Another Discrete BJT MC Preamp Design

[analog_guy]

Here is my latest design to throw into the ring. It is a 3-stage discrete MC preamp utilizing THAT304 matched PNP-NPN quad as input devices. The remainder of the circuit consists of a cascode VAS stage driving a 4-transistor output stage similar to that used in the OPA627 opamp. It simulates to the same En advertised by THAT Corp: 0.7-0.8 nV/sqrt(Hz). Distortion is in the -120 dB range. There are still a few tweaks to be done (like optimizing input stage resistances to minimize En). I have just about completed PCB layout and should be building the prototype by year's end.

[Joachim Gerhard]

when i open your document i can not see the circuit values very well but i like what i see.
should develop into an interesting project. i ordered samples of the THAT304 but never got them, so i am very interested what you experience.

[analog_guy]

Joachim,

The file is a 2003 SP3 Microsoft Word document. When you open it in Word, go to the toolbar and increase the magnification to approx 150% (it is expressed as a percentage). That should fix the problem.

Jeff

[Joachim Gerhard]

i did that but still i could not read the values. the resolution is not high enough on my mac.

[Joachim Gerhard]

i opened it under windows xp and now it works fine. so mac users may have problems.

[tiefbassuebertr]

Originally Posted by analog_guy

Here is my latest design to throw into the ring. It is a 3-stage discrete MC preamp utilizing THAT304 matched PNP-NPN quad as input devices. The remainder of the circuit consists of a cascode VAS stage driving a 4-transistor output stage similar to that used in the OPA627 opamp. It simulates to the same En advertised by THAT Corp: 0.7-0.8 nV/sqrt(Hz). Distortion is in the -120 dB range. There are still a few tweaks to be done (like optimizing input stage resistances to minimize En). I have just about completed PCB layout and should be building the prototype by year's end.

same circuit transfer to PDF

[analog_guy]

I noticed the V+ and V- pin numbers on the IC2 symbody are transposed. V+ should connect to pin 7, and V- to pin 4.

[analog_guy]

Originally Posted by tiefbassuebertr

same circuit transfer to PDF

Thanks for converting the image to a .pdf format. I don't have an Acrobat converter.

[Jan Dupont]

CutePDF - Create PDF for free, Free PDF Utilities, Save PDF Forms, Edit PDF easily.

[analog_guy]

After a few days in the shop the chassis for the preamp is nearly complete. Lacking access to sheet metal bending tools I elected to use thick material for the front and side panels and then tap/thread to attach the other panels. I could not find knobs to my liking so I machened the ones in the photo. (It helps to have a vertical mill, a drill press and a lathe). Once all the electronics has been installed and everything tested I'll disassemble the panels and send them to an anodizing shop