purchasing one of the orphaned pre-amp circuits from Jon. I used to really enjoy building circuits on pref boards, but I'm finding that I enjoy doing the enclosure work more now.

[Chuck]

The Grace Design V2, V3, Graham Patten DMIC-20, DMIC-24 and Denecke AD-20 are all instrumentation based microphone pre-amps. The instrumentation amp based pre-amps have been getting more popular over the years, because the instrumentation based microphone pre-amps require less discrete circuitry than op-amp based pre-amps and sound pretty darn good.

I started getting interested in these amps after reading this:
http://www.audiomasterclass.com/arc.cfm?a=giant-killing-$5-mic-preamp-its-secrets-revealed

An electronics hobbiest can build one of these pre-amps fairly cheap. Again, read the $5 mic Preamp article.

For me, with the parts I already have, plus the cheap batteries and the already designed, layed out, stuffed and tested circuit that Jon sells. I built a small, efficient very clean, 48v phantom powered pre-amp to run line-in to any of my recorders.

The unknown element is how it sounds... I know that it will be clean and efficient from my experience using the various other pre-amps based on the instrumentation amps.

I enjoy tinkering with electronics and view it as a fun thing to do. I anticipate that the pay-off will be a small, very clean, low noise, relatively cheap microphone pre-amp.

I plan to post recordings made with this pre-amp and report back my experiences good and bad. After building it, I have a good feeling about it.  


edited for spelling...

[page]

Cool, and no CNC work necessary what so ever.  It looks to me like there's no way to charge those batt's while in the box, correct?  Looks like you're having fun...

I haven't figured out how to charge the five 9v NiMH batteries while they are in the box.

While it's almost the size of the box w/ 15 cells; if you went to LiFePo4 3.2v cells at around $5 a cell, you could provide 48v at 1200mah instead of the 160mah ones your using and be festivus-maximus without a recharge...  Actually, building a double-decker box with a standard connector between compartments (say deans) inside would allow you to easily swap batteries/packs without opening the amp section. Just a thought, not sure how practical it would be.

[Kindguy]

T+

[Chuck]

Cool, and no CNC work necessary what so ever.  It looks to me like there's no way to charge those batt's while in the box, correct?  Looks like you're having fun...

I haven't figured out how to charge the five 9v NiMH batteries while they are in the box.

While it's almost the size of the box w/ 15 cells; if you went to LiFePo4 3.2v cells at around $5 a cell, you could provide 48v at 1200mah instead of the 160mah ones your using and be festivus-maximus without a recharge...  Actually, building a double-decker box with a standard connector between compartments (say deans) inside would allow you to easily swap batteries/packs without opening the amp section. Just a thought, not sure how practical it would be.

Yeah, for me having to recharge the batteries externally isn't a big deal. I don't do festivals anymore. I rarely do more than a couple of shows per month anymore.   I like having the batteries inside the case, because it's one less box and one less connection I have to worry about. I had thought of external 48v battery pack option. That's where I would be able to use an external DC input jack.

[page]

Yeah, for me having to recharge the batteries externally isn't a big deal. I don't do festivals anymore. I rarely do more than a couple of shows per month anymore.   I like having the batteries inside the case, because it's one less box and one less connection I have to worry about. I had thought of external 48v battery pack option. That's where I would be able to use an external DC input jack.

Probably cleaner, interesting idea though. 

Looks like a neat box.

[GDfan]

Looks fantastic. I want to hear samples.

[manitouman]

Nice work Chuck! Gold colored...have you named it yet? C3-PO? From the Colorado Crue thread it seems as if Jimna has been huffing your left over paint 

Can't wait to hear some pulls.