Recently bought some big PA Mosfet amps off a mate of mine.

[DrNomis_44]

Hey everyone,

I recently bought four big PA Mosfet amps plus two 8 Ohm 400 Watt Precision Audio PA speakers off a mate of mine, two of the PA amps I bought are a couple of Perreaux 6000C dual channel professional power mosfet amps which are rated to put out at least 300 watts, the other two amps are a couple of ARX SS1200VC professional series mosfet power amps, the two Perreaux amps seem to work fine, as do the two Precision Audio PA speakers, on the other hand, the two ARX amps need some work to get them working properly, but I always like a challenge.


Anyway, I'm also planning on retro-fitting Speakons to the four amps once the two ARX amps are working, the reason why I bought the amps was so I could build myself a small PA system.


I'll post some pics of the amps and speakers later on today so you can see what they look like, should make for an interesting diy/servicing project for next year.


Here you go, some pics:





A pic of the two ARX amps..




A pic of one of the Perreaux amps.




The two 8 Ohm 400 Watt Precision Audio PA speakers stacked one on top of the other.

[DrNomis_44]

I'll tell you something, my golly are those Perreaux amps heavy things to lug-around, I was just able to lift one up by one of it's carrying handles, I'm about 60kg in weight so I'm estimating that the Perreaux must weigh something like 50kg or so, the bulk of the weight is probably the dirty great big power transformer lurking inside.


All I need now is a good mixing desk that has stereo line-level outs, plus a stereo EQ, and some decent mics, and foldbacks, and I've got a full PA system.


One thing I will have to do is figure-out how to stop two furry felines from using my Precision Audio PA speakers as scratching-posts, without hurting or getting rid of said furry felines.

[Marcel]

Ahhh... the good old days....

Back in the '80's the big PA's of the day would have racks loaded with four of those sized amps.... I know the River Stage at Expo '88 had five racks of four per side(L+R) of Jands 920's. Each 920 weighed in at a little over 35kg..... And why would you have 20 x 450W RMS per channel amps per side you ask, well it was an active crossover 4 way system where they fed 16 three way Concord cabs a side slung in the air plus 4 Concord sub cabs on the ground each side, all plugged together using XLR's. I'll let you do the math on the total output of the system... Suffice to say that on the 5th night of Expo on Brisbane's South Bank they had Jimi Barnes playing, and they got noise complaints from Indooropilly the next day...lol

A side note for you Doc, they chose the 920's due to the way they go into clipping. Much softer clipping than the equivalent Perreaux or Yamaha's of the day. I had a bench full of all of those top high power amps and drove all to hard clipping with my Cro and N&D set attached, and a team of experts watching and making the final decisions on what amp would be chosen for the Expo contract..... I walked away with the thought in my head that a small sacrifice in THD at lower levels meant softer clipping at higher levels and hence less chance to kill speakers at full volume. And Hifi THD more often than not means hard and nasty speaker killing clipping..... Solid state and Tube guitar amps fit this thinking perfectly.....

Put your Perreaux into a dummy load and attach your Cro and see how it clips at high output.... A re-wired spiral resistance wire porcelain jug element in a cup of water makes an ideal 8 ohm 750W continuous dummy load. It will even quietly sing for you with over 200W into it and without a magnet at 1kHz, and make hot water for coffee at the same time ....lol

[DrNomis_44]

Ahhh... the good old days....

Back in the '80's the big PA's of the day would have racks loaded with four of those sized amps.... I know the River Stage at Expo '88 had five racks of four per side(L+R) of Jands 920's. Each 920 weighed in at a little over 35kg..... And why would you have 20 x 450W RMS per channel amps per side you ask, well it was an active crossover 4 way system where they fed 16 three way Concord cabs a side slung in the air plus 4 Concord sub cabs on the ground each side, all plugged together using XLR's. I'll let you do the math on the total output of the system... Suffice to say that on the 5th night of Expo on Brisbane's South Bank they had Jimi Barnes playing, and they got noise complaints from Indooropilly the next day...lol

A side note for you Doc, they chose the 920's due to the way they go into clipping. Much softer clipping than the equivalent Perreaux or Yamaha's of the day. I had a bench full of all of those top high power amps and drove all to hard clipping with my Cro and N&D set attached, and a team of experts watching and making the final decisions on what amp would be chosen for the Expo contract..... I walked away with the thought in my head that a small sacrifice in THD at lower levels meant softer clipping at higher levels and hence less chance to kill speakers at full volume. And Hifi THD more often than not means hard and nasty speaker killing clipping..... Solid state and Tube guitar amps fit this thinking perfectly.....

Put your Perreaux into a dummy load and attach your Cro and see how it clips at high output.... A re-wired spiral resistance wire porcelain jug element in a cup of water makes an ideal 8 ohm 750W continuous dummy load. It will even quietly sing for you with over 200W into it and without a magnet at 1kHz, and make hot water for coffee at the same time ....lol

I might actually do that one day, would be an interesting exercise, apparently Mosfets are supposed to behave in a similar way to tubes when driven into clipping, a mate of mine borrowed one of my Perreaux amps for a show in Humpty doo that he was sound-teching for, and he said that the Perreaux had a nice midrange to it.

If I wanted to, I could probably hook a Perreaux 6000C up to two Marshall quad boxes and then plug say a Marshall JMP-1 preamp into the line-ins of the Perreaux, and then I'd have a pretty good stereo 300 Watt guitar rig, makes my ears ring thinking about that.

Sounds like that Jimmy Barnes gig must have been a good one, must have been one of his post Cold Chisel bands.


Here's a bit of trivia for those who don't know what a Mosfet is:


Okay, the word "Mosfet" is short for Metal-Oxide Semiconductor Field Effect Transistor, think of it as a solid-state equivalent to a Tube.


Oh and thanks for reminding me about building a dummy load Marcel, cheers mate.


More trivia:


Pretty much any amp will go into clipping if you drive it hard enough, clipping is a form of distortion, when an amp is driven to the point where it starts clipping it can no longer reproduce the input signal faithfully because the output is limited by the power supply and the signal top and bottom is chopped-off, this form of clipping is known as hard-clipping and sounds very harsh to the human ear, the sound will have a "shattered-glass" quality to it, when a Tube amp is overdriven to the point where it starts to clip, it generates soft-clipping and the effect is less harsh or noticeable because the resulting waveform has more rounded corners to it, rather than the sharp corners generated by hard-clipping, the rounding of the corners sounds smoother.

[DrNomis_44]

Here's a pic of the insides of one of the ARX SS1200VC Power Mosfet amps:




If you look at the lower left-hand corner of the ARX amp, you'll notice a small circuit board, this board has the two amp volume control pots soldered directly to it, I am very strongly suspecting that a broken circuit-board track on the underside of the board is responsible for the amp not working, it powers up but no sound comes out no matter where you set the volume controls.

And yes, that huge hulking great piece of iron in the middle is the power transformer....no guesses as to why the amp is so heavy.


And here's a pic of the insides of one of the Perreaux 6000C power Mosfet amps, spot the huge power transformer:

[Fretworn]

I haven’t stumbled across Perreauxs for at least 15 years. They were practically indestructible, unless you are doing an outdoor gig on the Central Coast over the lake from one of the power stations........

[JimC]

Its a funny thing, there are more strange myths about clipping than just about any other subject in music technology... you can be sure if you hear someone mention both square waves and DC in the same or adjacent sentences then they have heard the myths and have been misinformed by them.
What really happens when a signal is clipped is that it artificially generates a lot of extra high frequency content, hence the harsh sound. Its exactly the same thing that a distortion pedal does, except that a typical distortion pedal has a tone circuits/filters in it as well to fine tune the end result.

[Marcel]

Its a funny thing, there are more strange myths about clipping than just about any other subject in music technology... you can be sure if you hear someone mention both square waves and DC in the same or adjacent sentences then they have heard the myths and have been misinformed by them.
What really happens when a signal is clipped is that it artificially generates a lot of extra high frequency content, hence the harsh sound. Its exactly the same thing that a distortion pedal does, except that a typical distortion pedal has a tone circuits/filters in it as well to fine tune the end result.

That is exactly what the "team of experts" were looking for when selecting the high powered amps for the Expo 88 contract. Most of the team had many years of experience in sound reinforcement at entertainment centers and very large venues around the country, and while there may have been some company allegiances in play there was consensus among the group as what was good and bad.

Interestingly by design the Jands 920 was flat to 12kHz and rolled off by 3dB at 20kHz, and 30dB at 50kHz.... Most of the others were flat to near 100kHz. It was truly weird putting 400W of super sonic audio (50kHz) into my jug element dummy loads in water and watching them boil in only a few minutes.

Also when back at 10kHz, 1kHz and 60Hz looking at the amplifiers output waveform when being overdriven by 6dB or 10dB and how the various amplifiers clipped. In some cases definite ringing could be clearly seen after the start of each sine wave cycle as it went into clipping indicating power supply inadequacies or basic amplifier design flaws. Some amps even had sharp kW sized spikes at the commencement of the flat top portion of the wave which came as a real surprise to many witnessing the tests. The Jands 920 was a simple all transistor amp that went into clipping much like any tube amp does with a gentle transition from sine wave to flat top with nil ringing.

For me it was just fun getting to blast the buggery and try and cause 'meltdowns' out of what was then the top range PA gear of the day, and still be able to walk away with my hearing intact. We boiled a lot of water.!!!

[DrNomis_44]

Pretty much all of these high-powered amps used negative feedback to reduce THD (Total Harmonic Distortion) to very low levels, and pretty much any amp has negative feedback included as standard, if the feedback was made positive instead of negative you would end up with a very powerful oscillator instead of an amplifier, hence the reason why it's very important to get the primary windings of the output transformer of a tube amp connected the correct way around for it to operate properly.

I've seen some power amp schematics where there's a network called a Zobel Network tacked on to the output where the speaker is connected, the Zobel Network acts as a snubber to stop high-frequency oscillations from occurring when the amp is driven into clipping.


I think my local Bunnings shop might have some of those electric kettle elements in stock, so next year I'll pay them a visit and see if I can buy enough to make at least two dummy loads.


For solid-state amps, driving an amp to the point where it is driven into clipping means a risk of burning-out the voice coil of a speaker, since when the amp is clipping on the top or bottom of the signal waveform, the power transistors will be in a state where they are fully conducting, which means that pure DC is being applied to the voice coil, usually at several amps if it's a high-power amplifier, it doesn't take long for the voice coil to burn-out and become open-circuited.

[Juddyman]

Ahhh... the good old days....

Back in the '80's the big PA's of the day would have racks loaded with four of those sized amps.... I know the River Stage at Expo '88 had five racks of four per side(L+R) of Jands 920's. Each 920 weighed in at a little over 35kg..... And why would you have 20 x 450W RMS per channel amps per side you ask, well it was an active crossover 4 way system where they fed 16 three way Concord cabs a side slung in the air plus 4 Concord sub cabs on the ground each side, all plugged together using XLR's. I'll let you do the math on the total output of the system... Suffice to say that on the 5th night of Expo on Brisbane's South Bank they had Jimi Barnes playing, and they got noise complaints from Indooropilly the next day...lol

A side note for you Doc, they chose the 920's due to the way they go into clipping. Much softer clipping than the equivalent Perreaux or Yamaha's of the day. I had a bench full of all of those top high power amps and drove all to hard clipping with my Cro and N&D set attached, and a team of experts watching and making the final decisions on what amp would be chosen for the Expo contract..... I walked away with the thought in my head that a small sacrifice in THD at lower levels meant softer clipping at higher levels and hence less chance to kill speakers at full volume. And Hifi THD more often than not means hard and nasty speaker killing clipping..... Solid state and Tube guitar amps fit this thinking perfectly.....

Put your Perreaux into a dummy load and attach your Cro and see how it clips at high output.... A re-wired spiral resistance wire porcelain jug element in a cup of water makes an ideal 8 ohm 750W continuous dummy load. It will even quietly sing for you with over 200W into it and without a magnet at 1kHz, and make hot water for coffee at the same time ....lol

Hey Marcel, would you know the specific THD of the Jands 920 and it’s specs? I’ve got two of these and was wanting to use them in my home cinema setup. My speakers are ar9’s and rated at 4ohms and need heaps of power! Thanks heaps Marcel!

Cheers
Hoa