Portable Guitar Preamp

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Introduction: 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.

Step 2: Layout

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

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.

Step 4: Finishing

Just put all the wires nicely and carefully into the box. Use some cable ties whenever suitable. Label the switches and knobs. Yes. I am a Dymo lover!

Step 5: Testing

Finally, the preamp was connected to my Peavey that has the preamp in/ out at the back. Remember to turn the volume all the way down before switching on anything, just in case.

I also tested the preamp using a pair of headphones and it worked fine for me, so it can also be doubled as a little practice headphone amp. A bonus.

The only problem I found with this preamp is that it has too much gain, so only about 1/5 of the volume knob is usable in my settings and adjusting the volume to level with others' instrument could be a problem. But I also doubted that it depends on the power amp, so I have not made any changes to the part values yet. Any suggestions on this are welcomed.

I hope that you would find this instructable useful!

3 People Made This Project!

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76 Comments

1
SB59
SB59

2 years ago

I'd try lowering your vol pot value to reduce gain. May need to also lower treb/mid/bass proportionately.

0
nburman
nburman

Reply 7 months ago

Thanks for the tip. I did that! I put a 500k on the volume and works a treat. I didn't change the other pots tho.

0
nburman
nburman

Question 7 months ago

I got it all finished but have a question. I've checked the wiring a number of times but can't get the low input working. No big deal, because the hi input works fine. Does that sound odd that one would work and the other doesn't?

Also, the case seems to be microphonic. I have tried insulating the circuit from the case. Tapping the caps makes a tiny squeal. Is that normal?

0
BiffS3
BiffS3

Question 1 year ago

Hi there,
Can you tell me what this section means, not what to put there?
Many thanks in advance

Preamp.jpg
0
nburman
nburman

Answer 7 months ago

Do you mean the three circles? That's one of the transistors. Orient them in the same way as the diagram to the right of the 'schematic'.

0
nburman
nburman

Question 8 months ago

Hi- Could you post a schematic please? I’m having trouble identifying a few connections. Thanks

0
nburman
nburman

Question 8 months ago on Introduction

Hi. I’m really looking forward to finishing this, but I have a question. What are the two input jacks for? And are they stereo? I’m confused because they seem to have three terminals.
thanks!

1
jthank
jthank

Answer 8 months ago

One is for low level input, another one is for high level.

0
nburman
nburman

Reply 8 months ago

Makes sense. Thanks! Which is which?

1
jthank
jthank

Reply 8 months ago

The one with the 1M resistor in parallel is high. =)

0
nburman
nburman

Reply 8 months ago

Super, thanks!!

Now I just have to get the circuit working. Hmmm.

0
parismintoninabox
parismintoninabox

Question 1 year ago

Great job! Can I ask you a question?The question is about pedal design. How do you get such cool volumetric labels? Could you tell us about these technologies or at least give a link how to make them? Is that a print?

0
Wickedweed
Wickedweed

Answer 1 year ago

Google and look up 'DYMO'

0
JamesP479
JamesP479

Question 2 years ago on Step 5

Hi, I'd like to build your layout. The jpg image here is too low resolution to discern. Could you please guide me to a higher resolution image of the layout?

0
Chris_xu
Chris_xu

4 years ago

Great job! It's really awesome. Put the switch and knob at different position allows everyone get the effect of Fender and Marshall!

Really appreciate for what you share!

p.s. Did you solve the "too much gain about 1/5 of the volume knob" problem? Can you show us how to fix it?

0
n0ukf
n0ukf

Reply 3 years ago

One way to reduce the available gain on the volume control would be to add a resistor above it (to the bright switch on the schematic), use a 500k resistor and drop the pot to 500k or even 250k. Play with the values til you're happy.

Has anyone tried this circuit with just 9v?

0
ArnavpunitJ
ArnavpunitJ

Reply 3 years ago

U can solve the "too much gain about 1/5 of the volume knob" problem

by using a potentiometer of 100k or 500k, the 1meg for volume is too much

becuase a larger pot changes its resistance more with a turn

a smaller pot changes its resistance less with the same turn

you can say you turn more to change less than the big pot

0
jthank
jthank

Reply 3 years ago

Thank you for your comments above. From time to time I received notifications from instructables on this project, and it always reminds me the fun I had with it. I hope I did inspire someone to build this and perhaps there is an even better version out there being used in gigs.

However, sorry that I wasn't able to respond promptly. I no longer have the time to pursue building projects like this. I'm also not playing guitar anymore so it makes more sense for me to concentrate on other stuffs. I'm still into DIY though, so perhaps I can come up with a project in another area in future.

0
ArnavpunitJ
ArnavpunitJ

3 years ago

U can solve the "too much gain about 1/5 of the volume knob" problem

by using a potentiometer of 100k or 500k, the 1meg for volume is too much

becuase a larger pot changes its resistance more with a turn

a smaller pot changes its resistance less with the same turn

you can say you turn more to change less than the big pot