Redesigning a cheap obsolete 8-track pre-amp.

[HHaase]

Ok, I know it sounds silly, but I'm looking to redesign a cheap old 8-track pre-amp used in 1960's/1970's arcade games.

It's a real oddball that uses 4 different tracks for various sounds within the game, switching them on and off depending on things happening within the game.

Problem is, the preamp circuit uses Motorola MFC8040 pre-amp chips. Noisy little freaks that haven't been made since prior to 1975 when 0.1% was considered to be low distortion. The original design only powers the preamp chips when the associated sounds are played. Kludgy, eh?

Anyway, I'm out of my league doing audio work. Normally I repair pinball circuitry. But I'd like to make updated pre-amp boards, and eventually amplifiers, for these old machines.

I'm slowly coming along in what I want to build for the circuit, and I'm leaning toward an op-amp style preamp. But I can't wrap my head around which would be a good choice. The tape head that it uses has an impedance of 3k ohms and an output voltage of .8mV at 1khz. The old preamp chip has a gain of around 80db.

Just keep in mind that I need four of them per pre-amp board, as I need to run 4 separate channels to keep it compatible with the rest of the machine. So cost is a factor, as is size. It's also not running the highest quality audio, we're talking cheap 8-tracks after all.

-Hans

[WSJ]

An NAB tape head preamp circuit can be found on page 26 of the
LM4562 data sheet. A quad op-amp could be used to keep the cost
a few dollars.

http://www.ti.com/lit/ds/symlink/lm4562.pdf

[HHaase]

I did see that one, and also am looking into the LM833. I think there's going to definitely be a point of diminishing returns for this regarding audio quality, given the source media.

My main question though is what factor the impedance of the tape head plays in all of this. I'm assuming I'd need to somehow factor that in when interfacing with the op-amp.

-Hans

[HHaase]

Actually, the more I dig into this project, the more I realize I'm wasting my time going in the wrong direction. In the late 60's, this was a fairly decent concept, but there's just too many more powerful modern options available out there. Why beat myself up over a 5 minute long endless loop 8-track that's just going to wear out and sound like crap again anyway.

I think I'll actually go with something based on a single chip multi-channel recorder/playback IC instead. I can get better sound quality, better longevity, lower cost and better reliability. But that's a concept for a different thread.

-Hans

[Pano]

I hope you do open another thread. Something like "Pinball Sounds, moving from 8-track to Digital". Supposing that you just want a modern system to play the sounds that were originally on tape.

[HHaase]

Originally Posted by Pano

I hope you do open another thread. Something like "Pinball Sounds, moving from 8-track to Digital". Supposing that you just want a modern system to play the sounds that were originally on tape.

Totally correct. If a simple reproduction was possible of the original boards, I'd probably stick with that, but there are just too many obsolete components in there. The only thing I'd be retaining would be the tape head and the tape.
They weren't exactly high quality tapes when they were new, and the short length on them means the tapes will wear far too fast. At that point why bother, if I can find a better audio source.

-Hans