20db mic pre

there are few situations where I need 10 to 20db mic pre gain [also no color needed]
Did anyone tired hooking up like a 1:7 or 1:10 mic input transformer only. for a 20db gain
would there be any problem with output impedance?

i guess there is product like this on the market http://www.realios.com/ra9033br1.html , just for an idea

A step-up transformer would probably make the source impedance too high for most preamps to load, no?

As meathands says, the reflected impedance at the secondary would be 10-20kohms.
If you intend to connect it to line inputs with 10-20k input impedance, you would lose 6dB because of the impedance-matching. Don't even think of connecting it to a standard loZ mic input.
You could DIY something, either with a 1:3-5 xfmr and a 5534, or a transformerless thingy based on a THAT 1510-12 chip.
I acknowledge the usefulness of a medium-gain mic pre, as I have made one myself; it's a tube-based single-stage push-pull with 30dB max gain.

yeah, I dont want to go to onother preamp - I need less gain not more.
how about input trx with 10k output, guess this is best worst case scenario

zmgwg said:

yeah, I dont want to go to onother preamp - I need less gain not more.
how about input trx with 10k output, guess this is best worst case scenario

Click to expand...

1:7 step up, 200R:10k, 17 dB gain in the xfmr, 6dB loss in matching, net result 11dB gain. No dynamic mic will love the 200R impedance.

hmm I see

I don't understand the question--almost any preamp easily adjusts down to 10 dB gain. So you want to design a preamp which only goes up to 20 dB?

Samuel

he wants to avoid buying a preamp if he can just use the input iron.

if you use a condenser mic with a built in preamp, then this idea might work,

modern inputs might not load down the 10K  secondary as much as the old stuff,

just grab some iron and try it, easy experiment to do, not much wiring, just the mic cable into the transformer and the transformer into the mixing board, you will need to work out the phantom power thing unless using a u47 type setup with the ext pwr supply,

you can get a 1:30 input from Tab/Tele  but it will cost about 300.

CJ said:

modern inputs might not load down the 10K  secondary as much as the old stuff,

Click to expand...

Modern or old, 10k is 10k, loads the same...

you can get a 1:30 input from Tab/Tele  but it will cost about 300.

Click to expand...

300 bucks for an experiment that's doomed from the start... 1:30 puts an 11 ohm load on the microphone; that's 20+dB loss right at the beginning.

I can understand this question if trying to do to it within the realm of antique technology, but not from a clean and modern standpoint.  The Realios product achieves it on one level, and it's obvious how to mimic it from a DIY standpoint.  A million options if looking at simple IC design. 

to further explain my question/situation: I'm aware of the fact that good preamp should provide gain in any situation, actually I have some preamps that can't go lower than 15db but if that was the case I'd probably just add a pad of somekind. I'm talking about multitracking situation where my good preamps are elsewhere and I use cheap preamp with pad and lowest gain and pad on a mic [ toms, btm snare etc.] having that gain from a quality transformer instead didn't seem like that of a crazy idea for me, but maybe it is... just spit balling here

@emrr
can you share some light

http://tritonaudio.com/index.php?sectionid=4&option=com_content&task=category&id=17&Itemid=33

Ah, I didn't realize you were hoping to run this right into a line input.

Just curious, what would you folks look at doing if you wanted to put something like this between a mic and a preamp for an extra 20dB or so? There is nothing in my limited knowledge that suggests how to do so while maintaining a mic-level output impedance.

@meathands check therecordingart's link

Ah, very cool! No schematic, though.

Also, re: your original question, the THAT1512 chip suggested by Abbey Road is a good way to go. Dantimax has very cheap and small kits for these, just check the datasheet for the proper resistor value for 20dB of gain.

therecordingart said:

http://tritonaudio.com/index.php?sectionid=4&option=com_content&task=category&id=17&Itemid=33

Click to expand...

Those sort of things never have terribly good noise specs, but if 20 dB is all you need....actually, it may not have the headroom if 20 is all you need. 

Meathands said:

Ah, I didn't realize you were hoping to run this right into a line input.

Just curious, what would you folks look at doing if you wanted to put something like this between a mic and a preamp for an extra 20dB or so? There is nothing in my limited knowledge that suggests how to do so while maintaining a mic-level output impedance.

Click to expand...

I own a Fethead and out of curiosity I tried to verify their statement that noise was immeasurable. In fact I found out that the EIN with a 200R source is about -118.5dBu (22kHz BW), which is not the best performance in absolute, but quite honorable for a 20dB gain stage. What surprised me the most was that noise would increase a lot with source impedance, suggesting that there was significant noise current, which FET's should not have.
Some discussions suggested that large geometry FET's such as the VLN (1nV/sqrtHz) 2SK170 exhibit significant noise current. The 2SK170 data sheet does not suggest that. Another suggested the circuitry used in the Fethead for biasing the FETs would be the culprit.
Not having the schemo makes all this subject to caution.
In my particular case, I use the Fethead in combination with a chinese ribbon mic that has an impedance of 2.2k (!).
Using the mic directly with any of my regular preamps (Focusrite 430MkII, Avalon 737, UA LA610, LA Classic Channel) results in a less than stellar performance, due to the loading of the mic, which, in addition to losing about 6dB right at the source, chokes the high-mid and treble response, the worse being the buzz due to this rather high impedance signal travelling along 20 meters of cable.
When used in conjunction with the Fethead, the upper response is restaured, and the buzz is gone.
Using Fethead with less idiotic microphones (SM7B, RE20) does not bring significant difference.
My conclusion is that, although the specs are not clearly substantiated with actual measurements, this product does help in some cases.
Two other products were discussed at the time:
The Cloudlifter
http://www.proaudioheaven.com/fr_home.cfm
and this
http://www.fullcompass.com/product/232756.html

what kind of mic are we talking about and what kind of mixing board?

In the late 50's-mid 60's I owned a pair of U47's branded for Telefunken (they were purchased in Germany for me. For 250 ohm impedance inputs they recommended using the 50 ohm tap on the output transformer & installing a resistive pad on the output from the power supply; I believe it was 15 db. I still have the mod sheet from Gotham Audio in my files.

If it were me I would open the micropone bottle & see if the output transformer is strapped for 250 ohms; if so reset it for 50 ohms. As you will be going from a lower impedance to a higher one there will be no frequency discrimination, just lower output.

I got the impression from something that was said tha you were having a problem with an original U47 but after quickly perusing the thread I did not find a mike type, my mistake. The pad will work tho without any side effects.