[guzzler]

I've always toyed with the idea of a VU meter and have finally got round to starting to design a PCB for it (it's done). It's more or less eye candy, but a little bit of usefulness would be nice (might even get round to including the PPM precision rectifier from the datasheet!). Using the LM3916, I've got a couple of questions:

1) If i use a trim pot to calibrate the input, does this present a variable impedance to the source (I will be using the VU in a PPA based preamp eventually, VU to go before the PPA board)?? If so, can I use a opamp set at gain=1 to buffer the signal, and what configuration would this need, eg resistors like R1/R2 from the META schematic??

2) Grounding; does the ground from the line level source go to the -ve rail on my VU board? The VU board and preamp board (PPA) will use seperate supplies, so I'm not sure if this is an issue!

edit: sort of the design I'm using, just no rectifier on board mine, so in mine, E becomes 0V from my 12V PS:

http://sound.westhost.com/project60.htm

thanks for reading!

g

 

[jeffreyj]

Are you referring to "VR1" on the schematic? If so, that presents a variable source impedance to the LM3915, but a constant load impedance to the monitored signal (50k). No additional buffering is necessary.

Yes, the signal ground would connect to the V- pin of the LM3915. The inverted triangle symbol on the schematic means "ground". Powering this circuit from a separate supply is not necessary if the appropriate measures are taken to filter/isolate the rails from the audio circuitry (specifically, putting a ~10R resistor in series with the signal ground for monitoring the audio and a separate, directly-connected ground for the power (which will be carrying the LED return current).

 

[guzzler]

. (doppel ganger!)

 

[guzzler]

thanks for the answer jeff! i'm still gonna have to power with seperate supplies; the VU wants a max 20V, whereas the PPA will probably eat up 24V, and i intend to have some digital circuitry as well, so the number of regulators kinda mounts up!

and yes, it was V1 i was referring to!

great, my design will work then! I knew the triangle was the ground, but I've never done anything involving two power supplies with virtual grounds and the like! thanks again!

g

 

[guzzler]

sorry, forgot another question re your post jeff...

should i put a 10R between signal ground and VU ground even if they are on seperate supplies??

g

 

[jeffreyj]

Quote:

should i put a 10R between signal ground and VU ground even if they are on seperate supplies??

Nope. Only if you intend to power them from the same supply, which I encourage you to look into doing. Use a 7815 to drop the PPA V+ rail down to something well within the LM3915's operating range (rather than just a hair's breadth within it!). In case you haven't looked at it, here's its datasheet:

LM3915

 

[guzzler]

hmmm... i'd rather stick with the dual supplies, just for the digital section as well as the VU meter, but i'll certainly look into using single supply! thanks for the help jeff, much appreciated!

g

 

[jeffreyj]

Ack - digital section?! As in, perhaps, an outboard DAC? You'll definitely want to either go with separate supplies, or keep the analog and digital grounds absolutely segregated on the pc board with a single (fat!) trace from each separately connected to a spur from the (last) reservoir capacitor! Heck, you might even need two supplies just for the DAC (some have separate power connections for the digital and analog sides of the chip).

Good luck! (you'll probably need it!

)