Mic Preamp concerns- power supply safety

[intergalactic]

I am starting an Eagle schematic for a THAT 1510/1512 design and I have a few questions.

(Maybe there is a FAQ for this?- feel free to link me to it. Please!)

I was wondering about human safety and IC safety for +/-15V and phantom power, especially if multiple preamps use the same DC-DC. Like 4 mic pres sharing the +15, -15 and phantom buses.

Are there practical noise concerns with this as well (should I source +/- 24v and locally linear regulate to +/-15v?) Do I need to isolate everything?

*nevermind this part* Also, the combined Neutrik XLR/1/4" TRS connector looks interesting, but doesn't immediately seem compatible with the THAT application note (140?) where a real switch is used to pad the line in (I am sure I am just being dumb)

 

[Speedskater]

Normally when multiple mic-pres share phantom power supply they also share +/- 15V DC power supply.  So the safety part is how you wire the AC section of the power supplies.

 

[abechap024]

and +15/-15 rails will in all probably not shock you. I believe 60v+ is when voltage starts becoming dangerous. But of course its always good to treat all voltage and current with respect!
AC

Also its probably a good Idea to put a 2 capacitors(~10uf 25v) at each chip from the +/- rails to ground , since your designing it yourself and the extra parts should not cost too much. This will help keep noise down you can also put a low ohm (1-10 ohms) in series with the power supplies to each chip to help isolate the chips from each other
AC

 

[leswatts]

Well i've done a good many THAT 1510/1512 pres...and use Eagle too.

The only hazardous part is the line side of an internal power supply.

If that's what your using, please be advised that you can get a lot of hum from a close by power transformer with mic pres.

Hum  60 db below max output is not uncommon at higher gains. It can be dealt with, but it's a bit of a trick. With the right layout,
you can get it down to the thermal noise level.

Easiest way is to use an external supply or wall wart.

Les
L M Watts Technology

 

[pucho812]

i would worry more about thge amps then the volts, a few milivolts can kill with enough amps.

 

[gemini86]

I've been hit with 2k volts before, didn't even phase me, it all depends on where the current is traveling.  That said, all voltage can be deadly and safety should never be overlooked. But if its for your own use, just don't be stupid and you should be ok.

 

[abechap024]

yes that's a good point. I was confusing my units....current and voltage are proportional. its the current that can kill. Any current above 50ma through the heart can cause deadly fibrillation. I'm glad I didn't have to learn that the hard way :

AC

 

[PRR]

> a few milivolts can kill with enough amps.

Through the skin, milliVolts makes microAmps and is utterly harmless.

(If your heart or brain are exposed, that's different.)

> I've been hit with 2k volts before, didn't even phase me

Oddly, 2KV _may_ be "less dangerous" than 130V. 100V-200V and typical skin resistance "tickles" the heart into uncontrolled beating which does not stop until the heart reaches total exhaustion. A 2KV slam may STOP the heart, and it may pick-up within a few minutes. However a huge shock is more likely to burn other tissue with odd aftereffects. This is not something you want to try. (FWIW: the one 3KV 30mA shock I took sat me down amazed that I was alive.)

> I believe 60v+ is when voltage starts becoming dangerous.

Opinions have changed.

Through the skin, Edison observed fewer dead workers around <100V machines than around >100V machines and set his delivery at 100V to ground. This is also roughly telephone ringer voltage.

What workers may accept (rare death) is different from what customers accept (NO deaths!). (However gas-light killed its customers too.) Improved insulation made 100V-120V "safe". Lesser insulation is allowed for lower voltage. 70V audio distribution is one limit, accepted as "class 2" similar to control circuits. Even this is a sharp tingle and "can" be fatal. When contact can't be prevented, or wiring is apt to be very sloppy, or the baby may chew on it, the limit is often 25V or so. (Cats can be seriously injured chewing 9V cords.)

48V Phantom and 48V telephone power are legacy of older days: Phantom is only used by "professionals" and you can't change the millions of legacy telephones (except that Cell Phone is killing the technology). Also the connectors are "usually" finger-safe or behind covers.

While voltages under 70-60-24V usually can't cause a dangerous shock through the skin, if you get your wedding-ring caught between starter battery terminal and engine block, it will GLOW white-hot and burn your finger to the bone. A 12V car battery can put 10V into 200 Amperes for cold-start and 6V 600 Amps 3,600 Watts into a few grams of metal.

You can be blinded by a short in a 208V 600A fusebox. The light is intense even on the other side of the wall. Such a short throws molten copper which can do harm.

The 15V 24V etc rails off a "small" power supply correctly wired are pretty darn safe. 48V Phantom does not kill people, though I would prefer not to grab the raw +48V while sweaty. A "small" power supply won't make more than a few Amperes into a dead short or a wedding ring: it may leave a burn-mark and blow-out a chunk of ring which will be hard to explain to your ring-partner.

The 120V/240V side is more dangerous. Contact is fatal-enough. Ground metal chassis to designated ground pin so that exposed chassis is "safe" and a cross-wire will blow your house fuse. In US custom, a dead-short appliance can burn-up its cord before the fuse blows, which may set fire to your notes, the desk, the drapes, and potentially the whole house. Be sure you know what you are doing here. Get qualified advice if needed. But millions of clueless idiots wire lamps and even houses, and mostly survive.