Just checking to make sure I'm thinking about this correctly. Would appreciate a sanity check.
Do I have that right?
Is there any significance to the grid stopper and grid leak being equal in this configuration?
- R138 and R140 form a voltage divider for local NFB.
- C103 bootstraps the stage to work around the resulting low 47K input impedance.
Do I have that right?
Is there any significance to the grid stopper and grid leak being equal in this configuration?
R140 defines the input impedance of the stage. R138 does neg. feedback. C103 is pos. feedback, maybe to linearize the output. They often came up with unusual methods for linearization purposes. Gird stopper could have different values, but for mass production its always good to have less different values I think and maybe that worked best for the circuits needed gain goal in this application.
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Cool, thank you. C103 definitely confuses me. It's such a tiny value (100pF). It seems like this would only pass very high frequencies. If I simulate this stage in LTspice, it doesn't make any difference whatsoever, regardless of frequency or input amplitude.
100pf in combination with the 47k should make a difference to the response.
I wonder if the model for the valve simulates the grid capacitance correctly or whether it is excessive and so the 100pF seems to make no difference in simulation.
I wonder if the model for the valve simulates the grid capacitance correctly or whether it is excessive and so the 100pF seems to make no difference in simulation.
Since the cathode follows the grid C103 should not do much untill very high frequencies. How high up do you simulate?
Adding C103 shifts the -3dB point from 54kHz to 32kHz. That's the only impact I can see. No impact on distortion - 2H at -62dB, 3H at -32dB, with or without it.
Maybe it's an attempt to squash a particular oscillation?
I'm using the triode-w1_dd 12AX7A-mz2 model.
Maybe it's an attempt to squash a particular oscillation?
I'm using the triode-w1_dd 12AX7A-mz2 model.
That's my model I made in Curve Captor waaay back in 2007. As you can see, I've increased the capacitance between adjacent pins.
Code:
* 12AX7A-mz2 LTSpice model
* Rydel model (5 parameters) mean fit error: .0353101mA
* Traced by Wayne Clay on 9/10/2007 using Curve Captor v0.9.1
* from Mazda Belvu data sheet
.subckt 12AX7A-mz2 P G K
Bp P K I=
+ ((0.001140727275m)+(0.0001108016536m)*V(G,K))*uramp((90.45081147)*V(G,K)+
+ V(P,K)+(54.12297213))**1.5 * V(P,K)/(V(P,K)+(3.220362399))
Cgk G K 2.3p ; 0.7p added (1.6p)
Cgp G P 2.3p ; 0.7p added (1.6p)
Cpk P K 0.53p ; 0.2p added (0.33p)
Rpk P K 1G ; to avoid floating nodes
d3 G K dx1
.model dx1 d(is=1n rs=2k cjo=1pf N=1.5 tt=1n)
.ends 12AX7A-mz2Ah, cool. Thanks for the updated model.
Using that, the -3dB frequency is 52.6kHz without C103 and 30.5kHz with it. Still well outside the audible range, so I guess I still don't get the purpose unless it's for stability purposes.
Using that, the -3dB frequency is 52.6kHz without C103 and 30.5kHz with it. Still well outside the audible range, so I guess I still don't get the purpose unless it's for stability purposes.
Try to see the whole circuit. It may have not a good frequency performance and they would tweak it. Not unusual. You show only a small fraction of the circuit. This always must be analysed in combination with the rest of the signal path. Its like medicine. The patient is ill, but the illness could be rooted in a different part than that, which show symptoms.
- 3dB @ 32kHz.
That single pole also causes - 1 dB at 16kHz.
Do that in a preamp, and then in an amp, and then in a speaker, and you get:
-3dB @ 16kHz.
But it does not matter for me now, I can not hear 16kHz anymore.
That single pole also causes - 1 dB at 16kHz.
Do that in a preamp, and then in an amp, and then in a speaker, and you get:
-3dB @ 16kHz.
But it does not matter for me now, I can not hear 16kHz anymore.
Yes.
Analyze this as a cheesy, VERY low performance Op Amp.
IF it were infinite open loop gain, like a "perfect" OpAmp, as shown and with no signal generator added , closed loop gain "would" be about 20X= 1M/47k
But open loop gain is around 50-60X ops, so real gain will be about , say, 15-18x
Being an inverting Op Amp stage, input will be more accurately measured in mA-uA than in Volts because we are approaching a current input stage.
Input impedance will actually be R140 in parallel with R138/stage gain.
So *around* 22k or so.
No, it´s too late for that.
Values and position shown make me think of an RF proof stage instead.
Such "short the input diode" capacitors are common in Pro equipment (microphone preams, mixers, etc.) which are used in huigh* RF present environments, thing the local radio van transmitting a Baseball game live, Boxing, Baseball, etc. where they have a transmitter antenna at the top of the van.
Where did you pick that schematic from?
No, sheer chance.
The "grid leak" is part of an NFB loop, making it too low would load driving stage very much, too high would make R138 impossibly high, sounds like a workable compromise value.
The grid stopper chosen guarantees killing any RF for good.
Again, both values look like seat of the pants chosen ... by an experienced designer.
Can we look at your question in a different way.
Are you wanting to learn about this circuit, but not going to use it?
Are you going to build this stage?
What is this input stage going to be used for?
What are the signal source parameters that will be connected to the input stage?
rms voltage?
output impedance?
What is this input stage going to drive?
rms voltage needs of the next stage?
Input impedance of the next stage?
Will negative feedback be used that includes this input stage?
(where the negative feedback loop wraps around [includes] this input stage).
Are you wanting to learn about this circuit, but not going to use it?
Are you going to build this stage?
What is this input stage going to be used for?
What are the signal source parameters that will be connected to the input stage?
rms voltage?
output impedance?
What is this input stage going to drive?
rms voltage needs of the next stage?
Input impedance of the next stage?
Will negative feedback be used that includes this input stage?
(where the negative feedback loop wraps around [includes] this input stage).