Ask Eddie: In Search of Semiconductors

Sep 1, 2011 9:00 AM, By Eddie Ciletti

THE QUEST FOR THE PERFECT DATASHEET

Polls


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Figure 2: Two schematic excerpts, same circuitry. The left version has actual op amp and transistor part numbers, the right version has factory part numbers.

Figure 2: Two schematic excerpts, same circuitry. The left version has actual op amp and transistor part numbers, the right version has factory part numbers.

BACK TO THE QUESTION
dbxpro.com has an extensive collection of “classic” and “vintage” manuals, and product manuals from that era often included schematics. The schematic downloaded from the dbx site differed from the one in my library, in that all of the IC op amps and transistors had factory part numbers, which are not at all compatible with cross-reference libraries and datasheets. The one in my collection had standard part numbers. Neither schematic has great resolution—I was not able to “decode” the LM308’s factory number. This is not a huge problem because the part numbers are usually on the components themselves, and so with the schematic and the unit side-by-side, you can sort it all out. (See Fig 2.)

The parts in question were the op amp OA11 (LM308) and transistor Q13 (dbx142081 = BC453). As mentioned in my blog, the National Semiconductor site had the original LM308 datasheet and a recommended replacement and its datasheet. But Kevin and I both had trouble finding any BC453 info.

I scrolled through Google’s search results until something promising showed up at alltransistors.com, which is a new site to me. There was no “official” datasheet, but the operating parameters in the table have nearly everything you need to know about the BC453, except for its pin-out, which cannot be assumed (alltransistors.com/transistor.php?transistor=23213). By the prefix we know it’s a European part—or is it? The manufacturer is listed as Toshiba. The BC453 is a silicon NPN transistor in a TO-92 package. At 30V max (between collector and emitter), it’s a low-voltage device capable of 300 milliWatts (mW) dissipation. It has a “forward-current transfer ratio” (aka, “beta” hFE or current gain) of 110 minimum.

I had no luck finding the BC453 on Toshiba’s site, but here’s where it gets easier. Most parts distributors—like Allied, DigiKey and Mouser—include datasheets on their sites. Their geeky search engines suck so avoid narrowing the search parameters too much; it’s better to have several pages to sort through than no pages at all. None of the suppliers listed the BC453, and the first narrow search on Allied yielded only an NTE 289. (NTE’s numbers do not relate to device manufacturer’s numbers.) Digi-Key provided at least three possible contenders—2N4401, 2N3904 and MPS-A06—that Allied also carried after I relaxed the search parameters. From Mouser, the BC-548 was added to the list. All have better specs than the original, beta being the one parameter that should be close to the original. All datasheets were downloaded into my library.

One final note about the dbx 165: The sidechain goes through two switches! On most signal processors, the audio signal path (AC) is pretty straightforward, but the sidechain comprises DC circuitry that is not as easily negotiated. (The sidechain is what turns the audio signal into a control voltage that can be manipulated by ratio, attack and release.) We know what audio sounds like through a 35-year-old switch or pot, but we can’t hear what happens to this important DC signal. From OA11 (LM308), the signal goes to OA10 (LM311 for attack/release) and then through the Auto-Manual switch (ouch-1), followed by OA14, OA4 and the master/slave switch (ouch-2) before the journey ends at the VCA.


Ask Eddie Ciletti whatever you want at blog.mixonline.com/mixblog/category/ask_eddie.






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