transformerless tube mic

[penquista]

i was reading about the s*e ele*ctr*oni*cs gemini and wondering if there have been similar transformerless designs before. the schematics i have seen were all with transformers.
any ideas?
pros and cons of designs of that kind?

c.

 

[tk@halmi]

analog has posted one or two. Search for his posts.

 

[12afael]

check this one too.
http://www.geocities.com/d_zeman/Projects/Micpream.htm

saludos

 

[Gus]

penquista

means a tube microphone without a transformer output I believe.

Have not seen one I would guess the output is a 12au7 CF KIND of like a schoeps PNP stage works

Maybe the front end is a 12ax7 gain stage to a cathodyne phase inverter to the push pull CFs output but that would have to much gain.

Maybe the front end is a cathodyne PI outputs from the plate and cathode to CFs 12au7 PP output. This would have about a gain of 1 like a lot of microphones

The above is a guess from the tubes used
http://www.seelectronics.com/Gemini.html

 

[Gus]

OK

I drew up a circuit. I don't have china 12a_7 curves so some of the resistor values(3) are first tries.

 

[API]

Calrec made one in the sixties.
See:
http://www.saturn-sound.com/Curio's/calrec_the_valve_years.htm

 

[alk509]

Hi Gus,

What would the output Z be on something like that? :?

Peace,
Al.

 

[gyraf]

[quote author="API"]Calrec made one in the sixties.
See:
http://www.saturn-sound.com/Curio's/calrec_the_valve_years.htm [/quote]

Most links and images on that page are missing.. :?

Jakob E.

 

[API]

Most links and images on that page are missing..

Yes, I noticed that, maybe he is updating or something.

Dennis

 

[rafafredd]

I drew up a circuit. I don't have china 12a_7 curves so some of the resistor values(3) are first tries.

Will you post it?

 

[Gus]

I would need to scan it.

This is a GUESS what could I do with the two tubes ?

It was a fast drawing

Lot of first guesses for china tubes I looked at curves of NOS ones

200V B+, 5.6 to 6.3 fils raised above ground would need to do some tests.

front end 1/2 12ax7 47K plate R
1.2K from cathode to another 47K to ground, 100uf or what ever bypass cap across the 1.2K (Look at a fender deluxe) Match the 47K if you want, the cap should drop the 1.2K out for AC balance

200meg to a 1gig, grid to 1.2K 47K node cathode leg

one side of the capsule to the grid other side ground.

film caps from plate and cathode nodes can be smaller because the 1 meg appears bigger than 1 meg to the first stage adjust to taste 1/2piRC

one to a cathode follower set about 3ma 1/2 12au7 470ohm cathode to a 33K to ground 1 meg from the 33k 470 ohm node to grid
cap couple out from node with maybe a 1meg pull down at the output of the cap, could be electro. Should be formed even with 48V phantom on.

same CF circuit using the other 1/2 of the tube hooked up to the cathode leg.

The front is a phase splitter like a shoeps fet but the cool thing it charges the capsule cardiod or omni. Look at AKG circuits.

200V seems to be common with china tube power supplies
looked at the 12ax7 12au7 curves and tossed some standard values at it .

3 ma because I like drive

I don't care to much for the DC points are because the signal might only be 1 or so volts

1st Phase splitter and capsule charge voltage 2nd CFs with some drive current. Maybe I am (whatever) but I don't think in terms of small signal output Z more in terms of advailbe current and voltage.

I am looking for at least 1V of input headroom and some drive from the push pull CF outputs.

BJT EF outputs would make more sense to me or even MOSFET SFs

Lets have some FUN again any ideas?

I have not built it.

 

[sismofyt]

[quote author="Gus"]any ideas?[/quote]

yeah, forget about it. designing a decent cf that can actually drive a lowimpedance mic input will be very hard. you may be able to do it on paper, but i really wouldn't wanna listen to it. ever

ECC82s are sucky cfs, even with that bias arrangement

if you want to have a tube but no output transformer, i'd much rather use transistors in the output stage .. FETS whatever .. very easy and probably quite nice sounding

cfs are really really hard to make sound good and i try to avoid them where ever i can, even when i have no restrictions regarding tube type, bias and B+

but what's wrong with transformers all of a sudden?

 

[Gus]

The first post is asked about transformerless all tube microphones pros and cons.

the
http://www.seelectronics.com/Gemini.html
used as an example uses a 12ax and a 12au

My post are more about what did they design?
You have two tubes no output transformer and need to make a transformerless tube microphone. How could you do it.


I often like simple 1 gain device and a transformer circuits If you read my post I did post about EFs and SFs

 

[sismofyt]

they probably did it like you decribed .. din't CAD (?) make a similar mic once? that big blue thing?

i can understand the idea.. sometimes it's nice not to have transformers in the signalpath, but trying to drive a typical (transformer coupled :roll: ) low impedance mic input with a transformerless tube mic is a bit silly, allthough i have no idea how that SE one sounds

there's also the neuheiser M147 / M149 way to do it..

my own ldc put's out an unbalanced linelevel signal, no transformers (or ECC8X :wink: ) anywhere, only two tubes and two caps into the converter .. pretty transparent or whatever you call it

i think that maglight mic is something similar?

that's pretty smart me thinks

allthough what to do with all them nice pres now? :green:

 

[Gus]

neuheiser, that is funny

 

[Larrchild]

neuheiser.. *holds ribs* :grin: :grin: :grin:

 

[analag]

This design is not hard to do...at all. If I was feeling as generous as I normally do, I would pop one up for all you's viewing pleasure. Look at a the old Williamson tube power amp schemo and see if you can't find the first clue, the rest should fall into place.

analag

 

[sismofyt]

you need to get laid / high or sumthing .. cheer the **** up dude

you may not find it hard to brew something up that'll satisfy your sim love, but in real life it's not easy to design a great sounding plate + cf follower for a transformerless mic. try it, you may be surprised :wink:

and i don't see what it has to with williamson's phasesplitter. the world _did_ exist before that design, ya know

here's a hug from all of us to you ..

:green:

 

[analag]

I was just about to post it...only to see this ********. F U too.
You think I need to **** with this or any other forum...watch this.

analag

 

[ioaudio]

[quote author="analag"]If I was feeling as generous as I normally do, I would pop one up for all you's viewing pleasure.[/quote]

come on analag... dont take it too serious...

nice bears, sismofyt