OP-AMPs in the power section, what's that all about?

[Feral Feline]

I've seen many circuits that only need one op-amp, but the circuit-designer used a dual op-amp IC (or 3 op-amps in circuit using a quad IC) and then sticks the spare op-amp in the power section.

Is there any benefit? Does it help regulate the power or condition it in some way? (often it goes to VB or VREF etc)
Or, is it just a way to keep the IC happy and keep the spare op-amp preoccupied like a problem child...?

 

[blackhatboojum]

I too would like to know the answer to this…

 

[Stickman393]

Have an example you could post? Let's get a little show and tell going here

 

[temol]

Here's an example. Combination of the resistive voltage divider and opamp provides low impedance Vref.

 

[danfrank]

Well, it's gonna be an unused op amp otherwise, so might as well put it to use. A typical Vb circuit/supply is at a fairly high impedance, the voltage dividing resistors are usually in the range of 4k7 - 100k ohms. By feeding this high impedance Vb voltage into an op amp, the Vb turns into a low impedance Vb. Low impedances are good in power supply design, high impedances aren't.
Oops, that's what temol said...
Lol

 

[Robert]

Sometimes you'll see opamp stages similar to temol's example but they have nothing connected to the output.

These are leftover unused stages with the inputs tied to Vref rather than left floating. This is just to stabilize the unused stages to prevent oscillations / noise in the other half of the opamp.

In this case they're drawn near the power supply for convenience, they're not actually part of the power supply (or anything else for that matter).

 

[benny_profane]

Here's an example. Combination of the resistive voltage divider and opamp provides low impedance Vref.
View attachment 13641

Isn't this essentially DC-coupling?

 

[danfrank]

Yes, a cap wouldn't let the DC Vref through

 

[fig]

These are leftover unused stages with the inputs tied to Vref rather than left floating. This is just to stabilize the unused stages to prevent oscillations / noise in the other half of the opamp.

So...why was a multi-stage opamp used? Guessing other specs maybe?
...and thank you ALL for the insights!

 

[benny_profane]

So...why was a multi-stage opamp used? Guessing other specs maybe?
...and thank you ALL for the insights!

I'd guess mostly to minimize the part numbers on the BOM/stock.

 

[Feral Feline]

Thanks for ALL the replies.

I figured it was for stability, in most cases.

Economies of scale makes sense, for the big manufacturers.




This is what I'm referring to:



This is a schematic for a DOD FX91 Bass Overdrive.

Like Temol's example, it looks like the power is going somewhere with the "VR" above and the "VREF_B" in Temol's case. \
The Hot Bod has 3 places where VR is supplied to the circuit (above R3 near the input/ below R4 to the left of IC1A and above R13 feeding the last op-amp before output), I had presumed the power-sections op-amp was feeding those 3 points in the circuit.

As a side note, it's interesting how many companies such as Boss always uses a hyphen in their pedal-nomenclature, for example Boss ODB-3, yet DOD never used hyphens such as the FX91 above (well, hardly ever, the DOD FX50-B is a rare exception to the rule).


So what's the deal with "VR" in this case?

 

[danfrank]

Vr is voltage reference, usually half of the supply voltage. This is because most pedals use a single 9 volt positive supply. All op amps need a Vref which is usually half the supply voltage. Sometimes it's called the op amp bias voltage. Pro equipment usually uses a bipolar supply (+/-9v or +/-15v or whatever) and in this case ground (0volts) becomes Vref, again, half the supply voltage.

 

[Feral Feline]

Thanks @danfrank. I understand VR is voltage reference, the half-voltage supply, but that half-supply can be had without the use of the op-amp in the power section itself (R16 & R17 in the FX91 above), correct? I mean, there's plenty of circuits involving op-amps in the main circuit that all get a half-supply without needing an op-amp "filtering" Vref.

As near as I can tell, having an op-amp in the power section does nothing. *Maybe.

The only reason it's there, "in the power", is because it's an extra op-amp that's not needed for the purposes of the main circuit but is still in the IC, so ... some minor kajiggery is used on the extra op-amp itself to keep everything stable, much like tying the unused inverters to ground in a 4049UBE circuit.


*Or does the "spare" op-amp alter the VR impedance, as per Temol's post? Then it's actually doing something worthwhile...?

 

[temol]

 

[Feral Feline]

Thanks Temol!

Gotta go work on my 5E3, will watch that when I get back.

EDIT:
Didn't get to work on the 5E3, but did get to order some more pedal parts and finally watched the vid!

THANKS AGAIN! @temol
Very much enjoyed watching that; I'll be checking out some of his other vids, too.

 

[danfrank]

Bingo!
In power supplies, low impedance is almost always better than high impedance