Understanding Power Supplies

[nano1981]

I have noticed that most preamp channels need the usual 24V or 48V in order to run.  Most power supplies I see that work with neve, opamps, or rca show the voltage within that range, however, how does amps or current play into the circuit?

Take this power supply from JLM audio for example.  It shows the 24v and 48v lead and on the transformer it says 1.25A for current.  In the description its says the power supply can support up to 4 neve style preamps.  I guess that is considered the "load" for the PS.  Say we hook the PS up to 4 neve style preamp boards and measured the current and not the volts at the leads, would they read 1.25 amps?  Or, would the amps be 1.25 amps divided by 4, so 0.3125 milliamps per channel?

What is limiting this power supply to only 4 preamp boards?

Also, if I wanted to buy a power supply for a random preamp I found on the web that operates on 24 volts, how am I to determine the amps or milliamps ?

Sorry for such a novice question.

 

[JohnRoberts]

I have noticed that most preamp channels need the usual 24V or 48V in order to run.  Most power supplies I see that work with neve, opamps, or rca show the voltage within that range, however, how does amps or current play into the circuit?

To use the popular water analogy, voltage is equivalent to water pressure or how much work the water could do... Current is equivalent to water flow or how much water is flowing at that pressure.

While you didn't ask watts are the product of voltage and current, so more of either results in more watts.

Take this power supply from JLM audio for example.  It shows the 24v and 48v lead and on the transformer it says 1.25A for current.  In the description its says the power supply can support up to 4 neve style preamps.  I guess that is considered the "load" for the PS.  Say we hook the PS up to 4 neve style preamp boards and measured the current and not the volts at the leads, would they read 1.25 amps?  Or, would the amps be 1.25 amps divided by 4, so 0.3125 milliamps per channel?

1 mA = 1A/1,000  so 1.25A/4 = 0.312A  or 312 mA

What is limiting this power supply to only 4 preamp boards?

for a given voltage output the current is generally limited by transformer size (and other things).

Also, if I wanted to buy a power supply for a random preamp I found on the web that operates on 24 volts, how am I to determine the amps or milliamps ?

The simplest is to measure one,,,

Sorry for such a novice question.

Good luck

JR

 

[nano1981]

Thanks for taking the time to answer all of that.  You have helped very much.  Thanks John.

 

[Bo Deadly]

I have noticed that most preamp channels need the usual 24V or 48V in order to run.  Most power supplies I see that work with neve, opamps, or rca show the voltage within that range, however, how does amps or current play into the circuit?

Actually op amps usually use a bipolar supply. Meaning +15 and -15. Descrete designs usually use unipolar like +24.

Say we hook the PS up to 4 neve style preamp boards and measured the current and not the volts at the leads, would they read 1.25 amps?  Or, would the amps be 1.25 amps divided by 4, so 0.3125 milliamps per channel?

Not quite. The transformer might be rated for 1.25A but the power supply in general might not be able to produce that cleanly. A Neve preamp has an output stage that has ~70mA DC going through it (which is rather unique actually). The rest of the circuit is use a lot less than that. So maybe in total it's 100mA. So the recommended 4 preamps is probably a little conservative. You could probably do 5 or 6 actually. But at some point you might start to get noise or some parts might start to get hot and burn out a lot sooner than you want.

Also, if I wanted to buy a power supply for a random preamp I found on the web that operates on 24 volts, how am I to determine the amps or milliamps ?

Like JR said, measure it. Either pass the supply through a DMM and measure mA or if there are smallish inline filter resistors, measure the voltage drop across them and use ohms law to compute mA.

Note that if you are in the market to buy a supply, ACDC SMPS are starting to be accepted as generally superior in almost every category so you might look into SMPS instead. Not sure if there's anything read-to-go or a kit for +24 and +48 (Sound Skulptor used to sell a +15, -15, +48 one) but they're so simple you could probably just clap one together.

 

[JohnRoberts]

Another perhaps not obvious point... You can safely use a PS with more current output than required, but too much voltage can and will damage components releasing their magic smoke.

JR

 

[Rob Flinn]

It is normal to over rate the Power supply by at least 50% on what is actually being drawn from it.

 

[PRR]

> on the transformer it says 1.25A for current.

"On the transformer" is an AC current rating.

You want DC. The DC current available may be "half" the AC rating. (Numbers like 1/1.6 or 1/1.8 apply in specific cases; 1/2 is close-enough for DIY.)

So this supply may be good for 0.6A of DC current.

I can build a mini-preamp to suck 2mA at 24V. "neve style preamp boards" covers a LOT of turf, some very thrifty, some pretty hungry. One much-loved type pulls over 100mA (0.1A) each. I figure this means 5 or 6, but "4" may be a safer bet for many potential power supply customers.

We usually supply power at a steady voltage. We usually take power by connecting loads which will suck as many Amps of current as needed to do the Power work that needs doing. My house gets 120V. When "everything is off", I really suck about 1 Amp of clocks, wallwarts, and other minor 24/7 loads. When we turn on the TV the current rises to 3 Amps. Well-pump sucks 11 Amps. The hot-water heater sucks 16 Amps (I coulda bought a 20 Amp). I used to have thousands of Watts of lamps, the more lamps the more Amps. (But LEDs are the same light with much less Amps.) When I came back from a week away and turned EVERYTHING (dry or cold or warm) on at once we hit 60 Amps, falling off as the water came up, freezer got cold, etc.

 

[nano1981]

  These examples are terrific.

I have started to take some basic electronics courses online and I just got a copy of the "Radio Designers Handbook" which really helps for audio specific circuits.

Since I have found this site I really am starting to become more passionate about learning.  I am just starting, but I really would love to get to the point to where I can design my own circuits and to truly understand electronic engineering. 

Thanks everybody