As some of you may remember, I conducted some experiments in building and rewinding ribbon transformers:
http://www.groupdiy.com/index.php?topic=18764
http://www.groupdiy.com/index.php?topic=19046
My main goal was to build an "overwound" transformer with a very high step-up ratio of at least 1:80 and use an active impedance converter to get back to a usual mic output impedance of 50-200 ohms. My rewound transformers work quite well. Now comes the electronic part. For my first experiments I used the Schoeps/Dorsey circuit. Actually, I only used the hind part, feeding the transformer signal into the coupling caps:
This works okay. The resulting active ribbon is about 12 dB louder than the same model using the stock transformer. Noise performance is about the same. As some commercial active ribbon designs are lower noise, I figured, there's room for improvement. So here's what I've come up with so far:
The front end is a FET input à la KM84 followed by a BJT phase splitter. The FET part is configured for an extra gain of about 12 dB on top of the transformer voltage gain. The resulting output is pretty high, maybe a little too high, about as high as the hottest condenser mics. According to my preliminary tests, noise performance (FET bias hand adjusted) is a little better than the Schoeps/Dorsey-derived ultra-simple design.
So everything looks fine. Nonetheless I'd be extremely grateful for some input from you guys. This design is a little unconventional, I guess, and I'm not really an electronics engineer. So there's a good chance I missed certain things. I'm sure there's stuff I should revise. Maybe my design is crap altogether. :? You never know...
Opinions anyone?
http://www.groupdiy.com/index.php?topic=18764
http://www.groupdiy.com/index.php?topic=19046
My main goal was to build an "overwound" transformer with a very high step-up ratio of at least 1:80 and use an active impedance converter to get back to a usual mic output impedance of 50-200 ohms. My rewound transformers work quite well. Now comes the electronic part. For my first experiments I used the Schoeps/Dorsey circuit. Actually, I only used the hind part, feeding the transformer signal into the coupling caps:
This works okay. The resulting active ribbon is about 12 dB louder than the same model using the stock transformer. Noise performance is about the same. As some commercial active ribbon designs are lower noise, I figured, there's room for improvement. So here's what I've come up with so far:
The front end is a FET input à la KM84 followed by a BJT phase splitter. The FET part is configured for an extra gain of about 12 dB on top of the transformer voltage gain. The resulting output is pretty high, maybe a little too high, about as high as the hottest condenser mics. According to my preliminary tests, noise performance (FET bias hand adjusted) is a little better than the Schoeps/Dorsey-derived ultra-simple design.
So everything looks fine. Nonetheless I'd be extremely grateful for some input from you guys. This design is a little unconventional, I guess, and I'm not really an electronics engineer. So there's a good chance I missed certain things. I'm sure there's stuff I should revise. Maybe my design is crap altogether. :? You never know...
Opinions anyone?