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#schematic
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:
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
Comments
Hi evereone!
same problem as peppelisme, I'm using 5v to feed the preamp, should I change any number (resitors, capacitors) in the circuit?
thanx in advance !!!
alfonso
I am going to replace the transistor and try again.
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"
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.
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
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.
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.
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.
dear Rand,
-
1
-
2
of 2 Next