Simple JFET Preamp for an iDevice Guitar Interface
Friday, September 23rd, 2011 at
7:30 pm
Following up on Part 1, it’s time now to get into the heart of the project:
In this video, I talk about how JFET’s work, then work up a circuit diagram (as shown at right).
Then, I prototype the circuit on the breadboard of my Radioshack Electronics Learning Lab, and finally play through the circuit to show how it sounds.
In Part 3, I’ll demonstrate how to assemble the circuit so that it fits entirely inside the jack.
Here are some useful resources and background reading:
- Great interactive simulation of a JFET
Some articles on JFET preamps:
- http://www.rason.org/Projects/jfetamp/jfetamp.htm
- http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp1.htm
Tillman’s FET preamp using the J201:
Some relevant forum topics:
- http://forum.harmonicdog.com/viewtopic.php?f=2&t=6&start=50
- http://www.diystompboxes.com/smfforum/index.php?topic=85913.0
- http://sgitornado.altervista.org/diyirig.html
Tagged with: Electronics • iPad • JFET • Preamp • Video
I ‘ve never really had any interest in plugging a guitar into an idevice but if that amplitube app is free I’m definitely building one of these.
@ibanez350mdx – yes, the free version includes a couple cabinets and a few effects, plus a simple tuner and an audio player with slow-downer speed control. It’s great for practicing. Then you can buy more cabs/effects a la carte or buy the whole thing for like $20. Enjoy!
@johnplanetz awesome. cant wait for part 3. I’m going to go to radioshack and then find my ipod touch.
yes! you’re back. missed your vids!
Hi, i try to build this buf, there aren’t any jfet except 2n5457. R2 must be near 10-30k. Thanks for Schematics.
U can dont damage you 3.5 4pin – 3rca cable. Use rca female-buf-6.3mm male cable.
Hi Dmitry,
Thanks for the source-resistor value suggestion for 2N5457- glad that worked well!
Yes, if you don’t want to cut the ends off an existing cable, using female->male adapters is a good idea.
-John
Hey John first of, I want to thank you for your patience explaining everything very well, I am building my own iDevice and I couldn’t find the MPF-102 transistor, can I replace it with a 2N5457?
thanks again
@OVProyect – yes, you can use a 2N5457, but you’ll need a bigger source resistor. Try some values between 10k and 50k, instead of the 1.2k I suggested for the MPF102. One person reported having luck with a 47k source resistor.
@johnplanetz Ok as I was very anxious about all this, I did it before I read your response because I didn’t think you would erite back so quickly. So it works with the 1.2 k resistor and it got rid of most of the problems although it’s still a little distorted, so I think I’ll have to go back to radioshack and purchase the 10 k one, thanks for the response John, cheers!
Apple should pay you. I hadn’t given the iPad any serious thoughts until hearing those amplitube samples.
hi what do you think about using 2n3819 ? It looks very similar to mpf102 .
@reppait – I took a look at the datasheet- looks like it should be no problem.
at 7:55 are the zeners inverted ? Maybe i’m wrong but your scheme looks different , cathode and anod are inverted
@reppait – You’re right- sorry for the confusion here- these were reversed from the schematic. However, it doesn’t really matter here. If you think about what these zeners are doing– current will flow freely in one direction, and if the voltage exceeds the breakdown voltage, it will also flow in the other direction. So it doesn’t matter which one comes first.
Thank you for these great tutorials, you’re a born teacher (high praise in my book). I built the active guitar lead into a Neutrik SilentPlug (http://www2.neutrik.com/uk/en/audio/204_826925229/silentPLUG_productlist.aspx), these are great in case you don’t know them, they have a sleeve which operates a reed switch by magnet to short out the input when withdrawn. This is particularly useful when you’ve increased the input impedance to 3 megohm, you can safely move the lead from one guitar to another without muting the amp, no more high level splats! Has anyone found a good source resistor for the J201? These are slightly lower noise and easier to find in England.
Hi John,
Thanks for the message.
Yes, those silent plugs are great. I made a bunch of cables for all my guitarist friends- silentplug on one end to avoid pops when changing guitars. Pricey connectors (around US$8). but totally worth it.
Cool that you fit a preamp inside one!
-John
Addendum.
Sorry, hit send too soon! I’ll now build the iPad interface and wanted to use your circuit with the J201, it was for this 2.8v circuit that I was looking for the correct source resistor.
For the J201, start with a 1.2K ohm source resistor. See how that works!
-John
what the hell …
hi.. i was just wondering on what capacitors to use? my friend told me that capacitors have specs like volts and farrad? sorry, im a little illiterate on circuits and the like.. thank you for the great tutorials..
This circuit doesn’t require any capacitors.
The resistors in this circuit can be 1/4W or 1/8W, carbon composition or metal film- doesn’t really matter. See the parts list in part 3 of this series:
http://www.planetz.com/?p=1536
If you’re looking for some general information on caps, you could watch my video here:
http://www.planetz.com/?p=667
-John
hi.. i have made a this jack but i would like to ask if i can use this jack and connect to my amplifier instead of speaker? i tried using it but it is grounded.. on my headphones its okay but on my amplifier it sound grounded.. thanks..
It should just work. I use it like that sometimes. I use a 1/8″ stereo mini adapter to dual RCA, then plug an RCA to 1/4″ cable into one side of that, and into my amp. Works fine.
John
Great vid. How about a vid showing how to build a Stratoblaster with a JFET that goes under a flat plate jack like with see on Alembic’s site?
@AxekickerDotCom – thanks for the suggestion. I’m planning a video on how to make a boost pedal. It would be pretty easy to mount such a thing below the pickguard of a strat, rather than in a separate enclosure. You could even make one that fits into a cable, as I did in my ipad cable, but you’d need to provide a 9v power source somewhere. See Tillman’s cable preamp. till. com/articles/PreampCable/
Nice One Bro.. wait is it ok to try BF245C or(J201 )?
OMG! I think building an iDevice guitar interface myself is awesome but you can even improve its tone quality!! You’re genius and I love your videos so much! They’re useful~
Thanks! As for improving the tone quality- using the mic/headphone jack isn’t the most high-fidelity way to make an audio interface for the iPad. For better fidelity, you really need an interface with its own A/D using the 30-pin dock. This would not be an easy DIY project!
can i use jfet bc547
bc547 is an NPN transistor, not a jfet
Hi John,
I made one of this and works fine, or almost. A strange thing happens.
Ampkit and amplitube works perfect with no bad sounds on iphone 3,4 and new ipad
GerageBand works perfect on iphone 4 but buzzes on iPad. I don’t know what happens here. Do you have some idea about? Thanks a lot for your work!
That’s strange- I’m not sure what that could be!
Hi!
Does this audio interface works with PC ?
I want to build an audio interface for PC, because if I use the Line-in on my Sound Blaster X-Fi it have a lot of “hiss” noise, I use AmpliTube 3 and Guitar Rig 4.
If your PC has a mic-input, it will probably work. But if you only have a line-in, then it won’t provide the necessary power for the jfet.
-John
I do have a Mic input on my sound card, but I don’t use it for my guitar, because it’s too noisy for me.
I still have some “hiss” noise when I use my Line-in.
Can I power externally the JFET ? Maybe from a 9v battery or power adapter with a voltage regulator.
Thanks for your answer in advance!
Daniel
Take a look at Tillman’s Preamp cable (as linked in my original post above). He uses a little battery box to power the preamp.
-John
Thanks for your help!
I’m going to build this interface, then I will test with my PC.
I just finished building my interface, and it worked out flawlessly!
As you said, I connected to the Mic input and it worked right away as I plugged in my guitar!
The noise issues is almost gone now, I still hear some noise in the background but it is much, much better than before! Also, the volume and the treble pots are working normally.
I’m going to build a battery box and then test this baby out using the Line-in.
Thanks for this awesome device!
Nice! That’s great to hear.
Some amount of noise is normal. This is not an ultra hi-fi connection unfortunately.
Enjoy!
John
Hey, do You know if BF256B would work as a replacement for MPF102?
I haven’t tried it, but the datasheet makes it look pretty comparable to the mpf102.
Double-check the pin orientation, I think the drain and gate pins are reversed compared with the mpf102.
John
hi.. i would like to make my second cable but my problem is 2N5457 ran out of stock.. can i use 2n5551?
2n5551 is an NPN transistor, not a JFET.
Your awesome… Thanks for the info and all, it’s a nice little project.
So I connected the cables without the transistor,resistors an diodes and my michrophone doesn’t work anymore (On my headset ie.) Aparently the input is damaged on my iPhone 4S. Any help would be greatly appreciated, Idk what might have happened and how to check what is what?
Thanks in advance…
I’d suspect a wiring issue with your cable/jacks. You should double check the jack wiring. See part 1 of my video ( http://www.planetz.com/?p=1510 ) where I show how to identify the cable wires with a multimeter, and hook it up with alligator leads…
To rule out the iphone itself:
– Is the built-in iphone microphone working ok?
– Does a standard pair of headphones work?
– Try a cheap wired headset- does that work ok?
-John
My headphones and built in michrophone works, it’s just the mic input on the iPhone that doesn’t work anymore. I’m not sure but I believe the mic input got fried in the process, i don’t know for sure.
I could use my headphones to make calls but now I can’t; I can hear but others can’t hear me. I tried other headphones and eventually resorted to using the built in michrophone on the phone (by removing the headphones) as they didn’t work either.
I’d try the project again but theres no input to work with. Do you know what could have happened and what I can do to possibly solve the problem?
Hi Wayne,
When I was prototyping my circuit, I made lots of mistakes, short circuits, etc, and did not have any problem with damage to the mic input on the ipad. My impression is that it’s a pretty robust circuit – but I suppose it’s possible you damaged yours somehow.
Have you used a multimeter to confirm that you still have 2.8V dc on the mic input? (See part 1 of the video where I show how to do that).
-John
Thanks a lot for the great videos! I followed your schematic and used a bf245c and it works great! Good job on putting it in such a small box. I put everything in a 5×3.5×2.5 cm box so I’m very satisfied
First of all, thanks John for the most detailed explanations on the net!
Anyway, amplivibe, I used a BF245b (there’s absolutely no MPF102s, J201s found here in singapore!), but I’m getting quite a bit of distortion still. Could you please share the R2 value you used, so that I have something to start trial and error with?
Much appreciated,
Darren
Hi Darren,
You could try installing a trimpot temporarily, in place of R2. That will let you easily experiment until you find a position that works. Then remove it, measure the trimpot resistance with a multimeter, and pick an R2 close to that value.
-John
Hi John,
I’ve tried playing with the R2 values using a 2k pot and then a 10k pot, but I still can’t get the bf245b to sound good, what i get is muddy and distorted exactly like an unbiased fuzz.
Could you suggest other replacements for the jFET? Or perhaps the key parameters of the jFET that I should look for in a replacement?
Much appreciated,
Darren
Hi Darren,
I would expect the bf245b to work ok. Are you sure there’s no other problem with your circuit? Are you testing this on a breadboard, so you have plenty of space to experiment, and can easily break down the circuit to the basics and test each step? Do you have another jfet to swap in, in case there’s a problem with your particular bf245b?
-John
John, thanks for this demo! I put together what I am calling the modular version. In my bin of “various AV cables” I found an iWhatever Red-White-Yellow RCA’s to TRRS, with yellow wired to the mic ring. I also had a R/W Male RCA to female 1/8th headphone Y cable.
So I thought why splice all these together, instead I built the circuit in a small Radio shack box with 1/4″ in and Female RCA out and everything plugs together. The headphone Y cable gets swapped for a RCA’s to Male mono 1/4″ for connecting to my amp.
what are your thoughts on going into a home stereo? I’ve heard it suggested that distorted waveforms can be bad for home speakers…
Cool, thanks for sharing! I wouldn’t worry about damaging your home speakers, unless you are playing at excessive volume (true for any kind of sound, not just distorted sounds).
-John
here is the direct link to my photo if the html didn’t work…
http://i1264.photobucket.com/albums/jj499/lichtaffe/Photobucket%2BDesktop%2B-%2BPauls%2BMac%2BPro/photo3_zps9dd21c58.jpg
That link doesn’t appear to work…
ok… try this:
https://www.box.com/s/4vfytpbu39hq60aq6f5x
Yes, that works. Looks nice!
Hey, I just wanted to say that I have successfully built this cable with the 2N3819 JFET instead. I kept all the other components (resistor values, etc) the same. It didn’t work in the beginning, but then it suddently kicked off, maybe it was something to do with the iPhone not detecting the line level input at first, dunno.
Anyway, it sounds great with the iPhone 4S.
Too bad it doesn’t work with the Macbook Air – the headphone sound is very weird and the input doesn’t work at all – Macbook does not detect it at all in the System Preferences.
Great news, you got it working!
I think the input on the MacBook Air is a line in, not mic in, so there’s no power for the jfet.
John
Hi John! I cant get MPF102, J201 or 2N5457, instead the guy at store got me the 2SK246, told me that was equivalent to 2N5457, can i use this one?
Tnx, for the great tutorial!
I don’t have specific experience with that jfet, but it’ll probably work ok. You may have to experiment with resistor values- so test on breadboard first. Good luck!
John
I have tested in a test board and i get no audio out, i tried several setups and nothing. On amplitube, the mic and out levels are up and down, even without playing any note.
Can u help me? Tnx.
I would double check TRRS connections… not all cables are the same… mine was different than the one in the video.
also, the MPF102 is regular stock item at Radio shack… that’s what I used.
But just double check your layout, if you are new to such projects, and I’ve only done a couple, It can take a bit of work to translate the 2D layout to the real world.
my 2cents.
Double-check the data sheet for the 2SK246- the gate, source, drain pins are in a different order compared to the MPF102, etc.
John
So I got some sound, but is very low, and i am just testing with no electronics. What is the setup on amplitube?
My settings are
In Level: -6
Out level: +5
Built In Mic: off
Latency: Ultra Low
No Feedback: Auto
If you’ve turned up the in/out levels in Amplitube and still only hear low levels, you need to experiment with your resistor values to increase the jfet gain.
-John
Hey i made one of these for my ipad and was hoping that it would work for my macbook pro however it is not detected by the macbook pro. i have one of those 2012 model macbook pro’s that only has a headphone jack that can detect mic input with the apple headphones (4pin headphones). do you think the macbook isn’t giving the 2.8v like the ipad and would require an external battery to power the jfet?
Should be easy to check with your multimeter in DCV mode- just plug in your cable, and check for a voltage across the various wires coming from the mac.
-John
I was just curious to know if it is possible to make the R3 adjustable via a potentiometer/knob if assembled in a project box.
I feel it would be useful when using more than one instrument.
What components would be needed, and how would it be wired?
You can easily replace any of the resistors with a trimpot or larger potentiometer. Just prototype on breadboard and experiment to find parts that work.
John
hi.. i cant find mpf102 jfet transistor.. please can you suggest another # transistor.. sorry for my english..
and the resistor is that half? thanks man. advance..
-philippines
There are some alternatives listed in the schematic, and also some others discussed in the comments here. Please take a look!
I’m not sure what you are asking about the resistors.
-John
i mean big resistor. you are using a 1/4 right? i bought 1/2 size.. is that ok?
This circuit is low power, so you can use resistors as low as 1/8W. The lower power-rating means they are physically smaller, which will help them fit better in a small enclosure (like the jack I show in the video).
Resistors with higher power rating like 1/2W or 1W resistors will work fine, but are physically larger.
-John
thanks john.. ill show you my project when done.
request
:can you do tutorial mini audio amplifier can play 80w of speaker?
hehe thanks john.
randy. PH
No plans right to do an amplifier tutorial…
I’ll look forward to seeing your finished project!
-John
Hello John.
2SK170 (LSK170) is a JFET used in hi-fi and hi-end audio.
Is a high-gain and very low noise JFET.Can be used in your plan?
I am interested to use the electric guitar in the microphone input of the PC sound card.This input generates a voltage that can power your preamplifier.It can?What do you think?
P.S. I do not write well in english.Am used a program to write you.
It’s probably ok – just check the data sheet, and give it a try.
John
Hi Dan,
I’ve successfully used the buffer circuit with a computer. The 3.5mm mic jack “usually” has a biasing voltage on it suitable for an electret condenser, typically 3 volts. The lower the voltage, the less headroom you get in the buffer. If your guitar clips the input to the buffer, this is likely to be why. Just reduce the output volume on your guitar and it should be fine.
John
Thanks for the tips John!
Dan, let us know how it goes.
-John
Ok.
Thank for this nice instructional vid. I was wondering if your circuit design can be used for pc as well without any modification? or if not, can you design something similar for pc. like having minimal parts and requires no extra power source. I wanted to build one for pc but all of the circuit designs I found requires extra power from a 9volt battery. Can you help me please. Thanks
If your PC has a mic-input, it will probably work. But if you only have a line-in, then it won’t provide the necessary power for the jfet. Check your PC’s input with a multimeter to see if it carries a DC voltage.
I built one of these in a workbox with perfboard and an extra 1/4″ TRS output. Works great! Way cool John!
how can i add a power supply to this circuit so that I can plug a head phones instead of an iDevice?
If you are asking how to make a standalone device that you can plug your guitar into and listen on headphones, you need to look elsewhere. E.g. see the generalguitargadgets headphone amp for a schematic.
Thanks for the demo, can I use this preamp for the Violin?
cheers
This circuit depends on the power-source from the iPad. To use this as an instrument preamp, you would need an independent power source, like a 9v battery. Search for Tillman’s powered preamp cable for an example project.
you’re the coolest dude on youtube.
you’re the coolest dude on youtube.
Hi johnplanetz, do you show the values of the resistors which you use in the video? Thank you in advance.
I discuss the schematic and resistor values at 5:52
+1 the coolest dude on youtube!
I don’t seem to find the transistor you used.
Could you point me out some alternatives that might work?
See the link below the video to my blog. I list some alternate JFETs in the schematic there, and there’s a lot of discussion in the blog comments about other JFET’s that people have used successfully.
Thank you so much from latinamerica. You have this amazing a simple easy way to explain things, especially electronics, here turn to be to expensive to buy an Irig.
Is it possible to adapt this device for using it with regular PC? Instead of 2.7V from iPad I could use 2x AA batteries. Would that work?
The microphone input on a PC often carries a similar voltage, to drive an electret microphone preamp. If you only have a line input, then yes, you could look into adding a battery box to power this preamp circuit.
Well I have both line and mic inputs, so I guess I’ll just measure the voltage on mic input and if it turns the same then Im off to build such preamp
I will also measure its impedance. If it will be for ex. 500k, then instead of 1M resistor I should just put 500k, right?
You won’t need to change the impedance in the circuit– that 1M resistor provides a high impedance input for your guitar.
Ah, right, its for guitar. I was thinking about line-in/mic impedance. Do I need to consider that, or just follow your schematic?
This circuit output is rather low impedance. You shouldn’t have an issue.
Great, added to my must-do list
Thanks for your help!
great circuit can i use direct box
can I use 2N5457 with the suggested resistor (e.g between 10k-50k) ?
typo* 2N5458
Yes, the 2N5458 should work as well. Just be prepared to experiment a bit with the source resistor.