I've recently come into the possession of a set of Piezo pickups for my acoustic guitar. I'm currently building a JFET preamp to bring down the impedance for plugging it into an amp, but I'd like to be able to do some home recording. From what I can tell, the general practice is to only use the line in input on a computer. Unfortunately, my laptop only has a mic in, and there doesn't seem to be a lot of documentation on using these.

Will a typical preamp suffice with an adjusted output impedance from the preamp, or are there special requirements in order to use the mic in without losing to much good sound?

 

Mic in is a lot more sensitive than line in. You'll probably have to add a volume control to turn it down.

 

Sorry for the late reply, but is that really it? I was under the impression that the mic input suffered from a whole mess of issues. I guess thats what i get for trusting the audiophool community hehe.

 

I'm working with no information about the preamp. The best I can do is to say that if the preamp gets the signal to arrive at "line" level, it is too high for a mic input.

Have you tried just plugging the mic into the mic input?

 

I know acoustic guitar pickups I've used are usually pretty low in signal output. You'd probably have to turn your mic volume all the way up to get a decent signal from it if used without a preamp. With a preamp, it will probably be over amplified as #12 said. And, in general, standard PC sound cards' mic inputs are not going to have the greatest sound quality as probably the "audiophools" have informed you.

If you could get a little boost out of your preamp, maybe a 2X gain that would probably get the best transfer from pickup to preamp to mic input in the sound card.

But you also have the A/D converter in your sound card which will, again, be less than ideal for a standard sound card.

If you want to spend a little money, you'd get a substantial gain in quality by purchasing a USB audio interface for recording, but that's just my opinion. Or better yet, use a condenser mic and no pickup at all! But I digress.

 

Here's a jfet preamp from a 9V battery. It was built, sold, and it worked.
Still needs a volume control to get it down to Mic level.
Leave out the 100uf capacitor and gain control. They just turn the (already too high) gain up.

 

Info

Sound card microphone input impedance and voltage levels:
Impedance: 1kΩ-20kΩ (Medium Impedance)
Level: 10mV to 100mV (Low to Medium Level), some have Automatic Gain Control

TRS ("stereo") type jack for Microphone input:
Tip - Signal Input
Ring - +5V bias current, typ source impdance 22k, sources ~225uA
Sleeve - Signal and bias Ground

Which piezo pickups are you using in your gee-tar? I've found them ranging in output impedance from 50kΩ to 10MΩ, matching this is as important for good sound as boosting it will be, otherwise some frequencies will come across a great deal louder than others, depending on piezo response curve and piezo resonant frequency (if Fo is in audio range).

From a quick search, condenser mics seem to be preferred, as that is what the MIC input of a notebook is wired for, hence the bias voltage.

 

That's why I designed that jfet preamp. Lots of input impedance in a low noise amplifier.

 

That's why I designed that jfet preamp. Lots of input impedance in a low noise amplifier.

If you could use silver paper/oil capacitors, gold plated circuit board, and "oxygen free" copper on the jacks and the patch cable (optional, $350), built in a Manhattan style board so response was DC to 1Ghz, you could get about $750 per copy, provided it came in a brushed anodized aluminum box, with a nice subdued knob (made from oak, of course).

 

Oh geeze. That stuff is so far removed from reality that I can't keep up with it!

ps, Remove the .001 cap across the drain resistor and I don't know how high the frequency range goes. (I can only measure up to 200KHz.

 

The best approach for quality records into your PC is to use a balanced input. I would use the Firebox or any of the M-audio USB or Firewire interfaces. The key is gain structure and impedance when dealing with any piezo pick-ups. my preference is the firebox, but cheaper units can be found.
http://www.musiciansfriend.com/pro-audio/presonus-firestudio-mobile-10x6-firewire-recording-system