LED bargraph issue

[druu]

I'm currently designing a mono compressor using the THAT4305 IC and also utilising the THAT Corp bar-graph compression meter design note for this design (https://www.thatcorp.com/datashts/dn112.pdf). The meter functions fine except the last LED only lights very dimly for the -20dB indicator and I'm not sure why.

I'm basically using the values taken directly from the dn112 design note but have adjusted some values according to real world values I currently have on the 4305 circuit.

Here is a sim layout of what I'm currently using:
https://tinyurl.com/25w5544j
I'm not getting the same mV readings as the design note but I was wondering if the real world +/- values of my power supply may be a cause here? I have also tried adjusting to the 27k at the end of the resistor chain in the sim above to 10k so that mV readings are as per DN but all this does is only permit the meter range to be 6 LED instead of the desired 7 string.

Any suggestions appreciated.

 

[RuudNL]

What is the supply voltage?
The voltage drop over a LED is approximately 2.1 Volt, so to light 7 LEDs you need at least a voltage of 7 x 2.1 = 14.7 Volt plus a bit for the current source.

 

[druu]

So I’m using a TC1044SCPA as a rail splitter, splitting the 9VDC to +/-4.5VDC but currently I’m getting +6/-5.5VDC. The falstad link shows the exact meter circuit I’m currently using.

 

[RuudNL]

So, the voltage is too low!

 

[Evertide]

So I’m using a TC1044SCPA as a rail splitter, splitting the 9VDC to +/-4.5VDC but currently I’m getting +6/-5.5VDC. The falstad link shows the exact meter circuit I’m currently using.

What happens without the comperators ? Do all the LEDs light up ?

 

[druu]

So, the voltage is too low!

I've completely overlooked this! Thanks for that, I've just tried the whole circuit with a 12VDC source and it works exactly as expected.

Any suggestions on how I can make this circuit work at the lower 9VDC source? Maybe I need to reduce the metering by one LED..

 

[druu]

Can anyone advise on how I can reduce the brightness of the meter? Is it a case of increasing the value of R21 in the current source?

 

[druu]

Can anyone advise on how I can reduce the brightness of the meter? Is it a case of increasing the value of R21 in the current source?

This seems to have done the trick, settled on 1k6 for some water clear super brights.