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INA217 Microphone Preamp

July 17th, 2016

This project incorporated a lot of firsts: my first working circuit powered by mains electricity, my first preamp and the first time I actually took some design initiative — I designed the (very simple) power supply and incorporated a clip detector into the circuit.  I opted for a “wall wart” input to supply phantom power for a couple reasons.  First, it kept the power supply design simple.  Second, it allowed me to eventually use the same supply to get power to multiple preamps, so long as I add the same input jack to all of them.

The preamp design is straight off the INA217 op amp’s datasheet.  The clipping indicator design was sourced from the Elliot Sound Products website. I source a lot of my parts from ebay, looking for multiples of the same item so I’m always building an inventory for future projects.  As a result, I have no idea how much the total supply cost is.  The expensive parts were the transformer ($24) and the case ($12).

Some other design considerations were to use a 1/4″ TRS jack instead of the standard XLR connector.  They’re cheaper, and I was intending from the get-go to incorporate the preamp into a patchbay setup.  Using a TRS connector doesn’t change its ability to take a balanced line.  What it does do, however, is allow the careless user to damage a mic by using a TS cable with phantom power.  For the single-engineer home studio this isn’t a problem.  It actually gives me a really good excuse to keep my roommate from using my gear.

The op amp’s datasheet does not specify a design for balanced output, and while that may be a minus on a commercial unit, it’s of little consequence to me since the pre will always be in my control rack with about a 3 foot cable run to the audio interface.

I opted for stripboard construction over more compact methods because I just wanted a working, reliable preamp.  For me, that requires plenty of soldering room.  I actually like the industrial look and size of it.  Perfboard irks me to no end and at the time I hadn’t yet worked up to etching.

For the phantom power supply, I found a 48v 375mA wall wart on ebay and rehoused it with a standard output power jack in the front and a standard mains IEC connector in the back.  To get the power to the pre, you can use a run-of-the-mill pedal daisy-chain cable.  With microphones typically consuming 10mA of power tops, there’s plenty of capacity in this supply.

Circuit design sources:


Stripboard Layout:

The schematic for project 146 specified a pot between pins 2 and 5 of the TL072, and in the notes, it is recommended that the pot be replaced with a 10k resistor for fixed-gain applications so I ended up going for the 10k resistor.

I had a hard time finding the right pot, but in the end — contrary to what’s recorded in the schematic — I went with a 2.5k reverse-log taper.

Build Photos:

Designing the circuit board.

Building the circuit board.

Putting the box together.

Rehousing the phantom power adapter.

Label of the phantom power adapter specs copied from the original housing.

The transformer.

Power supply detail.

Installed circuit detail.

Power section all put together.

Open box with front panel.

Open box with rear panel.

Top View

Finished Preamp:

Front panel: Pilot light, +48v input, phantom power switch, phantom power indicator, clip indicator, gain pot.

Rear panel: Input (TRS), Output (TS), power on/off switch, mains power jack.

Hooked up and powered on.

In the rack.

This pre definitely holds its own alongside the stock preamps in my interface, which was exactly what I wanted: simply more channels of clean gain. Considering the ART TubeMP retails at about $50, this maybe wasn’t the cheapest option. On the other hand, the DIY build is more elegant, not to mention repairable — should something go wrong.

The 1-watt power amp

March 1st, 2012

For a long time now, I’ve wanted to hack into a boombox, find the audio input on the circuit board and solder an input jack to it.  There are so many totally functional boomboxes out there that are useless for those who keep music on an MP3 player.  I tried this experiment a few years ago with no success.  Now I’m ready to design and build a dedicated recording studio in a spare room of the house, and part of that is coming up with a home-built power amp project.  Having only dealt with battery-powered voltages before, I thought I’d give the boombox hack another try–for practice.

My victim was an Emerson FM radio/CD player, bought for $35 at Target in 2004. Once I got it taken apart, I had to isolate the amplifier, which was combined with the FM radio on the same circuit board. After studying the layout I took a shot at analyzing where the division between the radio and amp was. I drew a line, scored it with a blade, and snapped it in half!

Finding the audio input to the amp was pretty easy because the CD player was a separate board and I could just use the points marked “R” and “L” that were wired to the second board. With some alligator clips and an MP3 player I could test if the broken-in-half-board still functioned as an amplifier–and it did.

I’m mounting everything inside of a cookie tin almost as if it’s a high-end standalone power amp. It’s not. It operates on 9 volts and outputs 1 watt judging by the specs stamped on the little 3″ speakers. It’ll end up being a funny little desk radio–probably about as loud as your average computer speakers. But I like rehousing things and I really need practice doing chassis work. Despite my attempts at precision, the tin got scuffed up because I was trying to use a dremel and a diamond bit to make the rectangular AC power input. It turned out horribly irregular so I made a plastic one to go over it.

The pictures below show all the components mounted into the tin. It still needs to be wired, painted (since I scuffed it up) and I need to make a lid and speaker enclosures.

Front View
Front panel: input jack, volume control, power LED and power switch

Top View
Top view: the large circuit board is the amplifier. Mounted on the right is a 9v battery, the power supply circuit and transformer.

Back View
Back panel: AC power receptacle and two banana jacks for each of the right and left speaker outputs.

Bottom View
Bottom view: I mounted the circuit boards using metal standoffs and then I used small nuts and bolts for the transformer.

I just set up this workshop in my basement. If you think the cat pictured here might be yours, sorry but he really likes it here.

I remember what this was like…

June 16th, 2008

I now have my laptop back and glass in my passenger side window.  Such simple things, but what an effect.  Now I can get back to work on my life.  Wait, do I want to do that?

I got a neck in the mail today.  $25 of Ebay.  A slightly strat-style design, but not looking too standard as far as fit.  (I lined the holes up to my Jackson and they’re off.)  Perfect for a new build, though.  I have to keep an eye on these.  Necks can go pretty cheap because they can all have slight differences.  But it seems that if you’re building from scratch, you just start with the neck and work around the differences.   New necks of standard dimensions are rarely seen under $100.

I have a show tomorrow.   As far as I know, they’re still looking for people to fill in the night.

Oh yeah.  I designed another guitar last night, after looking at Telecaster Thinline forum threads.  I designed a PRS-type body with a drip-shaped f-hole, that can be built hollow with a wood frame and masonite.  I really want to make it a 12-string, if possible.

Guitar Design

June 12th, 2008

I get into these phases which, if you’re on the periphery of my life, look like ruts, but they’re not. I get an idea and I want to run with it, and that gets in the way of what I’m “supposed” to do.

I’ve completed my guitar design, and now I have a general drawing of the whole thing, left and right sideview drawings (to show the contours), a wiring diagram and pickguard layout, and a rough routing diagram.

On top of that, I got another idea for a guitar design, and it threw off my sleeping schedule. I was laying in bed a few nights ago, trying to fall asleep, and it hit me. I stayed up sketching it out until about 4 a.m. I’m not saying what it is yet. I want to get the plans done first. It’s a unique idea that there may be a niche market for. It would also be a good one to start out with as a practice build–but that’s all I’ll say! I already bought a neck off of Ebay.

I’ve been harassing Geek Squad the best I can. I sent a customer complaint online and by phone, demanding my computer be done by Saturday, and I’ve been calling them constantly. I had to explain to one of them on the phone that it doesn’t need to take a week for something to get shipped from Minneapolis to Kentucky. Every time I buy something expensive it gets shipped two-day Fed Ex. Geek Squad, on the other hand knows they’re losing money when a repair job is under warranty and instead goes with the pony express. Figures.

Blog Update Complete

April 3rd, 2008

I’m finished installing, updating, configuring, coding, reinstalling, swearing etc. As you can see the new blog is up and running at its permanent location. Permalinks are on, RSS should be all set, and I can get on with my life. In the process of all this I got a little primer in PHP, SQL, XHTML and became a lot more familiar with CSS.

The theme you see here is an almost completely reconfigured version of the Kubrick theme that comes default with WordPress. Basic structure is the same. I changed all the images and most of the text styles and rollovers. I made custom bullets (the peach donuts) and made the formatting a little more dense.

Another cool thing is that WordPress allows you to edit the timestamp. That means I could add all the old posts dating back to when I started the Blogger version, and put the time stamp in the past so any permalinks from before should work, if there are any.

I’ll be phasing out LiveJournal shortly.