NOTE: Glen incorporated this material into a "QRP Quarterly" article, with many other re-imaginings of the R2 concept. He even called it the "R2A" -- these preamp ideas, a lower-noise, trimmable audio phase shift network, and a reworked audio section with a tunable SCAF audio filter. I will likely start with the audio quadrature, then move to the preamp/diplexer. -- K0JD From: Glen Leinweber Subject: R2 audio preamp, ASCII version Date: 16 May 1995 17:48:04 -0400 [edited by K0JD] Audio preamp- Roy Lewallan started us all using the 2N3904 as a grounded base audio preamp. Rick Campbell, in his first article (a classic) says he tried a common emitter audio preamp, and got great noise figure, but couldn't easily match the input to the 50 ohms that the mixer wants to see. The problem with the common base, is that you have to have about 0.6 ma collector current to achieve 50 ohms input resistance. Not enough collector current for a good noise figure. So my solution is to stick with common emitter, where you can use any collector current you want, and use some resistive feedback to bring the input resistance down to 50 ohms. Found out that the very common 2N4401 transistor gives pretty good noise figure when biassed at a few milliamps (a little better than 2N3904). Slapping two in parallel reduces the noise figure even further. Select the 2N4401 for high current gain... I found quite a few with beta > 200 (from Motorola). Measured the noise figure with 50 ohm soure resistance at 2dB By the way, the phase shifter schematic in the R2 contributes (according to PSPICE) a significant amount of noise. I'm scaling the R's and C's to improve noise performance (reduce R's, increase C's). Also going to use the LM837 quad op-amps in the phase shifter. They're cheap, low noise, and available from Digi-Key. This schematic should be simple enough to read if you squint. +12 volt DC about 8ma. ---------------------| | | | 100K / | | |/ | |----| 2N4401 | | |\| | + | \_____ | Audio Preamp 68uf/16v | | | Select 2N4401 Hfe > 200 | | .01uf | input Z = 50ohms | | | | Noise figure=2dB GND | | | Gain=60 1K | | ____________________________| GND / IN from Mixer | | | |/ | | | |___________| 2N3904 | | | | |\| |__________ 6.04k | | \ | | | | | | | | | | | / / |_______OUT | | 2.5mH | |/ |/ | | | | |______|_________________| | | 1uf |_22uf__| | | | | | + | |\| 2N4401 |\| 2N4401 1K | | | \ \ | 42mH | | |______ |______ | | | | | | | | | | 51 | | |+ | |+ | | | 2.48K 220 100uf 220 100uf | | | | | | 6v | | 6v | | | | | | | | | |___|_____________|_________|______|__________|______|______| GND Glen Leinweber VE3DNL leinwebe@mcmail.mcmaster.ca - - - - - - - - - - - - - - Date: 02 Oct 1995 16:53:52 -0400 About the diplexer, The new audio preamp maintains its 50 ohm input Z only down to a few tens of hertz, so the diplexer has a non- constant impedance load at the low end. The diplexer could be modified to terminate the mixer with 50 ohms right to DC. I suspect that DC termination into zero ohms is OK, maybe even a good thing. Something to check out, especially if you're interested in demodulating AM. I'll let you know if there's any difference. I should mention that this diplexer has been modelled, but untried so far. Could end up with some tweaks to component values. Those 42 mH coils are wound on small potcores. Rick likes to use prewound TOKO inductors, but they're a little lossy, and untrimmable. The handwound potcores allow me to match a pair very well (I have a good AC commercial MAXWELL bridge). The smaller inductors are wound single layer on high-permeability toroids, since they must have low capacitance up thru the radio band frequencies...FT37-77 perhaps. I'll be matching all the capacitors too...any diplexer sets, if not carefully matched, can cause phasing errors that'll foul up sideband rejection. It's going to be tough to match those 22uf caps...they'll have to be tantalum. The 42mH/1uf parallel tuned circuit has the same centre frequency and Q as the series tuned 2.5mH/22uf tuned circuit. The series tuned LC needs a 50 ohm load (the audio preamp) to get the correct Q. There should be no magnetic coupling between the 42mh and 2.5mh inductors (should have drawn it differently). This diplexer should have better behaved impedance at the higher radio frequencies than Rick's original. Rick's is easier to build. AUDIO/RF DIPLEXER Mixer_______________ Here's the mod for ouput |42mH |2.5mH 50 ohm termination ____|___ |_ all the way...DC to daylight | _) _) as Rick says. _|_ _) _) ___ _) _) |1uf _) _) | | | |_____| _|_ | ___ | |22uf R1 |____ to audio preamp 51ohm (50 ohm input Z) R1 | _|__ / / / R1 = 50 ohm minus 42mh resistance Glen Leinweber VE3DNL leinwebe@mcmail.mcmaster.ca