Portable Guitar Preamp

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.

Step 1 Schematics

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.

Comments

[Felol]

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.

[vincentn1818]

could you have a parts list of the merged circuit.
and better labels of what the component are(Like value of resistors and cap.)

[Thereyouhaveit]

i got a couple of questions more ,
where do the transistors go ?

[jthank]

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.

[Thereyouhaveit]

what's those white holes for ?

[skaar]

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.

[Thereyouhaveit]

Okay ! thank you . xx

[VAustin89]

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??

[casabonita69]

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)

[VAustin89]

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]

check esp, they have free amp circuits... there's also an open design amp somewhere, i forget, ax... something...

[swheeler3]

Hey how could I add a distortion option to this?

[Houdinipeter]

If you made this battery powered could you use it a headphone amp?

[-Syrus-]

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?

[Patodana]

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]

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]

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!

[Thereyouhaveit]

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 ?

[jthank]

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.

[mrmerino]

go back and buy a mono :|

[Thereyouhaveit]

Okay okay . xx

[Thereyouhaveit]

Where do the inputs go in from ? sorry , i'm a beginner , ( learnt quite a bit . )

[jthank]

the inputs are at the bottom left corner. The drawing shows where the tip and the sleeve should go. hope it helps.

[Thereyouhaveit]

Hey , i can't see what to label , i'm lost , mind to explain ?

[emihackr97]

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]

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-minik]

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]

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!

[no_key_bandit]

Awesome project. Could you wire an XLR connector in somehow and use this as a Direct box?

[jthank]

Thanks. Nice idea and good comments from the below members. Here is a nice page on DI:
http://zonaelektronika.blogspot.com/2010/06/direct-injection-box-for-recording-pa.html

Getting back to the original question: Could you wire an XLR connector in somehow and use this as a Direct box?
The signal from this preamp is unbalanced, so it cannot be used as a DI just by changing the wiring. However, an additional circuit can be built it to add that function, but then it is like putting a DI after this preamp. In that sense, I would choose to build a separate DI instead for its portability and usability.

Yet, it may be possible to build a DI with the same tonestack built in, which I think would be a better option.

[waynesl]

XLR connection means Lo-Z balanced. There are also some gain concerns, but the adjustments available in this project should be sufficient for that. happytobethemoon is correct in saying that either a transformer or an active splitter circuit should be used to get the signal from the unbalanced, high-impedance output of this preamp into the balanced, low-impedance line. If you use a transformer, connect the output of the preamp across the non-tapped winding, then the secondary goes centertap to pin 1, hot to pin 2 and cold to pin 3. If preamp output is too strong for the next stage, you can run a 2.2KΩ resistor in the line, and that to the wiper of a 10KΩ audio-taper pot. Then the two other tabs of the pot would go to the transformer primary, with one of them grounded. You probably don't want more than 1 or 2VPP on the secondary of the transformer at peak volume. There's some excellent gouge at: http://www.jensen-transformers.com/apps_sc.html  
A nice active DI circuit can be found at:  http://www.muzique.com/tech/opamp_multi.htm 

[allitode]

I may be wrong, but since a direct box goes from high impedance (instrument cable) to low impedance (mic cable), you would almost certainly need a transformer to make that work. I guess you could gut one and put it in the end or find an active DI schematic and go from the output here through that.

[happytobethemoon]

A passive DI uses a transformer to decrease the output impedance, but it also decreases the signal level. An active DI contains an amplifier, so the signal gain and input/output impedance can be chosen by the circuit designer.
A preamp and an active DI are basically the same thing. So, conceptually, it should work. The op-amp version of this should work for a mic, but I don't know about the transistor version (I'm not that good at circuits yet).

[rghoff]

This looks like a fun project. Any chance of getting some sound clips posted on here?

[computergeek321]

cool