I wanted to share a pic of the flat phono preamp protoboard.
This is the record path of the Flat Phono Transfer Preamp.
It has bipolar input instrumentation amps to provide a fully-balanced MM phono input.
An RIAA EQ de-emphasis network for full-range transfers is also included.
The relay switches between Flat and EQ'd transfers.
The output is also fully-balanced by THAT1646s.
An identical path will be provided for monitoring.
The monitor path also has switchable RIAA de-emphasis to permit non-destructively monitoring the record path during transfers with either flat response or with EQ.
On playback, the monitoring path can be used to monitor the effects of de-clicking both with and without EQ.
This permits de-clicking to be auditioned with RIAA de-emphasis without having to render the file with software EQ applied.
It will also be possible to simultaneously record and monitor two different sources.
The input stage of the monitoring section will have either 0 dB or +32 dB gain set by a jumper so that it may be used for playback or a second turntable input.
The input stage in the prototype uses NJM2114s as an instrumentation front end followed by a THAT1240 common mode rejection stage.
Without FETS its a copy of John's 1980-era P10.
The NJM2114 shown in the picture offers NE5534 noise performance in an NE5532/LM4562 dual footprint.
It is available from Mouser.
The NJM2114 has the high bias current typical of an NE5534.
An LM4562/LME49720 or LME49860 may also be used in this location.
Direct Coupled - No Electrolytics
The cartridge is direct-coupled to the op amp inputs.
Other than optional OutSmarts capacitors there are no electrolytics in the primary signal path.
OutSmarts caps are only really in-circuit when the output is connected unbalanced.
Servos are not used.
I considered using low noise NFETS but noticed during testing that the LSK170 and 2SK389s seemed to have significantly higher 1/f noise but only slightly better noise in the top octaves where cartridge inductance rises.
In the top octaves the FETs were a little quieter: the 100 Hz and 10 Hz 1/f noise was far higher with FETS.
When RIAA EQ is applied the difference is huge in the absence of A-weighting.
The FET advantages of low bias and noise current didn't seem to be worth the added cost and difficulty of sourcing components.
The LSK170/189 and 2SK389/LSK389/LSK489 are not inexpensive.
The BF862 is a fantastic part available for pennies but is available only in SMT.
The RIAA EQ circuit is based on John's excellent P10 design.
What I like about it is:
1) "Passive" 75 us pole at the input. HF gain falls forever.
2) 318 us and 3180 us breakpoints do not interact due to the location where feedback is taken.
3) It is inverting which eliminates CM-induced distortion.
4) The cap values are identical for the 318 and 3180 us breakpoints.
5) The combination of capacitor value and E96 resistor value hits all of the time constants spot-on.
I'll post a block diagram later today.