Power rails on opamp

[Noth]

I will follow general advice and use electrolytics instead! : )

 

[Johnny1234]

Hi,

I'm mounting an MCI JH600 preamp and I have some questions/doubts about the power rails.
For those interested, here is the circuit:View attachment 112857

The circuit is powered using +/- 18V. On the power supply section, we just have two 100 ohm resistors to lower this voltage a bit and a few filter capacitors.
When I power up the circuit with my lab power supply, I can see that initially the current is about 20mA on each power rail. After one minute, the current raises up to 180mA (especially the -18V) and the 100 ohm resistor starts to overheat.

1. Is this current growth "normal" ?
2. At 20mA, the power for 18V would be 0,36W so my 1W 100Ohm resistor is fine. But at 180mA this gives me 3,24W, hence the overheat. Now, I = U/R, thus 18/100 gives me those 180mA but to me it would mean that the opamp is shorted... Am I wrong somewhere?

Any advice would be appreciated!

100 ohm is way to large go to 10 ohm. The only reason these reistors are there is to fuse the circuit. I also would run the psu at +/- 15 volts measured at the op amp. Maybe +/- 16 or 17 volts tops. That's just my opinion. Best of luck with your project.

 

[xskyscrapers]

Are you talking about mounting the preamps in the channel? Or mounting the preamps in a rackmount? Please don’t rackmount any 600 series stuff haha

 

[RoadrunnerOZ]

100 ohm is way to large go to 10 ohm. The only reason these reistors are there is to fuse the circuit. I also would run the psu at +/- 15 volts measured at the op amp. Maybe +/- 16 or 17 volts tops. That's just my opinion. Best of luck with your project.

The 100 Ω resistors are common to all IC,s in the 600 console channel cards - likely both fusing and decoupling as well - but if you change the power rail resistors feeding the IC’s this may affect the bias relationship with Q1 transistor pair.

 

[Johnny1234]

The 100 Ω resistors are common to all IC,s in the 600 console channel cards - likely both fusing and decoupling as well - but if you change the power rail resistors feeding the IC’s this may affect the bias relationship with Q1 transistor pair.

But these were designed to have 36 inputs loading the PSU. May a good compromise would be a 22 ohm resistor.

 

[RoadrunnerOZ]

But these were designed to have 36 inputs loading the PSU. May a good compromise would be a 22 ohm resistor.

Then you’d maybe need to recalculate the divider network that sets the bias on IC 500 and IC 600 as that’s also derived from the +18V rails. The voltage at pins 4 and 7 of the IC’s will change whereas the bias voltages won’t which shifts them relative to supply voltage.

 

[Newmarket]

If the ones on that board are where + 18V got joined I’d just replace them but the caps may have fritzed as well so I’d run a supply test with the chips pulled. 5534’s are cheap enough and if C510, 511, 710 and 711 are tantalum I’d be definitely replacing them.

If the problem was/is a short from +18 to -18 then the chips powered from +/-18V should be fine. No current through them or voltage across them in that scenario. All stress is on the power supply side of things.

 

[Newmarket]

Adding ceramic decoupling caps habitually can actually cause lot's of problems. I have far fewer of those since I don't do that anymore. ;-)

What sort of problems were you experiencing ?

 

[RoadrunnerOZ]

If the problem was/is a short from +18 to -18 then the chips powered from +/-18V should be fine. No current through them or voltage across them in that scenario. All stress is on the power supply side of things.

Agreed totally, unless the short was intermittent in which case a chip could fry - tasty

 

[Newmarket]

Agreed totally, unless the short was intermittent in which case a chip could fry - tasty

How ? An intermittent short would simply be like turning the power on and off. Shouldn't damage the chip unless there is some odd power supply behaviour that produces >36V between the rails.

 

[RoadrunnerOZ]

How ? An intermittent short would simply be like turning the power on and off. Shouldn't damage the chip unless there is some odd power supply behaviour that produces >36V between the rails.

A hefty spike can cause chip failure if a cap fails. Unlikely but possible.

 

[Newmarket]

A hefty spike can cause chip failure if a cap fails. Unlikely but possible.

Dependent on power supply and load detail. Usually requires a significant inductive element to be involved somewhere along the line. eg "Load Dump" condition in automotive application. Or "Back EMF" spike from a badly designed relay stage. Not seeing a mechanism for it here but never say never.

 

[RoadrunnerOZ]

Dependent on power supply and load detail. Usually requires a significant inductive element to be involved somewhere along the line. eg "Load Dump" condition in automotive application. Or "Back EMF" spike from a badly designed relay stage. Not seeing a mechanism for it here but never say never.

Fair comment but I’ve seen failed caps in consoles taking out opamps when there’s been an internal short in a cap and then the short gets burned out resulting in a fairly hefty spike - especially when the cap is across one of the power rails right at the chip location.

 

[living sounds]

What sort of problems were you experiencing ?

Ringing, marginal stability, oscillation.

 

[Newmarket]

Ringing, marginal stability, oscillation.

We're the values the same or different ?

 

[living sounds]

We're the values the same or different ?

Sorry, don't understand your question.

 

[Newmarket]

Sorry, don't understand your question.

Okay. Were all your decoupling capacitors the same value ?

 

[Noth]

100 ohm is way to large go to 10 ohm. The only reason these reistors are there is to fuse the circuit. I also would run the psu at +/- 15 volts measured at the op amp. Maybe +/- 16 or 17 volts tops. That's just my opinion. Best of luck with your project.

I was actually wondering why the power supply to the preamp was lowered, after all this just sets the minimum/maximum output voltage for the opamps, and we'll never reach 18V (nor 15V) anyway.. I suppose there's might be some non-linearities when we're getting closer to the maximum/minimum capacities, but on a preamp we're always far from that..

Since most studio equipment runs on +/-18V (or +/-15V) and not +/- 3V, I suppose there's a good reason for that... !

 

[Newmarket]

Since most studio equipment runs on +/-18V (or +/-15V) and not +/- 3V, I suppose there's a good reason for that... !

+/-17V is fairly common. Loss of headroom is minimal. Gives a bit of leeway with the voltage accuracy. And reduces heat dissipation.

 

[Newmarket]

100 ohm is way to large go to 10 ohm. The only reason these reistors are there is to fuse the circuit.

They do also form a lpf with the associated capacitor. Maybe more useful to look at it in terms of allowing the energy storage function of the bypass cap to perform more effectively.