How much power to the preamp?

[drdagor]

So on solid-state, modern opamp preamps, how much current does the supply need? I know, I know... That's too vague, and the Bugle pre-amp runs just fine for months off of two bleeding 9 volt batteries. Stick on one lousy LED indicator light and you just blew the power budget.

But seriously... With +/- 15 volt rails running 6 opamps, I'm thinking 50 ma (1.5 WATTS?) ought to be plenty--like overkill. Throw in a simple 600 ohm headphone monitor and maybe 150 ma should do it. Is that the case or is more poop needed to keep those classy little pieces of silicon singing?

Thoughts, experience, disasters, happy endings?

[nigelwright7557]

Sounds like you have got it about right.

[AndrewT]

even opamps pass non constant currents when driving a line level load.
These variations may be many times the quiescent current of the opamp.

I think you should estimate the worst case current demand and then identify where that current must be supplied from.

Only after doing that can you design the PSU to suit the circuit.

[wintermute]

50mA for six opamps sounds a little optimistic to me, but I guess it will depend on the opamp! Datasheet should be able to help there. I had a figure in my head of 50mA for a single OPA2604 but I just checked the datasheet and it says PS +- 12mA (for both amps in the chip). However that is the PS current figure, it says the output current is +-35mA and short circuit current +- 40mA, so I'm not quite sure how it can output +-35mA of current if it only draws +-12mA from the supply....

Sorry if I've confused things!

Tony.

[AndrewT]

Hi Winter,
the quoted currents in the datasheet or for different operational conditions.
With Zero output current the opamp consumes ~12mA (6mA/amp).
If there is output current then this is drawn ClassAB from one of the supply rails.

If total maximum output current is 35mA then one rail could be passing 12mA while the other is passing 6mA+35mA ~ 41mApk with one opamp completely idle.
Then moments later the current draws could be swapped over so that the low 12mA current becomes 41mApk.
If both opamps are delivering half their maximum output current, the peak current draw on both supply rails would be ~41mA for one dual opamp.
Capacitive loads on the opamps can demand current that is far more onerous than a 10k line level load, i.e. one must allow for HF into cables.

[wintermute]

Thanks Andrew

Tony.