JBL 7510b, replacing front panel volume control

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kslight
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JBL 7510b, replacing front panel volume control

Post by kslight » Mon Aug 06, 2012 2:36 pm

Here is the schematic: http://www.jblproservice.com/pdf/Vintag ... manual.pdf
So I have a couple JBL 7510Bs, and I have read that you can remove the front panel pot and put in a reverse log pot connector to the d-sub terminal in back to make this function more like a traditional mic preamp. My question is, can I straight disconnect and remove the volume pot, is there something I need to put in its place on the circuit board?

Also, is it possible to have individually selectable phantom power on this, perhaps repurposing the front panel switch...?

Thanks

The Scum
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Post by The Scum » Tue Aug 07, 2012 10:16 pm

Without a little more detail about what's inside the preamp box on the schem (the 10-15785 thing), it's kinda hard to comment on what's going on inside it.

It looks like phantom maybe enters it on pin 7.

To remove and hardwire the gain pot, strap the wiper terminal to the output coupling cap (C24B?).

So much of that circuit is involved in the gating and compression, and the audio heavy lifting isn't really shown...it's hidden in the submodules. It might be easier to build something from scratch in the chassis, than to remove everything that isn't needed to make just a preamp.
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kslight
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Post by kslight » Wed Aug 08, 2012 4:52 am

Essentially what's interesting is that out of the box there is not a lot of headroom, but in addition to the front panel controls you have a D-sub connector on the back, which allows you to to step the gain down so it would be useful...you can either put varying values of resistors in there to achieve the desired result, or you can wire a reverse log pot in there for variable control. Jim Williams has said that he replaces the front panel volume with this reverse log pot connected to the "gain set" ports, which are the connections duplicated on the D-sub connector. I guess what I'm asking is if I can remove the front panel pot completely, or if I need to do something to prevent breaking the circuit where that original pot connects, because that is not where the new pot will connect at.

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Post by Jim Williams » Wed Aug 08, 2012 8:18 am

Those require a bit of work to become fuctional mic preamps. The output opamps in the cards are swapped, I use LME49720's and pull out the coupling caps. I yank out the volume pot and wire in a 2k reverse log gain pot. It's fed from the pcb or you can jack it into the D-sub connector. The output jack feeds the signal on the ring, not tip.

If the preamp cards are exposed, I pull out the quad TL082 opamp and use a LME49740 quad.
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kslight
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Post by kslight » Wed Aug 08, 2012 5:07 pm

Jim Williams wrote:I yank out the volume pot and wire in a 2k reverse log gain pot. It's fed from the pcb or you can jack it into the D-sub connector. The output jack feeds the signal on the ring, not tip.
Sorry to be a pain, but would it be possible to get a little more clarification? Am I accurate when I say you remove the original volume pot completely, and do not put anything in its place on the PCB where the volume pot was connected? And then the 2k reverse log pot goes in to the d-sub, or somewhere on the PCB? If I wanted to connect it internally what point would I solder to? Does it replace the connection of the original volume pot, or is it somewhere else?

Thanks

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Post by Jim Williams » Thu Aug 09, 2012 7:55 am

The volume pots are removed and shorted across. The electrolytic coupling caps are also removed and replaced with wires. It's not easy to find the pick-off points for the gain set, but they do come out at the rear connector. I have wired a 2k reverse log pot with some heavy buss wire and jacked the two wires into the d-sub connector where those jumpers are located. I also wired an extra wire to the pot case to the ground pins on the d-sub so the pot doesn't pick up noise when rotated.
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kslight
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Post by kslight » Thu Aug 09, 2012 4:18 pm

Jim Williams wrote:The volume pots are removed and shorted across. The electrolytic coupling caps are also removed and replaced with wires. It's not easy to find the pick-off points for the gain set, but they do come out at the rear connector. I have wired a 2k reverse log pot with some heavy buss wire and jacked the two wires into the d-sub connector where those jumpers are located. I also wired an extra wire to the pot case to the ground pins on the d-sub so the pot doesn't pick up noise when rotated.
So I can just put jumpers where the old pots were then? Is it necessary to remove the coupling caps if I'm not doing any opamp changes at this time? If so, which caps are they (sorry, I'm still trying to get my head around reading schematics, I do not have a professional tech background...just enough to be dangerous)?

Thanks Jim

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