Circuit Help

[codan]

Hi Everybody,

I have been making up a lot of different preamp circuits for electric guitar to learn more & to see what effects etc that i can get.

I have a very simple TL072 preamp circuit wired different than most that has an outstanding sound & effect to it but i have an issue that i can't seem to work out.

The input voltage from the guitar is around 200mV peak to peak & the output voltage goes to 4v peak to peak with the volume pot on the guitar turned up to 9.
The problem i am having is that i only get an output at two positions on the volume control pot on the guitar, i get an output signal when the pot is set on about 4 & about 9. I am not sure what value the pot is as i haven't taken the guitar apart to see.

In between these settings on the volume control pot there is no output signal from the circuit at all but when i get an output on the two settings mentioned it is an awesome sound.
I need to get it working on the full range of the volume pot but i can't seem to work it out.

Does anyone have an idea what may be happening without showing my circuit, i would like to keep this one to myself if you can understand, it is my first truly great sounding circuit.

The volume control pot works correctly & across the full range when not connected to this circuit.

Thank You

 

[Miles Prower]

"I have a very simple TL072 preamp circuit wired different than most..."

That's probably your problem right there. Looks like you inadvertently made a window detector. Use a DMM to see what's happening at the outputs. My guess is that it's locking up when it goes silent on you.

 

[codan]

Thanks Miles Prower,

I have an Oscilloscope as well, i am not sure what you mean by locking up, i am a newby at electronics.

Thanks

 

[audioguru]

I think the pot affects the DC bias voltage at the input of the TL072 maybe because a coupling capacitor is missing.
Post the circuit of the TL072 preamp.

 

[mneary]

The input voltage from the guitar is around 200mA peak to peak

200mA is unusual for an amplifier input. How did you measure it?

 

[codan]

Hi mneary,
Sorry it was a spelling error i should have written 200mV.


Hi audioguru,

The coupling capacitor is in place, i understand it's almost impossible to give advise without viewing the circuit but this is really something that i may be able to use to make some money & at my age it could be a start into something i really enjoy doing.
I placed the circuit into a small aluminium box yesterday & took it to a local music shop where they connected a very cheap guitar into it.
They were astonished at the sound, so much so they said if i can sort it out they would certainly purchase some foot pedals of me if i can put them together.
The sound is very much like a big tube amp but from a transistorised amp & makes the guitars sound very similar to how Carlos Santana has his equipment setup but at a small fraction of the cost. I am very excited about it all but i still have the same issue as mentioned.

I have a couple of questions i need some help with.

The circuit only works as mentioned at two positions on the volume control pot on the guitar.
So, i guess it is only working at two specific voltage levels, the voltage at the two positions on the control pot it is working at. Or is it the Current levels?
Is it possible that i could just set the volume control pot on the guitar to full--10 & then have a set value resistor in series on the input to the circuit so it will simulate the position on the volume control pot where the circuit works.
This way when the foot pedal is in use it's just a matter of settng the control pot to full & the volume can be set out of the foot pedal circuit.
Then again thinking about it not all guitars will have the same output from the pickups as i have so a set value into the circuit may not work for all??
If i could just get it to work just at a set level for the run of the mill guitar pickup outputs it would be great.

More questions,
What is a safe level regarding voltage & amps that i should restrict the output of this circuit at for use in transistor amps & also for use in tube amps. I am unsure what the safe limits are.

What is the average input impedance of the run of the mill tube amps.

I do need to check the input & output current but i am still unsure how to achieve this using my oscilloscope, i purchased a large .01m resistor yesterday as well to try to get some amperage values but i am unsure about how to achieve this, some advice would be really helpful. I have read a lot but i still don't quite get it?

Is there a basic variable frequency sinewave oscillator circuit on the net somewhere that would be suitable for testing this circuit, i may be able to find out more if i could simulate the pickup output with it connected to my circuit.
The output needs to be adjustable as well in the range of the guitar pickups output.

Thank You

 

[audioguru]

If you don't want to post the schematic then we cannot help you.

Many engineers modified transistorized amplifiers to sound like a big tube amp.
They cut the highs and lows like the output trandformer.
They add even and odd harmonics (distortion) like tubes do.

 

[codan]

Hi audioguru,

Yes i understand, after a sleepless night i have the circuit working & controllable from the volume control pot, i added a trimpot to the circuit & now it's fine?

Could i ask for some assistance regarding the other questions please?



What is a safe level regarding voltage & amps that i should restrict the output of this circuit at for use in transistor amps & also for use in tube amps. I am unsure what the safe limits are.

What is the average input impedance of the run of the mill tube amps.

I do need to check the input & output current but i am still unsure how to achieve this using my oscilloscope, i purchased a large .01m resistor yesterday as well to try to get some amperage values but i am unsure about how to achieve this, some advice would be really helpful. I have read a lot but i still don't quite get it?

Is there a basic variable frequency sinewave oscillator circuit on the net somewhere that would be suitable for testing this circuit, i may be able to find out more if i could simulate the pickup output with it connected to my circuit.
The output needs to be adjustable as well in the range of the guitar pickups output.

Thank You

 

[audioguru]

Hi audioguru,

i have the circuit working & controllable from the volume control pot, i added a trimpot to the circuit & now it's fine?

I don't know your circuit and i don't know what uou have done to it.

What is a safe level regarding voltage & amps that i should restrict the output of this circuit at for use in transistor amps & also for use in tube amps. I am unsure what the safe limits are.

without seeing the circuit and parts that are used then I don't know.

What is the average input impedance of the run of the mill tube amps.

i have not seen a tubes amplifier for about 44 years so i don't know what you see today.

I do need to check the input & output current but i am still unsure how to achieve this using my oscilloscope, i purchased a large .01m resistor yesterday as well to try to get some amperage values but i am unsure about how to achieve this, some advice would be really helpful. I have read a lot but i still don't quite get it?

The input current does not need to be measured because it is extremely low. The output current can easily be calculated.

Is there a basic variable frequency sinewave oscillator circuit on the net somewhere that would be suitable for testing this circuit, i may be able to find out more if i could simulate the pickup output with it connected to my circuit.
The output needs to be adjustable as well in the range of the guitar pickups output.

A Wien Bridge oscillator stabilized with a Jfet and a Phase-shift oscillator make good sine-waves. I have both types.

 

[codan]

Hi audioguru,

I found a few jfet stabilized Wein Bridge circuits on the net, i have attached one, is this suitable for my testing.

I was reading an article where they stated that all wave forms sound the same after heavy clipping no matter what the original shape of the wave form was like.

Is this correct?

Thanks

 

[audioguru]

Heavy clipping produces a square-wave. The Jfet in this Wien bridge oscillator prevents clipping if it is biased properly.

 

[codan]

Hi audioguru,

I have taken notice of your mention about that engineers add harmonics etc & i need to learn more about this.

I may be wrong but i thought a good way for me to learn would be to start filtering out low, mid & high harmonics to see the results myself on the oscilloscope, i have to start somewhere.

I have attached a circuit this time of a multiple feedback band pass filter with some values not entered.

My question is if you have the time of course, is how do you go about the design & component values for such mid, low & high pass filters.

Obviously there is a frequency range to consider, as an example for the electric guitar output, the frequency range of such i am unsure about at this stage.

As i take it.
On a guitar we have the low E string which has the lowest frequency & then the highest frequency would be the high E string on the highest fret.
So this is the frequency range i am working in & so to the design for filtering out certain frequencies?

Hope this made sense, i am eager to learn.

Thank You

 

[audioguru]

The bandpass filter you found is for a narrow range of two or three frequencies, not for a few octaves.
You need a highpass filter to cut the lows and a lowpass filter to cut the highs.
If you cut the high frequency harmonics then the guitar will sound something like a flute.

 

[codan]

Hi audioguru,

Ah, ok, is there anything wrong with simple RC filters or should i be looking into Opamp filters, i want to stay away from RL filters as i don't have good access to inductors unless i wind my own.

Everything i read about guitar harmonic frequencies relates to open string frequencies, but what is the frequency of the high E string whent fretted at the 22nd fret?
I have tried to have a look on the oscilloscope but the signal doesn't last long enough to have a good look & at this stage i can't record it to have a closer look.
Is it not important to know the entire frequency range of the signals i am trying to understand?

I think i have worked this out, i wasn't thinking of Octaves until you mentioned it.
The fundamental frequency of the high E string is 329.628Hz so one Octave which is the 12th fret on a guitar doubles the frequency.
So it just a matter of a simple calculation to find the frequency at the 22nd fret which is 1.826 Octaves.

Is this correct?

Also, you mention low & high pass filters but what about if i wanted to have a mid pass filter, is it just a matter of selecting the correct components for the two filters mentioned or is there a specific mid pass filter?
Or is this just called a Band Pass filter for the mid range.

Your a very knowledgable person & i have read a lot of your posts on this forum & others, it would be helpful if you could elaborate a little on the subject. I really need some advise.

EDIT:

I forgot to ask something that i need a better understanding of.

I have noticed on some trial pre amp circuits i have made up that sometimes there can be a very nice strong output signal
but instead of the signal amplitude decaying in a smooth linear way to 0v, it gets to X point where the output signal will shimmer or jitter or shake until it decays away.
I don't know the correct word for that.
It is quite noticable to the ear also.

What is the signal shimmer or jittering called in electronic terms?


Thanks Again

 

[audioguru]

Musical sounds produce harmonics up to higher than 20kHz. Distortion adds harmonics up to higher than 20kHz. Many people can hear as high as 20khz. So if you cut the high audio frequencies you remove important harmonics.

The 12th fret of the high E string on a guitar produces 659.2Hz, its second harmonic is at 1318.4Hz, its 3rd harmonic is at 1977.6Hz, its 4th harmonic is at 2636.8Hz, its 5th harmonic is at 3296Hz etc. A chart on the web shows the frequency at the 22nd fret.

Your circuits seem to have problems with the volume control and with stability which is not normal. You said that "you have a very simple TL072 preamp circuit wired different than most" and it produces problems so it is probably wired incorrectly. You don't post any of the problematic circuits so we can't help you fix them and produce the "outstanding sound" properly.

Maybe your "shimmering jittering shaking" sound is what we call "motorboating" which is caused by poor supply bypassing.