P
Phil Allison
Guest
Trevor Wilson wrote:
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** Did I ever tell you the story of the "Hertz" Mosfet amplifiers and the Australian Navy ?
Hertz amplifiers are ( were ?) large, heavy, lateral TO3 mosfet beasts intended for the live music and disco markets. The ones I saw did 800W comfortably into 4 ohms at any phase angle you liked.
The local agent for the product sold a couple to the RAN, for use with SONAR testing in the submarine labs here in Sydney. He had blithely assumed that this was a job the amps could do easily.
But he had not reckoned on the pure ham fistedness of the RAN technicians.
Live Rock music and disco material can be pretty hard on an audio amplifier, but not near as lethal as a half trained Navy tech with a signal generator that goes to 100kHz - PLUS no clue at all about how audio gear is normally treated and not treated.
The Hertz amps has response up to 100kHz but NO WAY was that allowable at full power !!! Even 20kHz is pushing it due to the limited power handling of the output ZOBEL.
PLUS, the Navy decided to build a "path panel" for all inputs and outputs - a flat aluminium plate covered in 1/4 inch jack sockets !!!!
The idea of using a sheet of insulation material never crossed their tiny minds OR that 1/4 inch jacks and plugs were not up to handling 800W safely - OR that jack plug tips connect signal to the ground circuit FIRST every time they are plugged in.
The Hertz amps did not last long, soon as the tip of a jack carrying the output from the amp touched the patch panel, the amp blew up. Full output current ( about 50 amps) passed along the thin wires and tiny PCB tracks from the input XLR to common ground and vaporised.
The other amp in the pair soon had burnt film caps in the Zobel network.
After some tedious repairs, I wound up adding 2.2 ohm resistors in series with the input ground and bypassing XLR pin 1 to chassis with a 35amp bridge rectifier wired as back to back diodes. I did both amps while I hade them in the workshop.
They both passed the "Navy test" after that.
Not much I could do about some idiot feeding 5Vrms from an audio gen into the input at 100kHz though.
Another example was loaned to some lunatic in the NT to do "magnetics " experiments with - I think he was trying to make a linear motor. He succeeded in blowing the output MOSFETs to bits.
The Navy later succeeded in doing much the same to one of theirs.
Hertz amps had no DC rail fuses, juts thermal breakers and zener diode current limiting on the MOSFETs.
No way, whatsoever were they SAFE to use in a SONAR test lab.
..... Phil
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** Did I ever tell you the story of the "Hertz" Mosfet amplifiers and the Australian Navy ?
Hertz amplifiers are ( were ?) large, heavy, lateral TO3 mosfet beasts intended for the live music and disco markets. The ones I saw did 800W comfortably into 4 ohms at any phase angle you liked.
The local agent for the product sold a couple to the RAN, for use with SONAR testing in the submarine labs here in Sydney. He had blithely assumed that this was a job the amps could do easily.
But he had not reckoned on the pure ham fistedness of the RAN technicians.
Live Rock music and disco material can be pretty hard on an audio amplifier, but not near as lethal as a half trained Navy tech with a signal generator that goes to 100kHz - PLUS no clue at all about how audio gear is normally treated and not treated.
The Hertz amps has response up to 100kHz but NO WAY was that allowable at full power !!! Even 20kHz is pushing it due to the limited power handling of the output ZOBEL.
PLUS, the Navy decided to build a "path panel" for all inputs and outputs - a flat aluminium plate covered in 1/4 inch jack sockets !!!!
The idea of using a sheet of insulation material never crossed their tiny minds OR that 1/4 inch jacks and plugs were not up to handling 800W safely - OR that jack plug tips connect signal to the ground circuit FIRST every time they are plugged in.
The Hertz amps did not last long, soon as the tip of a jack carrying the output from the amp touched the patch panel, the amp blew up. Full output current ( about 50 amps) passed along the thin wires and tiny PCB tracks from the input XLR to common ground and vaporised.
The other amp in the pair soon had burnt film caps in the Zobel network.
After some tedious repairs, I wound up adding 2.2 ohm resistors in series with the input ground and bypassing XLR pin 1 to chassis with a 35amp bridge rectifier wired as back to back diodes. I did both amps while I hade them in the workshop.
They both passed the "Navy test" after that.
Not much I could do about some idiot feeding 5Vrms from an audio gen into the input at 100kHz though.
Another example was loaned to some lunatic in the NT to do "magnetics " experiments with - I think he was trying to make a linear motor. He succeeded in blowing the output MOSFETs to bits.
The Navy later succeeded in doing much the same to one of theirs.
Hertz amps had no DC rail fuses, juts thermal breakers and zener diode current limiting on the MOSFETs.
No way, whatsoever were they SAFE to use in a SONAR test lab.
..... Phil