Troubleshooting homemade preamp....

[OneRoomStudios]

So I must have done something wrong since the preamp I built is acting up and not really working at all. I buzzes occasionally, and every once in a while it works if I shake things around a little and hold the mic in my hand (probably some grounding issues, huh?) Since this is the first thing I've built from scratch, maybe some of you could give me a hand trying to figure out why it's not working....

check the picture I made of the schematic that I used and let me know if I messed anything up when I simplified the TI schematic.

R1= 2.2K
R2= 2.2K
R3= 10
R4= 10K pot
C1= 0.1 uF
C2= 0.1 uF

Also, would a 12V battery be enough to power it or does it need the full 15V? If there's anything else you can think of as to why it might not be working, let me know, thanks.

 

[Outlaws]

Since this is the first thing I've built from scratch, maybe some of you could give me a hand trying to figure out why it's not working....

I am about to build a mic preamp in a month or so after I build a clean booster and compressor.

I just finished a fuzz pedal for my guitar and I had a simular problem.

Case in point:
The thing worked fine except the control was somehow set to fully clockwise is zero gain, and full counter clockwise is full gain. Also, when I rotated my volume and tone controls on my guitar, I would get scratching.
It was a simple fix. I had wired the pot backwards. lol I figure the scratching was just a bad solder joint.

My guess is that you didn't get a sold connection somewhere.

That and get a variable AC adaptor from Radio Shack. Its nice because you can flip a switch and get anything from 3v to 18v (if I recall). Plus they come with different size DC plugs for total connectivity.

 

[crazydoc]

What kind of mic are you connecting to the inputs, and how? You may be introducing a DC voltage at the output. I assume it doesn't need phantom power.

If you're using a balanced mic, ground pin one of the xlr and run pins 2 and 3 directly to the inputs of the IC (also pins 2 and 3). You may need to add R4 and R5 as in the TI schematic - I think they should be closely matched (1%). (I guess these are R1 and R2 in your schematic that you have in series with the inputs - they should go from the inputs to ground, not as you have them.)

If the mic is unbalanced, ground the shield and also pin 2 on the IC(- input), and connect the center conductor to pin 3 of the IC (+ input). You don't need R4 and R5.

You should probably have a DC blocking capacitor at Vout - this will pass the AC signal but keep any DC offset voltage from passing to the next stage. Try something like 25 to 100uF - don't worry about the polarity at this point.

Hope this helps.

 

[OneRoomStudios]

That and get a variable AC adaptor from Radio Shack. Its nice because you can flip a switch and get anything from 3v to 18v (if I recall). Plus they come with different size DC plugs for total connectivity.

Yeah, I have one of those, they're handy. Unfortunately this preamp is for mobile recording, so I need it to be battery powered.

What kind of mic are you connecting to the inputs, and how? You may be introducing a DC voltage at the output.

It's a battery powered remote recording shotgun mic (AudioTechnica ATR55) with a 1/8" phone plug.

If the mic is unbalanced, ground the shield and also pin 2 on the IC(- input), and connect the center conductor to pin 3 of the IC (+ input). You don't need R4 and R5.

So I should ground the negative input (tip?), and everything else should be ok right?

And just to double check, 12V should be enough right?

Thanks a ton for the help!


-Peter

 

[MOFO Pro]

12 volts will probably work... but probably not well... you don't want to starve the preamp circuit... will probably make the response sluggish...

Or induce noise or HUM

 

[mshilarious]

Well 12V is within spec for the INA217, but you'll have reduced headroom. If this pre is feeding -10dBV gear, it might be fine with the single battery.
Note that schematic has +/-15V, not just +15V. So for better performance, try two batteries:

http://www.prodigy-pro.com/forum/viewtopic.php?t=4913

 

[OneRoomStudios]

Note that schematic has +/-15V, not just +15V. So for better performance, try two batteries:

http://www.prodigy-pro.com/forum/viewtopic.php?t=4913

Good point... so the two battery method would be the easiest way to go, or do I need to worry about offsets?

And if anyone knows of a cheap source of 15V batteries, I'm all ears (preferably small ones since I'd like everything to fit into the small project box I'm using).

Thanks again everyone! I'm definitely learning a lot. I wish I had paid more attention in high school physics class when we learned about circuits and power. Oh well...

 

[crazydoc]

It's a battery powered remote recording shotgun mic (AudioTechnica ATR55) with a 1/8" phone plug. Could the battery be inducing a DC voltage?

No, it's output should be 0V DC.

So I should ground the negative input (tip?), and everything else should be ok right?

No, you should ground the sleeve. The tip should go to Vin+ of the IC and Vin- should be grounded. This is for normal polarity.

And just to double check, 12V should be enough right?

I'm not sure what your power supply is like. For this circuit you need a bipolar power supply - it calls for +/- 15V, but the IC can use anything from +/-4.5 to +/-18V. What this means is that you need a ground, a positive voltage above the ground, and a negative voltage below the ground. In your case the easiest way to do this would be with two 9V batteries. Hook the + terminal of one battery to the - terminal of the other - this is your ground potential. Then hook the + free terminal to V+, and the - free terminal to V-. This gives you a bipolar supply of +/- 9V, and should be plenty to run the preamp just fine.

BTW, if you have old 9V batteries, you can take them apart and use the old terminals as clips to hook the batteries to. Attached is a bipolar clip I made many years ago on a small piece of perf board.

Edit: I see there are other responses to your post since I started this post and did a bunch of honeydews around the house at the same time.

 

[crazydoc]

Good point... so the two battery method would be the easiest way to go, or do I need to worry about offsets?

No problem with offset voltage - see PRR's post in the link above.

 

[OneRoomStudios]

Wow, thanks for the detailed pic doc, I really appreciate it!

So just to double check, I shouldn't have any problems with offsets due to the slight voltage differance between the two batteries if I do it this way should I? If not then this should work...I'll have to run to radioshack (I wish there was a better electronics store around here) to grab some supplies, and hopefully I can finish it up tomorrow.

 

[Outlaws]

(I wish there was a better electronics store around here)

Check a little more. I don't know where you are, but I thought my only option was RadioShack, Frys, and the Internet. Then I stumbled onto the motherload for a guitar effect nut - Antique Electronic Supply. Found them on the net. Maybe not the best prices or the best selection on the net, but they turned out tobe in my city! And only about a 10 minute drive away.
Plus I found a few more smaller stores in town that I never knew exisited...just by accidentally surfing to their websites. They don't all have everything I need at once, but between the 5 stores, I can get basically anything I need and not pay $5 for shipping for a stupid pot or heavy duty DPDT.