Cascading LM3915s for 90dB VU-meter

[Black_Myke]

I want to make an 90dB stereo VU-meter...Muhwahaha!!

But there's a problem: in the datasheet of the LM3915 (www1.jaycar.com.au/images_uploaded/LM3915.PDF), there isn't a very clear method to combine 3 LM3915s for BAR mode. Ok, for "dot" mode, I know how to do it, but what I want is the posibillity to swich between "dot" and "bar" mode.

I know that if I want to cascade three LM3915s, I will have to:
1) Connect IC#3 directly to sound input, then amplify the signal by 30dB for IC#2, and again, another 30dB for IC#1. In that way, I'll have to know witch OpAmp (exactly) should I use for A1 (in the LM3915 datasheet from National Semiconductor, page 11/24);
2) Attenuate the signal to IC#1 by 60dB, for IC#2 by 30dB and IC#3 remains at full signal (without attenuation).
_________________________
project included in the Earhquake audio system (1100W rms).

[Black_Myke]

Seems like it's too complicated, so I'll just give you a screenshot of my ptoblem (from that PDF doc.).

All I want to know is what IC should I use for A1 (and A2, in my 90dB case). Should I use NE5534s?

[Mikael Abdellah]

*bump*

I too want to make an 90dB VU meter and I could really use some help with it. Has anyone here done it? What are the main problems? Wouldnt it be easier to have the signal directly to the "top" LM3915 and divide it with resistors for the two "lower" ones?

[yulquen]

go digital!

see this thread for different approaches:

http://www.diyaudio.com/forums/showt...297#post606297

--
Yulquen

[Mikael Abdellah]

i would if i could. Its for my tube bass guitar preamp. But after some simple math I realized that a 90dB VU meter is almost impossible to build. Despite my outrageous 0dB level the -90dB led would represent 0.000000009 volts. Thats just ridiculous. I have decided to build a 60dB with 2 LM3915. That might prove to be over my head even that since -60dB only will be a few fractions of a mV.

[Mikael Abdellah]

I have one more question regarding vu meters in general. I've been looking at the LM3915 pdf manual and there is a table with example voltages of each step from 0dB down to -27dB. What I would like to know is how to calculae the different steps. I know that they are using 1.412628902 but from where to they get that number? At first I thought it was the square root from 2 but that is 1.414213562. Close but no cigar. Where does the number come from?

[macboy]

The formula to calculate the ratio of two power levels, expressed in dB is:

dB = 10*log(Px/Py)

Where Px and Py are the two power levels. The actual formula is Bel = log(Px/Py) (which is to say that a Bel is defined as a 10x increase in power), but since we prefer to use deciBels, it is multiplied by 10.

However, since we always use dB to compare power levels, and we usually measure voltage levels, that formula isn't what we need. Remember that since Power (P) = Voltage (V) squared, divided by resistance, then Power is proportional to voltage squared.

Then

dB = 10 * log((Vx/Vy)^2) (where Vx and Vy are two voltages)
or
dB = 10 * 2* log(Vx/Vy)

dB = 20 * log(Vx/Vy)

If you solve for the ratio:

3 dB = 20 * log(X/Y)
3/20 = log (X/Y)
10^(0.15) = X/Y
or X/Y = 1.4125375446 (...)

That means that each 3 dB step represents a voltage which is 1.4125375 times greater than the previous step. Case closed? No.

That assumes that we are truly talking about exactly 3 dB. But usually, when we say 3 dB, we almost always really mean 2x the power (or 1/2 the power for -3 dB). Double the power actually means 3.0102999566398(...) dB. Double the power (from P=V^2 / R) is exactly sqrt(2) increase in voltage, or 1.41421356 (...), which is not exactly a 3 dB increase.

[Mikael Abdellah]

Now I am almost ready to test my first setup with 2 cascaded LM3915, but I there is one thing I can't understand. Assuming bar mode, I can't see how they are cascaded. Looks to me that it is basically 2 independent VU meters with different ranges or am I wrong? Also I'm not sure if the threshold of tha last few LED's can be much higher that V+? I would like the last led to indicate something between 10 and 12 volts, but I prefer to use a V+ of 5VDC since the IC will disspate too much heat otherwise.

[-ecdesigns-]

Hi Black_Myke,

I already build a nice digital VU meter years ago. It uses 50 LED's for each channel, multiplexed, driven by a microchip controller with build-in A/D converter. The scale indicators are multiplexed and displayed too, but with less brightness. The peak value is also displayed for a second, then slowly rolls back. I used five LED block's, each containing 10 LED's. Log scale was implemented in software.

I already designed a printed circuit board for it. So it's quite easy to build

I added a photograph to show two of these VU meters in action on the set I build.

[macboy]

I would like the last led to indicate something between 10 and 12 volts, but I prefer to use a V+ of 5VDC since the IC will disspate too much heat otherwise.

Attentuate the input and adjust the sensitivity accordingly.