tube mic pre with small-ratio input transformer

is there any known, good, interesting, approved, excellent-sounding circuit for a mic input transformer with the ratio 1:3 ?

-max

Hey i/o . I think You can take anyone of those we find around this forum. Look at NY-dave¨MILA for ex.I´ve build his earlier nonfeedback line amp with Tx 1:4 and its well sounding and the noise is not bothering me at all. The question is actually how much level you feed the Tx. With modern mic´s (read cond.)I dont think youll expect any trouble with noise. But of course if the recording situation is lute in a church 5 feet away of the mikes,well then I think you have to use another Input Tx. Cheers Bo

[quote author="ioaudio"]is there any known, good, interesting, approved, excellent-sounding circuit for a mic input transformer with the ratio 1:3 ?

-max[/quote]

Hi Max, I saw someone selling a bunch of certain 1:3 transformers on Ebay recently, and can confirm that they are probably not 1:3. Just in case you were thinking what I was thinking!

Roddy

I'm not sure there are many pre-existing designs, but NY Dave's MILA has good possibilities.

A 1:3 transformer has a 1:9 impedance ratio, so a 150 ohm source would have an impedance on the secondary of 1350, assuming a perfect transformer. In order for the electronics to add only 1dB of noise to the inherent noise of the microphone, it would have to have a total noise resistance of about 1350 / 4, or about 337 ohms. Assuming a bypassed cathode resistor as in Dave's design (or a negative bias supply feeding the grid), and ignoring the plate resistor for the moment, the main noise source would be the tube's own electronic noise. This is equal to 2.5 / gm, where gm is the transconductance in Siemenses. Turning the equation inside out, the gm required for an equivalent noise resistance of 337 ohms is 2.5 / 337 = .0074S, or 7400uS. That's high but not unattainable.

In reality, primary and secondary windings will have DC resistances; the secondary winding resistance gets added to 1350, and the primary winding times 9 gets added to 1350, so the actual impedance at the secondary is higher. That means the tube can be just a little bit noisier, which in practice would be perhaps the noise from the plate resistor. So if you design a tube stage with a bypassed cathode and operating conditions set to yield 7400uS (umho) of transconductance, the noise performance should be good.

Peace,
Paul

whay the 1:3?
did a box of them land in your driveway last night?

hey, thanks for suggestions!

paul, thank you very much for explaining the required math - that clears up a lot to me.
the transformers are ripped from a emt portable recorder (solid state) and they are huge, ready for signals up to 20dbU. so i think it should be possible to build a preamp without a pad before the input transformer.

-max

here´s the transformer guts, since i´m researching for building transformers i just couldnt resist :grin:
and what a nice surprise

ui30 core humbucking windings hmmm i am tempted to rewind it for a mic out (bv8, u47 transformer)
on the top the original wound in yellow tape, on the left side a ll1528 for comparison

I used transistor in front of a tube in such case.
Unfortunately I can't post a schemo since my server is down. I put some ice packs on top to prevent it from shutting down because of hot temperature here in California but a moisture condensate damaged my motherboard. I am going to rebuild it on PC with Linux, but I'm not sure if Linux can mount Sun's SCSI disk... It is UFS, but I've never tried before...

Wow. Triple bobbin on a dual leg core, should have good high end.

emt no corners cut
it seems they are prototypes - the laminations where hold in place only by the bobbin, the mumetal-shielding and some taping around the free ends of the core.
these are ui30 cores, according to http://www.waasner.de and i realize that the bv8 looks different in size ?

yes, you do not clamp Ni lams tight, as they expand with B field and if clamped, distort mechanically, which = electronic distortion.
Ask Ollie if you think I'm trash talkin.
cj

i wasnt talking with oliver, but with andreas grosser (on the phone since he doesnt care too much for internet-fora and stuff) about the bv8 and the different clones available - he gets his replacements made by haufe, but it seems getting more difficult to convince them to do small quantities...

anyway he also used olivers transformers and we were talking about the alloy, and if it contains cobalt or not, how to stack the laminations.
andreas is great and has a lot of knowledge which he was kind enough to share with me.
however i´m still trying to find information about the winding structure and turns count of a bv8 - it would be great to have some information like copper resistance and inductance of the coils - maybe somebody has the knowledge and is willing to share?

-max

i just realized that i went way off topic - sorry

wavebourn - i can upload the schematics - on twin-x´s server, or you could make your own account. thanks twin-x for hosting :thumb:

wavebourn did you really pack ice on your servers :shock:

-max

edit:wrong info

[quote author="ioaudio"]

wavebourn - i can upload the schematics - on twin-x´s server, or you could make your own account. thanks twin-x for hosting :thumb:
[/quote]

Friend of mine have Sun Uslta 60; he promiced to connect my HD there and tar the content on DVD, after that it will be accessible so I can share a schemo with you.

wavebourn did you really pack ice on your servers :shock:

Click to expand...

Yes, I did such mistake and found that overcooling is worse than overheating! :green: