Is this offset transistor?

[preciousmolina666]

guitar to tubescreamer = perfectly center symmetric. around 3 volts out. then after that i tried to put next chain is transistor common emitter circuit.then  this happen.

the 1st wave and 3rd wave are strumming. the 2nd and 4th are stacatto on Low E string.
bjt is 2N3904, Rc is 1k ohm, Re is 200 ohm, capacitors are 100uf
whats the problem

[merlinb]

whats the problem

There is no 'problem' as such. Discrete transistor circuits often clip in an asymmetrical way that can charge up the coupling capacitors, leading to slow changes in the average (DC) voltages that 'slew' the signal as you have discovered.

[preciousmolina666]

Thanks a lot i built it 3 times today my balls ache, if my next stage is opamp follower can he correct this?

[Fancy Lime]

Thanks a lot i built it 3 times today my balls ache, if my next stage is opamp follower can he correct this?

Only if you hook it up directly to your balls and feed it with more than 24 Volt.

[merlinb]

Thanks a lot i built it 3 times today my balls ache, if my next stage is opamp follower can he correct this?

Correct what? After passing through the imperfect transistor the signal is distorted slightly or a lot, depending on what you built. But that's what the signal looks like after it comes out, so there's nothing for an opamp to fix, it doesn't 'know' what you want, it just copies whatever you give to it. If you don't like distortion you're in the wrong game!

[PRR]

Guitar waveforms are not symmetrical. Tubescreamer clipping is not symmetrical. That's what happens. Nothing "wrong", nothing to correct.

[idy]

I think the poster is asking if he can use an opamp in place of the transistor stage to get rid of the changing bias. I think so. But I'm curious what the "sagging" signal sounds like...

[Rob Strand]

I think the poster is asking if he can use an opamp in place of the transistor stage to get rid of the changing bias. I think so. But I'm curious what the "sagging" signal sounds like...

I think what he is asking is will an opamp buffer following the *CE stage* correct the DC bias shift.

1) OP Reply 1: 

bjt is 2N3904, Rc is 1k ohm, Re is 200 ohm, capacitors are 100uf

Because:

2) Merlin reply 2:

"Discrete transistor circuits often clip in an asymmetrical way that can charge up the coupling capacitors, leading to slow changes in the average (DC) voltages"

So,

3) OP reply 3:

"if my next stage is opamp follower can he correct this?"

The answer is the yes.  The sag itself is a DC shift and the DC shift will be hidden at the output of the opamp.
However, the DC shift usually affects the sound and the question is does the OP think the sound is bad coming out out of CE stage? and does he want to fix the sound?

[merlinb]

The answer is the yes.  The sag itself is a DC shift and the DC shift

I doubt the answer is yes. Unless he was monitoring that signal with a DC-couple storage scope it looks to me like it has already passed through a few capacitors on its way to his sound card, so all DC has already been removed.

[Rob Strand]

I doubt the answer is yes. Unless he was monitoring that signal with a DC-couple storage scope it looks to me like it has already passed through a few capacitors on its way to his sound card, so all DC has already been removed.

I suppose it comes down to specifics:

Effect of 150Hz HPF:


Note the offset due to the fact the duty cycle of the waveform is not 50% (see zoomed section in inset).

[preciousmolina666]

my first version is my Capacitor in and out are 1uf . High pass at 1.6 Hertz. i test it, i strum the guitar the waveform is kinda symetric but when i play stacatto (Djent metal genre) its more on positive side. So i try to change the Cin and Cout to 100uf. when i play strum for 3 seconds its assending to negative side but when i play staccato its assending to positve side. my plan was to just amplify it. is there a problem in my biasing?

I doubt the answer is yes. Unless he was monitoring that signal with a DC-couple storage scope it looks to me like it has already passed through a few capacitors on its way to his sound card, so all DC has already been removed.

I record it  through Focusrite 2i2 soundcard which i think it has opamp on input

I think the poster is asking if he can use an opamp in place of the transistor stage to get rid of the changing bias. I think so. But I'm curious what the "sagging" signal sounds like...

its sound the same in me, but if i put a limiter on the next stage which is i usually do. it changes the character

Only if you hook it up directly to your balls and feed it with more than 24 Volt.

after building this my balls are now assymetric


UPDATE:
i tried to put signal generator 1khz and 1.8 volts its fine, but more than that the more volts the more it cut the positive side, i think im looking for a transistor that has more input voltage what specs look for that can handle 3-4volts