BV08 transformer capacitance, comparable to mic cable capacitance?

[ELS]

So it seems rather complicated. Maybe use a cathode follower? As far as I understand it would make things a lot simpler when you want to avoid a transformer but want to use tubes.

Well actually it's that I don't have any transformer that fits in my mic body, it's only 30mm ID, I previously used a gain-stage into a cathode follower which worked well, but I wanted better x)
I wanted to use a CF-Gain stage circuit, removing the need for very high resistance resistors. but that would give me a high output impedance so I was worried about high end roll off.
I do have one transformer that fits but it's not a very good one, frequency response is rated @ 160-6.3khz
distortion is also terrible:
100-200Hz 4%THD
200-400Hz 3%,
>400Hz 2%
It doesn't say at what drive levels.

I do like Roy Brown's records so maybe the 6,3khz response will suffice

 

[ELS]

Like this, the input impedance is around 1Gigaohm in theory.
But yeah without the OPT the cable capacitance will kill the high end.

 

[volker]

An anode follower using a 12AX7 is never going to work. The output impedance is way too high, the transformer for this scenario that works full bandwidth doesn't exist.

 

[Khron]

Would make a lot more sense to hang the transformer straight off that first triode's cathode (and go with some single triode / triode-strapped pentode, instead of a most-likely-noisy 12AX7).

Especially with condensers, one is rarely lacking for level, i think...

 

[ELS]

An anode follower using a 12AX7 is never going to work. The output impedance is way too high, the transformer for this scenario that works full bandwidth doesn't exist.

where do you see an anode follower?
and an anode follower setup works on any tube, it lowers the output impedance.

Would make a lot more sense to hang the transformer straight off that first triode's cathode (and go with some single triode / triode-strapped pentode, instead of a most-likely-noisy 12AX7).

Especially with condensers, one is rarely lacking for level, i think...

Well I used a 12AX7 in the previous circuit for this mic and noise wasn't a problem, but I used a FET source follower to drive the first gain stage since I didn't have high resistance resistors. so it had a low impedance input of the capsule's signal.

 

[Khron]

How much gain DO you need out of this circuit, though?

 

[volker]

where do you see an anode follower?
and an anode follower setup works on any tube, it lowers the output impedance.

Mixed up terminology, point still stands.

 

[Lampie519]

An anode follower using a 12AX7 is never going to work. The output impedance is way too high, the transformer for this scenario that works full bandwidth doesn't exist.

It does exist but you will have to change topology (going RF).
I am not sure if this would be the solution here though.
Maybe i should give it a try once just for fun…

 

[ELS]

Mixed up terminology, point still stands.

NO, IT DOES NOT. 12AX7's work fine as cathode followers, there isn't a single tube out there that doesn't, unless
you're driving the plate instead of the grid, so you have inverse Gm <1. this janky config wouldn't work great with a CF.

I just asked for the primary capacitance of the BV08 transformer, what don't you understand about the question?
I didn't ask for ideas for the circuit I posted, you just insisted that I post details about my entire rig setup so you can give a furfiling answer or something
the XY theory or whatever is such BS, it's like saying "oh it helps people answer the question because they can give an unrelated answer that they DO know instead of not gaining that 1 extra reaction score because they just don't have an answer.
The best answer was the first reply, which noted that the capacitance should be negligible, which I didn't want to agree because I have no idea still, what the capacitance is. If I did, I could clearly see if it is or isn't negligible.
Just saying "Don't worry about it" isn't much help.

 

[soapfoot]

I just asked for the primary capacitance of the BV08 transformer, what don't you understand about the question?

Speaking only for myself--

I'm struggling to understand because "what is the primary capacitance" reads a bit like "what is the length of a piece of string?" In other words, it feels like some necessary information is missing.

As several other posters mentioned, capacitance can exist between the primary winding and numerous other conductors (secondary, ground/core, etc) and it can also exist between individual turns, and it can also exist between individual layers.

I haven't yet seen where you've specified which of these you're looking to ascertain

 

[volker]

NO, IT DOES NOT. 12AX7's work fine as cathode followers, there isn't a single tube out there that doesn't, unless
you're driving the plate instead of the grid, so you have inverse Gm <1. this janky config wouldn't work great with a CF.

I just asked for the primary capacitance of the BV08 transformer, what don't you understand about the question?
I didn't ask for ideas for the circuit I posted, you just insisted that I post details about my entire rig setup so you can give a furfiling answer or something
the XY theory or whatever is such BS, it's like saying "oh it helps people answer the question because they can give an unrelated answer that they DO know instead of not gaining that 1 extra reaction score because they just don't have an answer.
The best answer was the first reply, which noted that the capacitance should be negligible, which I didn't want to agree because I have no idea still, what the capacitance is. If I did, I could clearly see if it is or isn't negligible.
Just saying "Don't worry about it" isn't much help.

Let me try one last time: I was concerned with the second triode in your schematic. As I was talking about driving the transformer, I thought that should have been obvious. I don't have an answer to your question, but it doesn't matter, because your topology is flawed to begin with. Neither a BV08, nor the other one you mentioned, or any other part you can find, will give a satisfactory performance if used that way with this tube.

 

[abbey road d enfer]

The OP wants to output signal directly from the tube to 15 ft of cable, which means about 750pF.
The problem is that the impedance that must be considered is the complete connection impedance, all in parallels, including the receiver's impedance and the cable's capacitance.
The impedance from the tube is either very high, if coming from the anode, or relatively low, if coming from the cathode.
The receiver's impedance is whatever the OP chooses to be.
One thing is using the cathode as output; in order to maintain a correct BW - let's say -3dB at 20kHz, the impedance resulting from paralleling the mic impedance and the receiver impedance in parallels must be less than 1.5kohm.
It can be done in many ways, like e.g. using the tube as a cath-follower, where the impedance would be close to that, then the receiver impedance can ve very high. The problem is that a cath-follower has limited gain.
Another solution is to use the plate as output, but then the impedance would be about 50 kohm, so an additional load uwt be added, of about 1.5kohm, which would result in decreasing gain by about 30 dB.
There are examples where a dual -triode is used, with the 1st half in common-cath mode, providing gain, and the second half in cath-follower mode, providing low impedance.
I would think it's the best choice for an all-tube xfmrless mic.
Anotehre option may be using a SS buffer after the tube.

 

[abbey road d enfer]

An anode follower using a 12AX7 is never going to work.

"anode-follower" ? Doesn't exist.

 

[Khron]

the XY theory or whatever is such BS, it's like saying "oh it helps people answer the question because they can give an unrelated answer that they DO know instead of not gaining that 1 extra reaction score because they just don't have an answer.
The best answer was the first reply, which noted that the capacitance should be negligible, which I didn't want to agree because I have no idea still, what the capacitance is. If I did, I could clearly see if it is or isn't negligible.
Just saying "Don't worry about it" isn't much help.

The XY problem isn't necessarily BS - it's just that sometimes people simply ask the wrong questions (or can't see the forest for the trees, as it were).

And you still haven't answered about how much actual voltage gain you need / want to get out of whatever circuit you insist on using.

Just a plain cathode follower (as in the left half of the circuit you showed in post #22 here) will give you the high input impedance you need, and a low-enough output impedance to drive just about any transformer (even a low-ratio like 2:1) to a suitable level. The resulting circuit would just not necessarily be quite as sensitive as most other condensers.

Using that full circuit (which is pretty much "back to front", at least as far as conventional wisdom goes) would still need a high-primary-inductance mid-to-high ratio step-down output transformer. That, to me, seems like a pretty convoluted hassle, if the aim is ONLY to not need a 100-1000 megohm resistor hanging off the first triode's grid.

 

[volker]

"anode-follower" ? Doesn't exist.

I don't know it it's just a Broskie term, but obviously what I wanted to write was just anode output. My bad. Let's not divert any more, OP is already a bit enraged it seems.

 

[abbey road d enfer]

View attachment 106973
Like this, the input impedance is around 1Gigaohm in theory.

Bootstrapping the grid resistor gives the correct LF response, but is not correct in terms of noise. The apparent noise with a 10Meg resistor (bootstrapped or not) is 20 dB higher than a 100 Meg, 40dB higher than a 1Gig.

 

[ELS]

Speaking only for myself--

I'm struggling to understand because "what is the primary capacitance" reads a bit like "what is the length of a piece of string?" In other words, it feels like some necessary information is missing.

As several other posters mentioned, capacitance can exist between the primary winding and numerous other conductors (secondary, ground/core, etc) and it can also exist between individual turns, and it can also exist between individual layers.

I haven't yet seen where you've specified which of these you're looking to ascertain

wouldn't "across the primary" imply that I mean the capacitance... across the primary?
The primary winding does not consist of the core, ground, or secondary winding.

The OP wants to output signal directly from the tube to 15 ft of cable, which means about 750pF.
The problem is that the impedance that must be considered is the complete connection impedance, all in parallels, including the receiver's impedance and the cable's capacitance.
The impedance from the tube is either very high, if coming from the anode, or relatively low, if coming from the cathode.
The receiver's impedance is whatever the OP chooses to be.
One thing is using the cathode as output; in order to maintain a correct BW - let's say -3dB at 20kHz, the impedance resulting from paralleling the mic impedance and the receiver impedance in parallels must be less than 1.5kohm.
It can be done in many ways, like e.g. using the tube as a cath-follower, where the impedance would be close to that, then the receiver impedance can ve very high. The problem is that a cath-follower has limited gain.
Another solution is to use the plate as output, but then the impedance would be about 50 kohm, so an additional load uwt be added, of about 1.5kohm, which would result in decreasing gain by about 30 dB.
There are examples where a dual -triode is used, with the 1st half in common-cath mode, providing gain, and the second half in cath-follower mode, providing low impedance.
I would think it's the best choice for an all-tube xfmrless mic.
Anotehre option may be using a SS buffer after the tube.

Or you can use an anode follower-ish circuit, an anode follower is a regular gain stage with unity feedback, it has comparable impedance to a CF, but the output is inverted. and the input impedance is low.

The XY problem isn't necessarily BS - it's just that sometimes people simply ask the wrong questions (or can't see the forest for the trees, as it were).

And you still haven't answered about how much actual voltage gain you need / want to get out of whatever circuit you insist on using.

Just a plain cathode follower (as in the left half of the circuit you showed in post #22 here) will give you the high input impedance you need, and a low-enough output impedance to drive just about any transformer (even a low-ratio like 2:1) to a suitable level. The resulting circuit would just not necessarily be quite as sensitive as most other condensers.

Using that full circuit (which is pretty much "back to front", at least as far as conventional wisdom goes) would still need a high-primary-inductance mid-to-high ratio step-down output transformer. That, to me, seems like a pretty convoluted hassle, if the aim is ONLY to not need a 100-1000 megohm resistor hanging off the first triode's grid.

yeah I'm not making a production model, just experimenting.
At first I thought I was the first one to think of this topology and was like "there must be a problem why nobody used a CF to bias the capsule"
few months later I came across schematics for some Sony tube condenser mics which do use a CF to bias the capsule


The reason I don't supply my demands for the design is because I don't know them yet, I want to first familiarize myself with other designs, such as the U47, that's why I asked the primary winding capacitance value.

 

[Khron]

Well then, especially if you don't yet 1000000% know WHAT you want... Why latch on so stubbornly to ONE parasitic parameter?

And i may or may not have already asked, what exactly led you to believe that the primary winding's (parallel) capacitance is THE deciding factor that limits the bandwidth, and nothing else?

 

[ELS]

Well then, especially if you don't yet 1000000% know WHAT you want... Why latch on so stubbornly to ONE parasitic parameter?

And i may or may not have already asked, what exactly led you to believe that the primary winding's (parallel) capacitance is THE deciding factor that limits the bandwidth, and nothing else?

1. because I didn't know what it was, and the other parts of the circuit I was familiar with already
2. well I don't know, and I didn't think it was the deciding factor, I just wanted to know what the capacitive load was on the tube.

 

[Khron]

Why would the capacitive load on the tube be so crucial then?

I promise i'm not being facetious or anything, just trying to get you to justify (at least to yourself) WHY you consider these details so crucial (when noone else does, or seem to do).

Some version of the M49 (M49b, by the looks of things) has a 600pF capacitor from plate to ground. You think the capacitance of the primary winding matters much there?

Point being, if wiser people than either of us assert that it's the leakage inductance that limits the high-frequency extension... What good reason would you have to insist on that difficult-to-measure (or even estimate) parameter?