D1 Transformer Coupled Active DI Prototype
Today I've assembled the first prototype of a Transformer Coupled Active DI that will be available as a kit soon. It is a simple, yet very good sounding classical design with a j-fet input stage chosen for its virtually unlimited input impedance (the input impedance of the device is set to 10Mohms) and a bipolar buffer/transformer driver. The transformer is my own original design developed and made particularly for this Active DI project...
The main ides of this project was to keep it as simple as possible, however the whole development process took lots of time - I have started this adventure in February, 2014. Of course, I couldn't dedicate all my time to this device, as I have lots of work to do with my RM-5 Ribbon Microphone manufacturing business, but still, a year is quite a lot.
Have to note, that most of the time was actually spent working on the output transformer design - prototype hand-winding (believe me, you really don't want to hand-wind over 1000 turns on a toroid core!), testing, re-winding, re-testing and so on until the best balance was found. I think we all can agree on the fact that an output transformer is the most critical component of such design, so this time investment does really make sense.
So, few words about the actual design now.
I was sure that I want a j-fet in the input stage from the very beginning, as one of the main benefits the active DI can provide is a very high input impedance, which can contribute a lot to passive pickups, be it guitar or bass. I have set the input impedance value at 10Mohms. The j-fet is a good-old 2SK246, which does not offer high gain (we barely need any anyway, as DI actually attenuates the signal), but offers good headroom for such type of device. In this particular circuit the j-fet is biased to have the input signal headroom of approximately 1.2Vrms. No negative feedback is applied.
The downside of such approach is the necessity to sort the j-fets by their Idss ("idle current"), group them, and to pair with different source resistor (Rs) values for each range. Not too good for serial production, but perfect for DIY, so every builder could benefit from the great sounding simplicity. The kits will come with j-fet/Rs pairs properly picked.
The next stage is a simple bipolar transistor emitter follower. The first idea was to use the j-fet only and to couple it directly to the transformer, but due to the high output impedance of the stage (around 1.5Kohms) the lowest end performance would be compromised this way or the output transformer would have to have the primary winding inductance value (read - turn count and core size) that wouldn't be practical in both - size and costs. So, the simpler way was to add a low impedance buffer to drive the transformer properly, plus it is actually very convenient to direct-couple the emitter follower to the j-fet stage, so there is no need for an interstage capacitor here.
Finally, the output stage is the output transformer, which is separated from the driving stage by an interstage capacitor. The transformer has a good long line drive capability due to the presence of a Faraday shield. The biggest natural advantage of the toridal design is the huge immunity to EMI, so the trafo does not need canning, it does not hum even when PCB is out of the enclosure (unless you put it right next to a laptop). The first tests have shown that I haven't spent the time designing the trafo in vain - this little beast does really sound good and the whole kit with the trafo and enclosure included should be available at the price similar to the price of a single Lundahl output transformer!