dacapitan said:
Hi
I am busy trying to bias the FET of my D-87 v1.3 (blue pcb)build using Matadors method - http://groupdiy.com/index.php?topic=46109.40
The way I have it setup is using my soundcard and "Soundcard Oscilloscope" - I have a Line Out cable with an alligator clip on the Hot (2) Pin going to R6 (Installed backwards) and an Alligator clip from Pin 1 to the Chassis of the Mic.
On the Line In Hot (2) Pin I have an alligator clip going to the Drain Leg of the FET on the mic. and from Pin 1 on the line in to the Mic Chassis.
I generate a signal a 1Khz and I see the Sine Wave on the Oscilloscope. I Increase the level from the signal generator and the output on my soundcard mixer but I cannot seem to get the signal to flatten at the top or bottom, I have also tried turning the Bias trimmer but that does not make any significant changes - I am not sure if the settings on the oscilloscope are wrong. I have never really used one before
I would appreciate it if you could please give me any advice as to what I might be doing wrong.
One thing I noticed was the level coming in to my sound card (from drain to scope) was very low
you don't need to install the R6 backward on the blue pcb ( the only model you don't need to) , so surely that will help out ,
basically you have to inject signal just before the FET coupling cap ,
I strongly recommend using a real sinetone generator to do this it is more convenient and you need to puh the signal quite hard ,
the first time you bias a 87 is can be tricky with the scope method as you are looking for a precise point of symmetry so you can experiment with a little befor getting use to it, I recommend to set the drain vltage at 11.5V to start and test the mic and afterwards bias it more precisely with it ,
Hope this helps,
Dan,