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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


Views: 19547

Tags: FET, MFP-102, Piezo, Preamp, electric CBG

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Comment by David McQueen on March 14, 2013 at 7:21am
Edit: It was the preamp, not the amplifier that blew
Comment by David McQueen on March 14, 2013 at 7:20am
Quick update. I tried plugging into a Traynor TVM50 last night and it immediately blew. It had been picked up used, so there was no manual. I did a search this morning and found that it as both a high and low impedance mode. From the performance of the straight disc piezo, I am guessing it was in low mode, especially since it was fine through my little Velleman amp (actually it was awesome).

I am going to replace the transistor and try again.
Comment by David McQueen on March 12, 2013 at 10:03am

Just a quick thank you Rand. I had been trying to build the original Tillman Preamp using subs for the 201 JFET and was getting grief and some what cool "Pink Smoke".

A note for anyone looking at this:

1. There is not much gain ... but that was not the goal.

2. It did tame the disc Piezos. I ran it into a little Velleman amp with a 3" .5 watt speaker and went from heavily distorted to to very clean.... Which was the goal.

I builtit up on a straight bread board and used conductive copper tape to imitate a half *ss'd pcb (may have lost some power that way). It took me about an hour.

I tried it on on a 4 string with 35 mm piezo, a 12 string with a 27 mm piezo and a six string with both a 27 mm piezo and a piezo belt consisting of a 35 mm, 27 mm and approx 10 mm piezo hooked up in parallel. On every one, it took the tone from muddy to clean.

I play with some people every Wednesday. So I will get to hook it up in line with a "real amp" then and compare the sound to my friends who have "real guitars"

Comment by Rand Moore on November 22, 2012 at 6:43pm

Hi Guiseppe,

I don't have much in the way electronics test equipment, certainly no oscilloscope. I just have a cheap VOM and a soldering iron, so my electronic testing capabilities are quite limited. So, I can't tell you what the voltage level is on the piezos I use, but they probably are very similar to the ones you are using. Your idea of a "few mV" sounds about right. My sources for piezo pickups are Radio Shack buzzers for 28mm diameter round piezo "pups" and CB Gitty for 20mm diameter round piezo "pups".

If you are an EE student, then you probably have access to a lot of different kinds of test equipment. Maybe you can replicate these pre-amp and practice guitar amp circuits and document for us here on CBN what the circuits are capable of doing, perhaps with 3 different frequencies of sine waves (bass, mid-range, and treble) of the audio range so we can see how the amplifier affects the input signal in terms of distortion, with different gain settings, and perhaps tone settings. Most of us on CBN have very little in the way of Electronics background, and such basic information would likely be very informative. Just a suggestion.

-Rand.

Comment by peppelisme on November 22, 2012 at 12:54pm

Hi Rand,

Thank you very much for your kind support!

I have to calculate needed gain, so I'm going to measure through an oscilloscope:

piezo voltage (It should be few mV);

maximum input voltage at mic connector of sound card (It should be 2V);

Anyway I'm going to buy some mini guitar amp (your Joyo JA-03, or others I've found on ebay like Bravo Audio...) and study their PCB as you suggested :-)

Batteries could be easily replaced by my USB voltage, or both :-)

Headphone level sound should be good for mic input sound card, because I loopbacked headphone output and mic input of my sound card, and I got a good sound...

Then I've to understand if the voltage level of my piezo is similar to your dulcimer one...

Do you have an idea of the voltage that can generate your dulcimer?

Thank you very much again

Regards

Giuseppe

Comment by Rand Moore on November 22, 2012 at 7:42am

Hi Guiseppe,

I found another article on the Internet that describes a fairly simple 1/2 Watt guitar amp that uses a FET on the input to match the impedance of a piezo Pickup and the FET then drives a LM386 OpAmp which provides the gain. Like my article, it uses all Radio Shack parts, so the article is more useful for Americans. But the circuit is typical of the kind of guitar practice Amp I was talking about in my previous posting. Still looking for this kind of circuit pre-built on a nice little PCB for cheap. Anyway the amplifier is called the "Noisy Cricket Mark II" and can be found at this URL:

http://www.beavisaudio.com/projects/NoisyCricket/MarkII/NoisyCricke...

I'd make this URL a link if I could find the tool bar for this little comment editor. Doesn't seem to be working today.

-Rand.

Comment by Rand Moore on November 19, 2012 at 10:12pm

Hi Guiseppe,

I checked with CBGitty, the CBG parts supplier that is hosting Cigar Box Nation, and found that they sell a couple of pre-amps for CBG builders.

The first is an "in-line pre-amp" which is to say the pre-amp circuit is contained in an extension of the phono jack, so there are no extra controls. It sells for like $20 (USD) each. You can find out more about it at this link.

The second pre-amp they sell has a volume control and built in 3-band equalizer. It also has a flush mounted battery compartment, so the whole preamp has a pretty big footprint. In one of the threads about this product, they say it will need a hole about 1 5/8" by 3 1/2" in the side of the box and that with the cover plate, the outside dimensions are 4 1/16" x 2 13/16". So, you need a pretty deep cigar box in order to house this control. However, everything is pre-wired, so you don't have to mess with soldering. It sells for like $15.00 (USD) each. More info can be found at this link.

As most simple pre-amp circuits are primarily designed to help match impedance as from a high impedence piezo pickup to a low impedance guitar amplifier, pre-amps often do not provide much or any gain (amplification). Because of this, you need to be careful of what you buy to make sure the circuit actually provides gain. Most of these circuits use a FET on the front end followed by an OpAmp or other usually multi-transitor amplifier. The circuit show in this discussion above is designed only for impedance matching. What might also work for you is one of the guitar practice amps. I recently bought a Joyo (Chinese brand) practice amp that plugs into the piezo pickup jack and provides just enough amplification for earphones (and no more -- to prevent people from damaging their ears). I was thinking one of these might provide the electronics you need. Seems like it cost me about $12.50 (USD). Here's a couple of photos:

And here it is plugged into one of my stick dulcimers:

I haven't yet looked inside to see what I can use inside my own CBGs, but it runs on 2 AAA batteries. Would be nice just to be able to buy the PCB. Maybe I'll buy another one some time and see if I can work it into an electric CBG.

I'd like to find a pre-assembled preamp PCB for real cheap that I can use. I'm thinking of looking on Taobao.com (a Chinese E-Bay-like website).  I'll continue to look for some more leads to post here.

-Rand.

Comment by Rand Moore on November 19, 2012 at 6:25pm

Hi Guiseppe,

While I have researched preamps on the Internet, experimented with a couple circuits and documented my results, I have not developed my work into an actual product. It basically was an experiment for my own education and most of the CBGs and dulcimers that I've built are purely acoustic. There are other people here on Cigar Box Nation that build primarily electric CBGs and can probably refer you to sources for either kits or pre-built preamps that they use in building their instruments. There are also a few electronics oriented web sites that have circuits and sometimes kits on the Internet that you can search for using Google. One site that I recall is runoffgroove.com.

Let me see if I can search CBN for you to come up with a list of preamp sources. Seems like a worthwhile list to have that other builders might like to know. I'll post another response in the next day or two.

-Rand.

Comment by peppelisme on November 19, 2012 at 10:18am

dear Rand,

I'm Giuseppe, an italian student.
Your post is very nice, and so very similar to my need:
I'm looking for a preamp for my application:
My sw records the sound from the mic input of the sound card, using a piezo as microphone.
The problem is that the voltage is too low (few mV), so I need a preamp, 
My constrain are:
piezo as input: few mV;
sound going to sound card mic input: 1 or 2 V max;
preamp supplied by USB port: 5V
Could you provide me your custom circuit for my need?
Thank you veru much
Regards
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.

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