oneoldude
Member
Hi all, I am a noobie here and am looking for help. I am trying to design a BJT Preamp for an ECM cartridge with which to test speakers. I use thru hole components and most of the designs out there require caps that are too big to get into the wand. So I came up with the following design that uses tiny caps and can be put into the wand. Yes it will fit into a 19 mm tube.
I have no training in this area so would like a review of my design and corrections and suggestions. Ideally, it would be great to have a design with reasonably low noise, decent distortion and wide bandwidth (down below 10 Hz). This is not a recording Mic so noise and distortion do not have to be vanishingly low. But lower is better. Also, gain should be such as to be able to get to 1V for line level inputs.
A condition that must be met is that I must use 12V DC to power the Pre.
R6 is a 10k Audio Taper pot in case things get too hot for Line In.
I have attached the schematic and the FR curve From LT Spice. The curve looks good with a low order rolloff and decent transient response on the low end. I would like the bass end extended an octave or so, but that is as good as I could do.
Could you all please take a look and critique what I have done? I do not know if my bias is set right, if the currents are ok for the application or if the gain is proper. I also have no idea about the inherent distortion of the design. Also I would like advice on R6, the Audio Taper Pot. I could go to 50K or 100K. What is the best here?
Any help or suggestions will be warmly appreciated.
Thanks.
Oh, one last thing, I do not know how to model an ECM in LTSpice so I simply used an AC signal source. I am sure that makes a difference, but it is all I could figure out to do.
Oh, one more last thing, I do not mind scraping my design entirely and go to a design with more transistors to get away from those emitter bypass caps. Of course I expect to keep the input and output caps and possibly some interstage signal caps. I was referred from a thread here to a thread about looking at Transistors as voltage controlled devices. There was a circuit there that by using two stages did away with the emitter cap. That is a wonderful idea, but way beyond my abilities. Hell, what I have posted here is way beyond my abilities. That is why I need help!
I have no training in this area so would like a review of my design and corrections and suggestions. Ideally, it would be great to have a design with reasonably low noise, decent distortion and wide bandwidth (down below 10 Hz). This is not a recording Mic so noise and distortion do not have to be vanishingly low. But lower is better. Also, gain should be such as to be able to get to 1V for line level inputs.
A condition that must be met is that I must use 12V DC to power the Pre.
R6 is a 10k Audio Taper pot in case things get too hot for Line In.
I have attached the schematic and the FR curve From LT Spice. The curve looks good with a low order rolloff and decent transient response on the low end. I would like the bass end extended an octave or so, but that is as good as I could do.
Could you all please take a look and critique what I have done? I do not know if my bias is set right, if the currents are ok for the application or if the gain is proper. I also have no idea about the inherent distortion of the design. Also I would like advice on R6, the Audio Taper Pot. I could go to 50K or 100K. What is the best here?
Any help or suggestions will be warmly appreciated.
Thanks.
Oh, one last thing, I do not know how to model an ECM in LTSpice so I simply used an AC signal source. I am sure that makes a difference, but it is all I could figure out to do.
Oh, one more last thing, I do not mind scraping my design entirely and go to a design with more transistors to get away from those emitter bypass caps. Of course I expect to keep the input and output caps and possibly some interstage signal caps. I was referred from a thread here to a thread about looking at Transistors as voltage controlled devices. There was a circuit there that by using two stages did away with the emitter cap. That is a wonderful idea, but way beyond my abilities. Hell, what I have posted here is way beyond my abilities. That is why I need help!
Last edited: