you do not change that value. with open loop (R3=infinity) OpAmp gain is huge. but adding resistors constraints that gain to some lower, fixed value.
this is how gain is calculated:
View attachment 369976

gain in image 22 (p6 of datasheet) is 5 because:
a) this is noninverting amp (signal is on input+)
b) resistor ratio is 2000Ohm/500 Ohm = 4 so total gain is A= 1+4=5
View attachment 369975
in the project you linked, gain is controlled by ratio of resistors R3 and R4. their ratio is 600/120=5 so overal gain is 1+5=6 (and not 5), likely just a typo. And after all, this is just an audio amp so gain 5 or 6 makes little difference. If this was an instrumentation circuit, such difference in gain would be catastrophic but in audio applications you have a volume knob and adjust it as you please. so of the signal is too high (for whatever is after this stage) you can choose to reduce R3. if it is too low and you need more gain, you can increase R3. if you want to follow that project as is, stick with the R3 value they used. buffer is just buffer so it an be ignored since gain = 1. rest of the parts in the feedback loop (R1,C1,C1a,R2,C2) are shaping frequency response.
View attachment 369977

The 6 instead of 5 might be also due to be "a little over 5", so 6. As you said, it doesn't really matter if its 5.5 or 6. They might just rounded up.

 

you do not change that value. with open loop (R3=infinity) OpAmp gain is huge. but adding resistors constraints that gain to some lower, fixed value.
this is how gain is calculated:
View attachment 369976

gain in image 22 (p6 of datasheet) is 5 because:
a) this is noninverting amp (signal is on input+)
b) resistor ratio is 2000Ohm/500 Ohm = 4 so total gain is A= 1+4=5
View attachment 369975
in the project you linked, gain is controlled by ratio of resistors R3 and R4. their ratio is 600/120=5 so overal gain is 1+5=6 (and not 5), likely just a typo. And after all, this is just an audio amp so gain 5 or 6 makes little difference. If this was an instrumentation circuit, such difference in gain would be catastrophic but in audio applications you have a volume knob and adjust it as you please. so of the signal is too high (for whatever is after this stage) you can choose to reduce R3. if it is too low and you need more gain, you can increase R3. if you want to follow that project as is, stick with the R3 value they used. buffer is just buffer so it an be ignored since gain = 1. rest of the parts in the feedback loop (R1,C1,C1a,R2,C2) are shaping frequency response.
View attachment 369977

This whole reply was what was looking for. Thx for going thru the analysis so meticulously!

I'm just interested in the phono section for now as it is bit novel.

About the obsolete AD745 opamp ....
In the original article about the preamp (2004), the AD745 was already obsolete, and the author noted Rochester Electronics as source.
it seems that Rochester Electronics still has 1000s of them. At reasonable prices. They've been around a while and RE's website notes:

Rochester Electronics is the world’s largest continuous source of semiconductors–100% Authorized by over 70 leading semiconductor manufacturers.

As an original manufacturer stocking distributor, Rochester has over 15 billion devices in stock encompassing more than 200,000-part numbers, providing the world’s most extensive range of end-of-life (EOL) semiconductors and broadest range of active semiconductors.

I have no prev experience with RE, but folks complaining of fake Chinese parts and no-longer-avail "Obsolete" parts might want to look into RE.
If anyone know more about RE, please let us know!

 

you can do that but if you do, get some spares... personally, i would skip any vintage part numbers and use modern parts. this way stock levels are not a problem, you can get replacement any time. also newer parts are there for a reason - better specs. if the old ones have reached "does not need improvement" level, there would be no need to design new parts. as suggested, NE5532 (and many others) will do nicely here.

 

I once attempted to purchase from Rochester Electronics, but they had a minimum order quantity which I couldn’t meet.
You may want to check if that restriction is still in place.

 

This whole reply was what was looking for. Thx for going thru the analysis so meticulously!

Sorry, but I thought it was well known those basic configurations. Next time it will help if you ask specifically what you are struggling. A good question is worth more than 10 good answers sometimes.

 

I once attempted to purchase from Rochester Electronics, but they had a minimum order quantity which I couldn’t meet.
You may want to check if that restriction is still in place.

Min order qty can be single units, despite the min. 25-100 discount category, because one can add single units to their SC . But there seems to be a min. $250 purch requirement, revealed at check out. I'll shoot them an email about how strict they are on that.

 

I have no prev experience with RE,

I do - Excellent.

ak