a guitar preamp(for monthly submission)

[drk]

hello, for this month topic im gonna build a preamp for my guitar. here is the schematic.


i've already build it on the solderless board and worked fine. 

it's gonna be used on a guitar so i will have a switch to turn the preamp on or off(to be able to use a guitar even without a battery)
im thinking putting the circuit after the volume pot, one doubt though, i can substitute the 1M resistor for the volume pot right?

another thing, the tone pot, should be before this circuit?(like pickups->vol.->tone->preamp?)

i'll be updating the topic as the work is done 

[BrianJ]

I would guess the tone pot needs to come before the preamp.  The low output impedance of the preamp may make the tone control less responsive. 

[drk]

i've built the preamp and works fine, just one problem, when i use the switch it does a loud 'poc'  . i think it has something to do with the output, cuz i forgot to put an output cap there.
if i put a cap in series it would stop the sound?

[MusicAudio]

i've built the preamp and works fine, just one problem, when i use the switch it does a loud 'poc'  . i think it has something to do with the output, cuz i forgot to put an output cap there.
if i put a cap in series it would stop the sound?

There's a good chance that adding an output cap would stop the pop, you should definitely add one - even if that isn't the reason for the popping. What you're hearing is the result of suddenly switching to a signal that has a dc offset, this is why we usually use pull down resistors. The 1k resistor between the emitter and ground of your second transistor tells me that the output is not at 0V when no signal is applied. Add your output cap and see if that solves your problem, if it doesn't then try a pull down resistor after the output cap.

[Gus]

You can remove 3 resistors and one cap that are not needed.

The 2nd stage is a bootstrapped emitter follower(EF) IMO you don't need that and unless you are driving a low input resistance device you can most likely remove the EF and connect a cap and 100k or so volume pot after the collector of the first stage

but if you want to keep it here is a way to direct couple an EF

remove the two 10ks and 4.7K and the .1uf move the 10uf to the 2nd stages output with a 100k volume pot or a pull down.  Direct connect the base of the 2n3904 to the collector of the 2n5088.  The 2nd stages 1k emitter resistor is on the low side.  Work out the DC current (ohms law simple rule 1V per 1K is 1ma, 4VDC across 1k is 4ma) that the battery will be drained at with just that section.  I would try a 10K.

Post if you have a question

[drk]

mike: though had something to do with the cap thanks

gus: im using this preamp right after the volume/tone pot. as the guitar will be connected to a high input impedance(a pedal or an amp), what you're saying is that it makes no difference?
i dont understand much about emitter followers, but i though it was a good thing to use in the end of 'chains'

didnt get the last part, i still dont get how we choose the emitter/collector resistors values. i've read that we choose the current that is going to flow through the transistor, and then calculate the resistor..
as it has no resistor on the collector, it means that the voltage is all dropped on emitter resistor??
if i use the 10k resistor, that would mean a 0.9mA current(9/10.000)??

btw, this changes in the 2nd part is to make it DC coupled right? this way it isnt needed any cap at the output?
thanks

edit: like this?

[Gus]

close

remove the 100K and the 10uf.  The collector voltage will bias the EF emitter node to the collectors voltage - about .6VDC

Reuse the 10uf as a output cap to a volume control or a pulldown resistor so you don't get pops if you switch the preamp in or out.

[drk]

ok, got it