Hi to All !
i have a big lack of knowledge about amps stability.
Assuming that an amp circuit is stable how could i know the effect of changing its gain on its stability ?
How a designer checks the stability of his/her designs ?
If there is a thread already discussing the topic please feel free to redirect me to it
Thank you very much indeed
kind regards
gino
i have a big lack of knowledge about amps stability.
Assuming that an amp circuit is stable how could i know the effect of changing its gain on its stability ?
How a designer checks the stability of his/her designs ?
If there is a thread already discussing the topic please feel free to redirect me to it
Thank you very much indeed
kind regards
gino
Hi ! yes ! i am a basic user Is there a way to see the stability of a circuit with LTSpice ? that would be very nice
To be honest i have already found an old discussion on the same topic Someone was pointing out the limits of using sim SW to check stability
i do not know really
To be honest i have already found an old discussion on the same topic Someone was pointing out the limits of using sim SW to check stability
i do not know really
This is how you go about it To generate the loop gain and phase plots from which you can determine stability :
1. In your LTspice schematic, short the amplifier input to 0 V
2. Insert an AC signal source between the output and the feedback resistor so that the feedback signal flows from the output, through the AC signal source to the feedback resistor.
3. Name the amplifier output Vo. Name the feedback resistor side of the AC signal source x
4. Set the amplitude in the AC signal source to 1
5. In the AC analysis dialog box, set the lower limit to 20 Hz and the upper limit to 100 MHz. Set the type of plot to ‘decade’ and the number of points per decade to 100.
6. Run the AC analysis and in the plot window, plot Vo/x (you can just select these in the dialog box)
7. This will give a plot of loop gain and loop phase
Post that plot with the schematic up here and we can then guide you through the rest of the process.
1. In your LTspice schematic, short the amplifier input to 0 V
2. Insert an AC signal source between the output and the feedback resistor so that the feedback signal flows from the output, through the AC signal source to the feedback resistor.
3. Name the amplifier output Vo. Name the feedback resistor side of the AC signal source x
4. Set the amplitude in the AC signal source to 1
5. In the AC analysis dialog box, set the lower limit to 20 Hz and the upper limit to 100 MHz. Set the type of plot to ‘decade’ and the number of points per decade to 100.
6. Run the AC analysis and in the plot window, plot Vo/x (you can just select these in the dialog box)
7. This will give a plot of loop gain and loop phase
Post that plot with the schematic up here and we can then guide you through the rest of the process.
Step by step instructions to generate the loop gain and phase plots, made by @minek123 , can be found here:
https://www.diyaudio.com/community/...att-with-tmc-and-laterals.406282/post-7540040
https://www.diyaudio.com/community/...att-with-tmc-and-laterals.406282/post-7540040
Thank you sincerely to you All for the very kind and precious help
I will try with a simple preamp circuit to start
in home listening i normally use very few watts Line preamps can provide up to 10Vrms with very low THD plus noise
I will have to add just current to those 10Vrms with a low gain power amp
I will try with a simple preamp circuit to start
in home listening i normally use very few watts Line preamps can provide up to 10Vrms with very low THD plus noise
I will have to add just current to those 10Vrms with a low gain power amp
Without any other knowledge all you can say about changing the gain of an amplifier that is stable is that it might become unstable. Reducing the gain of a feedback amplifier will reduce the gain margin of the feedback loop. Increasing the gain of an amplifier with less than ideal supply rail decoupling can destabilize it though unintended power-rail feedback paths. (applies to any amp, not just GNFB)
Hi thanks a lot This is exactly what i did some time ago The transformer started to vibrate and fume The sound was very weak
I touched the heatsink and burned my fingers
It is astonishing how little things can do all this damage Like changing a feedback resistor
Maybe it is one of the reasons for zero feedback amps
They will have maybe low power high distortion high output impedance but they will not have feedback resistors I guess
I start to hate feedback
HI thanks a lot Yes i have already simulated some diamond buffers taken here and there in the web or service manuals
But one of my main interests could be a diamond buffer so powerful to be able to drive speakers if this is possible of course
I could provide all the needed Voltage gain at line preamp level
I do not need many watts at all for my home listening I live in a flat unfortunately (ok better than under a bridge)
Some preamps have no issue in delivering even more than 10Vrms with very low THD and noise
I know that is a quite weird approach Usually preamps are used up to 2V out
Another strange idea would be to buffer the output of a chip amp ... but that is much more difficult
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Add AC Amplitude 1 to Voltage source.
You have only added text to output net. Right click on output net to label net to Vo.
Plot V(Vo)/I(x)
sorry but i am slow to understand How would it be the plot of a stable circuit ?
i realize now how trivial was my approach This issue of the stability never crossed my mind
And i have now a feeling that the range for stability is very narrow
I think i halved the closed loop gain when i destroyed a little Rotel RA 930ax Maybe some pieces were still good
However i do not know why but i have usually found cheap amps sounding quite hard and flat expecially in the midrange
I decided to mod the amp because i was not satisfied by its sound
The solution for me will not be to switch to tube because i cannot stand the bass from a tube amp Sluggish and weak
i realize now how trivial was my approach This issue of the stability never crossed my mind
And i have now a feeling that the range for stability is very narrow
I think i halved the closed loop gain when i destroyed a little Rotel RA 930ax Maybe some pieces were still good
However i do not know why but i have usually found cheap amps sounding quite hard and flat expecially in the midrange
I decided to mod the amp because i was not satisfied by its sound
The solution for me will not be to switch to tube because i cannot stand the bass from a tube amp Sluggish and weak
This is an excellent reference that gives a very good theoretical background of the subject and describes in detail the Tian probe method, what I've always used in LTSpice for stability analysis with good results: https://www.iceamplifiers.co.uk/articles/stability/stability.pdf
If you want to jump straight to this, it starts on page 56, "13. Using LTSpice to plot the open loop response". The circuit shown in fig. 28 is available in the included LTSpice Examples folder, located in C:\Users\[UserName]\AppData\Local\LTspice\examples\Educational
If you want to jump straight to this, it starts on page 56, "13. Using LTSpice to plot the open loop response". The circuit shown in fig. 28 is available in the included LTSpice Examples folder, located in C:\Users\[UserName]\AppData\Local\LTspice\examples\Educational
Hi ! sorry for the delay Here is the result
the circuit being a part of a commercial preamp should be stable
But how the graph tells that is a mistery to me 🤔
the circuit being a part of a commercial preamp should be stable
But how the graph tells that is a mistery to me 🤔
In that plot pane, click on "V(vo)/V(x)" at the top, this will show the cursor (a dotted line cross) in the middle of the graph and a small window will pop up showing you the reading of the graph at that point. Click anywhere on the vertical line (as you approach it the mouse cursor will change from a small cross to an arrow with a "1" next to it) and drag it right until the value in "Mag." reads 0 dB. The value you can read now in "Phase" is your phase margin.
Then you click again on the cursor vertical line and drag further right until "Phase" reaches 0º. The "Mag." value (ignoring the sign) is your gain margin. In this case it looks like you will have to simulate up to a much higher frequency to get there. Note that it's possible that the phase never reaches 0º, which means that, in principle, you could reduce the closed loop gain as much as you want without going into instability.
Then you click again on the cursor vertical line and drag further right until "Phase" reaches 0º. The "Mag." value (ignoring the sign) is your gain margin. In this case it looks like you will have to simulate up to a much higher frequency to get there. Note that it's possible that the phase never reaches 0º, which means that, in principle, you could reduce the closed loop gain as much as you want without going into instability.