Is there a way to set the level at which an Opamp rail saturates ?

[Vivek]

Suppose I have an Opamp running with 9V supply, biased at 4.5V and it's output rails saturates at 7.5V and 1.5V

Is there a way that I can get it to rail saturate at let's say 6.5V and 2V if I wish (besides lower the Vcc) ?

With an adjustable pot would be really nice.

Is there a way to have more graceful onset of rail saturation instead of running full steam ahead and then banging your head on a wall ?

[Mark Hammer]

This was purportedly a way to do what you suggest.

https://www.diystompboxes.com/pedals/Elektor_variable_fuzz.PDF

That said, I gather your focus is on clipping resulting from adjustment of headroom.  One way to do that is to employ an op-amp that can function on lower supply voltages, and vary that supply voltage: a don`t-raise-the-river-just-lower-the-bridge strategy.

Another strategy to deploy involves use of externally compensated op-amps, and adjusting the compensation cap to reduce open-loop gain for the intended bandwidth.

[Vivek]

This was purportedly a way to do what you suggest.

Wow, happy memories

My university in India used to import Elektor magazines.

I had built this circuit around 1980 and used it in my University band.

[Mark Hammer]

Scroll to the bottom, here, and reminisce freely.
https://worldradiohistory.com/Elektor.htm

[PRR]

Suppose I have an Opamp running with 9V supply, biased at 4.5V and it's output rails saturates at 7.5V and 1.5V
Is there a way that I can get it to rail saturate at let's say 6.5V and 2V if I wish (besides lower the Vcc) ?
With an adjustable pot would be really nice.

Isn't that what pots (voltage dividers) DO?

Is there a way to have more graceful onset of rail saturation instead of running full steam ahead and then banging your head on a wall ?

Less gain. (Don't use opamps.)

[Vivek]

Thanks Paul,

The pot keeps the existing symmetry of the clipping and scales it down

For example, suppose we get saturation 3 volts above bias point and 2 volts below bias point, we could use a pot to scale that by 50% and get clipped wave with +1.5 and -1 peaks

I was trying to figure out if it was possible to change the symmetry of the clip

Like if I was clipping at +3 and -2, and I want to now set it at +4 and -2 or if I want +3 and -0.5

[antonis]

I was trying to figure out if it was possible to change the symmetry of the clip
Like if I was clipping at +3 and -2, and I want to now set it at +4 and -2 or if I want +3 and -0.5

Alter bias..
e.g. +3/-2 -> bias at +0.5V, +4/-2 -> bias at +1V, +3/-0.5 -> bias at 1.25V, etc..

[Keppy]

Is there a way to have more graceful onset of rail saturation instead of running full steam ahead and then banging your head on a wall ?

Isn't that what diodes in the feedback loop are for?

[Vivek]

That said, I gather your focus is on clipping resulting from adjustment of headroom.  One way to do that is to employ an op-amp that can function on lower supply voltages, and vary that supply voltage: a don`t-raise-the-river-just-lower-the-bridge strategy.

Yes yes, that's what I want !!!

Like Merlin's Glass blower buffer that raises the rail voltage when the signal is bigger, but only in reverse.

I want to try to implement a time bound "sag" type thing. somewhat analogous to supply voltage drop on tube amps

by having an Opamp that is almost rail saturating and then lowering the Opamp Vcc when the signals get larger.

Please help me subvert Merlin's Glass Blower to do exactly the opposite of its intended purpose, with adjustable time constants for the Vcc change.


alternatively, dynamically change the settings on the transistors in the Elektor variable fuzz circuit


Or are there smarter ways to achieve the end goal ? 

[Steben]

That said, I gather your focus is on clipping resulting from adjustment of headroom.  One way to do that is to employ an op-amp that can function on lower supply voltages, and vary that supply voltage: a don`t-raise-the-river-just-lower-the-bridge strategy.

Yes yes, that's what I want !!!

Like Merlin's Glass blower buffer that raises the rail voltage when the signal is bigger, but only in reverse.

I want to try to implement a time bound "sag" type thing. somewhat analogous to supply voltage drop on tube amps

by having an Opamp that is almost rail saturating and then lowering the Opamp Vcc when the signals get larger.

Please help me subvert Merlin's Glass Blower to do exactly the opposite of its intended purpose, with adjustable time constants for the Vcc change.


Or are there smarter ways to achieve the end goal ?

It's brutal, but a higher signal will draw more current, meaning a classic sag resistor + cap structure works.

[Vivek]

That could be a nice way Steven, especially if I increase the load current by reducing the load resistor.

I will breadboard that and check it out

[Steben]

Do not forget it does not need to be an opamp that clips. Diode clippers can be biased as well in relation to the supply (voltage dividers). The loading of the supply by the opamp still functions that way, but it is not the opamps that does the clipping.

[Vivek]

That said, I gather your focus is on clipping resulting from adjustment of headroom.  One way to do that is to employ an op-amp that can function on lower supply voltages, and vary that supply voltage: a don`t-raise-the-river-just-lower-the-bridge strategy.

It's brutal, but a higher signal will draw more current, meaning a classic sag resistor + cap structure works.

Dear Mark and Steven,  Thanks for your ideas !!!

I tried to simulate it on LTSPICE.



I used a low load resistor R6, so that I can get a larger current, but within the range of the Opamp (I should have sent R6 to ground)

Then I added a sag resistance R7 and an integration cap C4

I have a signal source that feeds 1Vp sine waves from t = 1s till t = 2s



The green curve is the supply voltage that the Opamp receives. The blue curve is the voltage on load resistor R6.

After first 0.6 seconds the DC voltages have reached their stable state.

Then at t=1s, the input signal starts.

The Voltage at Vcc pin of the opamp start to fall

The output which was large and less distorted earlier soon gets smaller in amplitude and higher in distortion

The time where all this happens is first 100ms or so, since I read somewhere that is the time zone for "sag"


The amplitude and waveshape of first few cycles looks like this :


and the last few cycles look like this:



First few cycles and last few cycles on the same graph



Here is how things appear if the load resistor R6 goes to ground. A bit easier to understand.




Thanks once again. I owe you a beer when we meet !!!

[Vivek]

https://www.youtube.com/watch?v=gWC86zepZfo

[Vivek]

Question

How can I have different attack time constant than the release time constant of this sag ?

Suppose I want to simulate a power supply that drops voltage with time constant 75ms but charges up with time constant 15 ms. How to do that ?

Right now I have one resistor R7  going to integration cap and that goes to vcc pin of Opamp

Do I need to reduce R7 and then add one more "discharge resistor" before vcc pin of Opamp ?

[anotherjim]

Diodes?

[Rob Strand]

There's a big difference between:
- having adjustable clip levels
- changing the nature of the how an output stage saturates.
  (eg a CMOS output stage like a CA3130 clips quite differently to a LM741.)

A circuit like the Hotcake changes the clipping (of one polarity).
   
So does adding zener diode clippers.  You can even do an adjustable clipper with an adjustable VBE multiplier.

Even those aren't  really changing the nature output stage, they are replacing the thing that clips - by preventing the output stage from actually clipping.

[Vivek]

- having adjustable clip levels

It would be great to design a "sag" type of concept using modulateable Rubber diodes or other methods. I am more interested in this thread on variable clipping based on signal envelope


Here is a possible starting point for this idea



OCR and Google translate:


The new "fuzz
The oldest changing system
the timbre of the electric guitar is
fuzz layout. With the progress of electro
The proposed solutions are increasingly perfect
six versions of this layout. Primary
version, let's call it "static fuzz",
works on the principle of clipping - on
one fixed voltage level
the upper and lower halves of the run
electric generated by gi »
tare. |
| The new version of the system - "regulated
static fuzz "is shown in
fig. 1. The system works as follows
man. Electric guitar output waveform
successive
fading vibrations is
to an amplifier with an integrated circuit
nym US1. The amplifier has adjustable
gain in the range 1..100, for
by the power of the P1 potentiometer. T1 transistors
and T2 with potentiometers P2 and P3
create regulated voltage sources from-
very low resistance
output. Properly strengthened
guitar waveform is cut to
levels determined by
P2 and P3 tenniometers.
Let us consider such an example. It has
my guitar that when it hits
in the strings produces tension between
peak 200 mV. P1 potentiometer
we set such amplification that
the enhanced mileage had a value between
6V peak (gain 30).
Set the value with P2 and P3 potentiometers
reference voltage values ​​+1 Vi — 1V, |
and set the potentiometers P4 and P5 to
minimum resistance. So adjusted
The important attachment cuts the top and bottom
half of the mileage at approx
1.6V, the truncation will continue
until the value of the
the output waveform strengthens
The time will not decrease to 3.2 V.
From the value of the fixed resistance
the color depends on the tenniometers P4 and P5
sound; im the resistance value is
the bigger, the softer the bar-
wa sound.
The cut level of both halves of the
gu can be adjusted separately, co
obviously affects the sound
guitar sound.
After assembling the system it is necessary
selecting the position of the resistor slider
adjustable RS so that there is no
it is a distortion of the cooperating
power amplifier.
The second version of the header system, before
shown in Fig. 2, can be called: "fuzz
dynamic". Characteristic feature
system is that the distortion
the guitar run does not change its own
character with decreasing amplitude
vibration of the guitar strings. System
this one differs from the previous one in that
"| it uses circuits reproducing
_ | the shape of the upper and lower envelopes
_ | guitar run. They are for this purpose
US1 and US3 operational amplifiers
together with cooperating diodes D1i
D3 and C2 and C3 capacitors. Tension-
the envelope adjusts automatically
reference voltage level obtained
on the emitters of transistors T1 and T2.
So the reference voltage decreases with
with a guitar sound, co
causes approximately proportional
truncation of waveforms. The principle works
the system is the same as in the description
previous layout.
The PK1 switch can be changed
the type of work of the attachment with "fuzz dynamics"
"on" to "adjustable static fuzz".
The double potentiometer P1 is used for
regulation of the reference voltage in
if the system is switched to the latter
type of work.
Both systems should be powered from
symmetrical power supply stabilized
with an output voltage of +15 V.
The current consumption of the first circuit is
about 30 mA, the second - about 60 mA.
It is not recommended to use
terrestrial because it decreases
battery voltages change parameters
try "set" by potentiometers and at least
this change is not abrupt, however
the attachment reacts differently over time
"setting the potentiometers.
This enables the use of divisions at
knobs or potentiometers
what is necessary for quick
Attachment regulators lyes.

That isn't a VBE multiplier / rubber diode. It's just a plain shunt diode referenced to a DC offset, which is sourced from the emitter of a transistor circuit forming a voltage regulator. Two diodes and DC references are used for clipping negative and positive half waves.

[Steben]

I just keep getting the same reflexes that the most obvious way of recreating supply voltage drop is doing exactly what a tube amp does with a tube rectifier.
Get a variable current draw and use a resistance. That's what they do in semiconductor supply circuits.
And of course ... many many tube amps do not have that power supply sag, because they have semiconductor rectifiers out of the factory...

[Rob Strand]

It would be great to design a "sag" type of concept using modulateable Rubber diodes or other methods. I am more interested in this thread on variable clipping based on signal envelope


Here is a possible starting point for this idea

Just to be clear on the difference.   The Vbe multiplier stretches the diode characteristic so it's kind of distorting all the time, just less on low signals.  Whereas feeding the diode into  a voltages source, like your magazine example, the diode stays blocked until the voltage level approaches the clip point set on the voltage source.   That means the clipping is harder.   Just on the onset of clipping the diode characteristic will appear and provide a slight rounding to the clipping characteristic.

Also note on your example circuit, when the diode (say D1) start to conduct and "clipping" occurs the diode current is feeding into the 10k.  If too much diode current feeds into the 10k the voltage at the emitter of the transistor will rise up and that means the clip point will rise up.   As the diode current increases the emitter current decreases.  Making the emitter value low will fend-off more diode current.  The base-emitter  junction of the transistor will start to play a part in the sound.

If however you wired replace the NPN T1 with a PNP, wired the 10k to +15V, and wire the collector to -15V, then when the diode conducts the D1 diode current feeds into the emitter.   The diode cannot overcome the transistor like the existing circuit.  The voltage source then becomes more "stiff".    The base-emitter junction of still has in impact on the sound however in this case it is different: as the diode current increases the emitter current increases.

The subtle things which occur near the clipping point can affect the sound.

I don't know what variant will sound better.  You could play with the emitter resistor value.  The base resistors will also have an effect, as 100k is pretty high.

As for the sag with the Vbe multiplier, I'm pretty sure I've played with this in spice in the past.   It's so long ago I don't know if I got it to work or not.

The structure of the circuit you have looks OK to experiment.   I know I've posted some other forms of programmable clippers in the past on the forum.   Someone else (maybe Steben?) posted a link to a programmable clipper chip a while ago from analog devices; perhaps a year or two ago.   You will find they all use diodes and transistors in sort of the same way.

Here's the clamp chip,
https://www.diystompboxes.com/smfforum/index.php?topic=125169.msg1190389#msg1190389