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Portable Guitar Preamp

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The objective here is to build a portable guitar preamp that I can carry around and gig with different people. It can also replace the preamp section of my guitar amp, which is a Peavey, and borrow the sounds of Fender/ Marshall/ Vox from its tonestack.

 
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Step 1: Schematics

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I did not design the schematics myself. Since my objective is to make this preamp portable, I searched for the simplest preamp design and found this from www.redcircuits.com . This is called a "Solid-state Fender Blackface Preamp", which is a transistor version of the original valve circuit from the "Fender Blackface".

http://www.redcircuits.com/Page120.htm

I then borrow the tonestack from the Tonemender that is found at www.runoffgroove.com . The Tonemender is a booster with a very flexible tonestack, which can re-create the Fender, Marshall and Vox responses.

http://www.runoffgroove.com/tonemender.html

Details of these schematics can be found at the links above, which explain clearly what these circuits do and how they work.

Step 2: Layout

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Merging the 2 circuits found, I laid out the board of this little preamp. One thing to note is that pinout of the FET can be different, so it is wise to check the spec sheet from the manufacturer. If the pinout is the same as the ones I use, you should be able to build this project by using the same layout.

It took me some time to draw the layout (which was my first time), but it actually saved a lot of time for me to build the board.

I also matched the FETs by following the instructions found here:
http://www.nrgrecording.de/html/fetmachting.html

Step 3: Building the Preamp

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Not much to explain here. Just solder the parts according to the layout. It is always a good idea to test the board before putting the off board parts all together.

I used shielded wires for the signal path. More work and time is needed in soldering and grounding, but I think it surely worth it.

The preamp can be powered by 18 volt. The 18 volt battery clip is made by using some old 9V batteries.

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nikkram173 months ago

can this be AC powered? if not, can u make a revision of this in AC mode.. TIA :)

robroy071 year ago
I tried to build this project using vero board but it didn't work. Do you know if anyone has done this using veroboard ? Thanks
robroy071 year ago
I have some questions
1 I will be using a larger enclosure so that I can fit a mains transformer with 24 volt secondary and a filtered power supply with a 7824 regulator that I have. Can you foresee any problems ?

2 Is there any need to have all the offboard shielded?

3 I am confused with 2 input wiring. Obviously the jack sockets are switched but I am not sure how to wire them
I think this is a very good circuit and is just what I need
jthank (author)  robroy071 year ago
Hi,

1. I am not experienced with power supply. One reason I do not want to get into it is because of the high voltage. Second reason is that I did built some power supplies in some other projects and found that many unforeseeable issues may happen. For instance, shielding of the transformer is important or the magnetic field may interfere with the signal. You may want to put it as far from the circuit and wiring as possible.

2. It is always better to have them shielded.

3. You need mono jacks with switch. http://www.guitar-parts.com/catalog/jack-14-inch-female-choose-style-1. the middle one in the image of this link. Once you get the jack, check its connectivity with a multimeter by inserting a jack. You need to figure out which pin is the switch (one that will be disconnected from the tip when the jack is inserted). Once you figured out how this switch works (it is easy when you can and play around with it physically), you should be able to connect according to the circuit.

Good luck!
Ooops .... Will this work as a headphone amp?
jthank (author)  celticsmile1 year ago
When I tested this unit I use a headphone, so I would like it would work as a headphone amp. However, you will need a stereo jack for the output. Otherwise, you only get the sound from one side. Thanks.
With this work as a headphone amp?
billb1 year ago
Hi, very cool, what software did you use
to design your circuit board? THAMKS!!!
jthank (author)  billb1 year ago
you may try something like "diy layout creator" for simple layouts. Cheers!
martinnez1 year ago
try attenuator
juribe21 year ago
Is there a way to use a power source instead of using batteries????? thanks
jthank (author)  juribe21 year ago
Hi,

It was already routed to accept a DC power adapter. However, from experience, unless you have a very clean power source, batteries sound better.

Actual wiring depends on the DC jack you buy. You can use a multimeter to check for the connection.

You can refer to the wiring diagram for any guitar effects. They are all the same for the DC jack/ battery part.

http://www.generalguitargadgets.com/pdf/ggg_sgo_lo_wiring.pdf
Felol2 years ago
It will be awesome if you'll upload the layout in a higher resolution so we all can see the values of resistors and capacitors.
could you have a parts list of the merged circuit.
and better labels of what the component are(Like value of resistors and cap.)
i got a couple of questions more ,
where do the transistors go ?
jthank (author)  Thereyouhaveit3 years ago
transistors are those little circles (3 in a row, aligned vertically). There are 4 in pairs, and 1 next to the 4u7 before the output. The pinout follows the one shown on the right.
what's those white holes for ?
hmm, if you're still wondering, if you mean the ones near the jacks, they're the spots that off board components connect to the board, with wire.
Okay ! thank you . xx
VAustin893 years ago
i am planning to build an amp.. now i have found this one.. if i was to build an amp what are other circuits i need to create?? that circuit is where the guitar gets plugged in right?? and if i was to implement it on diy amp, how many watts could i get out from that circuit??
this is the circut where the guitar gets plugged in, but you would have your regular input and then for the output have the leads go to speakers. other circuts would be optional (like distortion or FX)
i have built a preamp now i need is a power amp for the speakers, i was wondering if you have an idea on how to build a power amp that is about 50 watts?..
thanks for the input..
skaar VAustin893 years ago
check esp, they have free amp circuits... there's also an open design amp somewhere, i forget, ax... something...
swheeler33 years ago
Hey how could I add a distortion option to this?
If you made this battery powered could you use it a headphone amp?
-Syrus-3 years ago
The way you are making 18V+ is not so good, if one battery goes out of juice, it will rapidly drain the other and back and forth, this can lead to leaking.

Why not up the voltage from a single cell? Or run at 9v?
Patodana3 years ago
Hey jthank, nice build! When you inserted in the original cicuit the tonestack from the Tonemender, you use -18v as Vref? It's possible that you post your adaptation of both circuits?
jthank (author)  Patodana3 years ago
Thanks Patodana. I used only the tonestack from the Tonemender, to replace that in the Blackface's original circuit.

The tonestack is the section I highlighted in red in the new photo. This is the part that shape the sound.

If you look at the Blackface's circuit, coming out of the 1st set of transistors, the signal goes into the tonestack, then the signal goes to the vol (P4) with or without the C6 (bright switch) to enter the 2nd set of transistors.

So instead of going into the original tonestack, in my version, the signal comes out of the 1st set of transistors to the Tonemender tonestack (the red part), then to vol (P4) and the 2nd set of transistors.

If you look at the Tonemender circuit again, the signal comes out of the 1st opamp, then into the tonestack, then into the 2nd opamp.

So if you look closely, both circuits are actually doing very similar things.

The tonestack is totally passive and it does not use any power. Vref in the Tonemender = Earth = 0V. Hope it helps!
Patodana jthank3 years ago
Thanks for your reply! I asked, just to be sure, since Tonemender's Vref it's around 4.5v and Blackface uses double rail voltage, so... I was a little confused (shame on me LOL)
In a couple of days I'll start to work on it. I'm interested on make a bass version, so I need to tweek the voicing in the tone section and some caps here and there, you know.
Thanks again for your kindly response!
I let you know as soon as I get positive results.
Cheers!
http://www.guitar-parts.com/pictures/918c/Jack-1-4-inch-female-Choose-Style.jpg
The guy at the store gave me the stereo plug , how do i wire it for mono ?
go back and buy a mono :|
Okay okay . xx
jthank (author)  Thereyouhaveit3 years ago
you can just use the tip and the sleeve, and do not connect the other pins. check with a jack and multimeter if you are unsure.
Where do the inputs go in from ? sorry , i'm a beginner , ( learnt quite a bit . )
jthank (author)  Thereyouhaveit3 years ago
the inputs are at the bottom left corner. The drawing shows where the tip and the sleeve should go. hope it helps.
Hey , i can't see what to label , i'm lost , mind to explain ?
Hi, I thought about doing the same 'ible, but with a TL072 instead of 4 transistors, it is also a small preamp, and it si also portable, but needs only 5 volts to work.
Ill post the link to m 'ible as soon as it's up.

P.D: Cool project, could you upload some videos or sound to have a listen on it???
jthank (author)  Emiliano Valencia3 years ago
I also had come across some opamp circuits, which surely would work the same way. One other reason for my choice of this circuit is I think the transistors would give a more tube-like sound than opamps.

Looking forward to your instructable!

P.S.: I will try to get some sound clips when I've got the time.
t-minik3 years ago
nice project !

for the big amount of gain you can remove the C8 cap and put the Jfet buffer like on the red circuit page,

just replace the output résistor by a 100k log pot and you get a master volume.
jthank (author)  t-minik3 years ago
Thanks for your comment. I actually had put the buffer, so adding another pot (possibly a trim pot) to replace the output resistor would be the solution for me!
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