BA2 OEP/99v Newbie- 47uF Cap Query

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kirkwylde
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Joined: Sat May 01, 2010 11:12 am

BA2 OEP/99v Newbie- 47uF Cap Query

Post by kirkwylde »

Hi All,

I have been trying to find some info on this to no avail- here or @ Prodigy Pro- so forgive me if this has been covered:

There are two 47uF caps which end up underneath the 99V opamp. Their pads are extremely close to those of several resistors (a 10k and a 51R each). I have soldered these in and there look to be shorts between the outside legs of each 47uF, and the nearest 10k to each, and the leg of each 51R closest to the IC socket (which has the jumper from 4 to 7). Sort of like 2 small triangles.... Forgive my electronics ignorance, but having checked the schematic- which is for the 99v/JLM14 config - I can't say for sure if this is safe.
circuit question.tiff
See this screen grab.

To me it looks ok. Having said that, I seem to recall reading that the second of the two 47uF caps is not required for the 99v/JLM14 config, and can't see it on the schematic, so I really don't know if either are going to be ok or safe. The pads themselves, even without solder, seem to be close enough to short anyway so maybe there is no cause at all for concern?

One further question: as the BA2 has variable impedance, what are the Rload/RZobel resistor values I need? I have failed to uncover this info in my hours of web trawling.

Any help will go a long way! Thanks in advance.

As an aside, my experience with DIY'ing is very limited. I have built a couple of stomp boxes and a small Tube amp designed by Bruce McBean. I really want to learn a lot from doing this project, but I also am under pressure to get this thing built as I need to use it on upcoming sessions, so a few short-cuts and prevention from frying an opamp or a resistor or three are going to be a vast help. (I'm still awaiting delivery of an Electronics textbook to help some more!)

Cheers,

Luke
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chrisp
Posts: 184
Joined: Sun Mar 04, 2007 11:46 am
Location: Sydney, Australia

Re: BA2 OEP/99v Newbie- 47uF Cap Query

Post by chrisp »

Hi there Luke
There are two 47uF caps which end up underneath the 99V opamp. Their pads are extremely close to those of several resistors (a 10k and a 51R each). I have soldered these in and there look to be shorts between the outside legs of each 47uF, and the nearest 10k to each, and the leg of each 51R closest to the IC socket (which has the jumper from 4 to 7). Sort of like 2 small triangles.... Forgive my electronics ignorance, but having checked the schematic- which is for the 99v/JLM14 config - I can't say for sure if this is safe.
Know exactly what you are talking about, and yes, those three legs are all on the same signal path so shorts are not disastrous. The trick is actually to mount all three parts before trying to solder any one of them, as otherwise its very easy to get solder spill into the plated holes next door, and that can be hard to remove. That said, any short is not good and you'll find with practice you'll get better at even fiddly places like this.
Having said that, I seem to recall reading that the second of the two 47uF caps is not required for the 99v/JLM14 config, and can't see it on the schematic
With a 99V, there is no "second half dual IC opamp", so you can scrub a few parts, depending on whether or not you are running an output transformer. If no output tranny, you need to link pin 7 to pin 4 (tying the second half opamp output to ground), and keep the 51R, 470uf and 10K load resistor. You can then take the out+ and out- sockets to your XLR. If you do run a transformer, you can omit all that second half opamp path (3 x 10K, 1 x 51R, 1 x 47pf and 1 x 470uf) and take out+ and 0V to your output transformer primaries. That said, having the parts in place does no harm. Its probably better to built 'em in unless you have a reason not to.
One further question: as the BA2 has variable impedance, what are the Rload/RZobel resistor values I need? I have failed to uncover this info in my hours of web trawling.
This info is in a table of the main JLM sites BA page http://www.jlmaudio.com/Baby_Animal_Mic_Pre.htm just above the output transformer wiring pictures.

Good luck with the build. You'll love the result.
Chris P
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I do lots of things. I believe eclectic skills are best.

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Joe Malone
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Re: BA2 OEP/99v Newbie- 47uF Cap Query

Post by Joe Malone »

The two sets of pads where the 10k,51R and 47pF caps join are all connected on the PCB so solderiing all three of these pads together is normal.
Joe :-)
JLM Audio
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