After using and living with the flat phono preamp and RIAA EQ monitor switcher for awhile I think I know where to put all the jumpers.
Pauls' suggestions, particularly the inverse RIAA EQ for dub plate cutting, really helped.
I tried to put in a lot of flexibility and make it a "Swiss Army knife."
Someone wanting a simple RIAA EQ'd phono preamp needs only to build that part of the board.
My application, for LP archiving using RAW flat transfer, requires the full circuitry.
This is the schematic for the RIAA EQ and Monitor switcher:
RIAA EQ, Inverse RIAA EQ and Monitor Switcher for the Flat Phono Preamp.
The design has two signal paths. One is for recording the second for confidence monitoring or simultaneous recording and playback.
An RIAA EQ section can be placed in either the record path or playback path.
Flat transfers can be made through the record path while non-destructively switching monitor modes from monitoring flat record, record with EQ or playback from the DAW output.
Having analog RIAA EQ for monitoring saves ears and permits later de-clicking, done in the RAW un-EQ'd domain, to be heard in context.
Final EQ of the transfered files is done in DSP.
The flat balanced input/balanced output moving magnet preamp can be remotely located and powered from the monitor switcher permitting long cables between the turntable and workstation.
The flat/wideband preamp output is received by a THAT1240 line receiver.
It is then AC-coupled into a variable gain inverting buffer.
Relay RY2 selects either flat or RIAA EQ recording mode. RY1 must also be in Record.
The operation of RY1 and RY2 are destructive.
The record out is available as an RCA connection or fully-balanced by a THAT1646.
The RIAA EQ section can be switched between the Record and Playback Monitor paths by RY1.
A post-EQ gain stage can be used to provide stepped gain trim and additional post-EQ gain if needed.
The DAW playback output is fed into the THAT1246-based line receiver.
RY3 selects either Flat or EQ'd monitoring from either the Play line input, Preamp Output or RIAA EQ output.
The operation of RY3 is non-destructive to recording.
J1 bypasses the balanced input stage. This is used when the flat preamp is in the same chassis as the EQ/Monitor Switcher.
When J1 is connected directly to the input, the THAT1240 on the EQ board and one THAT1240 on the flat preamp may be removed.
J2 (optional) permits the preamp gain to be raised or lowered in 3dB steps. It can also be a front panel switch.
J3 is a rumble/warp filter insert.
J4 is for alternate EQ. It can be used to bypass the RIAA stage and for inverse RIAA dub cutting allow the insert of a 50 kHz ultrasonic filter. (e.g. Neumann 2 pole SK)
J5A/J5B switch the RIAA filter to inverse RIAA to permit the board to be used for cutting discs.
J6 is an optional cartridge correction filter insert point. J6 may also be used to provide a return for alternate EQ.
J7 permits the post-EQ gain to be switched in 3 dB steps. J7 can also be used to sum cutterhead feedback with modulation.
J8 is a second playback cartridge correction filter insert. J8 can also be used for additional cutterhead EQ.
J9 selects the flat monitoring source. J9 can either be connected to the Playback line input or it can follow the RIAA EQ. J9 can also be made a switch. For transfers which are going to primarily be made without RIAA EQ, "RAW," then J9 should be jumpered to follow the RIAA EQ. If RIAA EQ'd transfers are the norm, placing the RIAA EQ in the record path, J9 should be jumpered to Play line in to permit confidence monitoring or simultaneous editing of files already recorded with EQ.
Board Stuffing Options
There are lots of them but these are the highlights.
The preamp gain can have an optional trim pot to adjust flat gain.
Gain is typically trimmed in the flat moving magnet preamp.
For moving coil preamps, fine adjustment of gain is not always possible.
A trim in this location allows minor amounts of "helper" preamp gain and adjustment of channel balance for moving coil carts.
If the flat preamp is located with the EQ/Monitor switcher the THAT1240 balanced input is not required.
The flat preamp can have one THAT1240 removed.
Due to signal inversion, the inverting output of the flat preamp and inverting input of the EQ board are used to maintain correct polarity.
This is also the reason the THAT1240 appears "upside down" with the + input connected to the - THAT1240 input.
The RIAA EQ shows optional capacitors bridging the 10 nF.
The 10 nF do not require them.
If precise 10 nF are not available, a combination of 6800 and 1500 pF can be stuffed.
The resistor values then become 11K8, 31K8 and 318K with links in the unused resistor locations.
For applications which do not require post-EQ gain trim it may be eliminated.
Two simultaneous signal paths allow flat transfers to take place while other files are de-clicked, edited and rendered with RIAA EQ in DSP.
Having two paths is a huge time saver for archival transfer because it allows to things to be done at once.
There will be no cigarette breaks for interns doing transfers with this box - if you got time to lean you got time to edit.