TL071 help

[mat]

Hello,

I'm building an overdrive pedal that has TL071 on the first stage and EF86 tube on another stage http://mattitee.kuvat.fi/kuvat/EF86overdrive+pedal/IMG_0815.jpg. I've been struggling with the TL071 to have a proper (half of the low V+) voltage on pin 6. First it was about 1.4V, then I built the IC section again with a fresh TL071 and tried it with a 9.3V fresh battery.  Now the voltages are:

pin1 - 0,2V
pin2 - 1,6V
pin3 - 4,2V
pin4 - 0V
pin5 - 0,16V
pin6 - 8,77
pin7 - 9,31V
pin8 - 0V

Pins 1,5 and 8 have no connection.

The power section: http://mattitee.kuvat.fi/kuvat/EF86overdrive+pedal/sasPWR.png

Everything worked when the pin6 was at 1.4V but the sound was not right. I'll try tomorrow again how it sounds with 8.77V on pin 6.

Any suggestion to where to seek the pin6 voltage problem much appreciated !

Mat

[Seljer]

To get 4.5V on the output you need 4.5 V on both inputs (pins 2 and 3), otherwise the opamp will do its thing and slam the output into the direction of whatever input is higher. The voltage on pin 3 (+) is higher than pin 2 (-), so the output swings to the positive rail.

4.2V on pin 3 seems reasonable, so figure out why pin 3 is so low. I'd check the resistance of the gain potentiometer first.

[mat]

Thanks for the quick reply ! I check the gain pot first thing tomorrow.

Mat

[iccaros]

@seljer good points

@mat  why are you looking for DC voltage on an opamp output? would you not look for AC voltage in the form of audio signal? at 4.7v AC in, that tube will be slammed, which is part of the point, but that tube is going to boost more, and then it will slam your amp, if you do not have a tube amp your input signal will be way too high.

[Gurner]

@mat  why are you looking for DC voltage on an opamp output? would you not look for AC voltage in the form of audio signal?

Cos the opamp 'output' (& input) DC needs to sit midway between the rails...in this case 4.5V (if it isn't at mid voltage, then the AC signal will likely clip)

A quick cursory glance indicates there's a discrepancy/irregularity between pin 6 (8.7V) & pin 2 (1.6V) ...I reckon pin 2 should have approx the same DC voltage as pin 6 ...it hasn't (are you sure your TL071 is ok?)

4.2V on pin 3 seems reasonable, so figure out why pin 3 is so low.

I reckon you mean "so figure out why pin 2 is so low"?

[Seljer]

(btw seljer...I reckon you meant "figure out why pin 2 is so low"?)

precisely that 

I had a similiar issue a couple of days ago, fixing the onboard active preamp of a bass. same symptoms on the opamp in the circuit and it took me three hours to discover that one of the pots was rotated a bit and was shorting on the shielding on the side of the control cavity.

[iccaros]

@mat  why are you looking for DC voltage on an opamp output? would you not look for AC voltage in the form of audio signal?

Cos the opamp 'output' (& input) DC needs to sit midway between the rails...in this case 4.5V (if it isn't at mid voltage, then the AC signal will likely clip)

A quick cursory glance indicates there's a discrepancy/irregularity between pin 6 (8.7V) & pin 2 (1.6V) ...I reckon pin 2 should have approx the same DC voltage as pin 6 ...it hasn't (are you sure your TL071 is ok?)

4.2V on pin 3 seems reasonable, so figure out why pin 3 is so low.

I reckon you mean "so figure out why pin 2 is so low"?

yes, but with that feedback design is he not looking for it to clip? I thought clipping was apart of the design, but I get your point, he is looking for idle voltage, but that would require a known feedback setting and  a known input. He has the input on the + side, but the pot could be bad, or way too big, I may be calculating wrong but gain would be  500K/2.2K + 1 right? gives a gain of 228? I would think a 10K pot to be better for this.  again I am wrong most of the time, but your AC signal would tell you how close you are to clipping with audio signal than the idle DC no?

[Gurner]

yes, but with that feedback design is he not looking for it to clip? I thought clipping was apart of the design, but I get your point, he is looking for idle voltage, but that would require a known feedback setting and  a known input.  

When you have a circuit problem, it normally pays to go back to basics & take quiescent DC readings. The DC in the feedback is blocked to ground (2u2 cap top left of the schematic)...therefore the feedback divider resistors doesn't affect the DC levels. The 'known input' is known (kind of like the opposite to what Donald Rumsfeld said!)...quiescently with a 9V supply it's a little under 4.5V (in this case 4.2v)

[iccaros]

Gruner thanks for taking the time, last one
Since this is positive feedback so it acts more like a comparator, should it not always be close to the rails unless there is ac signal?

[Gurner]

Gruner thanks for taking the time, last one
Since this is positive feedback so it acts more like a comparator, should it not always be close to the rails unless there is ac signal?

It's actually negative feedback in play...

http://mattitee.kuvat.fi/kuvat/EF86overdrive+pedal/IMG_0815.jpg

....the opamp's output pin (pin 6) loops back to the opamp's  negative input pin (pin2), so it's not acting as a comparator, but like an amplifier, the pot controls the amount of negative feedback (less signal fed back into the negative input pin = less negative feedback = more gain & vice versa) ....the DC level on the opamp output pin (pin 6) should be pretty darn close to the DC on pin 3.

[iccaros]

your right... I was thinking the whole time I saw it connect to the non inverting side, which is why all t he questions.

[mat]

The gain pot measures 2.2R to 532K in circuit.

Mat

To get 4.5V on the output you need 4.5 V on both inputs (pins 2 and 3), otherwise the opamp will do its thing and slam the output into the direction of whatever input is higher. The voltage on pin 3 (+) is higher than pin 2 (-), so the output swings to the positive rail.

4.2V on pin 3 seems reasonable, so figure out why pin 3 is so low. I'd check the resistance of the gain potentiometer first.

[Gurner]

Probably one of the following...

1. A short (or dodgy connection) in or around pin2
2. A faulty opamp.

If you can disconnect all the present existing connections on pin 2 and link pin6 directly back to pin 2 instead, that'll help establish the fault (with pin 6 to pin 2 it's a voltage follower, so you should get 4.2V on pin 6 & pin 2))

[mat]

(are you sure your TL071 is ok?)

I've swapped several fresh ones in but no help. Might there be something damaging them ?

Mat

[mat]

1. A short (or dodgy connection) in or around pin2

Seljer, Gurner, iccaros - Big Thanks ! There was no connection between pin2 and the other side of the 22pF cap 

Now the voltages are spot on.

I'll test the sound tomorrow - will let You know how it turned out.

Thanks again !
Mat

[mat]

iPhone sound sample (mindless noodling): http://soundcloud.com/mattit-1/diy-ef86od-pedal

Cheers,
Mat

[mat]

Btw. anyone can tell what does the 22pF cap do connected to gain pot. Is is high pass filter ? How does the sound change with a 50pF in there ? Yes, I know - should try it   Just thinking out loud  
Is it same as in tubescreamer ? From GEOFEX:
The small 51pf capacitor across the diodes acts to soften the "corners" of the clipped waveform somewhat. This softens the distortion somewhat. The action of the 51pf is most noticeable when the drive control is maxxed out, so it softens the distortion most when the gain (and distortion) is highest. Put another way, the capacitor's impedance goes down with increasng frequencies, so it starts cutting the gain of the stage when the impedance of the capacitor is equal to the resistance of the 51K resistor plus the setting of the drive control. This rolloff frequency is lowest in the audio range and most noticeable when the resistance of the drive control is highest.

Mat

[mat]

Btw. anyone can tell what does the 22pF cap do connected to gain pot. Is is high pass filter ? How does the sound change with a 50pF in there ? Yes, I know - should try it   Just thinking out loud  
Is it same as in tubescreamer ? From GEOFEX:
The small 51pf capacitor across the diodes acts to soften the "corners" of the clipped waveform somewhat. This softens the distortion somewhat. The action of the 51pf is most noticeable when the drive control is maxxed out, so it softens the distortion most when the gain (and distortion) is highest. Put another way, the capacitor's impedance goes down with increasng frequencies, so it starts cutting the gain of the stage when the impedance of the capacitor is equal to the resistance of the 51K resistor plus the setting of the drive control. This rolloff frequency is lowest in the audio range and most noticeable when the resistance of the drive control is highest.

Mat

Any insight for the 22pF cap?

Thank You,
Mat

[Seljer]

Its a low pass filter that cuts more high end as you turn the gain up. Increase the value of the resistor for more dramatic effect.

The gain of an opamp in an non-inverting amplifier arrangement (what you have there with your opamp) is 1+R2/R1, where R2 is the resistor between the output and the - input and R1 is the other one. The 22pF is in parallel with the gain pot, capacitors have an impedance that is inverse to frequency (so higher frequency = less ohms) so you can think of the total value of the 500k pot and capacitor in parallel being smaller for higher frequencies, and if R2 is smaller you have less gain.

The 2.2uF capacitor also adds some fixed low frequency rolloff (i.e. high pass filter) but its large enough to be below the range of a guitar (this capacitor is the one that cuts the low end in a tubescreamer).


Take note that in your setup all this filterting is happeing before your clipping stage

[mat]

Is R2 the 500k pot and R1 the 2k2 to ground ? Do You mean increasing the 2k2 will cut the highs on higher gain settings ?

Mat