TL072 Dual Op-Amp Boost to Single Output Project

[christominello]

I am trying to build a dual op-amp boost for use in a variety of volume level problems.
I would like a fairly significant amount of boost. I've breadboarded two boosts so far, but I have been dissatisfied by their meager output boost, and by the low headroom.
The next project I want to attempt to breadboard is this boost circuit from AMZ (http://www.muzique.com/tech/op1.htm).



If I jumper points 'A' and 'B', and also jumper the outputs to a single mono jack, would the parallel op-amps boost more than a single op-amp (TL071), or would jumping the outputs ruin the circuit topography?

Thanks for any help.

[dbp512]

You can't connect 2 opamps in parallel, since they will fight with each other and could destroy the chip. You can set one opamp as a master and the other as its slave, but I think that increases output current, not voltage. AMZ's super buffer uses a similar concept, but its designed to drive long cables, not increases output voltage. Try cascading one opamp stage into the next; that should increase output quite a bit. If you don't use opamps, you could try something like a MOSFET boost or mu-amp, both of which can produce quite a bit of volume.

[pappasmurfsharem]

Curious what do you need so much boost for?

You can adjust the gain of the microamp by changing 56K resistor to some higher value.
You could just change the 56k in the feedback loop of a MXR Microamp and get more gain?

As for headroom I'm not sure what specifically you could try except build for something that accepts higher voltage or using a Rail-to-Rail OP amp which gives you a little more headroom to work with.

Make note this is a Dual OPamp.
http://www.taydaelectronics.com/tlc2272-dual-operational-amplifier-general-purpose-pdip-8-tlc2272cp.html

[antonis]

I am trying to build a dual op-amp boost for use in a variety of volume level problems.

What kind of problems..??
From what kind of source(s)..??
To what kind of next stage(s)..??

I would like a fairly significant amount of boost.

10db..?? 100 db..?? 1kdb..??

[PBE6]

It is a bit curious that you're not getting the boost/headroom you want from just a simple non-inverting gain stage. A simple fix would be to increase your supply voltage by using two 9V batteries, a 12V or 18V power supply or a voltage doubler.

If you're looking for dedicated clean headroom, check out merlin's Glass Blower:

http://www.valvewizard.co.uk/glassblower.html

Lots of headroom with this one, so virtually all the coloration will come from overdriving the next stage and not from the pedal itself.

Don't forget though, tube amps get overdriven with about 1V on the input, so the headroom issue might not be from your pedal. To get around this you could try putting the boost in the effects loop instead of the input.


Sent from my iPhone using Tapatalk

[alfafalfa]

You can't connect 2 opamps in parallel, since they will fight with each other and could destroy the chip.

You should tell Barber fx , they clearly don't know what they're doing and they don't know how to stop.
There's no end to what some people are willing to do to improve their sound.   

http://www.barberelectronics.com/parallelopamps.htm

Alf

[Kipper4]

You can't connect 2 opamps in parallel, since they will fight with each other and could destroy the chip.

You should tell Barber fx , they clearly don't know what they're doing and they don't know how to stop.
There's no end to what some people are willing to do to improve their sound.   

http://www.barberelectronics.com/parallelopamps.htm

Alf

Right. This statement has me confused.....
Is what the op asking even the same concept as Stacked op amps.
I'm erring on the side of NOT.

I am trying to build a dual op-amp boost for use in a variety of volume level problems.

What kind of problems..??
From what kind of source(s)..??
To what kind of next stage(s)..??

I would like a fairly significant amount of boost.

10db..?? 100 db..?? 1kdb..??

This would help a lot to know this. N'est pas?


There are other simpler means of obtaining "fairly significant amount of boost."

[karbomusic]

The TLx series can handle an operating voltage of ~15 volts, that allows more boost/headroom than most any target device cares to receive in guitar land. Take that up a notch with something like the OPA2134 which is ~36 volts or so. In that respect, it becomes a power supply question more than anything.

So, likely would be great to know exactly which problem you are trying to solve and we can work from there.

[christominello]

Thanks for all the replies, and it has lead me to a solution for my needs.

Now that I know that running op-amps in parallel is not a good idea (connecting the two inputs in parallel and the two outputs in parallel), I've completely reevaluated the concept of the pedal. I decided to try and optimize the gain of a single op-amp, since the single jfet boost circuit I breadboarded last week gave about 10 dB boost, which was almost good enough, but not quite.
That image of the stacked op-amps in the link that alfafalfa posted was hilarious! Kipper4 nailed it though, that is not what I had in mind, but still, that guy is suggesting that people do this! Imagine someone taking this advice, and stacking 3 more TL's on their ICs in their Klon Centaur.

Anyway, since I don't know shit about circuits, I did what I usually do when I need to build an effect: head to muzique.com
I decided to build a buffered boost using a TL072 dual op-amp.
I combined this buffer circuit: http://www.muzique.com/lab/buffers.htm...



... with this boost circuit: http://www.muzique.com/lab/boost.htm...



... and it worked great! Sounds totally wonderful, muscular, and very, very well-balanced EQ-wise.
I wanted to know if I could get a bit more volume, so I decided to change the 14K potentiometer to a 50 k potentiometer, and to my untrained ear, I heard more volume boost. This could be illusory, because I did not test this with anything other than my ears. With the 50 k potentiometer in place, I tested the circuit using the VU meter on my cassette deck, and used a phone app ("Frequency Sound Generator") to generate a triangle wave at 220 Hz, and behold, well over 20 dB of boost from the fully counterclockwise (actually, just shy of fully counterclockwise) position to the fully clockwise position on the potentiometer, likely 30 dB. I would tell you exactly what the value was, but the VU meter on my deck is not extremely precise.

Before I vero this circuit and throw it in an enclosure, is there anything obviously wrong with this circuit?
Is the 50 k potentiometer a bad idea?
What about linear vs. log potentiometers with this circuit. I usually much prefer log potentiometers for volume/gain. I am new to variable gain potentiometers like in this circuit though, does a linear potentiometer work better than a logarithmic potentiometer in this situation?
Also, I've started using an 18 volt power supply to maximize headroom, but notice that in the boost circuit, it calls for 4.5 volts on the inputs and gain stage. I initially used a 9V power supply, so i made a simple voltage divider in the power section of the circuit (not shown) to get the 4.5 volts. Now that I'm using 18 volts, the voltage divider is putting out 9 volts. The circuit sounds fine, but is this a problematic situation for reasons I do not yet understand?

Thanks for the help, guys.

[PBE6]

With a 50k pot, the minimum gain is 1 and the maximum gain is 50, which is the same as saying a minimum of 0dB and maximum of 20*log(50) = 34dB. Either type of pot should work fine, with linear taper being slightly preferred for finer control over the louder end of the rotation.

As karbomusic noted above, the TL072 can only handle 15V safely so I would retract my earlier suggestion and instead stick to 9V with this opamp boost. The TL072 can get about 1V from either power rail, so a 9V supply gives you a maximum clean voltage of roughly 3.5V. That's plenty strong enough to overdrive just about anything following it. I suspect your original design used a low gain JFET, which was likely the problem and not the power supply per se.

With regard to your circuit, you could simplify things by getting rid of the buffer stage altogether. Non-inverting opamp gain stages have extremely high impedance, so the input impedance is set by the bias resistor (1M in your second diagram). This is plenty high enough to do the buffer's job without the buffer itself. If you decide to go this route, you can do something else with second half of the opamp like create an active tone control or buffer the bias voltage. If you don't do anything with it, make sure you tie it off properly by wiring it as a buffer with Vbias as the input.



Sent from my iPhone using Tapatalk

[PRR]

> Is the 50 k potentiometer a bad idea?

My favorite mix console had a 500K there. (It covered a LOT more gain than you should ever want in guitar cord work.)

The "bad idea" is when the pot wiper gets scratchy, gain can go to "infinity". An old pot that gets "a little scratchy" in other circuits, here it will get SCRATCHY. Nevertheless it is done a fair amount. We still don't know why you need this gain, but if you find a good gain and then replace the trimmer with a fixed resistor, it can't scratch.

> TL072 can only handle 15V safely

TL07x is rated 36V. Millions were used in +/-15V (30V total) systems. Many have been used at +/-17V (34V); I ran one on unregulated nominal 33V supply 24/7 for years.

There is not a guitar or line input in audio which won't be over-driven, even hurting, with even 15V supply, but you can run 36V total (regulated +/-18V) if you gotta be the biggest volt on the bandstand.

[balkanizeyou]

Also, I've started using an 18 volt power supply to maximize headroom, but notice that in the boost circuit, it calls for 4.5 volts on the inputs and gain stage. I initially used a 9V power supply, so i made a simple voltage divider in the power section of the circuit (not shown) to get the 4.5 volts. Now that I'm using 18 volts, the voltage divider is putting out 9 volts. The circuit sounds fine, but is this a problematic situation for reasons I do not yet understand?

The schematic calls for 4.5 volts, because it was intended to use with a 9V supply - if you want to use a 18V supply, you want the voltage divider to provide 9 volts so everything is fine in your case. The reason for that is that you want to have maximum available headroom, which means that the signal should be able to swing as much as possible - upwards and downwards. Obviously this circuit cannot swing the signal beyond the power rails, so by biasing the op amp to half the supply, the signal can swing (almost) 9v up and down, maximum available symmetrical headroom.

[ElectricDruid]

I decided to build a buffered boost using a TL072 dual op-amp.
I combined this buffer circuit: http://www.muzique.com/lab/buffers.htm...



... with this boost circuit: http://www.muzique.com/lab/boost.htm...



... and it worked great!

Nice work taking a buffer and a boost and combining them to get what you want. That's the way it's done - take the building blocks and build with them!
If you've just combined those two circuits above, then you've got an output capacitor (10u) from the buffer followed by an input capacitor on the boost. They both do the same thing, so you don't need both of them. Given the 1M input impedance of the boost stage, the capacitor could be as small as 22nF and you'd still get a -3dB point at <10Hz.

In fact, given that the first buffer circuit sets the bias at the midpoint (which is where you need it for the second circuit) you could get rid of both capacitors and the boost circuit's 1M input resistor too. That resistor sets the bias for the boost stage, but the signal coming into it from the buffer is already biased at the correct level, so there's no need. This is replacing an AC coupling of the two circuits with a (simpler) DC coupling of them instead. Saves a few parts.

HTH,
Tom