Does V+ == -(V-) have to hold?

[stelleg151]

I was thinking that although most people do +/- say 30VDC, that maybe the only reason that its done that way is because thats how the trasformer+rectifier output it. But looking at schematics I was thinking that really all that should matter is the difference between V+ and V-, at least for a simple GC setup like Brians. Am I wrong?

 

[analog_sa]

It will clip asymmetrically and at lower power.

 

[DcibeL]

As long as your common point is half way between the rails, I think you are okay. If common (ground) is not half way between the rails, then you will se a DC offset at the output, and will be forced to use a huge capacitor at the output to block that DC.

 

[hitsware]

As long as your common point is half way between the rails, I think you are okay. If common (ground) is not half way between the rails, then you will se a DC offset at the output, and will be forced to use a huge capacitor at the output to block that DC.

Wrong. the supplies can be assymetric, and the output will still quiess at 0 volts (as long as the minimum voltage for operation is maintained (at both rails))

 

[DcibeL]

Hmm..I'll take your word about this for now, but I will have to test this when I get the chance.

 

[Bazukaz]

I think hitsware is right.

If both output and input is referenced to one ground point, output should sit at ~0 V dc , even if this "ground" is not exactly half between the rails.
Try a simulator and you will see.

 

[pinkmouse]

Ok, lets work this out properly. Say you have an (ideal) chip amp with a gain of 20, and rails at +20V and -10V, and the signal is coming in as a 1V peak to peak sinewave. The input has no bias, i.e. it oscillates around the zero point, so the output signal will swing fine to the +20, but when it needs to swing negative, it will only be able to get to -10V before the wave is clipped.

To be able to swing rail to rail with no clipping, you need to bias the input up to a point where it is halfway between the rails, and reduce the gain so the amp doesn't clip, so in this case, you need to add an offset voltage to the input signal, and reduce the gain to 15. This bias voltage carries right through the amplifier, producing an output offset of +5V as well, so to avoid blowing up your speakers, you then need to remove this, which is why you would need a DC blocking cap on the output.

Does that make sense?

 

[DcibeL]

Yes, the bias voltage is required at the input in order for output to swing rail to rail. I must have had a brain fart this morning, I was trying to think back to when I learned single supply op-amps in school. I guess I am already loosing my memory in my old age .

 

[cpemma]

...I was trying to think back to when I learned single supply op-amps in school. I guess I am already losing my memory in my old age .

Maybe you're not ready for the knackers yard just yet - some opamps aimed at single-supply applications (eg, LM324, LM358) will swing down to ground but only up to 1.5-2V short of the positive rail, so there may be a point in setting their virtual ground a volt below midway to get the maximum swing.

Not hi-fi though.

 

[tiltedhalo]

V+==V-

Pinkmouse is very correct, for the amplifier to swing symetrically rail to rail the center has to be the center, see the point in center???

 

[stelleg151]

Thanks pinkmouse and the rest of ya.

Is it possible to do the offsetting and DC blocking without reducing the quality significantly?

 

[Bazukaz]

Maybe bridging ? In this case , dc offsets at both + and - outs would be the same ... Am i wrong ?

 

[pinkmouse]

Yes! Think about it.

 

[paulb]

Does V+ == -(V-) have to hold?

How much difference are we talking about here? A volt or two won't make much difference. DC feedback holds the output at whatever the input ground reference is, so the only downside is the asymmetrical clipping as discussed. For a small difference, not a big deal.

 

[AndrewT]

Hi,
if you set up a dual polarity supply to a chipamp (or any power amp) with ground references at both input and output then exact matching of +ve & -ve supplies is not essential.

Further, if you set PSU +ve=-ve and then measure the +ve clipping & -ve clipping you will find in most cases that the clipping level is not the same.

You could deliberately set up your supply with unequal voltages to ensure that the +ve & -ve clipping were the same. However the difference is likely to be (much) less than 1volt.

The normal operation of an amplifier with dual polarity supplies will not be affected by slight imbalances in Vrail voltages.

 

[anatech]

Any normal power supply from a transformer will naturally be symmetrical. You have to work to unbalance it.

If you are thinking of running from a car's B+ and generating a B-, I can see where your supplies may differ a bit. Should work though.

-Chris

 

[stelleg151]

Actually I was thinking of running two in a series off of a bipolar power supply. In other words like this (numbers approximated in V with respect to supply ground).

I-v+(30)
---->-------(15)
I-(v-)(0)
I-(v+) (0)
---->-------(-15)
I-v-(-30)

Basically I just want to know if its possible to skew the input voltages to this extent (for a supply of say +/-30VDC would need to skew to +15VDC and -15VDC for the top and bottom, respectively.

Is this doable without losing (significant) quality? Would the voltages be too unstable in a series?

Thanks.

 

[AudioFreak]

And that begs the question........ Why would you want to run 2 chipamps like that?

 

[peranders]

This situation may only occur if you have addtional stuff connected to one of the voltages but normally you have a dual secondary winding so this won't be problem.

I can think of one possible situation: You have two different transformers in you junk drawers and want to se them.

 

[stelleg151]

And that begs the question........ Why would you want to run 2 chipamps like that?

Im not really sure yet, I was thinking maybe if I find a transformer with too high of a voltage then I could just do this as a quick and easy to get the correct voltage running accross them. More of a theoretical question really, just to see if its possible.