transistor amplifiers im special

[the cracken]

I cant say Im new to electronics because im not. Iv done a national dip and i did pretty well but iv forgotten mostly everything!

iv read up on it but i still can find anywhere that say how i would go about picking the right component values for a class a signal transistor pre amp for my guitar.



**broken link removed**


are these all the components i need?
how do i choose these components? i have read about how they work and i know what they do and exactly how the transistor works but i don't know how to pick the values?

i know that the biasing resistors have to put a biasing voltage of half the input signal+the voltage drop of the transistor (0.6 usually). but what about the base to emitter current?

The input resistor is that only there to stop over current of the base?

the collector and emitter resistors is the only reason their there is to stop too much current going though the transisor?

im trying to make a preamp with a 2n2222 with no success. im using a spice program which is a pile of crap! its called LTspiceIV if you know of a better one for free please tell

but i was also wondering how you find out the transistor is good for the application?

thanks for any help you can give me

 

[Nigel Goodwin]

Don't use a single transistor, use an opamp - FAR better perfornance, easy to design, and trivial to set exactly how you want.

 

[the cracken]

Don't use a single transistor, use an opamp - FAR better perfornance, easy to design, and trivial to set exactly how you want.

opamps dont overdrive very well.

 

[Nigel Goodwin]

opamps dont overdrive very well.

Neither do transistors - almost every solidstate guitar amp out there uses opamps - wth plenty of xamples available on the net.

Check out this example:

**broken link removed**

Like most it uses diodes for overdrive.

 

[Hero999]

Please post a propper schematic rather than an ASCII. If you must use an ASCII then use code tags so it preserves the formatting.

It looks like a class A common emitter amplifier.

LTSpice is actually pretty good, it just has a very steep learning curve and is not a real time simulator: you have to run the simulation then view the results in the form of a graph, bode plot or a list of voltages at different nodes in the circuit. If you're looking for an interactive simulator which will show you changes in voltage when you flip a switch or alter the value of a pot. then LTSpice is not for you, you need a real time simulator for that. Unfortunately real time simulators are not so accurate as LTSpice because they're more optimised for speed.

 

[ericgibbs]

im trying to make a preamp with a 2n2222 with no success. im using a spice program which is a pile of crap! its called LTspiceIV if you know of a better one for free please tell

but i was also wondering how you find out the transistor is good for the application?

thanks for any help you can give me

hi,
There is nothing wrong with LTS for the simple transistor amp that you are considering.

What subject did you do your National Dip in.??

BTW: I can read your posted circuit OK, I dont understand what 'hero' means by 'your ASCII' image.???

 

[Nigel Goodwin]

BTW: I can read your posted circuit OK, I dont understand what 'hero' means by 'your ASCII' image.???

Likewise, it's not an ASCII image, it's actually a JPG (which is a poor choice anyway).

 

[ericgibbs]

hi cracken,

Look at this LTS showing a simple single stage 2N2222, Class 'A' transistor pre-amplifier.

 

[the cracken]

hi,
There is nothing wrong with LTS for the simple transistor amp that you are considering.

What subject did you do your National Dip in.??

BTW: I can read your posted circuit OK, I dont understand what 'hero' means by 'your ASCII' image.???

in electronics lol

 

[the cracken]

Likewise, it's not an ASCII image, it's actually a JPG (which is a poor choice anyway).

it was an ASCII image but it didnt come out right so i changed it

 

[the cracken]

hi cracken,

Look at this LTS showing a simple single stage 2N2222, Class 'A' transistor pre-amplifier.

View attachment 39333View attachment 39334

what i was trying to get at in my question was how do you choose the values of the resistors. i know the formula to work out the input output capacitor and it pretty much the same for all amps anyway. but R1 R2 R3 R4 i have no idea and the annoying thing is iv read load of shuff on it and i still cant work it out!
whats with c3 and c4 in this circuit?
and my spice program doesn't like fast frequency's (1000Hz) do you know why this is?

 

[ericgibbs]

what i was trying to get at in my question was how do you choose the values of the resistors. i know the formula to work out the input output capacitor and it pretty much the same for all amps anyway. but R1 R2 R3 R4 i have no idea and the annoying thing is iv read load of shuff on it and i still cant work it out!
whats with c3 and c4 in this circuit?
and my spice program doesn't like fast frequency's (1000Hz) do you know why this is?

hi,
This is the actual LTS *.asc file, open and run it with your LTspice, see if it runs OK.

Are you asking what the function of C3 and C4 is in an actual circuit.?

EDIT:
study this link and all will be revealed.

**broken link removed**

 

[Hero999]

im special

What's that, special needs?

Only joking, no offence intended.

 

[audioguru]

A transistor cannot provide the high input resistance needed by a guitar preamp and the gain of the circuits that are shown is too high.
The opamp preamp that was shown has an input resistance of 1M ohm and a gain that is high enough to produce very high distortion in the diodes.

Here is a guitar preamp that uses a Jfet for a high 3M input resistance and a gain that is less than 2.

 

[ericgibbs]

A transistor cannot provide the high input resistance needed by a guitar preamp and the gain of the circuits that are shown is too high.
The opamp preamp that was shown has an input resistance of 1M ohm and a gain that is high enough to produce very high distortion in the diodes.

hi agu,
If its my circuit you are referring too, I think you are about 1000 times too high with your 1megohm Zin calculation.! also it has a gain of approx 25...???

EDIT:
Or is it Nigels link.???

 

[audioguru]

hi agu,
If its my circuit you are referring too, I think you are about 1000 times too high with your 1megohm Zin calculation.! also it has a gain of approx 25...???

EDIT:
Or is it Nigels link.???

The input impedance of your transistor circuit is so low that it is like a dead short to a guitar pickup. Its very low impedance also changes the sound of the pickup.

 

[ericgibbs]

The input impedance of your transistor circuit is so low that it is like a dead short to a guitar pickup. Its very low impedance also changes the sound of the pickup.

I know the input impedance is low, its about 1200R, I clearly stated in my post my circuit is:

Look at this LTS showing a simple single stage 2N2222, Class 'A' transistor pre-amplifier.

I didn't claim it to be a guitar preamp.

As I understand it, the OP is asking about Class A pre-amps.

 

[the cracken]

hi agu,
If its my circuit you are referring too, I think you are about 1000 times too high with your 1megohm Zin calculation.! also it has a gain of approx 25...???

EDIT:
Or is it Nigels link.???

Now different people have different ideas but I'm a big believer in the theory that the current through the emitter should be between 5 - 10 times that of the base current. Let's pick a ratio of about midway, 7 i.e 0.7 mA for emitter current.

i dont like guessing whats the best way to finding what current you need?
thanks

 

[ericgibbs]

i dont like guessing whats the best way to finding what current you need?
thanks

hi,
When using a resistive divider to bias the base of the transistor you need to have sufficient current flowing thru the divider in order to avoid the small changes of current into the base from changing the bias point voltage.

Its common to choose between 5 and 10 times the required base current to flow thru the biasing resistor divider.

Its not guess work, its working with an acceptable range of divider currents.

 

[audioguru]

i dont like guessing whats the best way to finding what current you need?
thanks

The transistor has a current gain of about 200 so its input current is almost nothing. The voltage divider resistors that feed current to the transistor's base has a current that is 1/10th to 1/20th the emitter current.