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Build your own onboard preamp

rojo412

rojo412

Sit down, Danny...
Supporting Member
Feb 26, 2000
10,907
15,038
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Cleveland, OH.
#1
I am nearing the point of seriously considering to attempt to build my own preamp from scratch. I have had so much fun with wiring when it came to my latest project that I'd like to go the next step.

Does anyone know where to even begin this process?

I have a book on guitar electronics... from 1982. Not helping much. Wanted to find out what it takes to create one.
 
#3
How much electronics experience do you have?

You can design an EQ and buffer circuit, but if you had to ask about this, I think it would probably be easier to start with an existing preamp design off the internet and tweak with it.

FWIW, Personally, I would suggest a field-effect transistor based preamp as opposed to using opamps.
 
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#4
line6man said:
How much electronics experience do you have?

You can design an EQ and buffer circuit, but if you had to ask about this, I think it would probably be easier to start with an existing preamp design off the internet and tweak with it.

FWIW, Personally, I would suggest a field-effect transistor based preamp as opposed to using opamps.
Click to expand...

I have to agree with this,

A good starting point is Tillman's J201 preamp. Which you could modify if you wanted to try different FET's. You could then experiment with some Baxandall style tone circuits.

http://www.till.com/articles/GuitarPreamp/index.html

http://home.comcast.net/~stphkeri/NegativeFeedbackTone.pdf
 
#8
i_got_a_mohawk said:
I have to agree with this,

A good starting point is Tillman's J201 preamp. Which you could modify if you wanted to try different FET's. You could then experiment with some Baxandall style tone circuits.

http://www.till.com/articles/GuitarPreamp/index.html

http://home.comcast.net/~stphkeri/NegativeFeedbackTone.pdf
Click to expand...

Funny, I actually just built one of those two nights ago, just to see what it sounds like.

Whipped out my old breadboards and tested the circuit with a function generator and oscilloscope:
4612016318_546b1bef26.jpg

Then assembled it on a PCB to try with an instrument:
4607532849_2a210f86bc.jpg
 
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#9
rojo412 said:
I am nearing the point of seriously considering to attempt to build my own preamp from scratch. I have had so much fun with wiring when it came to my latest project that I'd like to go the next step.

Does anyone know where to even begin this process?

I have a book on guitar electronics... from 1982. Not helping much. Wanted to find out what it takes to create one.
Click to expand...

If you don't already have one I would also get a multimeter that can check capacitors. It saved me a lot of time and headaches when troubleshooting and checking new components. They also come in handy when checking a finished project for any voltage leakage through the output.
 
#10
BassLife77 said:
If you don't already have one I would also get a multimeter that can check capacitors. It saved me a lot of time and headaches when troubleshooting and checking new components. They also come in handy when checking a finished project for any voltage leakage through the output.
Click to expand...

Yep, check your capacitors before using them.

For example, if you get a shunt in the coupling capacitor after a transistor, that will give you 9V DC at the output, on top of the signal...

It's also a good idea to pick out the "good" parts from the ones with terrible tolerances.
 
#11
BTW, for anyone interested, here were the 1KHz sine, 20Hz square, and 20KHz square responses I got from the J. Tillman preamp.

4611721249_32fb567cb9_o.png


Not bad. Come to think of it, I was running the input fairly hot. I wasn't paying attention to the output amplitude on the function generator when I took the pics. The scope readings show somewhere around 250mV input, and about 600mV output. That's close enough to the 3dB gain increase it's supposed to provide, if I'm doing the math right. :hyper:

The preamp is really touchy to changes in power supply voltage, actually. When I had it wired up on the breadboard, I powered the circuit with a benchtop DC power supply. The preamp went nuts at the slightest change in voltage, if I went to adjust the power supply.
 
#12
line6man said:
BTW, for anyone interested, here were the 1KHz sine, 20Hz square, and 20KHz square responses I got from the J. Tillman preamp.

Not bad. Come to think of it, I was running the input fairly hot. I wasn't paying attention to the output amplitude on the function generator when I took the pics. The scope readings show somewhere around 250mV input, and about 600mV output. That's close enough to the 3dB gain increase it's supposed to provide, if I'm doing the math right. :hyper:

The preamp is really touchy to changes in power supply voltage, actually. When I had it wired up on the breadboard, I powered the circuit with a benchtop DC power supply. The preamp went nuts at the slightest change in voltage, if I went to adjust the power supply.
Click to expand...

looks interesting, for only $1.50 worth of parts that JFET preamp gives you a lot of bang for your buck. All I see is positive feedback about them on the web. I'm gonna order a J201 and see what it sounds like compared to the opamp and tube preamps I built. looks like the parts are very tweakable

JFET_Booster_Schematic.gif
 
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#14
BassLife77 said:
looks interesting, for only $1.50 worth of parts that JFET preamp gives you a lot of bang for your buck. All I see is positive feedback about them on the web. I'm gonna order a J201 and see what it sounds like compared to the opamp and tube preamps I built. looks like the parts are very tweakable

JFET_Booster_Schematic.gif
Click to expand...

Yeah, a capacitor from the source of the transistor to ground can give you a gain boost.

I tried adding one when I was breadboarding the circuit. It gave a huge gain boost, but I left it off, because I don't need a gain boost.

Btw, be careful about the pin layout of the transistor...
That diagram shows DSG, but the transistor I had on hand (an NTE458) goes DGS.
 
#15
line6man said:
Yeah, a capacitor from the source of the transistor to ground can give you a gain boost.

I tried adding one when I was breadboarding the circuit. It gave a huge gain boost, but I left it off, because I don't need a gain boost.

Btw, be careful about the pin layout of the transistor...
That diagram shows DSG, but the transistor I had on hand (an NTE458) goes DGS.
Click to expand...

I noticed the price difference between the J201 ($.39) and the NTE458 ($3.89). So I am assuming the NTE458 is better. Are there any dramatic differences betwen the two? At this newb level I can't translate the data from the spec sheets to the real world yet
 
#16
BassLife77 said:
I noticed the price difference between the J201 ($.39) and the NTE458 ($3.89). So I am assuming the NTE458 is better. Are there any dramatic differences betwen the two? At this newb level I can't translate the data from the spec sheets to the real world yet
Click to expand...

Christ! Is that how much they are charging for these things?
I doubt it if it costs much more than a penny to manufacture them.

Maybe the specs are tighter or something, but I really wouldn't worry too much about it. When you see all of the technical specs on data sheets, most of the time a lot of it really doesn't mean anything in a practical situation, especially with a simple low power circuit like this. It's just a lot of information they provide you with about the component in question.

I would have to compare the data sheets though to see how they differ.

I would just experiment around with a bunch of transistors to see what sounds good. Also, make sure you buy more than just one transistor. Just like with the capacitor values, transistor beta can vary all over the place and such.
 
#18
I built one of these and tried it externally with my Telecaster, it really sounded great! Nice and simple, and overcomes the tone-sucking effects of that big capacitor we call the guitar cable.
I used a 9 volt battery to power it and that worked just fine.
Buy several FETs for sure; 2 out of the 5 I bought worked great, 3 were not so hot.
 

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