How Does API use an LM3915 to get a range greater than 30dB?

It's my understanding that API uses the LM3915 for led metering in the API 3124 and API 512 preamps. The meters claim to go from -12 to +18, but to do this they would need to cover a 33dB range, as opposed to the 30dB range given by the LM3915 spec.

I've tried examining the 3124 schematic someone posted, but it's hard to follow due to all the unclear overlapping lines. How is this extended range being achieved?

EDIT: This is the schematic for which I'm referring to: http://www.classicapi.com/catalog/images/gallery/3124/1987-era-3124-schemo.pdf

Corrected, nevermind.

Just took a look, they jump 6 db on the bottom of the scale (-6 to -12) and 12 db at the top (from 6 to 18), so they might somehow be playing some games with additional resistance at the bottom and top of the chains.

Looking a little closer, they're only using 7 steps, so possible they're doing something like tying the top 2 and bottom 3 pins of the chains together or something along those lines.

It's always fun when looking at old hand drawn schematics to guess where and how many errors there are.

While the crossing schematic lines without junction nodes is a little ambiguous, there are a few things that do not make sense as drawn.

They only use some of the LED drive outputs but they treat the unused lines differently. Some are just floated while two are connected to the +unreg supply through different value resistors. There is no reason I can think of for doing this so I would look for alternate connections that make sense using those parts.

The designer may be using the LED drive output as a logic signal to shift meter input thresholds and extend the range. If I wanted to figure this out (I don't) I would look at how the circuit acts with the diodes and resistors connected to the other ends of R26 and R27.

have fun... when trying to read old schematics, most parts shown are really there, but not always connected as drawn.

JR

to get a range greater than 30dB it is possible to give a small negative voltage to Rlo pin (pin4) for example

scottlangendyk said:

...The meters claim to go from -12 to +18, but to do this they would need to cover a 33dB range, as opposed to the 30dB range given by the LM3915 spec...

Click to expand...

Hi,
the covered range is 30dB, not 33dB, so the LM3915 will fit just fine, IMO.
Regards,
Milan

moamps said:

scottlangendyk said:

...The meters claim to go from -12 to +18, but to do this they would need to cover a 33dB range, as opposed to the 30dB range given by the LM3915 spec...

Click to expand...

Hi,
the covered range is 30dB, not 33dB, so the LM3915 will fit just fine, IMO.
Regards,
Milan

Click to expand...

That's not the case, look at it like this, here's 10 steps of 3dB intervals starting from -12:

1: -12
2: -9
3: -6
4: -3
5: 0
6: +3
7: +6
8: +9
9: +12
10: +15

You would need an additional step to show +18. My question is how API does this.

Take a close look at what John said:

The designer may be using the LED drive output as a logic signal to shift meter input thresholds and extend the range

Click to expand...

As usual, John hit the nail on the head. 

Regards,
Jeff