Today I assembled my first preamp based on a MFP-102 FET using a circuit design I got from the Internet (see URLs listed below) and components I bought from Radio Shack last summer. It buffers a pair of Radio Shack piezo pickups to drive a 15 Amp guitar amplifier I have. It worked first time, but the two piezo pickups are very sensitive in spite of wrapping them in foam. I will be experimenting on how to make the piezo pickups less sensitive and where to best locate them on my CBGs. Here is the circuit design I used with all Radio Shack components:

Here's a photo of my "test" circuitry:



Here are 3 URLs related to the circuit design above:
http://www.rason.org/Projects/jfetamp/jfetamp.htm
http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp1.htm
http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp2.htm#schem...

I have also put together a Radio Shack parts list with current prices. Radio Shack has been slowly downsizing the components selection that they offer over the years, so hopefully they still have all the parts when you get around to building this circuit. You can check by going to the Radio Shack website (google: "Radio Shack" + "components"). Here's that parts list:

I'm told the specs for these FETs vary widely and that you should buy several in case the first couple are out of spec for this application. For out of spec FETs, you can still use them if you change the values for R2 and R3, but that whole topic is way beyond the scope of this posting. If you are new to FETs and the MPF102, you need to be sure to know which lead is which. Here's an image file that shows which lead is which for the MPF102:

Maybe this is a better picture:



If I had a working digital camera, I would take a photo and post it, but my solder side of the PCB makes me feel ashamed (it's so messy - workmanship-wise). I think I need to invest in one of those soldering "third-hand" helpers with the two movable alligator clips and magnifying glass to help see what I'm doing. A photo of the component side along with how the battery is connected would be good to have. Also a second photo of how I temporarily connected the circuit board and components inside my CBG would be helpful. So, I'll see what I can do to come up with a digital camera.

As the purpose of the preamp is to strengthen the signal from the piezo pickups so that you can use a long cable between the CBG and the guitar amp, I shortened the two leads on the piezo pickups (to minimize signal degradation); but I did so perhaps a bit too much, as it is difficult to reposition the pickups in different places on the back of the soundboard and have the preamp circuit in the middle. As I have it rigged right now, the two pickups are mounted on the neck inside the box with the preamp board between them. The 9V transistor radio battery is velcroed to the bottom back side of the cigar box. After the thin foam failed to minimize sensitivity, I added thick foam, and find that that seems to isolate it too much such that I barely get any gain out of my guitar amp (but no fingering noise). So, next time, I'll try wrapping it in 2 layers of thin foam. If that fails, I'll try hot glue, but that will make the installation permanent, so I'll likely hold off on that as long as possible.

Additional experiments will be to use this circuit on different instruments I've built to see how well it works with canjos as well as other CBG configurations. That's why I'd like to keep the circuit "portable". It will take some practice to see how I can get a good "electric sound" out of these instruments. Well, that's all for now.

-Rand


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Comment by Rand Moore on September 25, 2010 at 4:47am
Hi All. I didn't expect all the feedback. I guess this is a real popular topic.

First, to answer Naz Nomad's questions:

1.) I used three 1M ohm resistors in series because I could not find a 3.3M ohm resistor at Radio Shack. Had I more time, I could probably have found it at Fry's Electronics but thta was out of the way and I figured 3 M ohms is still in the ball park. The purpose of this resistor, as I understand it, is to present a high impedance input so as not to drain / distort the signal generated by the piezo elements.

2.) I am aware of the J201 FET and the good things said about it; but I needed to secure my components quickly before returning to China. I decided to collect the components the last week of my vacation to the States this past summer. The local Radio Shack (and the second closest one) were able to quickly meet my needs with what they had in stock. If I can find a local J201 FET supplier in China, I will likely switch. Right now I have six MPF102 FETs which I hope will tie me over till next summer. I also have three LM386 OpAmps, so I plan to do some more experiments in the direction of building a guitar practice amp.

As for Chester's suggestion, I have thought of doing a preamp in a small mint tin (Altoid boxes), but I am holding off on that trying to perfect by techniques, etc. with putting preamps into my CBGs. I did buy a couple boxes of Altoids and brought them back as well as another similar box entitled "Dante's Inferno Balls". These hot cinnamon candies weren't so good, but I love the box. I also brought back a dozen cigar boxes and several cans / tins for canjos including a cool "Gurkha" cigar tin which might make a popular instrument with the Gurka decendents in Hong Kong that originally came from Northern India and Nepal to provide security when Hong Kong was a British colony. Talking about a nitch market. I'll have to see what simple stringed instrument might be popular with them and see if I can do a cross-breed type of instrument that may appeal to them.

As for YeahDolt's question: As I understand it, you are using a headphone element as the pickup and are running it through your gutar amp and you can hear a fairly weak signal. You are asking whether this circuit will boost the signal so it's stronger for your guitar amp. Well, the design of this preamp is more for presenting a high impedance input for the piezo elements so that the signal strength is wasted away on a long guitar cable. The gain of this circuit is supposed to be negligible from what I have read elsewhere on the Internet. I don't have an oscilloscope or other means to measuring the gain of the circuit. You can give it a try, but I'd be prepared for modifying the circuit to provide additional gain. This is often done by adding a OpAmp down stream of the MPF102 FET and these circuits can be found on the Internet as well. I want to adapt one of these circuits at some point in the future to make a practice amp. I can probably come up with a circuit and URLs if you are interested.

Today I have been working on a couple of head/neck assemblies - one for a Tenor sized CGU and another for a Baritone sized CBU (wish I had a bigger Cigar Box for it). Well, that's all for now.
Comment by Chester Winowiecki on September 23, 2010 at 10:31am
This is a great circuit and good description of a build, good job! If you'd like to keep it portable and able to be used in all your instruments, you can build it into a mint tin or other small box with in and out jacks and wired to shut off when not jacked in. Here's a similar project that I've built as well:
http://www.scotthelmke.com/Mint-box-buffer.html
Comment by Naz Nomad on September 22, 2010 at 6:41pm
Ah, I see you answered my query in your 'edit' lol.

The J201 FET is a good one to use too, apparently less susceptible to 'noise', although I believe it's harder to get than the MPF102.

... and I agree, we don't wanna get into the realms of biasing the FET here ... ugh.
Comment by petey twofinger on September 22, 2010 at 6:33pm
i have one of those in my cbg , similar , uses the same fet . its really nice .
Comment by YeahDoIt on September 22, 2010 at 10:30am
I plugged my little headphones into a guitar amp and they act as a microphone. I put the headphones inside my CBG and get a nice sound through the amplifier speaker, but it is a weak signal. This preamp circuit could boost the signal from headphone microphones.
Comment by Naz Nomad on September 21, 2010 at 7:29am
Why not use a 3.3MΩ resistor in place of those three 1MΩ resistors in series?

... or is it all that Rat-Shack had in stock?

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