After having carefully coaxed a combination of Denon 103R
cart, SME tone arm, and Thorens TD124 with a new timber base to
start singing in my system, a client gladly took it home
to try with is Audio Research SP11 preamp.
Unfortunately, all he got from one channel of the
phono pre was some horrendous noise, and his Easter music
intentions were dashed.
He arrives back ehere with SP11, and I spend a couple of hours
deciphering what
AR had done back in about 1986.

There wasn't anything wrong with the tubes, since after replacing them
all,
the troubles remained. The L+R line stages work OK

I sat down to try to understand the schematic, which I have.

One channel of the phono amp has 8 devices in the signal path
with 3 x 1/2 6DJ8, 3 j-fets and two mosfets.
There are more NFB loops applied than in any other amp I have seen.
There are numerous other SS devices to protect the circuit
and its delicate devices from burning to a crisp.

But anyway, the cartridge signal
goes to an RCA socket, and there is switched impedance matching.

1st gain stage is an npn j-fet gate, Q1, in common source mode.

2nd stage is pnp j-fet, Q2, which is a source follower driving V1
cathode, a 1/2 of a 6DJ8, with 100k load and
mosftet direct coupled mosfet source follower output, Q3. Local NFB from

Q3 is fed to V1 grid.( its not a cascode circuit )

3rd stage is 1/2 A 6DJ8 in common cathode.

4th stage is another complex fet-tube-mosfet type like stage 2.

RIAA eq is performed around the 4 stage amp with a complex loop.

The operating voltages at the tube electrodes look about right,
so I think one of the fets is faulty, but damned if i know which one.
The board is a double sided board, and correlating the circuit digram to
what is in the box
is a nightmare, since no components are numbered, and the fets have
had their type numbers removed or painted over, and it doesn't help that

special trimming resistors are needed for the fets so the
DC settings of the amp will allow it to work, so just replacing
faulty fets may not fix the amp.

I'd better not say what i really think about this phono
amp section which is by far the most complex I have ever seen
where tubes are used, and its obvious that the possibility for
instability is high.

Does anyone have the list of fet types used in the phono stage of this
preamp?

Patrick Turner.