Re: Interpreting Noise Simulation Results
manauo,
?
Almost all the simulated noise in your circuit comes from the circuit itself.? If there was noise in the sources driving it (V1, V2, V5, V6), it is not present in the simulation.? Those voltage sources are noise-free.? Therefore, adding high-pass filter capacitors to reduce noise from the source, does not do you any good, unless the source itself had noise.? I think the only parts behind the HPF that generate any noise in this simulation, are the 0.6 ohm resistors, R9, R10, R21, and R22.? (Also the Rser of L1-L4.)? That's about the only benefit to be had from adding those capacitors.
?
On top of that, there is the increase in noise caused by making the source impedance (connected to the transistor bases) larger.? That is likely why you saw the noise density going up instead of down.
?
Not many people use SPICE's .NOISE analysis.? Consequently, many device models either lack the parameters needed to simulate it well, or the parameter values are badly wrong.? That was especially true of the JFET models that came with LTspice.? LTspice got those models from the company Linear Systems (not to be confused with Linear Technology Corp.), but someone at Linear Systems had inadvertently messed up their own SPICE models before giving them to LTC for inclusion in LTspice.? If I remember correctly, the mistake was 5 orders of magnitude (!) and applied to every one of their JFET models.? Fortunately your circuit has only BJTs, no JFETs, so it might be OK.? Should be OK for comparative simulations.
?
Andy
?
|
Re: Weird results DC operating point for Tube amplifier
Any tube sound this design gives you is weird tube sound. ?
toggle quoted message
Show quoted text
From: [email protected] <[email protected]> On Behalf Of Andy I via groups.io
Sent: Thursday, February 20, 2025 5:23 PM
To: [email protected]
Subject: Re: [LTspice] Weird results DC operating point for Tube amplifier? On Thu, Feb 20, 2025 at 12:32 PM, John Woodgate wrote: There really isn't any point in criticizing the design. It's deliberately weird. An LM386 makes a good headphone amplifier, but it might be difficult to sell for $130.
Yeah, but then it would not have that "tube sound".
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Re: PWM Timing Causing Shoot-thru
On Thu, Feb 20, 2025 at 04:20 PM, May wrote:
It is missing the symbol for IR2110.? The missing file is ir2110.asy.? Please add it to your .zip file and re-upload it.
?
Additionally, probing the gates of the FETs shows that there is somehow overlap in the HI and LO drive signals, despite my attempts to tune the voltage sources I am using to generate the PWM signals.
That is likely to be a cause of shoot-through current.
?
I am not sure if the shoot-through is an artifact of LTSpice or if I am simply wrong with how I have set the PWMs up.
I think it is unlikely to be an LTspice artifact, but something in the models might be responsible.? Large MOSFETs can have significant input (gate) capacitance which can mess things up.? If you probe the signals you generated, do they seem right or do they have the wrong overlap?
?
Andy
?
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Re: PWM Timing Causing Shoot-thru
Bruce,
?
Based on your comments, I think I have been able to successfully utilize the SD pin of the IR2110 to eliminate the shoot-through I was seeing.
?
Thanks so much!!
?
-May
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Re: Weird results DC operating point for Tube amplifier
On Thu, Feb 20, 2025 at 12:32 PM, John Woodgate wrote:
There really isn't any point in criticizing the design. It's deliberately weird. An LM386 makes a good headphone amplifier, but it might be difficult to sell for $130.
Yeah, but then it would not have that "tube sound".
?
?
|
Re: Weird results DC operating point for Tube amplifier
On Thu, Feb 20, 2025 at 12:17 PM, Jerry Lee Marcel wrote:
BTW, source-follower it is not. It's actually a common-source stage.
A source fiollower has about unity gain, this common-source has about 15dB gain.
No, it is a source follower.? Its voltage gain is unity.? Input applied to gate, output from source pin.? Almost the same output voltage, but shifted 4.2 V lower.
?
The overall circuit has voltage gain (~23 dB), but all of that comes from the 12AU7, in its common-cathode configuration.
?
I wonder why you saw only 15 dB gain, and why you saw that in just the MOSFET.
?
Not that the following has any significance - but I vaguely recall reading about operating valves (vacuum tubes) at much lower than "normal" anode voltages.? ?There were specialized miniature valves designed for it.? But it was also applied to more traditional valves too, ones like the 12AU7 that are capable of hundreds of volts.? It does not appear to be a sure-fire way to generate distortion, as this simulation demonstrates, if you trust the SPICE models.? I can't recall what were the advantages (if any) of using such low voltages - other than easier battery power.? Lower noise?? ?Longer life?? I dunno.
?
Andy
?
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Re: Interpreting Noise Simulation Results
Yes, an RC high pass filter on the input
will increase the noise. Although the capacitor doesn't itself add
any noise, it causes the effective input noise voltage to
increase, because:
Vn(tot) = √(En^2 + (In/2/pi/Cser)^2)
..where: En = I/P noise voltage density, and In = I/P noise
current density (neglecting the source resistance noise, which
often you can't do)
Remember also: "No attenuation before gain".
I should also mention (without seeing your schematic), that many
of the devices in the LTspice standard libraries don't have
realistic noise parameters, especially when it comes to 1/f noise.
--
Regards,
Tony
On 20/02/2025 20:17, manauo via
groups.io wrote:
toggle quoted message
Show quoted text
I don't have much experience using LTSPICE for noise
simulations, and I'm having trouble understanding some of the
results I'm getting. I'm investigating an amplifier circuit
(file "Amplifier Noise" uploaded to files/Temp).
?
The amplifier is a cascode differential pair, signal
frequency is 1MHz. Currently the circuit has no filtering
besides the inherent roll-off at high frequency. I'm
investigating whether adding AC coupling capacitors to the input
to filter low-frequency noise can improve the performance. The
inductance/resistance in series with each input represents the
output impedance of the previous stage.
?
When I simulate the original circuit with no coupling
capacitors, the result seems reasonable - relatively flat, input
noise is a fraction of the output noise. However when I simulate
the proposed circuit with coupling capacitors I don't understand
the result.
?
The input noise balloons at low frequencies - the frequency
range that the HPF of the capacitor is supposed to reduce, is
now much greater. The output noise is lower in terms of total
integrated noise, but still has an increased magnitude at low
frequency compared to the original.
?
The decrease in total output noise is coming from a decrease
in the high frequency noise, which makes sense because the
inductance of the source impedance and the base pull-down
resistor form an RL LPF. But the capacitor does not seem to be
reducing LF noise, even though an AC sweep shows the expected
HPF behavior.
?
Am I misinterpreting the results somehow? How is adding a
high-pass filter at the input increasing noise at the low
frequencies that it is supposed to be attenuating? What am I
missing about noise simulations?
|
Re: PWM Timing Causing Shoot-thru
You need to find a strategy to enforce some amount of deadtime between the signals. Tuning is not going to do the job unless it is predictable and repeatable, Analog methods tend to be unreliable.
?
BRUCE108
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PWM Timing Causing Shoot-thru
I am having some trouble getting the timing right on my PWM drive signals. What tells me my timing is wrong is the fact that I am seeing what appears to be shoot-through periodically.
?
?
Additionally, probing the gates of the FETs shows that there is somehow overlap in the HI and LO drive signals, despite my attempts to tune the voltage sources I am using to generate the PWM signals.
?
Any help is appreciated; I am not sure if the shoot-through is an artifact of LTSpice or if I am simply wrong with how I have set the PWMs up.
?
Thanks,
-May
|
Re: Interpreting Noise Simulation Results
At low frequencies, the capacitor's impedance is quite high. the
noise current generated by the active stages circulates in this
increased impedance, resulting in an increasing voltage.
Le 20/02/2025 à 20:17, manauo via
groups.io a écrit?:
toggle quoted message
Show quoted text
Hello,
?
I don't have much experience using LTSPICE for noise
simulations, and I'm having trouble understanding some of the
results I'm getting. I'm investigating an amplifier circuit
(file "Amplifier Noise" uploaded to files/Temp).
?
The amplifier is a cascode differential pair, signal
frequency is 1MHz. Currently the circuit has no filtering
besides the inherent roll-off at high frequency. I'm
investigating whether adding AC coupling capacitors to the input
to filter low-frequency noise can improve the performance. The
inductance/resistance in series with each input represents the
output impedance of the previous stage.
?
When I simulate the original circuit with no coupling
capacitors, the result seems reasonable - relatively flat, input
noise is a fraction of the output noise. However when I simulate
the proposed circuit with coupling capacitors I don't understand
the result.
?
The input noise balloons at low frequencies - the frequency
range that the HPF of the capacitor is supposed to reduce, is
now much greater. The output noise is lower in terms of total
integrated noise, but still has an increased magnitude at low
frequency compared to the original.
?
The decrease in total output noise is coming from a decrease
in the high frequency noise, which makes sense because the
inductance of the source impedance and the base pull-down
resistor form an RL LPF. But the capacitor does not seem to be
reducing LF noise, even though an AC sweep shows the expected
HPF behavior.
?
Am I misinterpreting the results somehow? How is adding a
high-pass filter at the input increasing noise at the low
frequencies that it is supposed to be attenuating? What am I
missing about noise simulations?
|
Re: Weird results DC operating point for Tube amplifier
That is not the subject.
The subject is that they try to justify their quirky "design"
with objective arguments.
Enough said now.
Le 20/02/2025 à 19:45, John Woodgate a
écrit?:
toggle quoted message
Show quoted text
RE; Yes, what? You asked, 'Or can they?',
and the answer is, 'Yes, easily'. Well, of course Bravo
wouldn't give figures that showed less than excellent
performance. The THD with 33 ohm load is admitted to be rather
high, and we don't know what the output voltage or power was
at that THD. That is why I gave the two pairs of amplifier
figures as an example.
On 2025-02-20 18:14, Jerry Lee Marcel
via groups.io wrote:
Le 20/02/2025 à 17:33, John
Woodgate a écrit?:
Yes, easily.
Yes what?
Consider, for example, these two
amplifiers:
a) Output power 30 W, THD 0.1%
b) Output power 35 W, THD 5.2%
Which would you buy?
I don't care, but Bravo Audio seems to care
about figures that are long accepted as performance
indicators.
They are the same amplifier. Similar
games are played with signal-to-noise ratio and frequency
response.
On 2025-02-20 16:05, Jerry Lee
Marcel via groups.io wrote:
Still they are publishing specifications that pertain to
typical audio performance, such as frequency response and
THD. They can't combine both quirkiness and normality.
Or can they?
Typical "I designed it that way because I could".
IMO it justifies disdain.
Le 20/02/2025 à 16:55, John
Woodgate a écrit?:
I suspect that the weirdness is
intentional. Weird designs have existed from? the
earliest day of DIY radio receivers, before
'electronics'? was in the dictionary. I recall a
report of a circuit that had the 2 V lead-acid cell
apparently in series with the antenna circuit.
Objective performance measurements are typically not
to be applied to such designs. DO a web search for
'Bravo Audio reviews'.
On 2025-02-20 15:36, Jerry
Lee Marcel via groups.io wrote:
Le 20/02/2025 à 15:27,
Carlo a écrit?:
On Thu, Feb 20, 2025 at 05:40 AM, Andy I wrote:
There is some feedback from the audio
signal into the heater voltage. Was that
intentional?? Or just an undesirable side-effect??
I don't expect it would have very much effect on
the heater's temperature (and from there to the
triode's characteristics), but it looks
undesirable to me. Should there be filtering?
Sorry, are you asking whether the audio signal
feedback into the heater voltage comes from a design
intentional choice ? Actually I don't know since I
took it from the schematic of a commercial audio
amplifier (Bravo Ocean).
It's extremely unlikely.
Heater temperature varies extremely slowly compared to
audio signals.
It could result in distortion at very very low
frequencies, definitely out of the audio band.
Now this design is weird from the start. Choosing to
power a tube circuit from 24VDC is a major flaw, unless
the goal is to create distortion.
--
OOO - Own Opinions only If something is true: * as far
as we know - it's science *for certain - it's
mathematics *unquestionably - it's religion
--
OOO - Own Opinions only If something is true: * as far as we
know - it's science *for certain - it's mathematics
*unquestionably - it's religion
--
OOO - Own Opinions only If something is true: * as far as we
know - it's science *for certain - it's mathematics
*unquestionably - it's religion
|
Interpreting Noise Simulation Results
Hello,
?
I don't have much experience using LTSPICE for noise simulations, and I'm having trouble understanding some of the results I'm getting. I'm investigating an amplifier circuit (file "Amplifier Noise" uploaded to files/Temp).
?
The amplifier is a cascode differential pair, signal frequency is 1MHz. Currently the circuit has no filtering besides the inherent roll-off at high frequency. I'm investigating whether adding AC coupling capacitors to the input to filter low-frequency noise can improve the performance. The inductance/resistance in series with each input represents the output impedance of the previous stage.
?
When I simulate the original circuit with no coupling capacitors, the result seems reasonable - relatively flat, input noise is a fraction of the output noise. However when I simulate the proposed circuit with coupling capacitors I don't understand the result.
?
The input noise balloons at low frequencies - the frequency range that the HPF of the capacitor is supposed to reduce, is now much greater. The output noise is lower in terms of total integrated noise, but still has an increased magnitude at low frequency compared to the original.
?
The decrease in total output noise is coming from a decrease in the high frequency noise, which makes sense because the inductance of the source impedance and the base pull-down resistor form an RL LPF. But the capacitor does not seem to be reducing LF noise, even though an AC sweep shows the expected HPF behavior.
?
Am I misinterpreting the results somehow? How is adding a high-pass filter at the input increasing noise at the low frequencies that it is supposed to be attenuating? What am I missing about noise simulations?
|
Re: 3 Phase Voltage Sense model
Thanks, Larry. I can't look at it at the
moment, but I'll try later.
John,
?
zip file now has the LMV324.asy and LMV324.cir files.
?
Larry
--
OOO - Own Opinions only
If something is true:
* as far as we know - it's science
*for certain - it's mathematics
*unquestionably - it's religion
|
Re: 3 Phase Voltage Sense model
John,
?
zip file now has the LMV324.asy and LMV324.cir files.
?
Larry
|
Re: 3 Phase Voltage Sense model
John,
?
Will update zip file and include the LMV324 symbol and the LMV324.cir.? Should be up there in about 5 minutes.
?
Larry
|
Re: 3 Phase Voltage Sense model
Andy,
?
Updated zip file so now the symbol in 3Phase_Voltage_Sense_Test.asc is calling 3Phase_Vsense.sub
?
Larry
|
Re: Weird results DC operating point for Tube amplifier
RE; Yes, what? You asked, 'Or can they?', and
the answer is, 'Yes, easily'. Well, of course Bravo wouldn't
give figures that showed less than excellent performance. The
THD with 33 ohm load is admitted to be rather high, and we don't
know what the output voltage or power was at that THD. That is
why I gave the two pairs of amplifier figures as an example.
On 2025-02-20 18:14, Jerry Lee Marcel
via groups.io wrote:
Le 20/02/2025 à 17:33, John Woodgate
a écrit?:
Yes, easily.
Yes what?
Consider, for example, these two
amplifiers:
a) Output power 30 W, THD 0.1%
b) Output power 35 W, THD 5.2%
Which would you buy?
I don't care, but Bravo Audio seems to care
about figures that are long accepted as performance indicators.
They are the same amplifier. Similar games
are played with signal-to-noise ratio and frequency
response.
On 2025-02-20 16:05, Jerry Lee
Marcel via groups.io wrote:
Still they are publishing specifications that pertain to
typical audio performance, such as frequency response and
THD. They can't combine both quirkiness and normality.
Or can they?
Typical "I designed it that way because I could".
IMO it justifies disdain.
Le 20/02/2025 à 16:55, John
Woodgate a écrit?:
I suspect that the weirdness is
intentional. Weird designs have existed from? the
earliest day of DIY radio receivers, before
'electronics'? was in the dictionary. I recall a report
of a circuit that had the 2 V lead-acid cell apparently
in series with the antenna circuit. Objective
performance measurements are typically not to be applied
to such designs. DO a web search for 'Bravo Audio
reviews'.
On 2025-02-20 15:36, Jerry Lee
Marcel via groups.io wrote:
Le 20/02/2025 à 15:27, Carlo
a écrit?:
On Thu, Feb 20, 2025 at 05:40 AM, Andy I wrote:
There is some feedback from the audio
signal into the heater voltage. Was that
intentional?? Or just an undesirable side-effect?? I
don't expect it would have very much effect on the
heater's temperature (and from there to the triode's
characteristics), but it looks undesirable to me.
Should there be filtering?
Sorry, are you asking whether the audio signal
feedback into the heater voltage comes from a design
intentional choice ? Actually I don't know since I
took it from the schematic of a commercial audio
amplifier (Bravo Ocean).
It's extremely unlikely.
Heater temperature varies extremely slowly compared to
audio signals.
It could result in distortion at very very low
frequencies, definitely out of the audio band.
Now this design is weird from the start. Choosing to power
a tube circuit from 24VDC is a major flaw, unless the goal
is to create distortion.
--
OOO - Own Opinions only If something is true: * as far as
we know - it's science *for certain - it's mathematics
*unquestionably - it's religion
--
OOO - Own Opinions only If something is true: * as far as we
know - it's science *for certain - it's mathematics
*unquestionably - it's religion
--
OOO - Own Opinions only
If something is true:
* as far as we know - it's science
*for certain - it's mathematics
*unquestionably - it's religion
|
Re: 3 Phase Voltage Sense model
ALL the .ASCs report 'Can't find model
LMV324', and there is no symbol in the schematic, just a blank
space. The readme.txt file doesn't mention the LMV324. I've just
now downloaded the ZIP and checked again. Same error message.
John,
?
Do you need to include LMV324 symbol and model, isn't that
taken care of when you generate the net list?? As mentioned in
the readme.txt file the 3Phase_Voltage_Sense_Circuit.asc has the
LMV324 and it simulates properly.? The goal was to create a
model of the circuit and then use that model with input voltages
(3Phase_Voltage_Sense_Test.asc).
?
Larry
--
OOO - Own Opinions only
If something is true:
* as far as we know - it's science
*for certain - it's mathematics
*unquestionably - it's religion
|
Re: Weird results DC operating point for Tube amplifier
Le 20/02/2025 à 18:31, John Woodgate a
écrit?:
There really isn't any point in criticizing
the design.
Maybe there is. The OP is probably on a wild
goose chase.
It's deliberately weird. An LM386 makes a
good headphone amplifier, but it might be difficult to sell
for $130.
On 2025-02-20 17:17, Jerry Lee Marcel
via groups.io wrote:
Yes, it's a circuit that works, somehow. However there are
many more efficient and stable ways to bias a MOSFET and to
provide voltage to a 6V heater from a 24V source.
BTW, source-follower it is not. It's actually a common-source
stage.
A source fiollower has about unity gain, this common-source
has about 15dB gain.
Le 20/02/2025 à 17:00, Carlo a
écrit?:
On Thu, Feb 20, 2025 at 07:36 AM, Jerry Lee Marcel wrote:
Now this design is weird from the start.
Choosing to power a tube circuit from 24VDC is a major
flaw, unless the goal is to create distortion.
I'm not an expert. However I believe the LM317 in "current
source" configuration actually fulfills two functions. First
it is employed to bias the source of the IRF510 mosfet in
common drain (aka source follower) configuration. Second
such a constant current is employed to drive the 12AU7
triode heater.
--
OOO - Own Opinions only If something is true: * as far as we
know - it's science *for certain - it's mathematics
*unquestionably - it's religion
|
Re: Weird results DC operating point for Tube amplifier
Le 20/02/2025 à 17:33, John Woodgate a
écrit?:
Yes, easily.
Yes what?
Consider, for example, these two amplifiers:
a) Output power 30 W, THD 0.1%
b) Output power 35 W, THD 5.2%
Which would you buy?
I don't care, but Bravo Audio seems to care about
figures that are long accepted as performance indicators.
They are the same amplifier. Similar games
are played with signal-to-noise ratio and frequency response.
On 2025-02-20 16:05, Jerry Lee Marcel
via groups.io wrote:
Still they are publishing specifications that pertain to
typical audio performance, such as frequency response and THD.
They can't combine both quirkiness and normality.
Or can they?
Typical "I designed it that way because I could".
IMO it justifies disdain.
Le 20/02/2025 à 16:55, John
Woodgate a écrit?:
I suspect that the weirdness is
intentional. Weird designs have existed from? the earliest
day of DIY radio receivers, before 'electronics'? was in
the dictionary. I recall a report of a circuit that had
the 2 V lead-acid cell apparently in series with the
antenna circuit. Objective performance measurements are
typically not to be applied to such designs. DO a web
search for 'Bravo Audio reviews'.
On 2025-02-20 15:36, Jerry Lee
Marcel via groups.io wrote:
Le 20/02/2025 à 15:27, Carlo a
écrit?:
On Thu, Feb 20, 2025 at 05:40 AM, Andy I wrote:
There is some feedback from the audio signal
into the heater voltage. Was that intentional?? Or
just an undesirable side-effect?? I don't expect it
would have very much effect on the heater's
temperature (and from there to the triode's
characteristics), but it looks undesirable to me.
Should there be filtering?
Sorry, are you asking whether the audio signal feedback
into the heater voltage comes from a design intentional
choice ? Actually I don't know since I took it from the
schematic of a commercial audio amplifier (Bravo Ocean).
It's extremely unlikely.
Heater temperature varies extremely slowly compared to audio
signals.
It could result in distortion at very very low frequencies,
definitely out of the audio band.
Now this design is weird from the start. Choosing to power a
tube circuit from 24VDC is a major flaw, unless the goal is
to create distortion.
--
OOO - Own Opinions only If something is true: * as far as we
know - it's science *for certain - it's mathematics
*unquestionably - it's religion
--
OOO - Own Opinions only If something is true: * as far as we
know - it's science *for certain - it's mathematics
*unquestionably - it's religion
|
Tony Casey
John Woodgate