As I've said before, the RIAA pre-emphasis that is required to convert constant amplitude recording is almost never published because most cutters never cared about it. Professional cutting amps had the EQ built in as a switchable filter, and the cutting engineer simply switched it into the circuit. The only time it was switched out was when certain testing was done on setting up the cutter head. Here are the RIAA EQ curves as applied to constant amplitude recording and playback:
Curve A is for recording, curve B for playback by a constant amplitude (crystal or ceramic) cartridge. Note that this curve looks a LOT different from all the other RIAA curves that you've seen published and that there is much less level deviation from flat. That's because the commonly-seen curves are playback curves and INCLUDE the 6dB/octave de-emphasis required to convert a constant velocity magnetic pickup to constant amplitude in addition to the RIAA de-emphasis. So, if you've already got your cutter head EQ'd to do constant amplitude recording, you need add only the above curve to do the RIAA pre-emphasis.
Now, some particulars regarding the application to home and semi-pro recorders: I'm not really too familiar with the early semi-pro recorders which is what I would call the Presto, Rek-O-Kut and similar units. These were designed primarily for use by radio stations to do air checks, record local commercials, etc. before the age of magnetic tape (i.e. pre WWII). There wasn't too much standardization of EQ curves back then, even among the companies that recorded for commercial, public release records (Victor, CBS, Decca, etc.). That's why you'll find about seven EQ playback settings noted on the early hifi preamps that date from the 1950 period. RCA had been using an EQ curve that they called "New Orthophonic" in the postwar period, and they managed to get the newly-formed Audio Engineering Society to approve the curve for general commercial release use around 1947. The Recording Industry Association of America (RIAA) was also really just getting going then, and they adopted this curve for their official use as well. Refer to the RIAA curves shown above: The EQ presented by this curve had the original intended purpose of reducing rumble from the record pressings (by rolling off the bass response below 50Hz during playback - which requires a corresponding boost during recording) and reducing high frequency tracing distortion (by reducing recorded treble levels by around 12dB or so above about 2kHz - which requires a corresponding boost during playback). These EQ changes are with respect to constant amplitude recording which was the basic de-facto standard going into this time.
So, in order to understand what part of the RIAA EQ you need to implement, you need to know how your cutter behaves with no EQ. Crystal cutters are essentially constant amplitude cutters, but they have a mechanical resonance in the audio range which causes their response to fall off at higher frequencies. Magnetic cutter heads tend to have a pronounced resonance peak in their response below which they behave as constant amplitude and above which they behave as constant velocity. Most makers of cutting systems that used non-feedback magnetic cutters used some form of electronic EQ to try to make their cutters behave as constant amplitude over as much of the frequency range as possible. This usually meant that they had to EQ a notch or dip at the cutter resonance, then add some treble boost above the resonance. This EQ will be built into the maker's recording amp which matches the cutter head model, and you probably can't switch it on and off. In addition to this fixed basic cutter EQ, the amp may also have switchable RIAA (also adopted by the NAB - National Association of Broadcasters) EQ.
Long story short(er), if you want to make records that can be played by most people and sound correct on their home record players - all of which built since about 1947 are nominally designed to conform to the RIAA playback EQ, you need to implement some EQ in your recording that produces flat frequency response when played back with the RIAA de-emphasis. Figuring out academically the specific EQ that you need to add to your particular cutter is probably a waste of time due to all the variables, including the age of your particular unit. Your best bet is to attach a 10-band or higher resolution graphic equalizer to your cutter amp and use it produce the flattest frequency response you can get when playing back test cuts on a good quality stereo turntable equipped with a good modern magnetic cartridge played thru an accurate RIAA preamp. Use an audio signal generator (and AC voltmeter on playback) capable of covering the entire audio frequency range, particularly from about 30Hz to at least 15kHz. Set your recording levels to produce about -20dB from your standard loudness for these tests. You DO NOT want to record very high frequencies at levels above this for two reasons - you run a very real risk of burning out your cutter head because you will need to add a lot of treble boost to achieve flat frequency response, and even the best modern playback cartridges cannot handle high levels at high frequencies, so even if you manage to cut high levels, you cannot play them back accurately so you can't tell if your recording is correctly EQ'd. You'll need to make several tests of spot frequencies throughout the audio frequency range - record some spot tones over the whole range of frequencies, play them back into the AC voltmeter and measure the output level at each tone, adjust your recording EQ, repeat the test - until you come up with fairly flat results, i.e. the same output voltage on the voltmeter at all the test tone frequencies. It's tedious and time consuming, but it's the only way to get the result you want.
