Buffer (Low output / Q: adding a volume pot?)

[Woolly Mother Mammoth]

Just built this with a TL082 (original TL072):



Problem, low output.

Pins:
1= 7.91
2= 7.91
3= 7.91
4= 8.10
5= 7.72
6= 7.72
7= 8.23
8= 9.30

http://circuits.datasheetdir.com/175/TL082-Q1-pinout.jpg TL082
http://pinout-circuits-images.dz863.com/54/TL072.jpg TL072

[gritz]

Pin 4 should be 0V.

Remember also to terminate the unused opamp in the TL082 properly (if you've not done so already). Connect it's inverting input to it's output and connect the non-inverting input to 4V5 (directly - i.e. without a resistor - is fine). Leaving it all floating may cause oscillation, latchup or other weirdness.

[thelonious]

Hmm... pin 4 should be grounded. You should be getting 0 volts there. And pin 3 should have close to half supply (4.6v).

With the battery disconnected, do you get continuity from the negative contact on the battery snap to pin 4?
Are both 100k bias resistors for sure 100k?
Is the chip socketed? Have you tried replacing the chip?

EDIT: Oops! Gritz beat me to it

[Woolly Mother Mammoth]

Pin 4 should be 0V.

Remember also to terminate the unused opamp in the TL082 properly (if you've not done so already). Connect it's inverting input to it's output and connect the non-inverting input to 4V5 (directly - i.e. without a resistor - is fine). Leaving it all floating may cause oscillation, latchup or other weirdness.

Yep, I have connected the invert (pin 2) with the output (pin 1). By "connecting the non-inverting output to 4v5 " you mean to the 1 Meg, 47uF and the two 100k? Which should lower the volt down to 4.5v right?

Hmm... pin 4 should be grounded. You should be getting 0 volts there. And pin 3 should have close to half supply (4.6v).

With the battery disconnected, do you get continuity from the negative contact on the battery snap to pin 4?
Are both 100k bias resistors for sure 100k?
Is the chip socketed? Have you tried replacing the chip?

EDIT: Oops! Gritz beat me to it

The 100's are 100k, for sure. Double checked it! Will check pin with battery/power supply disconnected.


Edit:


Ground wasn't connected to board.  

[thelonious]

Ground wasn't connected to board. 

Glad you found it! We have all done stuff like that...

[Woolly Mother Mammoth]

Feeling like such an idiot, starting a topic and all.. Thanks for your time guys I do appreciate it!

[gritz]

Feeling like such an idiot, starting a topic and all.. Thanks for your time guys I do appreciate it!

Yeah, glad you found it. I realise now that my "helpful answer" was a bit brief. I got distracted - I guess the gals are right about what they say re: men and multitasking...

Yeah, by connecting the non-inverting input to 4V5 I do mean connecting it to the junction of the two 100k resistors and the 47u capacitor. That will keep it out of mischief.

[Woolly Mother Mammoth]

Yeah, glad you found it. I realise now that my "helpful answer" was a bit brief. I got distracted - I guess the gals are right about what they say re: men and multitasking...

Hahaha, yeah you got that right brother!

[Woolly Mother Mammoth]

Oh, still got one more question. Could I add a volume pot (to lower the volume, not boost) at the start or end of this circuit without interfering with the buffering?

[thelonious]

Could I add a volume pot (to lower the volume, not boost) at the start or end of this circuit without interfering with the buffering?

Well... others can comment better than I on how it will affect the output impedance, operation of its job as a buffer, etc., but you could replace R4 and R5 with a 100k pot. You'd connect the 1uF to lug 3 of the pot, output to 2, and ground to 1.

[Woolly Mother Mammoth]

Could I add a volume pot (to lower the volume, not boost) at the start or end of this circuit without interfering with the buffering?

Well... others can comment better than I on how it will affect the output impedance, operation of its job as a buffer, etc., but you could replace R4 and R5 with a 100k pot. You'd connect the 1uF to lug 3 of the pot, output to 2, and ground to 1.

Thanks! I'll try that and see what happens.

[Woolly Mother Mammoth]

I replaced the 100k and the 560Ω with a 100k pot and the buffer did not do the job that well when the pot was turned down a bit, at least for me.

[thelonious]

You need input from somebody who knows more than me. What I can say, though, is that you're trying to get it to do something that is not strictly in "buffer" territory. The volume control is going to interfere somewhat with its buffering, which is what you're hearing. I imagine that the proper way to do this would be to use the other side of the TL072 as a 2nd buffer stage and put the volume control in between the two stages.

[Woolly Mother Mammoth]

You need input from somebody who knows more than me. What I can say, though, is that you're trying to get it to do something that is not strictly in "buffer" territory. The volume control is going to interfere somewhat with its buffering, which is what you're hearing. I imagine that the proper way to do this would be to use the other side of the TL072 as a 2nd buffer stage and put the volume control in between the two stages.

Hey, at least we tried! Your right about this, what I'm trying to achieve goes against the buffers purpose..

I got the buffer placed last in the chain and I can't help but wonder what a pot at the start of the circuit would do? Maybe even "outside" the circuit? Could it be like turning down the volume on some other pedal placed earlier in the chain?

[Gurner]

Replace R4 with your 100k pot...the wiper becomes your output (& R5 the 560R goes on the wiper)

Pot Wiper turned to the bottom....no signal

Pot Wiper turned to the top....max signal.

Use a log pot else the sweep won't seem linear to your ear).

that 540R resistor is just to protect the opamp output against shorts.

[Woolly Mother Mammoth]

Replace R4 with your 100k pot...the wiper becomes your output (& R5 the 560R goes on the wiper)

Pot Wiper turned to the bottom....no signal

Pot Wiper turned to the top....max signal.

Use a log pot else the sweep won't seem linear to your ear).

that 540R resistor is just to protect the opamp output against shorts.

I did like thelonious suggested but unfortunately it seemed (to me) like the buffer did not do its job that well when the volume was turned down a bit. The loss-of-treble-business went right back up again.

Well... others can comment better than I on how it will affect the output impedance, operation of its job as a buffer, etc., but you could replace R4 and R5 with a 100k pot. You'd connect the 1uF to lug 3 of the pot, output to 2, and ground to 1.

[thelonious]

I did like thelonious suggested but unfortunately it seemed (to me) like the buffer did not do its job that well when the volume was turned down a bit. The loss-of-treble-business went right back up again.

I think what is happening is that as you turn the volume control down, you're adding more series resistance to the output, which is messing with your highs.

What would likely solve your problem and give you back a real buffer would be to connect the second half of the TL072 like the first and put the volume control in between the two halves. You wouldn't have to duplicate the Vref resistors and filter cap, just duplicate the coupling capacitors.

Another option is that you could try putting a bright cap across lugs 2 and 3 of the pot. The value of the pot contributes to what size bright cap you would choose, so you'd have to experiment with values. Smaller pots need larger value caps and vice versa. Maybe clip a 1000pF (1n) cap across your 100k pot and see what happens? Or start w a 470pF and work your way up.

...but the better way to deal with this is to wire up the other side of the op amp, because option #2 will shape the tone... and most of the point of a buffer is to pass the signal untouched as much as possible.

[Gurner]

I think what is happening is that as you turn the volume control down, you're adding more series resistance to the output, which is messing with your highs.

If that's the case, then a simpler (& quicker) fix would be crank up the value of the 1uf output until there was no audible treble rolloff with the pot set low. (try a 47uf or even higher)

[Woolly Mother Mammoth]

I will try out both your ideas and see which works best.

Thanks guys, I owe you one!