Guitar to AV Receiver Interface

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Introduction: Guitar to AV Receiver Interface

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

Designing

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.

Cutting
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.

Bending
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.

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    47 Comments

    Thanks technovative, just curious, figured I'd ask, perhaps there is a guitar interface project here somewhere?

    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.

    PS, just joined today, my first question here (of many)

    1 reply

    Welcome to the community. This circuit isn't a good choice for guitar to pc interfacing.

    What about having a AcousticGuitar with piezo pickup?? Would this work as Preamp?? or just for a guitar with a magnetic pickup with preamp??

    1 reply

    I can't say with certainty because I haven't tried it, but I think it should work.

    Can you adapt it to be a Guitar to USB interface?


    This way I can plug my guitar on my computer

    2 replies

    If your computer doesn't have a 3.5mm line or mic input jack, you could get a USB audio adapter. Then you would use a 3.5mm plug as the output from the preamp instead of RCA plugs like I did.

    Very good! I´ll try to use a P2 jack to output and a P10 jack to capture signal from my guitar.

    Another question (sorry, I´m really newbie on this and I dont have knowledge on eletronics):

    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.

    Ttks in advance.

    Thanks, if you do make one come back and share how it turns out.

    Congratulations!!! Great ible' by the way!

    1 reply

    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

    1 reply

    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!
    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?

    3 replies

    If you want to plug into a PC's microphone jack all you should need is a 1/4" to 1/8" 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