Question about biasresistors

[Rutger]

Hi,

I have a (simple) question about biasresistors. I want to add a simple opampbuffer after a preamp (for impedance matching). The preamp runs on a dual power supply and so does the buffer. Is it necessary to add a biasresistor to ground at the + side of the opamp? And why, or why not?

It seems to me that once the bias is set in a circuit, you're done. But I might be wrong...

[J M Fahey]

The Op Amp needs to "see" a voltage reference, it can't be left "floating", it needs a DC path to ground (which *is* a voltage) or to some defined point.
So if it's DC coupled (straight or through a resistor) to the earlier preamp, fine, you're done, but if there's a series capacitor in the path, it blocks DC so you need to add that resistor to ground.
It may be a high value, with modern Op Amps you can easily run 1M.

[Rutger]

Thanks Fahey! So only when the opamp is 'isolated' by series capacitors, a new bias is needed.

[Rutger]

So I've installed the buffer at the end of the preamp and changed the powersupply of the preamp so the IC's would all run on +/- 18V. I did the last thing wrong, and measurements told me the there's +/-25V on the IC's. I thought the IC's should be smoked out but turned the amp on anyway.... and it works! 

But how? Datasheet tells me a 4558 can run at +/-18V max.

[J M Fahey]

You got lucky, but don't count on it.
I get my +/-15V the simple way: a couple resistors from the amp (usually +/-42V) rails, with a couple 15 or 16V Zeners.
So far so good, but a couple times the phone or the bell rang when I was finishing an amp, with the owner just by me and in a hurry (they always are ) and I forgot to put them in.
I build boards in a very definite sequence and Zeners go last.
Worst thing is that the amp tests fine, sounds good, owner pays and leaves.
3 days to 1 month later he calls: the amp turns on, pops, *slightly* hums (meaning it's alive ) .... but it does not "play".
Sure enough, some dead TL072
Measured rail voltage is what you found, around +/-25V , so I *guess* at that voltage they "Zener" or something ... which of course needlessly increases internal heat generation big way.
Maybe not even warm to the touch, but enough to cook something inside the tiny chip.

[Rutger]

Ah, so I just got lucky... it sounds great though  :cheesy:

I somehow calculated the resistor in the powersupply the wrong way. The powersupply is a 'traditional' one, with elco's and resistors. Can you help me out calculate the right value of the resistor that will make it (just below) 18V?

I have a 30VAC/18V trafo, after the rectifier I get +/-27,5V DC and I need to get it down to 18V. It powers a preamp with two 4558 chips in it (4 opamps).

[Rutger]

please...?

[Enzo]

I wouldn't count on resistors.  If I needed +/-18v supplies and already had +/-27v supplies, I'd probably go get a 7818 and a 7918, 18v voltage regulator ICs, and add them to the circuit.  The data sheet on each should show a sample basic circuit.   Actually, unless I have a reason to go to 18, I'd probably just use 15v regulators - I stock them in greater numbers than the 18v ones.

[phatt]

I wouldn't count on resistors.  If I needed +/-18v supplies and already had +/-27v supplies, I'd probably go get a 7818 and a 7918, 18v voltage regulator ICs, and add them to the circuit.  The data sheet on each should show a sample basic circuit.   Actually, unless I have a reason to go to 18, I'd probably just use 15v regulators - I stock them in greater numbers than the 18v ones.

Humm,, I've had some trouble getting my head around this kind of problem a few times myself?

So Q?

How much voltage *Above* the regulator can you go?
Sure I know you need a few volts Above reg for normal operation but dropping from 28VDC down to 15 VDC seems a big ask.

You often see a series drop R of ~ 220R even 470R.

Some circuits might use 2 watt and some might use 5 Watt.

But I've noticed some circuits the drop R runs very hot,, where as others may run only warm.

That mongrel Fender Performer 1,000 I worked on a while back was an oven. :duh
The Drop resistors had De-laminated the copper tracks. 
The Zener diodes where also way too Hot.

Having worked on enough equipment to make a comment then this seems like a common failure in a lot of SS gear.

I guess if Big names still can't get it right then maybe it's not so simple. Head scratch?
Phil.
Ed, I suppose What I should ask is; What is the maths I need to punch in to get a cooler result?

[Enzo]

A standard 7815 or whatever data sheet will tell you the max input voltage, offhand I think it is 35v.  One can ALSO add a series resistor before the regulator to dissipate some of the excess.

Yes, thet will need to bo on a heat sink - the chassis will do.

When I said I do not rely on just resistors, what I meant was I would not simply stick resistors in series with the 25v to get it down to 15v.   I was implying we needed some regulating part, like a 78/79xx regulator or a zener.

Dropping from 25-30 down to 15 is pretty darn common actually.  The Hot Rod DeVille drops from 33 to 16, for example.

You calculate your dropping resistor to fit your circuit, just as yuo would calculate the dropping resistor for an LED.   If you have 35v and a 15v zener to power, your resistor must drop 20v.   Ohm's Law will tell you how many watts, and thus how large a wattage resistor.

Zeners also need a minimum amount of current to regulate well, and that is where the 1w or 5w zener comes in.  Remember all the zener current is from the raw supply to ground - none of it goes through the load.  The load current is in addition to the zener current, coming from the power source.  You select a 5w zener for better regulation.   Your resistor will limit the current available to the zener.

COnsidering that almost all the amps we see have zeners in them, it is inevitable we will see zener failures.  But put that in context.  We see flat tires on cars all the time, but is a flat tire a common failure?   How often do you get one?   You need to see the difference between "This fails all the time" and "WHEN something fails, it is often this."  Those are very different.   In the overall market, we do see Fenders with zener circuit problems more often than we should, but it remains that the MAJORITY of those amps never have trouble with it.    The Fender engineers made an error - they underspec'd parts in that circuit on amps of that era.  Other Fender amp had no issue, and even the much maligned Crate amps rarely give me zener issues.  We can't just paint the big boys with a broad brush of can;t get it right because of a few failures.  Even Hank Aaron struck out now and then.

[Rutger]

Well, it's a bit too late for regulators now. It's an existing powersupply that I only need to mod slightly, so I just need to replace the dropresistors for another value.

I found out I did the math totally wrong (and too complicated), so here is a new calculation.

The datasheet tells me that one 4558 needs 3,5 mA with a Vcc of +/- 18V, times 2 makes it 7 mA. The voltage needs to drop 9,5V, so the resistor will need to have a value of 9,5 / 0,007 =1357 Ohm. The heat dissipated will be 0,007 x 9,5 = 67 mW. So I'll pick a 1k5 resistor of 0,25W for each rail.

Am I right?

[Roly]

@Rutger - nobody, but nobody, depends on the current consumption spec of an op-amp to drop the supply voltage to it.  For a start it depends on what the op-amp is driving, and if its output is lightly loaded its supply draw may be a lot less, and thus the applied voltage much higher.  Current is only drawn from one of the supplies when the output is on that side, so the current draw is quite unstable and depends on the signal.

That why zeners and 3-pin regulators get used.

One way to approach this is to work out the worst case current draw, then select a zener and series resistor to pass this current (plus a bit for safety/error).

Say you figure out your preamp will draw 10mA on each rail, worst case, make that a 12 or 15mA assumption, then using the difference between the available supply voltage, say 24, and the desired preamp rail, say 15, work out the required resistor and zener power.

24 - 15 = 9 volts

9 volts drop at 15mA

R = E / I = 9 / 0.015 = 600 ohms (560 or 680)

P = E * I = 9 * 0.015 = 0.135 so anything over a quarter watt

Zener power (worst case);

P = E * I = 15 * 0.015 = 0.225 so a 250mW would be cutting it a bit fine, so go 1 watt.


@phatt - 3-pin regulators have a maximum supply input of 35 volts, current of 1 amp, and temperature of 75°C, require a minimum difference of 3 volts between input and output to keep regulating, and can dissipate between 1 and 2 watts without a heatsink.

The two things they are not protected against are excessive input voltage, and reverse voltage from output to input.

Enzo mentions what is sometimes called an "economising" resistor between the raw supply and the regulator input.  This doesn't really economise on anything, except perhaps required heatsink size, but serves to shift excess power dissipation from the regulator to the resistor.  The value is chosen so that with lowest raw supply and highest load current the input voltage to the regulator is still comfortably above the minimum headroom (V_out+3V).  This will generally be a power resistor and will get pretty hot.

Designers often seem to underestimate just how hot zener regulators and their series resistors will get under worst case conditions, but this isn't helped by the fact that zeners, like diodes generally, will typically fail short circuit, placing the entire incoming supply across their series resistor and cooking it and everything nearby.

[phatt]

Hi Enzo,,
          Thanks and yes I see your point. :tu:
I guess I just expect too much. xP
All I know is I would not be able to sleep if I knew some poor sod had paid top dollar for MY stuff up because I did not do my work correctly.

I've seen inside enough gear to know a lot of it is cheap and nasty and quite obviously little is done to road test what they make.

I guess only being an amateur I can't pass judgement but Even I know you don't lay a power resistor flat against the pcb,, yet I see it done a lot.
IMO If some of these brand names manufactured medical equipment they would likely spend a lot of time in court.

Rant over.

Hi Roly,
                   Yep yep that helps.

Still a bit misty on where it all goes too or from? 

Am I right in assuming the Lighter the load *After* regulation the MORE heat the Zener (or Reg chip) has to dump too ground?

I got some of the maths you posted,, but forget what "E" means.
Bahh,, not to worry mate,,,  maybe I just need some bedtime reading again to refresh my brain cells.

Interesting that I picked up on basic transistor circuits without much help but did not take long to realize that Regulated Power supply stuff was not so simple,,stumped me a few times. 

one more,,,
I've often noticed a fair bit of gear has a dedicated lower voltage for preamps.
Am I right to assume this is a better alternative?  Adds to build cost I guess.

Hey Rutger,, Sorry to slightly derail this for You but as you can see I'm a little blurred on the workings as well.
Carry on,
Phil.

[Enzo]

The zener has a set breakdown voltage.  Your raw supply drops through a resistor to the zener.  The load current never goes through the zener.  In a zener regulated supply, all the zener current flows through the dropping resistor. any load current also flows through that resistor.If the load lightens, it doesn't do anything to the zener, it just means the resistor dissipates less heat.

[phatt]

Arrh huh!! Thanks Enzo   dat makes sense. :tu:
Where would I be without help from chaps like you?  :dbtu:

I must have done something right with that Performer 1,000 as it gets flogged pretty hard every week and shows no sign of problems.
I moved all the drop R's off the PCB and onto the chassis.
Phil.