Shaker amp and when caps get too big

[Toby Paddock]

Some more info on the shaker amplifier I mentioned.  
Sorry, this got kind of long.

There are 92 19,000uF caps (1.75F total).  On startup, there are 
resistors limiting the inrush current for a few seconds.  Then relays 
short the resistors.  The rails are at +/-32VDC.  There is nothing 
modern about it, it's just a big linear amp (the new ones are switchers).  
The transistors are screwed into square tubing with cooling water 
flowing through.  It's a little weird because they are not insulated.  
Unscrew a transistor and water runs out.  A de-ionizer cartridge 
keeps the conductivity of the water low.  Each transistor has 
0.5 ohm series for load sharing.  Each group of 6 transistors goes 
through a 30A fuse.  If a transistor fails (shorts) the fuse blows 
and takes that group off-line. 

It drives an electromagnetic shaker.  Pretty much a speaker with 
an 18 inch dia voice coil. Instead of a cone, it pushes a metal surface 
that the test article is bolted to.  In fact, it's a little bit like a *vintage* 
speaker in that instead of a permanent magnet, it has a DC coil to 
provide the DC magnetic field.  The field coil is made out of copper 
tube with cooling water flowing through it and runs about 250-400 amps DC.

Is it true that old tube amps ran the B+ supply through the DC field coil of 
the speaker to give the magnetic field AND the coil's inductance 
smoothed out the B+ ripple?  

This one doesn't actually use a moving voice coil.  The AC drive coil 
is stationary and the moving armature is a big fat aluminum ring.  
The AC field induces some oh-my-God big currents in the ring and 
then the ring wants to move in the DC field.  
Advantages: no electrical connection to the moving 
armature and no lightweight moving voice coil to rip itself apart.  
Disadvantages: will not work down to DC and I don't think it's very 
efficient.  Drive coil and armature are cooled by a large blower on the 
roof pulling air through it.  I think Advent made a car speaker with a 
coaxial tweeter driven suspiciously like this.

There is no air enclosure to work against so it doesn't put out very 
good bass audio.  It can get painfully loud, but the bass response 
is not that great.  The spec'ed range is 5-2000Hz.  Max stroke is 
1 inch.

An accelerometer is mounted on the shaker to provide feedback to 
the controller.  Some can be mounted on the test article to 
see resonances.

250,000 watts is the full load INPUT power.  Actual amp power is 
probably, let's see... about 60V x 750A... only about 45kW pk maybe. 
The rest goes to the DC field, pumps, blowers, and 
steam belching out of the cooling tower.  

Wait a minute, I just thought of something.  If all the transistors are 
the same, how can you have + and - drive?  I think I've got 
something wrong here.  One *problem* is that it's so reliable I 
haven't had to poke around in it's guts often enough to 
remember the details.

I put up a couple pictures of Mackie mixers I abused.  
Test photos used by permission of Mackie Designs www.mackie.com.

CFX20, shake axis up-down.
http://www.seanet.com/~tpaddock/images/mackie_cfx20_v.jpg

d8b, shake axis front-back:
http://www.seanet.com/~tpaddock/images/mackie_d8b_h.jpg

I started a page about the vib lab, but never finished it.  
Not very much to see:
http://www.seanet.com/~tpaddock/vib_lab.html

The shaker is a Unholtz-Dickie T1000. UD webthing is at:
http://www.udco.com/tseries.htm

The controller is a Spectral Dynamics (GenRad) 2550B:
http://www.sd-corp.com/2550br.htm

 - -- -  Toby Paddock