ReadMePR

There are a several preamp related schematics available on this
page. Note that all schematics on these pages are (c) Copyright,
1996-1998, Steve Bench. All rights reserved. You may use or
build these FOR YOUR OWN use freely. You may NOT build and/or sell
for commercial profit, any of these without a royalty agreement
in place with the author.

You may place these schematics on your web page if you credit the
author.

Note that high voltages are present in these circuits. The author
IS NOT RESPONSIBLE for ANY injuries, damages, etc. associated
with construction, or use of these supplies.


There are a number of RIAA Phono Preamps presented here. As low
signal levels are present, some microphonic control is important.
See my microphonics section m_fonix.txt for more information.

RIAA1.GIF presents a 417A/5842 based RIAA preamp using passive
RIAA equalization. This is potentially a very quiet preamp, but
the quality of the 417A/5842s noise performance varies. I've
found the "gold pin" variety Raytheons (including some labeled
RCA) work very well. The 417A sound is quite nice, although I
consider this preamp to be in the experimental stage yet. A very
well filtered power supply is important for this preamp. It
uses 185 volts (170 to 200 is OK) at about 70 mA, and uses 290V
(275 to 350 is OK) at about 5 mA. SUPPLY3.GIF in the power supply
section is appropriate. 1 kHz gain is about 37 dB.

RIAA2.GIF is the same supply but omits the 6SN7 cathode follower
stage. As the output resistance of the 417A second stage is
pretty low anyway, this allows the opportunity of eliminating
one more device from the chain. Also appropriate for folks who
do not like cathode followers. Also, since the CF output is
eliminated, the 290V supply is not needed. Gain at 1 kHz is about
38 dB.

RIAA3.GIF presents a different RIAA preamp approach. This uses
6DJ8/6922 tubes for the gain. It also uses a feedback RIAA
low frequency compensation and passive RIAA high frequency
compensation. As usual, the power supply filtering is important.
The supply listed in SUPPLY1.GIF is appropriate here, if you use
2 individual supplies to power each "half" of the preamp. Gain
at 1 kHz is about 38 dB.

RIAA4.GIF modifies the RIAA1.GIF schematic by replacing the input
tube with a 12AY7 (I used a 6072A). This provides even lower input
noise level. Both sections of the 'AY7 are paralleled, which lowers
noise an additional 3 dB. Gain is essentially identical to the
original one, at lower noise (0.33uV in my setup). Power supply
current is reduced from 70 mA to 37 mA. Note that the RIAA compensation
component values change, because of the difference in plate resistance
of the 12AY7(s). This schematic also demonstrates the effects of
changing tube types on circuit values.

RIAA5.GIF is a 12AY7/6072A input stage similar to above. However, that
is where the similarity ends. The plates are tied to the cathode of
a 6DJ8 operating as a cascode. This lowers the voltage amplification
of the 6072, and with that, the Miller capacitance seen by the input
is vastly lower. The second stage of the preamp is the "other half" of
the 6DJ8. This preamp REALLY SOUNDS GOOD. It is, by far, my favorite
of the group. The degeneration at the cathode does raise the noise
a little, so this preamp hits about 0.5uV equivalent noise. The
degeneration also eliminates any auditory effects caused by paralleling
both tube sections (see the miscellaneous section of this web page).
Gain is slightly higher (38.5 dB) than the others above, but 0.5 VOLT
input before overload is still maintained. If you're not sure which
to build, BUILD THIS ONE! Power is similar to the others, but uses
less current. 185 volts at 10 mA is nominal, but anything from 170
volts to 250 volts works great. Slightly higher gain and slightly lower
distortin at 250 volts. Current consumption will be higher at higher
voltages. Figure on 16 mA at 250V, and a gain of 40 dB.


There is also a complete preamp on these pages shown in Lineamp.gif
and the corresponding lineamp.txt description. This provides
information on the line section of the preamp as well as "integrating"
the power supplies and phono section into a full preamp.