BA questions: VU meter, variable impedance

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lontas
Posts: 5
Joined: Mon Jan 10, 2011 10:47 am

BA questions: VU meter, variable impedance

Post by lontas »

Howdy,

I'm building a single BA with JLM14 transformer, LME49860 opamp, fet DI, and VU buffer+meter.

1) The table in the BA build thread says RL should be 2k2 + 50k pot for variable impedance, but my kit came with a 100k pot. Is it okay to use this instead of a 50k pot?

2) I want to power the VU buffer/meter off the BA board if possible. Is it okay to tap into the opamp + supply, since that will be less than 30v which is the max of the VU buffer?

3) Can I also tap into the opamp supply to power the led inside the 34mm VU meter (along with the right series resistor)?

Thanks! Looking forward to using the pre (and I can't wait to show off the custom case I'm doing, too)!

Chris

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Joe Malone
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Re: BA questions: VU meter, variable impedance

Post by Joe Malone »

lontas wrote:Howdy,

I'm building a single BA with JLM14 transformer, LME49860 opamp, fet DI, and VU buffer+meter.
Ok remember the fit the #BD681 and a #zener of 39v 400mW so you do not blow up the LME49860 which can only do +/-22v max.
(If you mention the LME49860 when you ordered in the comments box the guys would of added the extra parts for free.)
1) The table in the BA build thread says RL should be 2k2 + 50k pot for variable impedance, but my kit came with a 100k pot. Is it okay to use this instead of a 50k pot?
We always provide a dual 100k log pot with centre detent so it can be used for all combinations. To make 50k join both pots in parellel (front 3 legs to back 3 legs).
2) I want to power the VU buffer/meter off the BA board if possible. Is it okay to tap into the opamp + supply, since that will be less than 30v which is the max of the VU buffer?
You can power the VU buffer from the 48v rail by using a 10k series resistor between the 48v and +V on the VU Buffer PCB so the power in on the VU buffer will be about 24v to 30v.
(DO NOT try to tap into the half rail on the BA PCB)
3) Can I also tap into the opamp supply to power the led inside the 34mm VU meter (along with the right series resistor)?
You can power the VU meter led from the 48v rail by using another 10k series resistor between the 48v and +meter led terminal on the VU meter. And connecting the -meter led terminal to 0v.
(IF meter is to bright lit use larger value resistor like 20k, 34mm meter led terminals are the small terminals on the outside of the Larger meter terminals.)
Thanks! Looking forward to using the pre (and I can't wait to show off the custom case I'm doing, too)!
Cool let me know if you want me to serve the photo's on our website when you are finished
Joe :-)
JLM Audio
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lontas
Posts: 5
Joined: Mon Jan 10, 2011 10:47 am

Re: BA questions: VU meter, variable impedance

Post by lontas »

Awesome, thanks Joe! This takes care of everything I can think of (for now...heheh).

Chris

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