LINEAMP.TXT

This text describes the "line amp" stage (lineamp.gif) schematic
on this web page. The purpose of this line amplifier is not to
present the end all-be all Hi-Fi preamp, but rather to present
various principles that can be applied to your own designs.

Pretty much all preamp functions are accomodated, so that you can
add or delete those items that you want to include or exclude from
your design.

The power supply shown in the power supplies section of these web
pages (supply3.gif) is the appropriate power supply for this
preamp. The phono section schematic is separately shown on these web
pages in RIAA1, 2 or 3.

SPECS:
As shown, the bandwidth of this preamp is within 0.3 dB from about
10 Hz to about 50 kHz. Line amp max gain is about 20 dB. Distortion
is about 0.25% at 30V RMS output, and less that 0.1% at 10V RMS
output, anywhere in the audio band. Noise level is about 80 dB below
1 volt for the line section, and 80 dB below 10 mV input level in the
phono section. I view this preamp as being in "test state", meaning
that I'm not fully happy with the sound as yet; as I mentioned, this
is intended to present principles, rather than a "completed" work.

DESCRIPTION:
The input section is pretty much standard. The first unusual piece is
the use of a relay (K1) to switch between record monitor (tape, DAT,
CD recorder or whatever) and other inputs. This allows you to place
the relay near the direct signal points rather than dragging the hot
audio leads to another switch. Saves a little on noise, and on wiring
capacitance.

The volume is a 1M audio taper control. "Loudness" compensation is
shown, again implemented via relay rather than a volume/loudness
switch. Notice that the low frequency "pole" shifts as the control
position is changed, providing a "closer" match to "loudness": as the
volume is turned down, more and more "bass" is added. Treble compensation
is also provided, and it too follows the curve, initially providing
more compensation then less as the volume is reduced, as shown in
Fletcher's work. The advantages of this scheme are accurate following
of the curve, and tapped controls not needed. The disadvantage is
there is no real "0" volume level. Those not wanting loudness can
short R20, remove K1, C4, C12 and R6, and the corresponding components
for the other channel.

The input stage is a 417A/5842 run at a gain of slightly less than 4.
This is a inverting op-amp feedback configuration. The gain is set
primarily by the ratio of R24 to R21. This provides a low impedance
output, so the output of this stage "could" be the line amp output,
if tone controls are not to your liking. Notice the 4 separate grid
stopper resistors on the 417A/5842; one on each grid pin. This keeps
the inductance of the grid leads low, combined with the resistor de-Qing,
this stage is STABLE. This is always a good idea with this very high gm
tube.

The tone controls are also a feedback from a 6SL7/6SN7 combination.
I have found this combination of tubes... the 'SL7 providing the
gain driving a cathode follower 'SN7 to be particularly low in distortion
with the open loop characteristics providing very low distortion AND
sounding good at almost any desired level.

Tone controls are linear taper pots, which is a convenience of the
feedback design. "Flat" is, of course, the center of the pot, and
is set up by the 125k and 499k resistors shunting part of the pot.
Notice there is a 2:1 resistor ratio... the plate side resistance
is twice the grid side resistance, providing a "flat" gain of about
2. The "plate" side is coupled from the cathode follower output, and
provides a DC path to ground at the preamp output. The treble
frequency characteristics can be changed by altering C1 AND C2, and
the bass frequency characteristics can be altered by C5 AND C6. As
shown, the tone effects are reasonably subtle, I don't like "heavy
handed" tone controls.