Small transformer based preamp, passive?
« on: September 24, 2019, 04:57:11 AM »
Dear All,

For live sound, I use on some instruments a pair of tube SDC, which is working great so far.
As I need almost always around 22db of gain at the FOH desk, I was wondering if I could build a small stereo preamp, if possible totally passive, transformer based, with no phantom and a fixed gain of around 20/25db.

Do you think this is possible?
Have you already seen such design?

The best would be with a small footprint, and not too heavy as I would have to carry it in plane as part of my cabin luggage while on tour.

I have searched the web, but nothing so far...

Thanks a lot for your answers,


Re: Small transformer based preamp, passive?
« Reply #1 on: September 24, 2019, 09:11:28 AM »
That would be just a transformer alone.  Consider your impedance consequences. 

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Re: Small transformer based preamp, passive?
« Reply #2 on: September 24, 2019, 12:58:22 PM »
Thanks, I will look into it


Re: Small transformer based preamp, passive?
« Reply #3 on: October 21, 2019, 02:17:34 AM »
SDC mics of the tube variety usually have a 'mic transformer' on their output ...   something like a 4K:600  'stepdown'  more or less, which converts the mic tube's high output impedance down to something low-ish (impedance wise) with the comensurate step down in signal (amplitude, which is quite tiny)  and with some ever-present [small]  loss of power and freq_resp (insertion losses).

Now -  that quite tiny signal,  usually goes into a mic preamp,  which is a low-ish impedance input  ...    prolly something like 1K5 .. 2K5  ohms   or so -  which  then goes to the mic preamp's 'active' gain section.

- which would typically add something like 20dB .. 55dB of gain   (on a potentiometer of some sort)   ....   before pushing it out, [with some good current gain  behind it], to a 600 ohm impedance output  ...   which is a typical  'line output'  ..  in the 'vintage world'  :)

Unfortunately it takes the addition of some amount of power to do this in a reasonable way.  :-X


I suppose you would like to connect your SDC to a circuit of   transformer(s)   for some 'free gain'  [passive]  and so on ...   it can be done, 'in a sense'  but  with many  radeoffs  of 'this-that-and-the-other'  ....  and certainly with more  detailed info required on what network you want to drive  etc.

I think if you have sufficent 'grunt' before the 'passive preamp'  you can carve and whittle your way to some  'free gain' in terms of signal  amplitude, and still be left with  something that might be connectable to a modern mixer, for example.

'Passive Preamps' are more accurately called 'Passive Attenuators' imho   ;D   

ANyway, there is actual stuff out there on 'passive mixing networks' and 'passive impedance converters'   and so on.

You could try and get a hold of 'The Audio Cyclopedia Howard Tremaine ..  it's a great resource for 'vintage audio techniques' and does address  these topics  ... of 'passive attenuators', 'audio signal transformers'  and 'impedance matching networks'    ...   in a reasonably understandable way  ..  for an EEng at least!


Apologies if I ramble a bit or talk up-down to your original post.
I ping therefore I am


Re: Small transformer based preamp, passive?
« Reply #4 on: October 24, 2019, 01:59:47 PM »
I think the point, and what the other posters were getting at, is that a transformer will give you voltage gain, while reducing the current (ie drive capability).   So the short answer is no, you can not build a truly passive preamp to fit your needs.  However it isn’t too big of a deal to build a preamp with fixed gain... this can be done with a single tube if you want to keep it all tube.... NYDs one bottle pre comes to mind...

Another thought might be to build a new power supply for the mics with a preamp in it and a switch that changes the output from mic to line level, that way you have everything in one box. 

"Sometimes you have to play a long time to be able to play like yourself.” - Miles


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