Phono Preamp
Here is the schematic for one channel of my preamp:
Background
For the last several months I had been listening to mostly CD's thru my 2A3 Amp with much joy. Unfortunately, my CD collection is not
vast. I do, however, have a 600+ LP collection which has remained un-tapped for the last 3
years. The burning desire for another tube project coupled with a longing to hear Velvet
Underground '69 thru my new 2A3 Amp led me to embark on this
Phono-Preamp design.
Design Details
Starting Point
This preamp design started out as a conventional three stage cascade with feedback
RIAA equalization. I decided on 6SL7's running into 6SN7's. I figured it would be a good
match (electrically and asthetically) for my 2A3 Amp
After thinking it through I thought I might just try passive RIAA. (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.
Design Development
I knew that I wanted to stay with the octals. But the simplicity of the RCA-19 circuit
also had its appeal. At first I was simply going to sub a 6sl7 for the 7025 in the RCA
circuit and be done with it.
(btw: I checked the effect on RIAA parameters -it is actually more accurate with the 6sl7
!!!)
But 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).
I finally settled on the 6sl7 cascaded into a 6sn7 gain stage. At first I kept the RCA
passive shunt circuit, but placed it after the 2nd gain stage to keep noise down. This
unfortunately led to overload of the 2nd stage at high frequencies. If you have been
following the development of this project you've seen the various strategies I've tried to
get around this problem. However, I think I've finally settled on the most straightfoward
approach shown in the schematic, above. The HF break is placed between the first and
second stage.
Circuit Features
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. This stage is ac coupled to the
second gain stage made up of the 1st half of the 6sn7. The 2nd half of the 6sn7 became the
requisite cathode follower.
The cathode resistors of the gain stages are un-bypassed (for now) since I don't think
I'll need the gain. My 2A3 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 coupling caps are already probably too
small (breaks at 1Hz and 5Hz).
The RIAA equalization is split into two halves. The 2130 Hz pole is created by C3 (330pF)
and the combination of R4/R5 and the output impedance of the first stage. This should
solve the high frequency overload problem in the second stage. The 50 Hz/500 Hz pole/zero
is via C4 and the combination of R9/R10 and the output impedance of the second stage. 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. I couldn't just make R4 1 Meg or more
because then the effective input capacitance of the second stage would start to become
significant compared to C3. trade-off and more trade-offs...
I used the LF network to dc couple the second stage to the output cathode follower. I also
changed the output coupling cap to 2uF to keep well away from the low end of the RIAA
characteristic. This will feed into the 100k pot of my "passive pre-amp" (I
HATE the term, but that seems to be what everyone's calling it these days) section
This arrangement laid out nicely on my 17 X 8 plate. I used the same general layout as i
used on my 2A3 Amp . The only problem I had now was a 6 x 8
void between the power transformers and the preamp tubes. Hmmmmm........
Enter the Regulated Power Supply
The power transformer I used had been set aside for a bench supply based upon a few
tube regulator circuits from Glass
Audio . So I put two-and-two-together...
I laid out the regulator circuit's tubes in a row perpendicular to the parallel columns
formed by the octal pre's. The circuit uses (2) 02B's, a 6AU6, and a 6W6 for a pass tube.
The Raw 330V input comes from a *cough* silicon diode bridge (the only SIN in this
project!?). The regulated output is 250 VDC.
Other Features
The preamp tube filaments are fed with regulated DC. A single Radio-Shack transformer
feeds two linear regulators (one for each channel).
Construction Details
As I mentioned, I built the entire amp on a 17" x 8" aluminum plate. This
plate will drop into a nice burly maple box that my brother, Rich, is going to make
to match the one he built for my 2A3 Amp . I cut the tube
socket holes using a hole-saw on a low speed power drill. I use WD-40 as a
"cutting-oil".
Performance
Simulated:
The following data was obtained from Saber simulation tool (sorta like PSpice). I
adapted and tweeked some of the PSpice tube model for Saber. So far its done a pretty good
job...
Gain:...24.1dB @ 1KHz.....44.4dB @ 20Hz.....5.2dB @ 20KHz
Deviation from RIAA Standard:
...-0.2 to -0.4 dB...from 20 to 1kHz
...-0.4 to -0.1 dB...from 1kHz to 5kHz
...-0.1 or less from 5kHz to 50kHz
Of course this is all based upon nominal values. I intend to screen/select the components
(especially C3 & C4) and adjust R4 & R9 as required to make up for the tube
parameters.
Email your comments/suggestions/critiques/questions to Bob. D. at rdaniela@astro.ge.com