THAT1512 (INA217) based preamp and input transformer


2009-12-17 10:26 am
There are several discussions on the forum about INA217 or the equivalent THAT1512 chips, but I didn't see anyone trying to implement an input trafo.
The idea is to replace the old DC blocking caps with an input transformer.
I thought to use a transformer with 1:3.5 ratio from Cinemag (CMMI-3.5C) implemented in the way you see on the schematic I attached. This gives me a +9.7dB voltage step-up achieving 70dB of max gain of the amp (max gain of THAT1512 is 60 dB).
With R5 and R6 of 15k the input impedance should be of 2k (differential input impedance seen by the microphone). I don't use the "t-bias" recommended by THAT datasheet because with the transformer there's no longer the problem of matching the DC blocking caps.
My knowledge about this topic is limited, so maybe I'm doing something wrong in the choice of the trafo or in the circuit. This is why I really need your critics and comments!


  • THAT1512_trafo.png
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2009-12-17 10:26 am
Old post, but I just saw it. still there?

The 1512 voltage and current noise is optimised for a pretty low impedance, perhaps 150 ohms. With that transformer it will see over 2K, so there will be some noise.

We can do a noise/DR analysis if you wish.


Yes, still here! ;) Thank you for the answer.
An analisys would be very useful for me to know if there will be too much noise with a 1:3.5 transformer.
The guys at FiveFish studios are selling a THAT1512 preamp with an 1:8 input transformer, and this made me think I could implement the 1:3.5 with no problems, but I'm not an expert.
Beside the noise problem, do you see any other problems to consider (CMMR, input impedance, etc...) or something to be solved in the schematic? I've already designed a PCB with this schematic and I'm going to test it, but I didn't etch it yet... I'm happy to receive suggestions!
Thank you,
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2008-07-08 3:24 pm
Hello Ettos,
I know I'm 3 years late with this post,but was wondering if you managed to complete this preamp? If you did, what problems did you encounter & what additional mods did you make to this circuit to work properly?

If you'd kindly share any problems or solutions you may have had would be of immence help to me, as I am about to embark on a similar project.