Power rails on opamp
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Johnny1234
Well-known member
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
New member
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
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
Well-known member
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
Well-known member
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
Well-known member
What sort of problems were you experiencing ?
Agreed totally, unless the short was intermittent in which case a chip could fry - tasty
Newmarket
Well-known member
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.
A hefty spike can cause chip failure if a cap fails. Unlikely but possible.
Newmarket
Well-known member
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
Well-known member
Ringing, marginal stability, oscillation.
Newmarket
Well-known member
We're the values the same or different ?
living sounds
Well-known member
Sorry, don't understand your question.
Newmarket
Well-known member
Okay. Were all your decoupling capacitors the same value ?
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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
Well-known member
+/-17V is fairly common. Loss of headroom is minimal. Gives a bit of leeway with the voltage accuracy. And reduces heat dissipation.
Newmarket
Well-known member
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.
