VU meter amplifier/rectifier.

[mbates14]

I have a flourescent VU meter board, and it has BA658s onboard the unit. This meter requires an external amplifier/rectifier time constant to drive it.

I remember the LED VU chip BA6124 had a built in high gain rectifier. So i built a circuit around this IC and the amplifier actually drives this VFD nice. problem is, i only have one IC. and im actually wasting it, as im not using the bult in 5-LED scale. just using the amp/rect section.

Does anyone have an equivalent circuit of the amp portion of this IC?

I tried building my own LM324 driver/rectifier and i couldnt get it to work at all. I need something that works on single supply.


any ideas? thanks.

[DigitalJunkie]

Rod Elliot has a couple rectifier circuits.
http://sound.westhost.com/appnotes/an001.htm

[mbates14]

like i mentioned before. those circuits are designed for split-rail. i need something for single supply.

[Steerpike]

The LM324 is specifically designed for single supply, although it's AC performance isn't really wonderful - so its not great for actual signal work. For a meter driver shouold be perfectly adequate.
Maybe if you post your circuit we can see why it's not working.

[mbates14]

circuit is pretty crude. but here it is:

it worked in one design but not the next. its weird.

[Steerpike]

The problem is tying the op-amp + input directly to one of the power supply rails. No op amp really likes this.

The easiest fix - without changing your basic circuit - is to create an artificial 'ground' reference for the op amp at half the power supply voltage (R5,R6,C5 on my diagram) and then reference both op-amp inputs to that voltage (R7, R8). This also means you need to DC isolate the op amp circuit from the rest of the circuit on both sides by means of two 47uF capacitors (C2, C3).

This diagram ought to illustrate the point.

(Don't pay any attention to me mistakenly labelling R2 as 47k)

[mbates14]

yea circuit is on a breadboard so altering a circuit isnt a factor.

With those 47uf caps, wonder what the frequency response would be? The first circuit when it did work properly, (barely) the VU meter only responded to the bass frequencies. probably because of the positive input issue?

[mbates14]

on a lighter note. what schematic program do you use? thanks. I like the way that looks. hehe.

[Steerpike]

With those 47uf caps, wonder what the frequency response would be

Well the ones on the output are the ones of concern; the input ones are loaded by the op-amp, it's bias- and feedback resistors - which is a load slightly less than 100k.
The output is loaded by *about 1k*, so using the formula:
f = 1/(2Pi.R.C) gives a turnover frequency of around 3.3Hz.

Since electrolytic caps are only specified to around +/- 50% tolerance or worse, it's pointless trying to be TOO fussy with the maths.


My schematic drawing software is Orcad V3. Old, yes, but then I prefer my Garrard 301 over an Ipod, so don't try to talk me into something 'modern' that wants 2GB of memory.

[mbates14]

well it gets the job done. so who cares what version it is? hehe.