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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.

Electronics Projects

June 8th, 2009

Digitizing vinyl and mixing my next album has inspired me to build the CMoy headphone amplifier.  I’ve been taking regular walks to the bar with a set of headphones and a cheap MP3 player loaded with in-progress mixes.  Since these are unmastered tracks, and thus uncompressed audio signals, the little USB-charged MP3 player doesn’t have the juice to bring the tracks up to normal listening level.  The more complex the track, the greater the build-up of transient peaks and the quieter the mix.  Vinyl tracks are also uncompressed, which means there’s more difference between the loud parts and the quiet parts.  The CMoy amp adds an extra 18v of power, which means I can plug in the MP3 player and leave it at its default volume, using the amp to drive the headphones.

CMoy Headphone Amp
I’m almost done.  The tutorial I was using suggested a dual pot with a built-in power switch, which I had to order separately from the other parts.  This particular pot works so that when you turn the volume all the way down, the power clicks off.  It’s a wise feature considering that you can seriously damage your hearing with this amp.  This way, it’s always turned all the way down when you power it up.  I tested it out before I housed it (in the standard Altoids tin) and it got pretty loud with one 9v battery.

I took another look at the looper project I’ve had put off to the side for about a year:  three toy sound samplers with pitch shifting that you can hack into and add to a simple mixer circuit to create a lo-fi, multi-channel looping effect.  I drew up a wiring diagram, planning on using a chunk of steel stud to house it:
bent looper

I still have the power supply, mixer and one channel hooked up to a breadboard. Last I checked, it worked, and I haven’t touched it since:
Toy guts on a breadboard

Three Channel Live Looper
I decided instead on an enclosure from an old whatever-the-hell-it-was from ax-man.  You can buy old machines for $5-$10, take the guts out and use the case for new stuff, which is cool because electronics enclosures are really expensive.  The drawback is that it’s harder to get a space efficient design since the housing was meant for something else, but I’m starting to realize that I like things to be life size, sturdy and clunky with plenty of soldering room.  The last looper circuit I built, I think I fried the chips because everything was crammed too tight on the board and I couldn’t troubleshoot it.

All the knobs are where the pots will go.  The nuts represent toggle switches, the plastic grommets (LED holders) represent LEDs, the washers represent momentary switches, and the pots represent foot-switches.

Solder on PCB

June 5th, 2008

When I got home today, I went to get my mail and there was a blank circuit board in there from General Guitar Gadgets! (I built a looping pedal way back when, and decided to do the point-to-point soldering myself. Then the project didn’t work. So I ordered a printed board for it.)

Soldering can have quite a calming effect–watching as a board gets populated with components and forcing yourself to be patient. It was a nice surprise–I forgot that I ordered it!

I never want to build circuits on perfboard again! Of course for one-offs its practical, but it was so easy rebuilding the looper on the PCB. The solder just glides effortlessly onto the copper, and it didn’t fall apart every time I heated up a lead. I shouldn’t talk because I haven’t tested it yet…

My Cheap Rackmounting Idea

April 25th, 2008

One of the most expensive parts of any project is the case, often the most expensive. I read somewhere about using metal studs to make enclosures for pedals, and that got me thinking. I went and got some supplies today and threw this together. Its a new housing for my headphone splitter (which will eventually house other things too since there’s plenty of room.)

Cheap Rackmount
I bought a blank rackspace panel from Guitar Center for $6.49. They’re meant to just cover up empty spaces in your rack, but that’s kind of silly. I got some machine screws and nuts, and drilled the two holes in the panel, and two holes in a 17″ section of aluminum stud and bolted them together. Studs are a couple bucks per 8′ section, so this is totally the cheap way to go for simple projects.
Cheap Rackmount
Then I drilled more holes to mount the jacks and the switch. This got frustrating with my cordless drill because those panels are 1/8″ steel. I need a drill press. I also should have drilled all the holes and then bolted the panel to the stud, because a bunch of burr got caught in the middle.
Cheap Rackmount
I drilled a little too high for the input jack. I had to snip a little piece so the jack wouldn’t short out. The whole enclosure acts as ground for the circuit.
Cheap Rackmount
Installed. Nice blank mystery box. Obviously this only works for projects that don’t take up much space, and that aren’t heavy, but “real” rackmount enclosures start around $60 last I checked.

Another Looper Project

April 25th, 2008

I can’t get shit off my mind tonight. I entered that place in my head where I just start navigating the same paths over and over again, getting nowhere. I decided to start soldering. I found the project here. It’s a looper that uses these little voice recorder toys. You take them apart and wire them up to a simple mixer circuit and 5v power supply. I’m building a case out of a piece of aluminum 2×4 stud.

Here’s the board I made tonight, making up the power supply and mixer circuits. A 9v battery powers the mixer and a 5v regulator, which powers the toys. There will be all kinds of offboard wiring going from this board to the toys and all the pots and switches. I built the mixer on a breadboard this morning, and it works. I have some other ideas I want to try with it. One of them involved housing my old DOD distortion pedal in a new case along with a feedback loop device I built, and I could add the mixer circuit to it to blend in the dry signal.

This is one of the toys still in the packaging. They’re fun. Get them at Target for 6.99, even if you don’t circuit-bend. You can warp the speed of whatever you record. Currently this one makes a high squealy cat noise I made in the car after buying these.

DIY Headphone Splitter Project

March 24th, 2008

I completed a simple project today. I needed a headphone splitter, mainly because I use a set of headphones that are attached to a long home-made cable, so that I can record in the kitchen, or the other side of the room etc. It gets a little messy dragging that long cord with you when you go back to the board to do rough mixes and whatnot. With a splitter, I can keep one pair of phones by the board while the other is free to roam. Also, in case I ever record with another person it would be helpful if we could both hear the mix! So yesterday I found these simple instructions for a passive headphone splitter. Not a distribution amplifier, which I would like, but they’re more complicated. You don’t get any gain and there’s no volume controls or anything, but it’ll work. I went to RadioShack for the parts and threw it together this afternoon.

Front and Back with the components installed. I used a metal plate that I got at AxMan (Surplus store) when I realized that the box I bought was too small to house jacks and those huge resistors. Everything is just open in the back. The resistors are non-inductive 8-ohm 20W, and I used hot glue to hold them in place.

I used a switch to cut the ground coming from output 2, as an on/off. It doesn’t work, presumably because I didn’t fully insulate the jack from the metal plate. I think I need a DPDT switch to cut the signal from each channel rather than cutting the ground.

Here’s the wiring. Pretty simple. I still managed to fuck it up, however.

I installed the thing under a shelf, which is where my monitor will be when I get it.

There it is hooked up. I tried it out just checking into the mic, but have yet to record with it. It seems like it’ll do what it’s supposed to.

I have yet to finish any complicated projects, but I really enjoy drilling holes, soldering and designing the layout and housing and everything. For this I used regular old silver paint marker. I like it.

The other thing about this project is that if I need to (or want to) I can keep expanding it pretty easily if for more outputs. I have several of those black plates from AxMan!