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!

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62 Discussions

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?

3 replies

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?

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

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.

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

hi man
I'm a bit confused about the parts so can you add a parts list too.
Thank you

This is a great instruction. Can you do one for a tube? I have a 12Fs.Q8 and several 12AV6 that I would like to put to use. Also, one extra 6J1 and two 6N3. Any info would be nice. Have a great day.

Im a newbie in electronics, is this possible guitar>audio interface>computer line? should I use a power supply to power this before using or can I just plug the interface directly to the computer?

There should be an easier way to do it. Meaning, a better lay-out sketch/drawing. More detailed. One problem with smd's is insufficient mounting options. The industry needs to try harder. You are messing with guitar player's that want to play more and fidget less. Things like Tl072's, 4558's, TDA2030a's. Need plugs. Even if they must adapt a tube base to do it, or use an octopus harness and screw lugs. I prefer fiberboards and eyelets over phenolics and pcb's I realize a lot of work went into what you've done so far. It is a device I truly want to explore. I need it simple because I believe once I understand something I can make it simpler for the next person. I was raised on 60's chevy's and they had become so simple it was going to bankrupt them. Tv's, phones, and laptops can be built to support the landfills but not your guitar amps. Even the solid states in your pre-amps should be formatted for handwiring and long lasting serviceability. I just replaced a crumbling and spongy flakewood baffle board on a 70's Vibrolux. When I want to hook an iphone to an amp on stage and pray I do not lose the phone signal ( or whatever)- well, I will see if my cash flow will allow me to keep some throw-a-way landfill bound PCB and flake board expendable ( collateral damage) short-life expectancy throw-a-way guitar amp around. So, until this pre-amp can be laid out and assembly made simpler, I must request that you make them and offer them for sale. I realize the great potential of a solid state pre-amp through a tube power amp. As did Music Man. I would rather prefer the solid state pre-amp be independent from the rest of the amp. I respect what you've done - make it available to buy !

There should be an easier way to do it. Meaning, a better lay-out sketch/drawing. More detailed. One problem with smd's is insufficient mounting options. The industry needs to try harder. You are messing with guitar player's that want to play more and fidget less. Things like Tl072's, 4558's, TDA2030a's. Need plugs. Even if they must adapt a tube base to do it, or use an octopus harness and screw lugs. I prefer fiberboards and eyelets over phenolics and pcb's I realize a lot of work went into what you've done so far. It is a device I truly want to explore. I need it simple because I believe once I understand something I can make it simpler for the next person. I was raised on 60's chevy's and they had become so simple it was going to bankrupt them. Tv's, phones, and laptops can be built to support the landfills but not your guitar amps. Even the solid states in your pre-amps should be formatted for handwiring and long lasting serviceability. I just replaced a crumbling and spongy flakewood baffle board on a 70's Vibrolux. When I want to hook an iphone to an amp on stage and pray I do not lose the phone signal ( or whatever)- well, I will see if my cash flow will allow me to keep some throw-a-way landfill bound PCB and flake board expendable ( collateral damage) short-life expectancy throw-a-way guitar amp around. So, until this pre-amp can be laid out and assembly made simpler, I must request that you make them and offer them for sale. I realize the great potential of a solid state pre-amp through a tube power amp. As did Music Man. I would rather prefer the solid state pre-amp be independent from the rest of the amp. I respect what you've done - make it available to buy !

Can I replace 2N3819 with BF245A or any other transistor?

Everyone else seems to understand what you have done, because they have not asked any questions.

I am presuming that you have taken the tone circuit from the middle of the tone circuit on the FET circuit and swapped it with the tone circuit on the op-amp circuit.

Do you know how different it would sound if you used the op-amps in the final cicuit? Maybe with a few mods?

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

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

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

1 reply

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?