Your phono may be capable of outputting 18V but I strongly doubt it’s output is getting anywhere near to that level. What is the cartridge output and what is the gain of your phono? If you do the math, V out is never more than 1-2V and can be a little higher on peaks but not 18V. Unless there’s a huge mismatch between cartridge and phono, like driving an MC phono with an MM cartridge. In that case, you’d overload the phono inputs before anything else.
Yeah agree with @lewm this is a case of worrying about the wrong spec. 18V max output is actually good - that means it’s way "over-engineered", with gobs of headroom built into its output stage. You’ll never approach that 18V output limit. You’ll probably never even breach 2V. It would take an epic mismatch of cartridge output level to gain setting (way too high in both cases) to get over a few V of output - and you’d have way way more problems from the mismatch itself, than from worrying about what you’re feeding into the downstream preamp stage. If the phono's max output was 100V it would make no difference to your preamp - only that the phono output stage (and PSU) is even more over-engineered in that case - and at that point, the overengineering is a needless waste of resources & budget that will negatively affect the overall design.
Wrong spec. Hilariously wrong. The high max is probably explained by the phono stage being overbuilt with headroom to handle a wide range of cartridges. Ultimately the problem is in confusing max output with actual output. Like when a guy worries about using a 300 watt amp with 100 watt speakers. Neither spec is of any use at all!
What you care about is that in actual use the amp is going to see about 1-3 volts from the phono stage. No worries.
Your question raises a more useful subject, the proper use and misuse of specifications. The vast majority of audiophiles the vast majority of the time will be well served to ignore pretty much all specifications. At least until you know way more than you do now. When you know enough to never even think of asking a question like you did then you will know enough to pay attention to specifications.
What will happen then however is you will know there are only a very few specifications worth any attention at all. And even then only cursory. Speaker sensitivity, you want to be at least 92dB. Because then your amp choices are many and easy. Cartridge output you want to be at least .3mV, because then your phono stage choices are many and easy.
Pretty much all the rest you can take a pass. This approach frees up tremendous time and energy to be spent studying what really matters, the way people say the stuff sounds. You haven't mentioned that. Even though that is all that really matters. See how much time I just saved you?
4mV (0.004 V) into 40dB gain is 0.4 V
Multiply by 10x for each 20dB, OR multiply by 2x for each 6dB, so:
42dB = 6db + 6db + 6db + 6db + 6db + 6db + 6db = 2x * 2x * 2x * 2x * 2x * 2x * 2x = 128x.
Using these 2 rules of thumb you can quickly convert from dB's to gain ratios and vice versa. Just guesstimate whenever a dB value lands between a multiple of 20 or 6. Impress your friends with quick mental conversions! Like Rainman lol. Got a 16x SUT? that's a little less than 10x * 2x but more than 10x which would put it between 26dB (20 + 6) and 20dB, so...maybe 24dB? Yep that's it!
smrex, You actually raised an interesting question which caused me to think further after responding based on my gut instincts and having done a few gain calculations for phono stages in the past. 40db of gain is 100X voltage gain. So the output of your phono stage with your 4-mV cartridge is 0.4V, at 1kHz. However (I was thinking), that calculation is for 1kHz, where the RIAA curve is a flat line. As you go up in frequency, the voltage output of any cartridge goes up because MM, MI, and MC cartridges are velocity-sensitive. The faster the stylus moves, the more V is created. On the other hand, above ~2kHz, the RIAA correction filter is attenuating the signal by about 6db/octave. So I began to think more deeply about just what is the max voltage output of any phono stage, and at what frequency. I consulted a guru friend who sometimes posts here and is "in the business". He cited some studies that show that the max V output from a cartridge occurs at about 7kHz. When you account for RIAA correction at 7kHz and based on a reported observation cited by my friend, the peak voltage (at 7khz) can be 24db above the "standard" at 1kHz and 5cm/sec, or in this case 16X0.4V, or 4.8V, at 7kHz, for your particular system. Your integrated amplifier is not being overdriven by the phono stage.
A sensible line stage has the volume control ahead of the gain in the line stage, so you really can't overload the line stage even with 18 volts, although the volume control would be tricky to use.
So with the phono section no worries.