Have you ever wanted to amplify your electric guitar by plugging into an AV Receiver (home stereo)? Ever tried rigging up a 1/4" mono instrument plug to RCA patch cable? If so you most likely were disappointed when it didn't work. Most modern consumer grade AV equipment only has output power amplification which has to be fed with Line-Level Pre-Amplified signals. Musical instruments with passive pick-ups have a high impedance low signal level output that has to to be gradually stepped up with pre-amplification.

Article Features:
  • Simple two transistor stage pre-amplifier circuit, with a high impedance input stage suitable for musical instruments with passive pick-ups and an output stage with enough signal level to feed AV equipment.
  • A brief description of the processes I used to fabricate the clear acrylic case I made.

Step 1: Make the Circuit

The input stage of this amplifier circuit is a Common Collector amplifier which has high input impedance, low output impedance, and high current gain. The output stage is a Common Emitter amplifier which has medium input and output impedance, and high power gain. The input and output is decoupled from the 3V DC supply power by C1, C2 & C3, R1 & R3 control the base current of Q1 & Q2, and R2 & R4 control the output current of each transistor stage respectively.

I used point-to-point construction for this circuit, however it could be built on a PC board if desired. I used aluminum duct tape to form power rails on the inside of the acrylic case. Since the tape can't be soldered I used small pieces of tape to fasten the connection points to the components. It was challenging to make reliable connections with this method. Alternatively copper tape, or wire could be used.

Parts List:

I used male RCA plugs as the output termination because I wanted a compact inline connection, alternatively female RCA jacks could be used with patch cables. I connected the RCA output plugs in parallel to feed both the left and right channels of the AV receiver.

AV RCA phono plug on center spacing on my receiver is 13mm with a 10mm panel mount hole.

Step 2: Custom Acrylic Battery Holder

I didn't have a 2 AAA battery holder so I made one using the same 2mm thick clear acrylic sheet scraps that I made the case out of. The method that I came up with to form spring terminals for the AAA cells is my favorite part of the project. As you can see in the close up image I soldered zinc plated springs to 8-32 zinc plated hex nuts to form the terminals which were then through bolted to the acrylic tray with 8-32 X 1/4" SS undercut flat-head screws. These were all parts that I happened to have on hand. I used strips of aluminum duct tape as conductors.

Step 3: Custom Acrylic Case

I worked for a marine products manufacturer for 9 years and learned much, including thermoplastics fabricating techniques. I had some 2mm thick scrap acrylic sheet on hand and decided it would be put to good use for this project.

One Piece 5 Sided Tray From Formable Sheet Stock


The technique used to design a 5 sided tray from one blank of acrylic is similar to the technique used for sheet metal forming. First determine the inside length and width dimensions desired, then determine the desired inside height. Multiply the inside height by 2 and add it to the inside length and width, this will give you your overall blank size. If you want the outside corners of the height dimension to be flush when formed you have to multiply the material thickness by 2 and add it to either the length, or width when doing the layout to cut the blank into a cross forming the tabs that will be folded forming the sides. I designed the case to fit over the battery holder forming a complete enclosure, and allowing easy access.

Since I don't have access to a laser cutter I used hand tools. Acrylic can be cut with woodworking tools such as saws and routers. Thin sheet plastics typically can be cut with a score-and-snap technique, which I used for some of the cuts. I just used a sharp razor knife to score the cut lines several passes, then I held the score line against the corner of a table and with a quick but careful application of force snapped off the piece. For some cuts I used a hacksaw with a fine toothed blade.

I used the homemade small strip heating fixture seen in the image to heat the blank along the bend lines until the plastic is soft enough to bend. For critical tolerances a bending fixture can be built to ensure that the bends are precise. For this application I just did it by eye. You'll notice in the images for the blank that there are holes drilled at the corners of the bend intersections. These are relief holes to remove material that would interfere with forming the compound corner and eliminate a sharp point. A nice feature of this bending technique is that the formed corners are radiused.

Step 4: Final Thoughts

This was a fun and useful project. The pre-amp works quite well interfacing my guitar with my AV receiver. The pre-amp has enough gain that guitars with "Hot" pick-ups may overdrive the pre-amp into some distortion at high guitar volume.
<p>Thanks technovative, just curious, figured I'd ask, perhaps there is a guitar interface project here somewhere?</p>
<p>It's been a few years, has anyone thoroughly tested this as a guitar interface for the PC? I know it's been mentioned, but not much details.</p><p>PS, just joined today, my first question here (of many)</p>
Welcome to the community. This circuit isn't a good choice for guitar to pc interfacing.
<p>What about having a AcousticGuitar with piezo pickup?? Would this work as Preamp?? or just for a guitar with a magnetic pickup with preamp??</p>
<p>I can't say with certainty because I haven't tried it, but I think it should work.</p>
<p>Can you adapt it to be a Guitar to USB interface?</p><p><br>This way I can plug my guitar on my computer</p>
<p>If your computer doesn't have a 3.5mm line or mic input jack, you could get a <a rel="nofollow">USB audio adapter</a>. Then you would use a 3.5mm plug as the output from the preamp instead of RCA plugs like I did.</p>
<p>Very good! I&acute;ll try to use a P2 jack to output and a P10 jack to capture signal from my guitar.<br><br>Another question (sorry, I&acute;m really newbie on this and I dont have knowledge on eletronics):<br><br>Or maybe, can I weld a USB port as the output? So this way I can send the signal to my computer via USB port, use some app on my computer like Amplitube, and use the headphone output from my computer to plug my headphone. This way every efect that I apply using Amplitube or Garage Band, I can hear on my phone.<br><br>Ttks in advance.</p>
awesome,,would love to make it
Thanks, if you do make one come back and share how it turns out.
Congratulations!!! Great ible' by the way!
Thank you.
So i built it, then i read the comments. I built it down to a board with an 1/8th inch 3' output. Put it into a nice little project box because i have a few other projects going atm as far as enclosure fab is concerned. I did this to pass through to my pc as some have mentioned. I did it for this reason because unlike some im running a passive electric through and with just the 1/4" to 1/8" there was little to no volume passing through aside from static. So after the build i tested and got much better results. I think next im going to attempt some of those modifications KrisblueNZ mentioned
Glad to get some feedback from someone who successfully replicated the circuit. If you you do try the mods KrisBlueNZ suggested, please come back and share how the results compare to the original.
Nice 'ible. Please give information on how you made the thermoplastic heater to bend the material, including how you powered it, what type of wire to make the heating element, and how long it takes to heat up the material. I too make a lot of things with thermoplastic, using a heat gun. This gives mixed results, as you can't concentrate the heat on just the joint. By the way, sources of thermoplastic include old computer case side panels, cd jewel cases, and TV cases. I have made several knife sheaths from computer case side panels, with excellent results. The last mini 30 watt audio amp I made has a clear case made from a cd case.
Is it possible to replace the circuit board or cable intestine. Must be near everything except the power supply?
Great job! <br>Actually, I don't know much about electronics but I'd like to build this circuit. Do you think it could be used to connect a guitar to a PC sound card? <br>
If you want to plug into a PC's microphone jack all you should need is a 1/4&quot; to 1/8&quot; adapter plug. The microphone input should be pre-amplified. I recommend that before you attempt this connection that you reduce the microphone level to 50% in the audio settings, then connect and check the level. I don't recommend the circuit featured in this article for connecting to a PC, you shouldn't need it, and if you use it you may damage your PC.
well, Mic in on a pc its not apropiate (sorry for the bad english), to record a guitar on pc you need to conect to Line, (assuming the guitar its conected straight up to the PC)and the guitar needs its signal to be warm (preamp), of course souncards nowadays manage better, but the signal for instrument its manage better on line in input, about yout armp shouldbe enought to warm the signal for the line in on a pc sound card, pluss it solve the mono to stereo problem.
gonna try this previo myself, ill post results when im done
Please do.
Many thanks for your advices. I still plan to build this interface but just for the fun. As I mentionned before, I don't know much about electronic and I see this tuto as an opportunity to lear. :-)
Would this work with a bass guitar? Don't really play often and would rather just use my 1000watt 5.1 surround system then shell out a couple hundred for effectively a foffee table/ footstool 80% of the time.
The amp circuit should work for your bass.
This is a good circuit for DIing your guitar, but there can be problems playing a guitar straight through into hifi speakers. Hifi speakers are not designed to be used for the raw sound from a guitar. The main problem is that you can generate too much high frequency in the signal and this can blow the tweeters. When guitars are recorded for playing through hifi speakers or a PA, the signal is compressed to stop this being a problem and to retain tone. Guitar amp speakers combos don't use tweetera. You might also find it sounds tinny if fed straight in without going through an effects box or guitar amp emulator with a little compression. <br> <br>You can search the internet for details or find some info here: <br>http://www.mindspring.com/~atlantatubeamp/id15.html <br>
best use of aluminum tape ever!
Thanks, I do hope to come up with a better way to terminate components to the aluminum circuit traces since soldering isn't an option. I think that using the aluminum tape could be a viable, economical, &amp; versatile alternative to PC boards for small, simple, &amp; unique circuit construction.
I remember back in the days, that Radio Shack had those 100-in-1 electronics kits and there were small springs bolted to a board and components attached to the bolts. When you needed to make a connection between components, you would bend over the spring, open a gap, slip a wire in the gap, then release the spring to close the gap onto the wire.
I still have a 160 in ONE kit! The spring terminals are a good idea except it is common to end up with intermittent connections, in particular with sensitive RF circuits. I will try some conductive paint, since I'm expecting to receive a couple of jars soon.
Nice work. <br>Instead of building your own pre-amp, can you use an old tape recorder as a pre-amp, by pressing the record button, plugging the guitar into the microphone input and for a home tape deck, plug the line-out into the A/V amp? If you only have a portable tape recorder, there might not be a line-out, so could you use an impedance matching cord and plug it into the headphone jack, then to the A/V amp?
The microphone input on tape recorders sends the signal to an amp stage and then to the magnetic record head. If the tape recorder has a monitor function that further amplifies the microphone signal and sends it to a speaker, or a line out, then you could do as you suggest. When connecting a line level source such as a headphone output to a power amplifier, no impedance matching is necessary, if the level is to high you can reduce it with the devices volume control.<br> <br> Another option for using a cassette deck is to insert a <a href="http://www.ebay.com/itm/BLACK-CAR-CASSETTE-TAPE-ADAPTER-FOR-Apple-iPod-NANO-TOUCH-MD-CD-MP3-PLAYER-PDA-/360677994459?pt=US_Cassette_Adapters&hash=item53fa156fdb" rel="nofollow">cassette adapter</a> and use an <a href="http://www.ebay.com/itm/New-and-nice-1-8-3-5mm-female-to-1-4-6-5mm-male-plug-stereo-adapters-golden-HK-/350815861132?pt=US_Audio_Cables_Adapters&hash=item51ae41398c" rel="nofollow">1/8&quot; to 1/4&quot; adapter</a> to connect to the guitar press play and adjust the volume. With this method you will only have one channel amplification.
Thanks for the explanations and alternate setups. <br>I thought that cassette adapters use a stereo tape head and has 2 channels. <br>Isn't one channel enough to amplify a guitar?
Cassette adapters are stereo, but you're plugging a stereo adapter into a mono jack on your guitar. Yes one channel is enough.
Pretty slick, I like it!
The real arbiter of whether the amplifier is &quot;correct or proper&quot; is the distortion measurement, which will be significant for the design as it stands. Of course, some guitarists like a bit of distortion :-)
For the input stage to have high input impedance, you need to increase R1. With 10k input impedance, it will attenuate the guitar signal dramatically. Also the bias for the first stage needs to be properly controlled, so that both positive and negative excursions will be amplified. I suggest changing R1 to 1M and adding a 2M2 resistor from Q1's base to ground. Also increase the emitter resistance, unless you use a transistor with high current gain (e.g. BC548C). You will need less gain in the second stage since there will be much less loading on the guitar signal. <br>The second stage also is not properly biased. The best option is an emitter degeneration resistor (between emitter and 0V) with a bypass capacitor across it (and a resistor in series with that, to reduce the gain), and a resistor from base to 0V to stabilise the bias. You will be able to increase the collector load resistance too. <br>Nice construction.
I knew it was only a matter of time until someone would tell me some of the things I did wrong with this circuit. ;) Obviously I'm not a formally trained engineer. I'm sure that the efficiency and functionality of the circuit can be improved greatly. My goal was to create a simple circuit that functioned to my satisfaction with as few components as possible, and I've accomplished that. I appreciate you sharing your suggestions so that I and other visitors can consider them.
Nice work! Not so much a criticism as a suggestion, you could use a long-tailed pair configuration to give you REALLY high input impedance. If you take the output from the second transistor you don't really need a resistor in the collector of the input transistor so you can use the same number of components. Bias it with a resistor divider on the base of the output transistor. In fact you could get rid of one of the capacitors. I'd be inclined to add a buffer stage though to improve the output isolation.
I have studied conventional transistor amplifier design and am aware of some of the points mentioned here. Again, I wasn't concerned with a &quot;by the book&quot; or datasheet design approach. I tested a push-pull design that used a more conventional topology, but I didn't like the way it sounded. I appreciate these suggestions being posted for the benefit of those who might want to refine and improve the design, or to, in the view of some, correct it or make it proper.
&quot;Circuit Board...? We Don Need No Stinking CIRCUIT BOARD!!!&quot;. <br> <br>I Love your values, building from rails. I like other shapes too, blocks, cylinders etc.:)
The casing looks great! Neat job!
wow really impressive
wow really impressive

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Bio: Exploring the cosmos one synapse fire and one mouse click at a time.
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