Re: 4000 watt Audio Amp to drive Tesla coil (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 10 Sep 2007 21:48:20 -0700
From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: 4000 watt Audio Amp to drive Tesla coil (fwd)

Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Mon, 10 Sep 2007 10:57:28 -0700 (PDT)
> From: Timka <tomasdoc2002@xxxxxxxxx>
> To: tesla@xxxxxxxxxx
> Subject: 4000 watt Audio Amp to drive Tesla coil
> 
> Hello all,
>    
>   Did anyone ever used and audio amp to drive Tesla coil before?
>    
>   I came across a few mosfet amps on EBay, new but relatively cheap. The one I was looking at is rated at 4000 watts bridged output into 4 Ohms load. Now, I figured if I would use copper tubing, the resistance wouldn�t be a major factor after all.
>    
>   Another good thing is that I can feed anything into it via signal generator or even audio to see how coil behaves.
>    
>   Now, nasty things that I worry about:
>    
>   1) It would really suck if I ended up blowing final mosfets due to TC secondary kick back and frying internal circuitry, which would be a total fiasco and probably would puzzle a lot of clerks at the return department. Ad states that amp employs overload protection but whoever designed them back in China never envisioned a million volts kickback from an ordinary speaker, so I'm not holding my breath there. Any suggestions here?
>    
>   2) What kind of power supply is needed? In a car amp works off alternator and use around...... around a lot of amps! In fact, not every alternator can pull a 4000 watts amp as the most of them is in the area of 120 amps on average. Here is quick math for 4K watts amplifier:
>    
>   1 channel with 4000 watts RMS per channel doubled to the account for amplifier inefficiency (4000 watts X 2 = 8000 watts), then divided by the average output Voltage of an alternator, 13.8 volts (8000 divided by 13.8 = 580 amps). Since the average signal requires about 1/3rd of the average power in a tone used to drive TC, dividing by 3 (580 amps divided by 3 = 193 amps). This is the result of the amplifier's approximate average current draw.
>    
>   Soooooooo, how and where in the world do I get a source of 200 constant amps at 13.8 volts? This is not your average power supply you can plug into a wall. I was thinking of getting a car booster/starter a.k.a battery charger, it's the size of small fridge, with 2 fat clammed cables, weights about 60lb and probably employs a very large transformer you can use to kill your mother in-law that can be plugged into a wall. Another ad claims that this car starter can give an instant 300 amps surge or so but how about using it for longer than instant or lets say 10 min? Can it pull this amp? Of course I can add a few 2 farad audio caps in between, but would it be enough?
>

You don't.. 4kW RMS is probably a bogus spec. (something like "peak 
music power", which has no real meaning)  For one thing, to put 4kW into 
a 4 ohm load takes, let's see.. P=E^2/R 4000*4 = E^2 -> 
Vrms=sqrt(16,000), or about 128 V.

If you buy a "real" multi-kW audio amp, they tend to run off 120 or 240V 
supplies at tens of amps.  Check out the products from, e.g. Techron or 
Crown.  I have a couple Techron 7560s at work, and they're a pretty 
beefy unit.. rack mounted, about a foot tall, etc.  They'll comfortably 
put out a couple kilowatts at 120V, and you can bridge them.  They're 
basically a giant power op amp.  We use them to do things like drive big 
voice coil positioners on shaker tables, simulate power supply 
transients, etc.