Improving the preamp?

[The Basement]

I have an old Yamaha RBX375 with the original electronics. The preamp is often said to be the downside of this bass and, somewhat, holding it back. I'm wondering if it would help if I replaced the components on the circuitboard with newer parts - keeping the same values? For instance, replacing the carbon resistors with metal film and the original op-amp (NJM4558) with a TL072.

Or is the issue with this preamp more the actual circuit being too simplistic and unrefined?

I realise this is a hard question to answer to most people, but perhaps there are some electronics wizards on this forum. Or other RBX375 owners with experience of replacing the preamp with an aftermarket preamp. What was the nature of the improvement?

 

[Snaxster]

I have an old Yamaha RBX375 with the original electronics. The preamp is often said to be the downside of this bass and, somewhat, holding it back. I'm wondering if it would help if I replaced the components on the circuitboard with newer parts - keeping the same values? For instance, replacing the carbon resistors with metal film and the original op-amp (NJM4558) with a TL072.

Or is the issue with this preamp more the actual circuit being too simplistic and unrefined?

I realise this is a hard question to answer to most people, but perhaps there are some electronics wizards on this forum. Or other RBX375 owners with experience of replacing the preamp with an aftermarket preamp. What was the nature of the improvement?

Hello. If it were mine, I would keep the original preamp intact while replacing it in the bass with a new preamp. That approach lets you or a future owner easily revert to the stock configuration.

That's what I did with my 1990s Sukop, for example: replaced but kept the original electronics. The original preamp sounds wooly, warm, and muffled. The modern replacement sounds open, clear, and detailed.

The sound of the original might be objectively less good than a modern alternative, but only the original will let the instrument sound as it did originally. That could be important to a future owner, especially a collector.

 

[A9X]

For instance, replacing the carbon resistors with metal film and the original op-amp (NJM4558) with a TL072.

Not really. MF resistors won't change the tone, but they might lower the noise fractionally, but I doubt it will be audible. Both of the mentioned opamps are pretty awful in terms of SOTA performance, but again whether that's audible in a major way is dubious but I can't say definitively as I don't have the circuit in front of me.

I'd remove the original pre, and keep it or sell it, and replace it with something else.

 

[dwizum]

I concur with @A9X.

In order to get significantly better performance, you're going to need to fundamentally change the circuit. Replacing parts with similar spec "better" parts isn't likely to do much - at best, possibly, a small difference in noise. But usually, when people say "this stock preamp is crap," it's about the frequencies, boost/cut levels, q factor, etc - and not the noise. You won't change any of those properties by swapping for same-value components.

And if you're willing to swap components, why not just build from scratch? There are a handful of very well documented preamp circuits floating around the net. You have a lot of choice in overall design and performance. And it won't be much harder than swapping components in an existing preamp. Significantly cheaper than buying off the shelf, too.

 

[The Basement]

Thanks for all your input. It seems like the concensus is to not mess with the original preamp. Perhaps I will look into building a Sadowsky clone preamp for this bass, keeping the same knob layout.