OT - Positive / Negative supply rails

[Malc C]

Quick question for you more experienced guys.

In my search for a decent bass filter, I've noticed that a lot of the active filters use op-amps such as the TL 082, TL 084 etc, and have +ve and -ve supply rails. My question is that if I used a +5ve and 0v rather than +5ve and -5ve would this have an adverse effect on such circuits, example attached ?

[paulg]

Hi

As the output will only be able to swing between 0v and +5v you would have to ensure the input signal was biased such that it never goes negative (WRT 0V).

[slipstick]

If you're talking audio, you'll have to capacitively couple the signals in and out of the circuit because input and output are no longer ground/0V referenced. Plus 5V is not really enough voltage. TL08x output only swings to within about 1.5V of each rail so you'd get less than 2V peak-peak out, possibly a lot less. The specified power supply is 7V minimum so somewhere around 9-12V would be more practical.

BTW that's not a very good bass filter cct, what happened to the "bass beat extractor" you were building ? Which you might notice uses the same TL08x op-amps on +12V with no negative supply required.

Steve

[Malc C]

Guys, thanks form the feedback.

Yes we are talking audio for the filtering. The filter is part of a three channel sound-to-light thingy I came acorss whilst searching for decent beat extraction for my lighting project. The full schematic can be found http://www.amarkham.com/andrew/light_master.htm

Steve, the base beat extraction that outlined in that other thread is working well. I modified a couple of capacitors / resistors and managed to get a decen result, have a look http://www.micro-heli.co.uk/discolights2.avi

I was just wondering that as I was using an audio input which I presume will be both negative and postive going signal, having a 0 to +5v supply might be the reason I experienced strange results when I built the complete filter in that other thread yesterday. - So now I'm really confused, would 0 to +12v be better, or -5v to +5v be perferable ?

[Comatose]

You could use a max232 type chip to make +10v and -10v from your 5v signal, at a cost of a buck or so. It can't really supply any real power (few milliamp) but its fine for driving an op-amp. You'll need to put bigger decoupling caps on the outputs of the max232 if you go that way.

[Acetronics]

Hi, malc

I've looked at your files :You play it yourself "pro fashion" ... :D

For your supply question ... if you want to make it easy for modules interconnection, always have a symmetrical supply : you will keep a neat 0v common reference !!!

How ??? Once the circuit built, have a look to ICL 7660 or LTC 1044 ... which are voltage "inverters" ...
At design stage ... get the -v from a mains 2 secondary windings transformer ... and a couple of 78xx/79xx ;) of course !!!

Alain

[slipstick]

I was just wondering that as I was using an audio input which I presume will be both negative and postive going signal, having a 0 to +5v supply might be the reason I experienced strange results when I built the complete filter in that other thread yesterday. - So now I'm really confused, would 0 to +12v be better, or -5v to +5v be perferable ?
Symmetrical supplies are easier because you can get away without capacitors on the input.

If you built the circuit exactly as drawn you had two problems.....
1. +5V is too low, the op-amp would not work properly (minimum supply voltage is 7V as I said before and 5V is less than 7V ;)). Hence 12V would be better from that point of view
2. Without a decoupling capacitor on the input you may have been messing with the input signal and only seeing a partial signal. It depends whether whatever you were driving it with had a floating output or was ground referenced. You tell me ;).

Steve

[Chippie]

why not use a 7660 to generate the -ve rail?

[Malc C]

Thanks again for the input. I've used 7660's to provide a -5v for a few circuits, but didn't think about it in this case. As most of the stuff I've been playing with only needed 0 and +5v I use a small switching PSU to drive them. I'm still a tad confused though, as both the original bass beat extractor used +12v and 0v and I assume that when the NE555 is triggered its output (logic 1) will also be at 12v. If so that will mean I will have to come up with a means (voltage divider ??) so that the level is dropped to 5v so it can be fed to the PIC's input.

Thanks again

[ElectroLawndart]

You could use a max232 type chip to make +10v and -10v from your 5v signal...

THAT'S AN AWSOME IDEA!!!
I've be looking for a simple solution for the + and - supplies that some opamps require. Thank's Comatose.

Dart

[Malc C]

You could use a max232 type chip to make +10v and -10v from your 5v signal, at a cost of a buck or so. It can't really supply any real power (few milliamp) but its fine for driving an op-amp. You'll need to put bigger decoupling caps on the outputs of the max232 if you go that way.

This would give me a better voltage range than using a 7660. I assume that there will be enough power to drive the two op-amps in this schimatic (http://www.rcgroups.com/forums/showpost.php?p=5650744&postcount=8)

Could you post a suggested schemaic on how to use the MAX 232 chip from the 5 volt single rail supply ?

[xtal]

if input is AC coupled you might be able to put a large cap from R5[10k] to ground
in lieu of grounding R5.....I seen this done in other circuits....