Whats the use of an asymmetric virtual gnd (opamp circuit)

[markusw]

I finally depotted my faulty Bartolini NTMB bass preamp. See http://diystompboxes.com/sboxforum/viewtopic.php?t=33100
When reverse engineering the circuit I found that the virtual ground differs from V/2: the resistor divider consists of a 30k (4 band colour code) and a 25.6 k (5 band colour code).

Any ideas whats the purpose of this asymmetric virtual ground :?:  :?:

Thanks for your help in advance,

Markus

[gez]

Haven't looked at the schematic, but usually when you get odd divider values it's because the output of the op-amp isn't symmetrical (look at data sheets and you'll often see that there's less headroom in one direction).  So, if you want max swing both ways (titter ye not) you adjust the bias accordingly to compensate for this.

[markusw]

Thanks for your reply!!! Sounds logical.

Markus

[puretube]

or: sometimes the pos half of the circuit (above phantom ground) draws more current than the neg half (below ph.-grd,) - the divider compensates for that, to stay midways.

(one of the pleasures of revengineering   )

[edited] like in circuits with dual (+/-) supply, when one of the supplies needs to deliver more current then the other (for whatever reason...)

[markusw]

Thanks for your answer!

or: sometimes the pos half of the phantom ground draws more current than the neg half - the divider compensates for that, to stay midways.

How may this happen??

Markus

[gez]

or: sometimes the pos half of the phantom ground draws more current than the neg half - the divider compensates for that, to stay midways.

How may this happen??

When the collective input bias currents of the amps connected to the divider (or whatever else is connected to it) draw enough current to have a loading effect, i.e. when current is diverted away from the lower resitor in the divider so its voltage sits lower than half Vcc (do a quick ohms law calculation   ).  To compensate for this you can set the divider so that its 'centre tap' is higher than half supply, so when loading occurs the amps will bias up where you want them to.

Apologies to Ton for stepping in here (think that's what he meant)... :oops:

[Gladmarr]

An asymetric voltage divider in a virtual ground circuit can also be used to remove any DC offset.  I have no idea what the circit is here, so I don't know if there is a DC offset.  I suppose it is possible, since these are bass pickups, that the preamp is meant to go all the way down to DC.  That means there would be no coupling capacitors before the first opamp, so there could be a DC offset.  Or maybe I'm just nuts.... :roll:

[markusw]

Thanks a lot for your help!!

An asymetric voltage divider in a virtual ground circuit can also be used to remove any DC offset. I have no idea what the circit is here, so I don't know if there is a DC offset. I suppose it is possible, since these are bass pickups, that the preamp is meant to go all the way down to DC. That means there would be no coupling capacitors before the first opamp, so there could be a DC offset. Or maybe I'm just nuts....

There is a coupling cap, so I don't think this is the case. I also checked the data sheet of the MC33178  http://www.onsemi.com/pub/Collateral/MC33178-D.PDF. At +/-15V supply, 600 ohm loadR: Vout+ = 13.6V, Vout- = 13.0V. Does this make sense? Unfortunately I could not find any data on +/- 4.5V. Is it likely that difference increases with lower supply voltage?

Edit: Does it make more sense if the headroom compensation is meant to be for the second opamp since it is configured as inverting stage?

By the way here is the schemo.



Markus

PS: Please forget about R10.

[puretube]

why not just measure V+ & Vref...  :?:

(in situ)

[markusw]

Ahm, I don't think I can measure anything in this unit. It looks like recovered from a plane crash  :wink: But I will check it in my clone.

Markus

[Gladmarr]

You found a Bartolini preamp in a plane crash?  Don't tell the FAA.