The input resistors for these networks (R4 and R9) were made large in value in order to swamp out effects of rp and mu variations in the tubes. The 50 Hz/500 Hz pole/zero is via C4 and the combination of R9/R10 and the output impedance of the second stage. This should solve the high frequency overload problem in the second stage. The 2130 Hz pole is created by C3 (330pF) and the combination of R4/R5 and the output impedance of the first stage. The RIAA equalization is split into two halves. I have a few 2u /200V polyprops and a couple of Infinity Caps that I'll eventually try. The coupling caps are already probably too small (breaks at 1Hz and 5Hz). (I think i used 1000u/25V ) Since myĢA3 Amp has about 200mV sensitivity, I was trying to avoid the large value (read: electrolytic) caps needed to stay sufficiently far away from the first 50Hz break of the RIAA. The 2nd half of the 6sn7 became the requisite cathode follower.Īlthough not yet shown in the schematic, I wound up having to bypass the cathode resistor of the first gain stage. This stage is ac coupled to the second gain stage made up of the 1st half of the 6sn7. Rather than share a single 6SL7 between the two channels I decided to parallel the two sections in each channel in hopes of reducing noise. The HF break is placed between the first and second stage. However, I think I've finally settled on the most straightfoward approach shown in the schematic, above. If you have been following the development of this project you've seen the various strategies I've tried to get around this problem. This unfortunately led to overload of the 2nd stage at high frequencies. At first I kept the RCA passive shunt circuit, but placed it after the 2nd gain stage to keep noise down. I finally settled on the 6sl7 cascaded into a 6sn7 gain stage. (btw: I checked the effect on RIAA parameters -it is actually more accurate with the 6sl7 !!!)īut that was still a little sparse (and I already had my heart set on using a 17 x 8 footprint to go with my 17 x 13 amp). At first I was simply going to sub a 6sl7 for the 7025 in the RCA circuit and be done with it. But the simplicity of the RCA-19 circuit also had its appeal. I knew that I wanted to stay with the octals. (Even a better match - no feedback!!!) I noticed a neat RIAA preamp circuit in the back of the RCA Tube Manual RC-19 that used a single 7025 per channel. I figured it would be a good match (electrically and asthetically) for myĪfter thinking it through I thought I might just try passive RIAA. This preamp design started out as a conventional three stage cascade with feedback RIAA equalization. The burning desire for another tube project coupled with a longing to hear Velvet Underground '69 thru my newĢA3 Amp led me to embark on this Phono-Preamp design. I do, however, have a 600+ LP collection which has remained un-tapped for the last 3 years. Unfortunately, my CD collection is not vast. Last Update - 2/19/95 Here is the schematic for one channel of my preamp:įor the last several months I had been listening to mostly CD's thru myĢA3 Amp with much joy.
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