A Tube Mic

[Guest]

[quote author="Sonic"]Capacity of a cap with a membrane 25мм-80-90pF;
Cap with a membrane 32мм-120-135pF.
Data are resulted approximately as especially exact device at hand is not present.
Gaugings of capacity of a cap on LOMO MKE-271 have shown 35pF, but the resistor on shutter FET KP303 by face value 560M there is used.[/quote]

In such case differences for 1 Giga Ohm and 100 Mega Ohm will be inaudible. Since usually output from a first FET is taken from it's destination input capacitance will be much higher because of Miller effect; that means even lower impedance. Load of a capsule is mostly capacitive.
In my case ( http://www.groupdiy.com/index.php?topic=260576#260576 )a source of a FET is bootstaped by NFB while it's destination is loaded on a small dynamic resistance of B-E junction of PNP transistor so an effective capacitance is much less that means higher level of signal on a gate that leads to better S/N ratio. Again, it is an example how to kill many ducks by a single bullet.
Something similar happens in tube mics when a capsule is connected to a cathode follower so load capacitance is very low, but in such case resistances of resistors are more critical.

 

[Sonic]

Thanks for the link. Your method of connection of a cap is a little bit unusual to me. In the transistor designs I use a method similar RODE NT2, or MXL 603. If to connect the loading resistor by such principle, that, actually, resistance of the loading resistor considerably increases.

 

[Guest]

[quote author="Sonic"]Thanks for the link. Your method of connection of a cap is a little bit unusual to me. In the transistor designs I use a method similar RODE NT2, or MXL 603. If to connect the loading resistor by such principle, that, actually, resistance of the loading resistor considerably increases.[/quote]

I used 1GOhm resistors, they are more than adequate for audio frequencies (i.e. 500 MOhm if to consider them paralleled).

 

[Guest]

[quote author="Sorr"]The subject is about a Tube Microphone, not 2SK series fets and crappy Rodes

[/quote]

Since input capacitance of a tube cathode follower is relatively low resistors should be calculated depending on capsule's capacitance and desired low frequency response.

 

[Sorr]

Marik and Wavebourn, thank you for your replies.
I am using the Peluso CEK-12 capsule with a 12AY7.
I will check the capsules capacitance.
Sorr

 

[SUPERMAGOO]

which would be the transformers that could be used for this project? sowter if it can be better because I have to buy at these days

 

[rodabod]

which would be the transformers that could be used for this project? sowter if it can be better because I have to buy at these days

As was mentioned, a ratio between 5:1 and 10:1 should work ok. Sowter have plenty of mic input models which you can flip around. I'd maybe go for one of the ones that is stated to handle +12dBu. Or you could go for their very fancy valve mic output transformer.

Rowan, out of interest, you loaded down the HF at the output with that 500pF cap; did you select this by calculating the value using Rp, or did you try by ear or measurement? I'd like to try this with some other designs that use bright capsules...

Also, does the addition of the 500pF cap not increase distortion somewhat? The valve output is going to see 1K at higher frequencies.

Cheers,

Roddy

 

[SUPERMAGOO]

This would be a good option?

http://www.sowter.co.uk/specs/9995.htm

 

[analag]

[quote author=rodabod] Rowan, out of interest, you loaded down the HF at the output with that 500pF cap; did you select this by calculating the value using Rp, or did you try by ear or measurement? I'd like to try this with some other designs that use bright capsules...

Also, does the addition of the 500pF cap not increase distortion somewhat? The valve output is going to see 1K at higher frequencies.

Cheers,

Roddy
[/quote]

I saw caps connected this way in some of the old designs, I tried it and it helps to tame the frizziness associated with some of these capsules we strip from the cheapo mics.

analag

 

[rodabod]

I saw caps connected this way in some of the old designs, I tried it and it helps to tame the frizziness associated with some of these capsules we strip from the cheapo mics.

Thanks. Does anyone see what advantages/disadvantages there are of using this method compared to bypassing Ra with a cap to reduce HF gain?

 

[3nity]

I'm getting near building this mic just waiting for some things to show up!
This question goes out to Analag or Mattias...Resistors are 1/4 watt?
I did ordered a 6AU6 tube any objection? output transformer will be a 1:4 Cinemag.
Thanks.

 

[Gus]

Power in a DC circuit is voltage x current or I^2 R or V^2/R

Figure out the max voltage drop across each resistor in question, calculate the current then calculate the power pick a resistor with a higher rating.

http://en.wikipedia.org/wiki/Joule%27s_laws  

One 6au6?  Good luck finding a low noise one.

 

[3nity]

should i buy more 6AU6?
thanks

 

[MagnetoSound]

I saw caps connected this way in some of the old designs, I tried it and it helps to tame the frizziness associated with some of these capsules we strip from the cheapo mics.

Thanks. Does anyone see what advantages/disadvantages there are of using this method compared to bypassing Ra with a cap to reduce HF gain?

Isn't it the same thing, when Ra has 1uF behind it?

 

[Rossi]

output transformer will be a 1:4 Cinemag.

That may be a little low for a plate out circuit. Try it, if you already own that tranny, but if you're about to buy one, better get a somewhat higher step down. Analag's 5:1 tranny is already on the low side.

 

[rodabod]

I saw caps connected this way in some of the old designs, I tried it and it helps to tame the frizziness associated with some of these capsules we strip from the cheapo mics.

Thanks. Does anyone see what advantages/disadvantages there are of using this method compared to bypassing Ra with a cap to reduce HF gain?

Isn't it the same thing, when Ra has 1uF behind it?

It's my understanding that the effect is actually the same, but because it sees a path to ground either directly, or in the case of bypassing Ra, via B+. I think I see what you mean about the path through the 1uF though; it's effect would also be the same, but I have a feeling it would also be the same without that 1uF cap there.

Bypassing Ra means we can draw dynamic curves for different frequencies, but I'm not sure how something similar would apply to tying that cap from the plate straight to ground. At the moment, the only way I can see of re-calculating the gain is to consider the loading of that cap on Rp. Maybe there is a better way.

Roddy

 

[3nity]

Thanks Rossi.
I own that transformer already..ièm gonna try it if not i will buy something higher.
Thanks.

 

[rodabod]

That may be a little low for a plate out circuit. Try it, if you already own that tranny, but if you're about to buy one, better get a somewhat higher step down. Analag's 5:1 tranny is already on the low side.

Agreed, but it might not be too bad:

http://www.tubes.mynetcologne.de/roehren/daten/as_triode_overview_e.pdf

Depends on if things change significantly at lower plate currents. It's slightly more tricky to see in that region:

http://www.tubes.mynetcologne.de/roehren/daten/ef94pentode_as_triode.gif

 

[Gus]

It is not all about the transformer ratio there are other things to take note of.

 

[Rossi]

Yes, of course. Above all, the transformer inductance will be a determining factor, if the tranny is usable.