Looking For Small Instrument Pre Schematic?

[Vikki]

After installing a couple of quite expensive pickups in my acoustic guitar i've been a bit disappointed with the result. The pickups do not have a preamp built into the guitar and i.m using a 30 foot stereo cable, i think there's a chance the capacitance of the cable is loading the pickups, one pickup is a Sunrise magnetic which has quite a low output and i have 3 piezo pickups superglued under the bridge plate. I have a dual channel blender unit at the far end of the cable which is quite a high quality unit. I'm looking for a circuit of a small(perhaps FET) dual channel preamp that i can build into a belt or strap mounted box that would run off battery power, any ideas appreciated.
Vikki(uk) :sad:

 

[rodabod]

Hi,

What is it about the sound that you are dissapointed with?

Does the pickup sound too hard or metallic?

I can't imagine long cables being too much of a problem if the pickup was designed for passive use.

I have a Fishman magnetic single-coil pickup (active) on my acoustic. The sound is fairly electric and sometimes a bit brash - but that's what magnetic pickups sound like on acoustics - you only get string sound.

I'd maybe consider one of those Gordon Giltrap / Session preamps which have tone controls, etc. since a bit of eq. might do the trick.

With my old Schaller passive pickup, I used a Behringer DI box which helped for certain inputs - eg. mixing desks.

 

[Vikki]

Hi
I think the mag pickup is loosing high end, i know the mags don't have a great high response and notice on the Sunrise website there's a buffer box unit that boosts the signal by 16db. I was looking at that simple single FET preamp cable circuit may be that will do the trick. The piezo seems to have loads of output.
Vikki(uk)

 

[adrianh]

Vikki;

Have you tried a simple FET amp.

More info here;
http://www.till.com/articles/GuitarPreamp/

This is neat too; http://www.till.com/articles/PreampCable/index.htm[/img]

If you wire it correct with a 3 conductor 1/4 inch phone plug you can switch the battery negative if you connect the battery to the "ring"
connection on the jack so when a 2 conductor guitar cord is inserted the
battery neg is connected to the circuit ground. i.e. when the guitar is
unplugged the circuit is not draining the battery.

 

[Vikki]

Thanks Adrianh.
Thats the circuit i was thinking about having a go at, i'll have to try and find some FET's, seems to be hard work getting parts here in the uk everything seems to be out of stock.
Vikki(uk) :grin:

 

[NewYorkDave]

The good ol' J201, one of my favorite JFETs for audio projects.

Add a cathode... whoops, I mean source bypass cap to lower output impedance and increase gain. 22uF or higher should do nicely.

If you get really stuck with finding the J201 in the UK, PM me and I'll drop a couple in an envelope for you.

 

[Scodiddly]

Here's my usual buffer for piezo pickups:
http://www.scotthelmke.com/mbb.html

 

[Vikki]

Thanks everyone
Looks as if Farnell have the J201. I'll build a few of these circuits to experiment with.
Thanks again.
Vikki :grin:

 

[Bo Hansen]

Vikki,

If you only need a impedance converter, (with out gain) take a look on this very simple circuit I design for a other application. (I think it was to get a HI-Z input on a measurment card for a guy here on the forum)

http://web.telia.com/~u31617586/#simple%20FET%20impedans%20converter

This FET source follower will help the HI-Z piezo pickups to feed some meters of cable with out gain or treble loss.
It will work with any types of N-channel JFET transistors, and draw only 0,5 mA from a 9 volts battery.

Important: be sure that you adjust the 10k preset resistor so you get approx. 4,5 to 5 volt (half of battery voltage) between ground and the source pin on the JFET.
This is adjusting for the bias operating point, to get best output swing and symetrical clipp.

--Bo