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Q&A, discussion, and information for the labels covered by The Guitar Gallery (Specifically and exclusively guitars made by Matsumoku up to 1987)
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PostPosted: Fri 20 Apr 2012 11:20 AM 
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Virtuoso
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Joined: Sat 30 Apr 2011 08:55 AM
Posts: 231
Location: Pori, Suomi
Hi all! It has been a while since I have been active however I have managed a hell of a lot. In addition to having made my own SB Black n' Gold I tribute nearing the finishing post (no joke intended) I have successfully replicated the first edition BB Noisekiller pre from the old old old SB-1000s. This repro has already resurrected two basses for customers with dead pre's which is awesome of itself, and I have made headway on making a small run of BBs to keep on stock for other repairs.

As part of this I decided to play around with the op-amp chip used in the pre-amp, substituting it for alternative modern ICs with higher specifications than the original JRC4558.

Why would I do this? Well, a little background history on semiconductor manufacturing might help. Back in the seventies when the BB was originally made, the cleanliness and overall accuracy of the wafers that went into making the end product was nowhere near where it is today. Well, mostly. Cheap fabs still continue to turn out junk here and there (sometimes falsely branding their "product" as familiar names...bar stewards...) however there is a night and day difference in quality in the "real deal" stuff like Burr Brown, Linear, TI, Analog, STM, etc. etc. to those back in the seventies. Especially the 4558!

But isn't the 4558 revered by people who would sell their left nut for a "vintage correct" Tube Screamer? It certainly is, for better or for worse. So why do I want to consider altering the design to one that doesn't use such a marvellous chip? Because it isn't marvellous. In evolutionary terms, the 4558 is a drooling noisy poo-flinging high current consumption monkey with no thumbs. It is truly stone age. Operating in its non-linear zone the 4558 soon heads into the area where its rubbishy slew rate (maximum rate of signal voltage swing) which consequently dampens off the top end frequencies, pleasantly in the case of the Tube Screamer. Using a negative aspect of the chip. In a pre-amp such as the BB, the 4558 is certainly operating within its linear zone.

The differences between 70s op-amps and the same chip manufactured in a modern fab is stupendous. The near-perfect output of modern chips drastically reduces the noise induced by dirty imperfect wafers to the point where the other passive components in the circuit create more noise than the amp itself. My initial expectations on trying modern 4558s in a shootout against pin-for-pin alternatives was that no difference will be discerned. At unity gain, the slew rate of a 4558 is sufficient within the audio range at pickup output levels. That said, my ears and my music equipment isn't the same as everybody else's and any differences will only be positive in comparison to the original circuits.

The lineup consisted of:
RC4558
TL062
TLE2072
TLC2272
NE5532
OPA2277
OPA2107

The report is somewhat of an anticlimax. I could discern no difference between any of the chips. In hindsight I decided to reject the TLC2272 on the basis that the datasheet lists the maximum rail to rail voltage as 16v. The 18v supply of the BB circuit would likely fry these chips or at the very least shorten their operating lifespan considerably.

The newest BBs I am making are now using 1% tolerance metal film resistors, high quality polyprop caps and Nichicon electrolytics to help lower the noise floor further. To be fair, the silence of these pre-amps is what I would term, inky silence. :D

The advantages of the alternative chips are of course a lower power consumption in comparison to leaky old components like comparatively junky ICs and failure prone explosive Tantalum caps. Always a good thing. If anybody wants a new 10-pin BB to upgrade/resurrect their SB-1000 or even convert a passive bass to active, PM me for details. Just waiting on a batch of potting boxes to arrive and they'll be ready to go. :loon:

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"It looks just like a Telefunken U47" - Frank Zappa


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