Hi folks,
I have been searching for an answer to the following question:
What is the purpose of a cathode bypass capacitor in a White Cathode Follower buffer?
My rather limited understanding of tube-land would indicate that a cathode bypass capacitor is employed when the is a need for gain augmentation and/or output impedance reduction.
If I remove the cathode bypass capacitor in the following circuit, would I need to pay attention to any other component values?
My goal is to mostly drive low impedance cans.
Specifically, a 38Ω load (Sivga Luan pair of headphones).
My higher impedance cans will be driven by my other amps, so I don't really need for this Morgan Jones mini-tube headphone amplifier to be compatible with cans above 60Ω.
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Anyway, back to the original questions:
A. If I remove C3 completely, do I need to consider altering any other values in the circuit in order to not upset the balance of the WFC?
B. What's the 'mystery magic' of having a cathode bypass capacitor in what's essentially a unity gain buffer?
With respect,
Nick.
I have been searching for an answer to the following question:
What is the purpose of a cathode bypass capacitor in a White Cathode Follower buffer?
My rather limited understanding of tube-land would indicate that a cathode bypass capacitor is employed when the is a need for gain augmentation and/or output impedance reduction.
If I remove the cathode bypass capacitor in the following circuit, would I need to pay attention to any other component values?
My goal is to mostly drive low impedance cans.
Specifically, a 38Ω load (Sivga Luan pair of headphones).
My higher impedance cans will be driven by my other amps, so I don't really need for this Morgan Jones mini-tube headphone amplifier to be compatible with cans above 60Ω.
------------
Anyway, back to the original questions:
A. If I remove C3 completely, do I need to consider altering any other values in the circuit in order to not upset the balance of the WFC?
B. What's the 'mystery magic' of having a cathode bypass capacitor in what's essentially a unity gain buffer?
With respect,
Nick.
The "bottom" half of V2 is fed current-sensing positive feedback via C2, so its contribution to total output will be changed if cathode degenerated. Into a high impedance load this would amount to R5 in series with reciprocal of Gm (maybe 100R-ish) instead of just the 100R-ish. Into an almost dead short 38R, who knows? LTspice could probably guide you, but why do it?
All good fortune,
Chris
All good fortune,
Chris
All grids are in danger here !
They should all be protected with a 1n4148 or similar diode between grid and cathode to protect dangerous
voltage occuring at power on.
R4 is quite close to the reciprocal of Chris's estimate of the transconductance of the bottom triode. The only reason I could think of why this is so, is that the output triodes are supposed to work in push-pull. In that case, R4 has to be increased to 390 ohm when C3 is removed.
Or, one could say that both halves operate in parallel. When I asked "Why do it?" I really meant "Why try to drive 38R with a button base receiving valve intended for small signal operation?" Just seems Quixotic.
All good fortune,
Chris
All good fortune,
Chris
Hey, I've been kicking this circuit around a bit myself, the original MJ circuit is not really suitable for the low-Z headphones. Check the article on Headwize, they explore options to lower the output impedance. I also floated this question over on the Headphone sub and got someone to model an interesting version using 6N6P as outputs. Higher current drives 35R easily.
There's also a threat to your equipment: if one of those output caps fails, you may get a large DC voltage on the headphones. There are ways to prevent this (DC servo ?) but I don't know how to implement. Maybe someone on this thread is familiar?
w
There's also a threat to your equipment: if one of those output caps fails, you may get a large DC voltage on the headphones. There are ways to prevent this (DC servo ?) but I don't know how to implement. Maybe someone on this thread is familiar?
w
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One issue I have with the original circuit is the fact that P1 and the input source are part of the negative feedback loop. You will have different nFB levels depending on P1 position.
Hi, what is an app you use for Spice simulation? I use LTSpice but your is something else...
You'll have to ask @lineup he was kind enough to whip up a schematic in Post #16 of This thread over on Headphone Systems.
Thanks for the replies folks.
A lot of new info to digest.
By the way. have used the design as seen in the schematic with low impedance cans for quite a while and it performs pretty well.
At least at the low levels I'm using it. It's a high gain design, even with the feedback applied and sounds surprisingly good.
Anyway, back to the bypass cap issue.
I want to remove it mostly to save some space.
It takes a 2200uF there (or 2 x 1000uF).
Also, I generally prefer the sound of un-bypassed stages.
So, if I understand correctly, it can be bypassed.
In fact I quite like its effect on lower impedance headphones (boost the bass which I prefer).
So, would you recommend getting rid of it?
Also, is it required using a higher R sensing at the top if I get rid of it?
Think I read something similar in Broskie's writing somewhere.
Actually, using formulas by Broskie et al was considering lowering the R sensing on top to a value of 100Ω.
I'll use it to drive almost exclusively Sivga Luan headphones(38Ω) and I get 80-100Ω results for 40Ω load and 2600Ω R internal, 0.012 gm (for 6N23P).
A lot of new info to digest.
By the way. have used the design as seen in the schematic with low impedance cans for quite a while and it performs pretty well.
At least at the low levels I'm using it. It's a high gain design, even with the feedback applied and sounds surprisingly good.
Anyway, back to the bypass cap issue.
I want to remove it mostly to save some space.
It takes a 2200uF there (or 2 x 1000uF).
Also, I generally prefer the sound of un-bypassed stages.
So, if I understand correctly, it can be bypassed.
Merlin, I'm totally fine with slightly higher output impedance.
In fact I quite like its effect on lower impedance headphones (boost the bass which I prefer).
So, would you recommend getting rid of it?
Also, is it required using a higher R sensing at the top if I get rid of it?
Think I read something similar in Broskie's writing somewhere.
Actually, using formulas by Broskie et al was considering lowering the R sensing on top to a value of 100Ω.
I'll use it to drive almost exclusively Sivga Luan headphones(38Ω) and I get 80-100Ω results for 40Ω load and 2600Ω R internal, 0.012 gm (for 6N23P).