In my experience designing and building amplifiers, if used in an audio capacity op-amp based pre-amps should use a fully regulated power supply rather than a simple diode fix as the "buzz" at the output will be considerable and has to be heard to be believed. A few commercial amps I have come across, mainly older designs, tried the Zener and resistor method in their pre-amp stages and it was always unsatisfactory albeit cheap.1024MAK wrote: ↑Sun May 30, 2021 3:17 pm In this application we mainly need voltage gain rather than power gain. But the amplifier must be a low noise type. The voltage gain has to be in the order of about 5600!
That kind of gain would get you to about 2.8V AC.
A 74LS14 input needs 2V DC to ensure it sees a logic 1 input. But it’s possible to bias the signal from the amplifier so that a 2.8V AC signal when positive goes above 2V DC, and when negative does not go below 0V.
I’m guessing that the idea is that there should be no modification to the ZX81, right?
A possible source of a suitable DC power supply would be to use a splitter cable, and use the +9V DC (nominal) supply from the Sinclair PSU. Use a 1N4001 diode (or similar) followed by a suitable electrolytic capacitor to further smooth the ripple out.
That should provide a suitable power rail for any audio amplifier chips.
Mark
All my stage and guitar amps have separate regulated supplies at the pre-amp stage and are very quiet as a result, so would have thought low noise a priority here that's why I suggested the the audio grade NE5534s in the first place.
Power op-amps however are more tolerant supply wise and use no regulation, hence my suggestion rather than a need for a gain in power but it should be noted that once a pre-amp has done its job it will need to be fed into a power amp type device to trigger the zeddies loading sequence if that is the route he is going down, in the manner of a cassette players earphone output. A pre-amp on its own will never feed the input of the 81 no matter what it's voltage gain without the speaker driving "thump" that ear sockets produce hence the devices made for loading the spectrum that surface from time to time featuring power amps not voltage ones.
The banked pre-amp stages I use kick out at at least 3 p-p but wouldn't even power the smallest of speakers as the current is small however when that signal goes through the power amp stage the current goes up the voltage down in relative terms and as the zeddy is usually fed from a speaker driving source then power must come somewhere into the equation rather than pure voltage.
If however he just wants to feed a chip of some sort with 5v rather than the zeddy directly itself then yes agreed no actual power gain as such needed only voltage.