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*it would be very handy knowing how much current such a mic pre is going to draw. To do this, would I just take voltage/resistance at the opamp somehow and figure current requirements using ohm's law?*Sadly, no. Transistors in circuits work very different at 10V-40V than they do with the 1.5V (or 0.2V) from an ohm meter.

If it is entirely chips (power goes only to chips, or chips and high-value bias resistors), then you can estimate the idle current by looking up the chips. TL072 eats about 3mA, 5532 eats about 8mA. Ideally you find a specification for the same power voltage as you will use, but most chips eat nearly the same current at any useful voltage. Maybe rising 30% from 8V to 40V.

When an amp drives a load it takes more power. Load a 1,000 Watt amp with speakers, hit a power-chord, the lights dim. You can approximate this, for common chip topologies, for sine-wave test tones, as a resistor across the power rails with a value

**6 times the load resistor**. If a perfect (zero idle current) amp drives a 600Ω load to clipping, the total supply rails see something like a 3,600Ω power drain. If you happen to be using +/-18V rails (36V total), then it works out 36V/3,600Ω= 0.010 Amps or 10mA.

So a chip-only mike amp will eat 5 to 20mA idle, plus another 10mA if you do sine-wave tests with 600Ω load. (Speech/music power drain will be much less.)

Many mike amps have a pair of input transistors that eat more power. Usually 1mA-2mA each, up to 4mA total, but a few run richer. Usually the emitter resistor runs from a power rail to near ground (base is grounded and emitter is 0.6V offset). So if you find two 15K emitter resistors, and +/-18V supplies, figure 18V/15K= 1.2mA each, 2.4mA total for these transistors.

You may have LEDs and other frills. One design I looked at recently, the LED ate as much juice as everything else. Relays can be real power pigs.

Back to the power transformer. Just for illustration (and easy math), say it is 10V and 1A. As you know, the "10V" is the RMS of a bent (sine) wave, and our rectifier will charge up to the Peak voltage which is √2 or 1.4 times higher. So we get 14 volts. The rectifier loss is 0.6V (or 1.2V), so we are down to 13V (don't sweat small fractions). The filter capacitor is never big enough for perfectly steady DC: 120 times a second it charges to 13V, then between peaks it discharges. We typically pick a capacitor size to give about 1V of ripple. And what we really care about is how low it goes. So now we are down to 12VDC from a 10V AC winding.

**But** that "10V" will go up and down with utility-company voltage, and with load. Small transformers rise 20% when un-loaded. Hot summer days drag the wall-power down 10%. My power company (apparently) does not own a voltmeter. So that estimated 12V DC could be 14V or 10V, with 1V of 120Hz ripple.

If we need power cleaner and steadier, we regulate. The regulator can only waste excess voltage, it can't fix low voltage. The regulator itself needs 2V to 4V to stay alive. So we can get maybe 8V or 10V of solid regulated DC from a "10VAC" winding. Since some of those losses are fixed-voltage, and we can usually over-size the transformer (so it never runs at full rated load and sag) at low cost, we can expect to design for good regulated 18VDC from 18VAC windings.

There is no "free lunch" in that √2 or 1.4 times higher raw DC voltage from the rectifier and capacitor. The downside is that the peaky current wave heats and sags the transformer. A winding rated 1A AC can only give about 0.6A DC before heat or sag become real problems.

Worked example. Amplifier to drive 8Ω to 20V peak sine wave. Assume the amplifier wastes 4V on peaks, we need +/-24V DC rails. Assume that we are not too concerned with ripple: no regulator. But we do allow 1V of ripple-dip, 1V for rectifiers, we need 26V peak AC from the transformer winding. 26V peak is 18.4V AC RMS, we want 2*18V (or 36VCT) winding.

At full sine-wave power the amp will act like a resistor of 8Ω*6 or 48Ω. With +/-24V rails, 48V total supply, this is 48V/48Ω= 1 Amp DC. The transformer winding must be rated 1/0.6 or at least 1.6 times higher: we need 1.6A winding rating.

Most chip-based mike preamps eat more like 30mA. Rack four of those, 120mA DC. Times 1.6, 192mA or 200mA or 0.2A AC needed. So yes, a 0.5A winding is generally ample for a few mike preamps. And for total

*regulated* DC voltage of 24V-48V with simple rectifiers (not doublers) we can rule-of-thumb say that VAC=VDC: we need 18VAC to rectify to 25VDC +/-20% that we can regulate to solid 18VDC. So the spec looks like 2*18VAC at 0.2A. This is 36VAC and 0.2A, 36V*0.2= 7.2VA. Transformers are priced by the VA, the maximum energy they can handle. And trannies smaller than 10VA are not cheaper (it costs more to make them too small), and 25VA trannies are not much more dollars than 10VA trannies. So for mike-amp type supplies, always super-size the transformer: you get more raw DC for better regulation, and it will run cooler, for only a buck more.

Rectifiers must be rated for 2.8 times the maximum transformer voltage. These days, 500V rects cost the same (in DIY quantity) as 50V, so buy a bag of 500V rects. The rectifier must be rated for the Amps of the

**transformer** (not the load). The rect sees big spikes from the transformer to the capacitor, not the average DC out of the cap. Big transformer, big spikes. A 1N1006 rectifier rated 1A is barely big enough for a 1A winding. If you miss this point, it will work for a few hundred turn-ons and one day die at switch-on. So if you are close, always round-up the rectifier current rating to the next bigger size.

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*rectifier diodes themselves can have a nasty very-high-frequency snap*bcarso is quite right. But if you are just starting to build power supplies, this is a frill. Excellent audio can be made with $0.07 1N1005 diodes. Build your first few supplies with generic parts. Tweak, listen, tweak. One of those tweaks will be $7 rects instead of $0.07 rects. But it is discouraging to make beginner mistakes with expensive parts. And you may find that, in some circuits and systems, fancy rects don't make a difference. Or if they do, trying the ordinary rect first gives you a comparison point.

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*Keith (SSL Tech) has a +/-18V w/phantom supply from a 2x20V mains transformer design that is very popular around here. http://www.beatbazar.com/guests/ssltech/kps-1/index.htm *And whenever possible, plagiarize! There is nothing new under the sun, and no shame in following the paths of others, who followed paths worn by those before them. Be sure what you are stealing is appropriate to the job, and that you understand the basic design, and adapt as needed, but plagiarize! (Only: call it "research".)