Opamp Power Supply

[teknoman2]

What are the benefits if we increase the power supply in a opamp?
Feeding an opamp with bipolar 15v dc comparing to standar 9v dc.
Increasing power supply I think will affect headroom but is there any audible difference?

[drolo]

In usual circuits handling audio as we deal with, there should not be any audible difference. The opamp will be able to amplify larger signals before it clips.
In other applications like LFO's and so on ... not sure what the increase in headroom does, I guess a wider sweep of the LFO signal.
I have never really felt the need to have such a headroom with guitar signals, at least passive ones.
These are just my own observations of course.
The only situation where I would call for such a headroom is if I wanted to hit a tube amp with a very large (clean?) signal.

[teknoman2]

Exact to the point my friend,
I was thinking increasing power feed of the opamp for an LFO circuit.
So increasing headroom will be a positive effect in LFO cisrcuit or still we wont hear any difference?

[drolo]

I think it will depend on what the LFO is driving. The question is: Will the thing that is being driven by it profit from the increased LFO signal (can it handle the increase?)

[teknoman2]

If I understand correct probably it will.
The LFO will drive 4 stages of two OTA chips (LM13700).
Maximum rating of TL062 and LM13700 are similar from what I saw in their datsheet.
Do you think driving those chips with bipolar 15v dc supply will benefit the effect?

[RobA]

For the op-amp, it depends on the op-amp you use. Some op-amps really prefer having lots of headroom and working from a bipolar supply.

For the LM13700, lots of circuits are easier to setup using a bipolar supply, but you have to be careful because it changes the current flowing through the control pin and that alters the response of the OTA.

Another thing that going to a bipolar supply gets you is being able to drop all sorts of coupling caps, which is basically good for the audio but also really helps in doing filter circuits.

For most of guitar stuff, just going to a +/-9V would be good.

[drolo]

I'm not very experienced with OTA's but although the datasheet only shows examples of application with -/+ 15V, the maximum input signal is limited before it distorts (or before it distorts a lot).

In this article you can see a graph of how the distortion increases with signal voltage (figure 5) :

http://www.nutsvolts.com/uploads/magazine_downloads/11/May%202003%20Ray%20Marston%20-%20Understanding%20And%20Using%20OTA%20OP-Amps.pdf

In this article, RG Keen mentions that the maximum voltage at the amp bias input is limited to 2 diode drops, so the added voltage of the LFO will perhaps not add much:

http://www.geofex.com/Article_Folders/VCA%20Applications.pdf

You have to watch out for the resistor value to limit the current though

The data sheet is not very clear to me as it does not mention the minimum supply voltage not the maximum input voltage. Unless its what they listed as : Differential Input Voltage ±5V.

I have just been reading on OTA's lately for a potential project but have not yet actually worked with one ...

I'm sure someone with more experience in OTA's will be able to shed some light on this :-)

[teknoman2]

My main thought is to modify the ROG trivibe using bipolar supply using LT3463A which gives +/- 15v but also I can use a voltage doubler TC1044 or LT1054 that gives more current.
Probably I will use the second option cause its more cheap and common.
Also I am thinking add another lm13700 (two phase stages) for more "real and deep" univibe effect.
I have to socket the cap of each stage and found the correct ratio between each stage something like 15:1 (like original Univibe has)

[teknoman2]

I'm not very experienced with OTA's but although the datasheet only shows examples of application with -/+ 15V, the maximum input signal is limited before it distorts (or before it distorts a lot).

In this article you can see a graph of how the distortion increases with signal voltage (figure 5) :

Thanx for all the replies mate.
I believe we can handle 0.2% of THD and it is not too critical.
Imagine that MNxxx BBD's has 0.5% THD and the results are great.
Correct me if I am saying something wrong.

[RobA]

My main thought is to modify the ROG trivibe using bipolar supply using LT3463A which gives +/- 15v but also I can use a voltage doubler TC1044 or LT1054 that gives more current.
Probably I will use the second option cause its more cheap and common.
Also I am thinking add another lm13700 (two phase stages) for more "real and deep" univibe effect.
I have to socket the cap of each stage and found the correct ratio between each stage something like 15:1 (like original Univibe has)

There are several synth phase shifter schematics that are designed around LM13700's running at either +/-15V or +/-12V that you could adapt. You could add on the LFO shaping section from the Tri-Vibe and be most of the way there.

[teknoman2]

There are several synth phase shifter schematics that are designed around LM13700's running at either +/-15V or +/-12V that you could adapt. You could add on the LFO shaping section from the Tri-Vibe and be most of the way there.

I was trying to find some other desings applications of lm13700 using +/-15V but I could find anything on the net.
Can you please point out some links?

Thanx RobA,
one more time you ve been very helpfull.

[RobA]

There are several synth phase shifter schematics that are designed around LM13700's running at either +/-15V or +/-12V that you could adapt. You could add on the LFO shaping section from the Tri-Vibe and be most of the way there.

I was trying to find some other desings applications of lm13700 using +/-15V but I could find anything on the net.
Can you please point out some links?

Thanx RobA,
one more time you ve been very helpfull.

The only one I can remember off the top of my head is the Eight Stage Phase Shifter from MFOS, http://www.musicfromouterspace.com/analogsynth_new/PHASESHIFTER2007/PHASESHIFTER2007.php

[teknoman2]

Great mate,
I will take a look of this one.
Also thanx to Drolo attachements I found out that we must limit the current input of the OTA via a bias resistor if we would like to use each two diodes in series on pin2 and pin15 (diode Bias) to eliminate THD.
ROG's schematic uses 3k9 for a maximum 2mA current, so if we feed the OTA with 15v a 6k8 resistor gives the same current.