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Old 9th April 2019
Here for the gear

Theres nothing wrong with trying using resistors to match your impedances if you are the patient, careful and mindful type, but the common way is to use the gain knobs on a mixing console, or a direct box/preamp.

A potentiometer is a variable resistor, by definition, so u could just throw a 100 or so ohm pot on the cassette outs, in parallel to the audio signal, and adjust it close to wide open to get around that 50 ohms. You could just as well try a 8 or 6 ohm resistor in parallel to audio signal as a temporary fix. This essentially would be adding gain into the interface so you dont have to push either the tape main outs or the input volume sliders past unity (about 4/5 up).

Proper gain would adjust the input resistance- make the input more inviting and accommodating to the current. Using resistors in parallel on the tape audio out would draw more current out of the output stage of the tape, and on the factor of 600 ohms to 50 ohms, it could cause the audio output circuit to overheat- thats doubtful to damage any components but would introduce distortion , so, if u go that way, use potentiometers so you can bring it right to barely distorting, then bring it back clean.

You just dont want any of your volume sliders wide open- thats asking for background noise.

A preamp/direct box with ground lift would be the best for avoiding em interference type background hiss, but its not necessary in every building. In a home setting with no bad ground, a stable power grid, and not nextdoor to a radio broadcast tower- you just have to keep your input cables from running parallel to any power cables. The natural noise floor of 24 bit recording isnt gonna be audibly noticeable, even at -10, but if you crank all the ins and outs till 11 you will introduce noise; hence a mixer or preamp with gain, to match impedances. Personally, id use a small, high quality mixer in front of your interface- but thats only because my real preamp / direct box seems to have died last week... A pair of potentiometers and a small project box sounds fun, as well. Just make sure you account for the total amperage, wattage- not just ohmage. Its doubtful youd burn out the pot just attenuating a -10 db line level audio signal; however, if you exceed the wattage rating of a pot or resistor, youre askin to start a fire. If the output stage of your tape player uses all 2(+) watt resistors, and decent capacitors, you should be fine overdriving it a little with some parallel resistors or a potentiometer- its those mosfet ic''s that burn out first, and your tape player shouldn't have any of those

. You are looking at approx 600 ohms,-10db,for a microphone or guitar input, and around 50 ohms,+4db, for professional electronic instruments. And we havent touched trs yet. The actual outboard recording and top line PA hardware uses trs , three conductors, 1/4 inch, like an xlr mic cable, that cancels out em interference and noise, and rates at +12 db continuous- thats four times louder than the +4 ts 1/4", at the same noise floor. TRS is only used for serious pro hardware recording and the top PA gear because the sheer amount and lengths of cables in these settings need special precautions against picking up em interference.

None of my instruments- synths, samplers, drum machines, guitars) have trs ( looks like a stereo 1/4 jack), because the unbalanced guitar cables ( mono 1/4") and RCA pairs arent near long enough to run into so much em interference.

My bass amp has all unbalanced ins, but it has a balanced record out- because that record out could possibly have to run 100 yards back to the recording setup, alongside 30 other audio signals, all running back to the sound booth together.

Idc about dbv, dbw whatever. Decibels are defined on subjective opinions. One decibel is hardly noticeable, 2 decibels are very noticeable, and an increase of three decibels is perceived as a doubling of sound pressure/ volume. If u could reach +4 db nominal, your inherent noise floor would drop fourhold- but even at 4x noise floor at -10 is not going to be noticed in 24 bit, unless you are cranking your tracks to peak at 140 db in an arena hall.