> pair of high-mu triodes in a push-pull mic preamp. That way, each tube > can > have its own 10 meg grid leak to allow for a 20 megohm load on the xtal.
It's possible to duplicate a similar circuit using an FET differential amplifier with dual FETs in a single package (e.g., 2SJ109), or a JFET-input instrumentation amp (e.g., LT1102). Both would perform best when using a bi-polar supply to economize on parts count and avoid having to use half-rail biasing technique. In each case, input Z is 2x the single-ended amp for a total of 22 meg-ohm. The circuit could be installed in the D-104 mic head, with shielded twisted-pair running to a balanced-input speech amp. Seems like there are other factors that limit the practical input Z to ~10 megohm for a crystal cartridge. The source Z and slight shunt resistance internal to the crystal cartridge may be the limiting factor. I've used a series input R of 10-meg in front of my single JFET's gate and the response is pretty much limited to about 50 Hz with or without the added R -- with the only noticeable difference being lower output level and increased noise. With the first fundamental of the deepest male voice being greater than 70 Hz, it's kinda' hard to justify a response that extends any lower for any form of voice communication, including AM and ESSB. Anything lower than 70 Hz unnecessarily increases power duty cycle and produces other unwanted artifacts like room rumble and deep breathing sounds. Paul, W9AC ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/amradio@mailman.qth.net/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Post: AMRadio@mailman.qth.net To unsubscribe, send an email to amradio-requ...@mailman.qth.net with the word unsubscribe in the message body. This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
